POSIVA OY

Working Report 2000-41

Updated compartment model for near-field transport

in a KBS-3 type repository

Timo Vieno

Henrik Nordman

September 2000

Toolonkatu 4. FIN-00100 HELSINKI. FINLAND

Tel. +358-9-2280 30

Fax +358-9-2280 3719

-trr ENERGY Research organisation and address VTI Energy, Nuclear Energy P.O. Box 1604 FIN-02044 VTI, FINLAND

Project manager

Timo Vieno

Diary code

ENE4-34T -2000

Project title and reference code

Kaytetyn polttoaineen seka voimalaitos- ja purku­jatteen loppusijoituksen turvallisuustutkimukset II/2000 45POSN A002 NOSU00317

Report title and author( s)

Customer Posiva Oy ToolOnkatu 4 00100 HELSINKI

C~ntact per~~". _ wpl l. k~/M~' A1mo HautoJarvi /

Order reference

9624/00/ AJH

Report identification & Pages ENE4/27/00 27 p.

Date 6.9. 2000

UPDATED COMPARTMENT MODEL FOR NEAR-FIELD TRANSPORT IN A KBS-3 TYPE REPOSITORY

Timo Vieno & Henrik Nordman

Summary The new compartment model unifies the modelling of radionuclide transport in the buffer and backfill in the cases of a small hole in the canister and of a severely damaged canister in a similar manner as in SKB 's SR 97 safety assessment. The modelling artefacts caused by the overly conservative presentation of the near-field in the small hole case in the TILA-99 safety assessment are erased. Simplifications are unavoidable when the 3-dimensional transport situation is modelled with a compartment model. The most significant uncertainties are, however, not related to the transport within the buffer and backfill, but rather to the boundary conditions at the interfaces with the canister and rock, and to the groundwater flow conditions in the backfill.

TILA-99 and SR 97 employ similar approximations to derive transfer coefficients from the canister into the bentonite and from the bentonite into rock fissures intersecting the deposition hole. There are some minor differences where TILA-99 uses more conservative approximations than SR 97.

The effects of the model modifications on release rates of radionuclides as compared with TILA-99 are nuclide- and scenario-specific. The overall release rates into the biosphere and dose rates are affected significantly only in scenarios with a very high flow and transport of groundwater in the near-field and geosphere (vhflow-scenarios of TILA-99). In the small hole (SH) scenarios the new model results in equal or lower maximum release rates. The reduction is caused by the delaying and diluting effects of the larger amount of bentonite taken into account in the new model. The effect is most significant for nuclides with a relatively short half-life. In the case of a severely damaged (DC) canister, the maximum release rates of solubility-limited nuclides and anions are decreased, but those of non-solubility-limited nuclides with a moderate or high sorption in the bentonite are increased. The latter effect is caused by the smaller amount of bentonite in the short release route into the rock surrounding the deposition hole. The overall effect is an increase, at most by a factor of two, in the maximum dose rates in the damaged canister scenarios with a very high flow of groundwater.

Distribution

Posiva (the report is intended to be published in Posiva's Working Report series)

Principal author or Project manager

Timo Vieno ?i~; 16-· Senior research scientist Approved by ·

c::;_ :.~. t/~., sepp{t[J{r Research Manager, Nuclear Energy

R~U-Heikki Raiko Group Manager, Nuclear Waste Management Availability statement

Confidential

The use of the name of the Technical Research Centre of Finland (VTT) in advertising or publication in part of this report is only permissible by written authorisation from the Technical Research Centre of Finland

Working Report 2000-41

Updated compartment model for near-field transport

in a KBS-3 type repository

Timo Vieno

Henrik Nordman

VTT Energy

September 2000

Working Reports contain information on work in progress

or pending completion.

The conclusions and viewpoints presented in the report

are those of author(s) and do not necessarily

coincide with those of Posiva.

------------------------------

UPDATED COMPARTMENT MODEL FOR NEAR-FIELD TRANSPORT IN A KBS-3 TYPE REPOSITORY

ABSTRACT

The new compartment model unifies the modelling of radionuclide transport in the buffer and backfill in the cases of a small hole in the canister and of a severely damaged canister in a similar manner as in SKB' s SR 97 safety assessment. The modelling artefacts caused by the overly conservative presentation of the near-field in the small hole case in the TILA-99 safety assessment are erased. Simplifications are unavoidable when the 3-dimensional transport situation is modelled with a compartment model. The most significant uncertainties are, however, not related to the transport within the buffer and backfill, but rather to the boundary conditions at the interfaces with the canister and rock, and to the groundwater flow conditions in the backfill.

TILA-99 and SR 97 employ similar approximations to derive transfer coefficients from the canister into the bentonite and from the bentonite into rock fissures intersecting the deposition hole. There are some minor differences where TILA-99 uses more conservative approximations than SR 97.

The effects of the model modifications on release rates of radionuclides as compared with TILA-99 are nuclide- and scenario-specific. The overall release rates into the biosphere and dose rates are affected significantly only in scenarios with a very high flow and transport of groundwater in the near-field and geosphere (vhflow-scenarios of TILA-99). In the small hole (SH) scenarios the new model results in equal or lower maximum release rates. The reduction is caused by the delaying and diluting effects of the larger amount of bentonite taken into account in the new model. The effect is most significant for nuclides with a relatively short half-life. In the case of a severely damaged (DC) canister, the maximum release rates of solubility-limited nuclides and anions are decreased, but those of non-solubility-limited nuclides with a moderate or high sorption in the bentonite are increased. The latter effect is caused by the smaller amount of bentonite in the short release route into the rock surrounding the deposition hole. The overall effect is an increase, at most by a factor of two, in the maximum dose rates in the damaged canister scenarios with a very high flow of groundwater.

Keywords: near-field, transport, compartment, model, spent fuel, repository, KBS-3

UUDISTETTU KOMPARTMENTTIMALLI LAHIALUEEN KULKEUTUMIS­ANAL YYSEIHIN KBS-3 -TYYPPISESSA LOPPUSIJOITUSTILASSA

TIIVISTELMA

Lahialueen uudistettu kompartmenttimalli yhdenmukaistaa radionuklidien kulkeutumi­sen mallinnuksen tapauksissa, joissa kapselissa on pieni reika tai suurempi vaurio. Mallia on otettu SKB:n SR 97 turvallisuusanalyysista. Uudella mallilla valtetaan TILA-99:n pienen reHin mallinnuksen ylikonservatiivisuudesta aiheutuneet pienet mallinnusvaaristymat. Yksinkertaistuksia joudutaan aina tekemaan, kun 3-ulotteista kulkeutumistapausta mallinnetaan kompartmenttimallilla. Merkittavimmat epavarmuu­det eivat kuitenkaan liity kulkeutumiseen bentoniitissa ja tunnelin tayteaineessa, vaan reunaehtoihin tayteaineiden rajapinnoilla kapselin ja kallion kanssa seka pohjaveden virtaukseen tunnelin tayteaineessa.

TILA-99:ssa ja SR 97:ssa kaytetaan samanlaisia approksimaatiota johdettaessa radionuklidien siirrostekijoita kapselista bentoniittiin ja bentoniitista kallioon. TILA-99:n approksimaatiot ovat erain osin konservatiivisempia kuin SR 97:ssa.

Muutosten vaikutukset radionuklidien vapautumisnopeuksiin vaihtelevat nuklideittain ja skenaarioittain. V apautumisnopeuksiin biosfaariin j a annosnopeuksiin lahialuemallin­nuksessa tehdyt muutokset vaikuttavat merkittavasti vain skenaarioissa, joissa oletetaan voimakas pohjaveden virtaus seka lahialueella etta kallioperassa (TILA-99:n vhflow­skenaariot). Pienen reHin SH-skenaarioissa uusi lahialuemalli tuottaa yhta suuria tai pienempia vapautumisnopeuksia kuin TILA-99:ssa. Vapautumisnopeuksien pienenemi­nen johtuu suuremman huomioon otetun bentoniittimaaran viivastavasta ja laimentavas­ta vaikutuksesta, jolla on merkitysta etenkin lyhytikaisten radionuklidien vapautumis­nopeuksiin. Pahasti vaurioituneen kapselin DC-skenaarioissa liukoisuusrajoitteisten nuklidien ja anioinien vapautumisnopeudet laskevat, mutta sellaisten ei-liukoisuus­rajoitteisten nuklidien, jotka sorboituvat ainakin kohtuullisesti bentoniitissa, vapau­tumisnopeudet kasvavat. Viimeksi mainittu johtuu pienemmasta bentoniittimaarasta lyhimmalla vapautumisreitilla kallioon. Yhteisvaikutuksena on enimmaisannosnopeuk­sien kasvu, enimmillaan kaksinkertaiseksi, sellaisissa pahasti vaurioituneen kapseleiden skenaarioissa, joissa oletetaan voimakas pohjaveden virtaus.

A vainsanat: Lahialue, kulkeutuminen, kompartmentti, malli, kaytetty polttoaine, loppusijoitustila, KBS-3

--------------------------· --

1

TABLE OF CONTENTS

Page

Abstract

Tiivistelma

1 BACKGROUND AND SCOPE ....................................................................... 1

2 COMPARTMENT MODEL .............................................................................. 6

3 TRANSFER COEFFICIENTS: TILA-99 vs. SR 97 .......................................... 9 3.1 From canister into bentonite . . . .. . . . . .. . . . .. .. .. . . . . . . .. .. . . . . .. . . . . .. . .. . . . . . . . . .. .. . . . . . . . . . 9 3.2 From bentonite into rock fissures ........................................................... 11

4 BEHAVIOUR OF THE MODEL: STABLE ELEMENTS ................................... 15

5 RADIONUCLIDE TRANSPORT ...................................................................... 21

6 DISCUSSION ................................................................................................. 25

REFERENCES . .... .. .. ..... ... .. .. .. .... ..... .... .... ...... .. .......... ..... .. ...... ........ .. .... ................. ... 26

2

1 BACKGROUND AND SCOPE

Near-field transport modelling in TILA-99

In TILA-96 and TILA-99 safety assessments (Vieno & Nordman 1996, 1999) near-field transport of radionuclides was simulated with the REPCOM model (Nordman & Vieno 1994). For the copper-iron canister, two cases were analyses: a disappearing canister, and a small (5 mm2

) or large (1 cm2) hole through the canister wall. The compartment

models used in these cases are illustrated in Figure 1-1.

Tunnel Section 1

r----------- -~

I ( Tunnel Section N ) I I I

_ _L _____ ~ I I , I I ' I I ', I L - - --- - - - - --".- - .J

QTDZI '1. (QTDzN)

- Canister interior

liWid Bentonite

c=J Tunnel backfill

- Canister interior 1•• Bentonite

c::J Hole

Figure 1-1. T/LA-99 compartment models for the "disappearing canister" (left) and "hole in canister" (right) cases.

In the disappearing canister case the model consists of five main blocks: • water volume in the canister interior • bentonite buffer around the canister • bentonite buffer above canister • backfill in the top of the deposition hole • backfill in the tunnel. In the TILA-99 modelling, the bentonite buffer around the canister was divided into several compartments in the radial direction and the bentonite above the canister in the vertical direction. The three other main blocks were modelled as one compartment each. The compartment model is basically one-dimensional, therefore the cylinder shell compartments around the canister and the first slab compartment above the canister interact only via the water compartment representing the canister interior.

In the disappearing canister case, there are three escape routes from the near-field model into the geosphere: • from the bentonite around the canister into the rock fissures intersecting the

deposition hole (QF in Figure 1-1) • from the backfill in the top of the deposition hole into the excavation damaged

rock zone (EDZ) below the tunnel floor ( Qoz) • from the tunnel into the rock or EDZ (QTozt). The releases via the three routes are summed up to form the total release rate from the near-field into the geosphere. Transport by advection and diffusion along the tunnel

3

could also be modelled, but in the reference scenario it is conservatively assumed that radionuclides are released from the first tunnel compartment directly into the geosphere (QTn = 0).

The transfer from the outermost parts of the near-field into the geosphere is modelled by means of transfer coefficients. The outermost shell compartment in the bentonite around the canister, the backfill in the top of the deposition hole, and the tunnel section above the deposition hole are modelled as mixing tanks and the transfer coefficients associated to them represent the equivalent flow rates of groundwater through them. The derivation of the transfer coefficients is presented in detail in Section 11.6 of TILA-99 (Vieno & Nordman 1999) and will be further discussed in Section 3.2 of the present report.

If there is just a small hole in the canister, the mass flow rates in the near-field are efficiently limited by the small size of the hole. In this case, the buffer does not affect significantly steady state mass flow rates but functions mainly as a delay component. The transport model was thus simplified to include only three main components (Figure 1-1): • water volume in the canister interior • hole through the canister wall • bentonite between the canister and rock. Of the bentonite, only a hemisphere with a radius of 35 cm (thickness of the bentonite between the canister and rock) is taken into consideration. The modelled bentonite volume comprises thus less than 1% of the total volume of the bentonite in the deposition hole. The bentonite is divided into hemisphere shell compartments, the discretisation being most dense at the mouth of the hole. The transfer from the outermost compartment in the bentonite into the rock is modelled by means of the transfer coefficient Qp, which has the same value as in the disappearing canister case. This means that in the hole scenario, all groundwater flow in the rock around the deposition hole is assumed to take place in a single fissure just opposite the hole in the canister. The modelling assumptions and compartment data are presented in detail in TILA-99.

Shortcomings of TILA-99 modelling

The TILA-99 approach where the "disappearing canister" and "hole in canister" cases are modelled with different compartment models involves some shortcomings. The disappearance of the copper canister has been considered as unrealistic and overly conservative (Ruokola 2000). On the other hand, in cases where the canister wall is badly damaged but, of course, not totally absent, the model involves a too large volume of bentonite in a direct contact with the canister interior, which may be a non­conservative feature for some sorbing nuclides. In the "hole in canister" case, the small amount of bentonite taken into account and the use of the QF transfer coefficient at the outer surface of the hemisphere leads to an overly conservative transient behaviour. The modelling artefacts are discussed in more detail in TILA-96 and TILA-99.

Near-field transport modelling in SR 97

SKB too has applied a compartment model, COMP23, to simulate near-field transport of radionuclides in the KBS-3 deposition hole (Romero et al. 1995, 1999, Lindgren &

4

Lindstrom 1999). The division of the near-field into blocks of compartments in the SR 97 safety assessment is shown in Figure 1-2. The same model is used in the case representing a small initial hole in the copper canister and in the case where the hole through the canister wall is assumed to be so large that it no more limits the transport of radionuclides. SKB' s model consists of eight main blocks: water volume in the canister interior (B 1), bentonite around the canister at the level where the hole is located (B3), bentonite around the canister elsewhere in the deposition hole (B4 ), bentonite above (B5) and below the canister (B8), backfill in the top of the deposition hole (B6) and in the tunnel (B7), and rock between the deposition hole and a nearby fracture zone (B9). The bentonite blocks B3, B4 and B5 are subdivided into several compartments as indicated in Figure 1-2.

Fracture Zone

D Water

[Jj Bentonite

~Rock

11 Crushed rock -bentonite

I I I I I I I I I 87 I I I I I I I

86 -+Q2 J3

--------------85

--------------

GD 11111

r+Q1 11111

.::.:.:.:

83 1

-84 ---

88

89

' Q4

Figure 1-2. Division of the near-field into blocks of compartments in the SR 97 analysis ( Lindgren & Lindstrom 1999 ).

5

The transfer from the canister interior into the bentonite (from B 1 to B3), from the bentonite into rock fissures intersecting the deposition hole (Q1), from the top of the deposition hole (Q2) and from the tunnel (Q3) into the excavation damaged zone, and from the deposition hole through a block of intact rock into a fracture zone (Q4) is modelled by means of transfer coefficients. The releases via these routes are summed up to form the total release rate from the near-field into the geosphere.

SKB' s analysis differs from TILA-99 in that it does not assume any significant flow of groundwater through the backfill in the top of the deposition hole and tunnel. Consequently, the release of radionuclides from the backfill blocks is limited by the diffusion resistances and the boundary layer resistance between the stagnant water in the backfill and the water flowing in the rock fractures. In TILA-99 the release from the backfill blocks is governed by the groundwater flow rates through them. Moreno (2000) has performed sensitivity analyses with the SR 97 model and has found out that the maximum release rate is increased at most by a factor of two (for Cs-135 with the high diffusivity in the bentonite) when a high ground water flow rate ( 1 m3 /year) through the tunnel backfill is assumed.

Aims and scope of the present study

The main aim of the present study is to explore whether the near-field transport modelling in the cases of a small, initial hole and of a canister damaged in a more severe way could be unified in a similar manner as in SKB' s model. In addition, Chapter 3 will include an intercomparison of the transfer coefficients used to model the transfer from the canister interior through the small hole into the bentonite, and from the bentonite into the rock fissures intersecting the deposition hole.

The behaviour of the new model will be illustrated by means of pulse and solubility­limited inputs of a set of stable elements in a similar way as in Appendix I of TILA-99. Finally, some of the scenarios of TILA-99 will be recalculated with the new model.

The present study is limited basically to the geometrical features of transport modelling in the buffer and backfill. It deals with water-borne transport of radionuclides, and with the following processes only: chain decay of radionuclides; solubility-limits which are applied only in the water volume representing the canister interior; sorption, diffusion and advection in the buffer and backfill; and the transfer coefficients as discussed above.

6

2 COMPARTMENT MODEL

The new compartment model is shown in Figure 2-1 and the compartment data are presented in Table 2-1. The new model differs from the DC model of TILA-99 only as concerns modelling of the transfer from the canister interior into the bentonite and modelling of the bentonite buffer itself.

r----------- -~

Tunnel Section 1 I ( Tunnel Section N ) I I I

_l _____ ~ I

I " I I " I I "" I l---------~-- ...J

~ (QTDZN)

Canister interior

Bentonite B

Tunnel backfill

Figure 2-1. The new compartment model.

The transfer from the canister interior through a hole into the innermost compartment in the bentonite block B 1 is modelled by means of an analytical transfer coefficient. The transfer coefficient takes into account the two "hole effects": diffusion in the hole through the canister wall and diffusion into the bentonite from the mouth of the small hole. The analytical equation for the transfer coefficient, which will be presented in Chapter 3, leads to equivalent transfer with the compartment model used in the SH case of TILA-99. In the case of a severely damaged canister (DC), the water inside the canister is assumed to be in contact with the innermost compartment of the bentonite block B1.

The release from the canister is transferred into the innermost compartment of a 35 cm high cylindrical bentonite block (B1) around the top of the canister. Also the release from the bentonite into rock fissures is concentrated in the same block by applying the transfer coefficient Qp in the outermost compartment of B 1. The radial diffusion through block B 1 is modelled by means of a large number of cylinder shell compartments.

7

Table 2-1. Compartment data.

Canister interior • fuel: Olkiluoto 2.14 tU, Loviisa 1.44 tU • water volume: Olkiluoto 0.700 m3

, Loviisa 0.425 m3

• number of compartments: 1 • transfer into the innermost compartment of bentonite block B1

small hole (SH): transfer coefficient (see Chapter 3} damaged canister (DC): contact

Bentonite block B1 • height: 0.35 m • inner radius: 0.53 m • outer radius: 0.88 m • number of compartments (in the radial direction): 22 • diffusion length to blocks B2 and B3: 0.175 m • transfer into the geosphere from the outermost compartment: QF

Bentonite block B2 • height: 0.01 m • area: 2.4 m2

• number of compartments: 1 • diffusion contacts with the first compartment of B5 and all compartments of B1

Bentonite block B3 • height: 0.01 m • area: 1 .55 m2

• number of compartments: 1 • diffusion contacts with the first compartment of B4 and all compartments of B1

Bentonite block B4 • height: Olkiluoto 4.44 m, Loviisa 3.24 m • area: 1 .55 m2

• number of compartments (in the vertical direction): 19

Bentonite block B5 • height: 1.49 m • area: 2.4 m2

• number of compartments (in the vertical direction): 9

Backfill in the top of the deposition hole • height: 1.0 m • area: 2.4 m2

• number of compartments: 1 • transfer into the geosphere: Ooz

Backfill in the tunnel • number of tunnel sections modelled: N = 1 in the present study • volume: 1 00 m3

• number of compartments: 1 • no diffusion resistance in the tunnel, i.e. the diffusion distance between the backfill

compartments in the deposition hole and tunnel is 0.5 m and diffusion area is 2.4 m2

• transfer into the geosphere: QTDZ1

• transfer into the next tunnel section: On1 = 0 in the present study

8

The other parts of the deposition hole and the tunnel section above are modelled with compartments constructed basically in the vertical direction. They are coupled with the radial compartments of B 1 by means of thin interaction compartments B2 and B3 above and below of block B 1. B2 and B3 provide an alternative route from the innermost parts of B 1 to its outermost parts. However, the diffusion distance along this circuitous route is similar as in the direct radial route, because radionuclides must diffuse in the vertical direction and the height of block B 1 is equal to its thickness (35 cm).

Block B4 represents the bentonite in the lower part of the deposition hole. This dead­end block with no escape points into the geosphere acts as a reversible store for radio­nuclides diffusing from upper parts of the bentonite buffer. The bentonite block B5 above the canister provides a release route into the tunnel backfill and further into the geosphere as in TILA-99. Blocks B4 and B5 are divided into several compartments, the discretisation being most dense in the vicinity of blocks B 1, B2 and B3.

In the case of the small hole, there is an obviously erroneous feature in the model: The release from the small hole is distributed into a compartment, which goes all around the canister. As a consequence the concentration in the bentonite outside the hole is reduced. The large concentration gradient increases the release rate from the canister into the bentonite. In the bentonite the lowered concentration is counterbalanced by the increased diffusion area, but obviously the overall effect is a reduced transport in the bentonite in the transient phase. The final release rate into the geosphere is, however, governed by the transient coefficient Qp at the outer surface of bentonite and the concentration in the outermost compartment of the bentonite. Qp is usually based on the estimated flow rates around the whole deposition hole, and not on the local flow rate in the rock just opposite the hole in the canister. The basic reason for these difficulties is, of course, that simplifications need to be made when the 3-dimensional case is modelled with the !-dimensional (or, at best, quasi-2-dimensional) compartment model.

9

3 TRANSFER COEFFICIENTS: TILA-99 vs. SR 97

In this chapter we present the equations used to derive the transfer coefficients from the canister into the bentonite and from the bentonite into the rock fissures intersecting the deposition hole, and compare these equations with those used in SR 97.

3.1 From canister into bentonite

In the case of a severely damaged canister the water inside the canister is assumed to be in a direct contact with the first compartment in the bentonite block B 1. In the case of a small hole, the transport resistance from the canister interior into the first compartment of B 1 is provided by two factors: 1) diffusion through the hole in the canister, 2) diffusion from the outer end or "mouth" of the small hole into the bentonite.

The mass flow rate by diffusion through the hole can be calculated straightforwardly

where

Ah is the area of the hole (m2)

Dh is the effective diffusion coefficient in the material filling the hole (m2/s) Llx is the length of the hole (m) Ci is the water-phase concentration inside the canister (Bq/m3

)

C 1 is the water-phase concentration at the outer end of the hole (Bq/m3)

Qh is the resulting equivalent flow rate through the hole (m3/s).

(3-1)

In the small hole scenario of TILA-99, the area of the hole was assumed to be 5 mm2

and the length 5 cm. The hole was assumed to be filled with water (Dh = Dw = 2·10-9 m2/s), except for cations in the non-saline water scenarios in which case the effective diffusion coefficient in bentonite was higher than that in free water and, therefore, the hole was assumed to be filled with bentonite for cations (Dh = De = 5·10-9 m2/s).

From the outer end of the hole diffusing species are spread out spherically into the bentonite. At the distance of r the mass flow rate is

2 dC Jb =-2·1!·r ·D ·­

e dr

where

De is the effective diffusion coefficient in the bentonite (m2/s) C is the water-phase concentration in the bentonite (Bq/m3

).

(3-2)

Equation 3-2 can be rewritten as

dr J ·--=-D ·dC b

2 2 e

tr· r

10

(3-3)

Due to conservation of mass, the mass flow rate Jb is in a steady state constant in the hemisphere. By integrating the left side from the inner radius (radius of the hole) rt to r2 and the right side from the concentration at the mouth of the hole C 1 to C2 we get

(3-4)

where

r1 is the radius of the hole in the canister (1.26 mm in the TILA-99 SH scenarios) r2 is the distance from the hole into bentonite (m), for example to the centre point of

the first compartment of the block B 1 C1 is the water-phase concentration at the mouth of the hole (Bq/m3

)

C2 is the water-phase concentration at the distance of r2 in the bentonite (Bq/m3).

Typically r2 >> r1 and thus Equation 3-4 can be written as

(3-5)

where Qm is the resulting equivalent flow rate (m3 /s) from the mouth of the hole into the bentonite.

The mass transport resistances are combined according to the "resistors in series" principle to obtain the mass flow rate from the canister interior into the first compartment in the bentonite block B 1.

(3-6)

where

Ci is the water-phase concentration inside the canister (Bq/m3)

Cm1 is the water-phase concentration in the first compartment of the bentonite block B 1 (Bq/m3

) (note: corresponds to C2 in Equations 3-4 and 3-5).

In TILA-99 the transport through the hole (Qh) was calculated as presented above in Equation 3-1. At the mouth of the hole the transport was calculated by means of a very dense compartment discretisation, and thus, the analytical expression for Qm was not used in the TILA-99 calculations.

One should note that outside the canister, most of the resistance to diffusion is concentrated close the mouth of the hole where the diffusion area is still very low. The

11

mass flow resistance may, therefore, decrease significantly if the diffusivity in the bentonite close to the hole is increased, for example, due to effects of corrosion products. In the above equations, this would correspond to an increase of r1 in Equations 3-4 and 3-5.

SR97

SR 97 (Lindgren & Lindstrom 1999) employs similar transfer coefficients as presented above. There is, however, a difference: In SR 97 the equivalent flow rate Qm is by a

factor of J2 lower than in TILA-99. The difference is due to that in the SR 97 derivation of the transfer coefficient the surface area of the first hemisphere is set equal to the cross-sectional area of the hole, whereas in TILA-99 the radius of the hemisphere is equal to the radius of the hole. Intuitively one may say that the reality is obviously somewhere between these two, SR 97 being closer to the reality, but maybe slightly on the non-conservative side.

3.2 From bentonite into rock fissures

In TILA-99 the transfer of radionuclides from the outermost compartment in the bentonite into the geosphere is modelled by means a lumped transfer coefficient (equivalent flow rate). The derivation of this lumped parameter takes into account the estimated rate and characteristics of the groundwater flow in the rock fissures around the deposition hole. The derivation is based on the phenomena and mathematical expressions discussed below, but these equations are not explicitly included in the REPCOM model. To a large extent, also SR 97 employs a similar approach of lumped parameters.

The transport resistance from the bentonite into the rock fissures is provided by two factors: 1) transport from the bentonite into the narrow mouth of the fissures, 2) boundary layer (film) resistance between the stagnant water in the bentonite and the groundwater flowing in the fissure.

Transport into the mouth of the fracture

In a similar way as in the transport from the hole into the bentonite, most of the resistance to the diffusive transport into a narrow fracture intersecting the deposition hole is located nearest to the mouth of the fracture where the diffusion area is decreased. Figure 3-1 shows an example of the concentration profiles based on steady state solutions derived by Neretnieks (1986).

12

Distance from rock surface (m)

0.35

0.3

0.2

0.1

0

-------C=l.O--------

b=o at fissure mouth

Figure 3-1. Concentration profiles in the bentonite at the mouth of a fracture intersecting the deposition hole (based on Figure 5 of Neretnieks ( 1986)).

COMP23 employs for the transport resistance an approximation derived by Neretnieks (1986):

(3-7)

where

o is the aperture of the fracture (m) De is the effective diffusion coefficient in the bentonite (m2/s) Ar is the diffusion area (m2

) which is set equal to the area of the fracture (27tr2·0, where r2 is the radius of the deposition hole).

Fx,o/0 is a factor.

Neretnieks ( 1986) evaluates the value of the Fx,o/0 factor with variations in the input parameters. It is found out that that a good approximation to the exact solution is provided by

Fx,o/8 = 1 -1.35 ·log( 8/ a) + 1.6 ·log (dl a) (3-8)

in the regime 10-6 < o/a < 10-1 and 0.03 < d/a < 1

where

a is the height of the compartment in contact with the fracture (m) d is the thickness of the bentonite (m).

13

In COMP23 the above transport resistance is presented by a means of a bentonite plug at the mouth of the fracture. The plug has a length corresponding to (Fx,o/0)0 and an area corresponding to 27tr·O.

TILA-99

TILA-99 employs for Qr an approximation based on simplified modelling by Hautojarvi (1990):

Q - 1!2 . ( rl + r2) . De f- In(~/ hv)

and

where

De is the effective diffusion coefficient in the bentonite (m2/s) 2bv is the aperture of the fracture (m) r1 is the radius of the canister (m) r2 is the radius of the deposition hole (m)

(3-9)

S is the fracture spacing (m) in the rock (or the height of the compartment in contact with a fracture).

Intercomparison

With the values of r1 = 0.53 m, r2 = 0.88 m, d = r2 - r1 = 0.35 m, S = a = 0.35 m, and De= 1·10-10 m2/s, the SR 97 approximation (Equation 3-7) results for a fracture aperture of 0 = 2bv = 50 J..Lm in an equivalent flow rate of Qp = 2.8 litres/year, whereas the TILA-99 approximation (Equation 3-9) gives 4.7 litres/year. For a fracture aperture of 500 J..Lm, the SR 97 approximation results in an equivalent flow rate of 3.6 litres/year, whereas the TILA-99 approximation gives 6.3 litres/year. In our example dla = 1, which according to Neretnieks (1986) means that the SR 97 approximation overestimates the equivalent flow rate at most by 60% as compared to the exact solution of Neretnieks (1986).

The TILA-99 approximation gives somewhat lower values for the transport resistance (higher values for the equivalent flow rate) from the bentonite into the mouth of the fracture. However, only with rather high groundwater flow velocities in the fracture this resistance becomes significant as compared to the other resistance in the system, the boundary layer resistance between the stagnant water in the bentonite and the groundwater flowing in the fracture (see Table 11-16 of TILA-99). The relative importance of the two resistances in SR 97 will be illustrated below.

Transfer from bentonite into the groundwater flowing in fractures

The mass transfer from the stagnant water in the bentonite into the groundwater flowing in a fracture intersecting the deposition hole is limited by the boundary layer (film) resistance (Neretnieks 1982, Hillebrand 1985, Nilsson et al. 1991). SR 97 and TILA-99 employ the same approximation for this resistance. With the TILA-99 notations, the equivalent flow rate is

14

(3-10)

Dw is the diffusion coefficient in water (m2/s) u is the velocity of water in the fracture (m/s).

(Note: There is a conflict between Equation 7.2.1 of Andersson (1999) and the (unnumbered) equation on page 8 of Lindgren & Lindstrom (1999). In the latter one the equivalent flow rate is by a factor two lower than in the former one, which corresponds to Equation 3-10 above, and is, in our opinion, the correct one. In the COMP23 calculations, Lindgren & Lindstrom (1999) do, however, not use this equation, but a lumped parameter as explained below.)

Lumped parameters

In COMP23, the transport resistance Rr is presented "explicitly" by a means of a bentonite plug at the mouth of the fracture, as explained above. In SR 97, the equivalent flow rate Qr through the plug has a value of 3.5 litres/year (based on Table 3-14 of Lindgren & Lindstrom 1999) if De= 1·10-10 m2/s. The boundary layer resistance is represented in COMP 23 by means of a lumped parameter Qb1 = Aq112 where q is the groundwater flow rate (m3/(m2/year)) in the rock around the deposition hole. In SR 97, the equivalent flow rate Qb1 has at Aspo a reasonable value of 1.3 litres/year and a pessimistic value of 180 litres/year, at Finnsjon the reasonable and pessimistic values are, respectively, 0.95 and 79litres/year, and at Gidea 0.19 and 7.9 litres/year (based on Table 3-14 of Lindgren & Lindstrom 1999). As the two transport resistances are in series, the resulting overall equivalent flow rate from the outermost bentonite compartment into the rock ranges from 0.18 litres/year (Gidea reasonable) to 3.4 litres/year (Aspo pessimistic). The derivation of the parameters for the transfer coefficients is discussed in more detail in the SR 97 data (Andersson 1999) and calculations (Lindgren & Lindstrom 1999) reports.

TILA-99 combines the two transport resistances into a lumped parameter according to the "resistors in series" principle. The values of the two resistances and their combinations are calculated over a wide range of parameter values based on the ground­water flow and transport analysis (Table 11-16 of TILA-99). Based on these analyses, the combined transfer coefficient from the outmost compartment of the bentonite into the rock is then selected for the various site-specific scenarios as presented in Section 11.6 of TILA-99. In the TILA-99 scenarios the equivalent flow rate ranges from 0.2 litres/year (ns50- and sal50-scenarios) to 5 litres/year (vhflowns-scenarios).

In the following chapters, the reanalyses of the TILA-99 cases with the new compartment model are made employing the TILA-99 transfer coefficients from the near-field into the geosphere. For the transport from the bentonite into the rock this implies that all of the groundwater flow around the deposition hole is now concentrated around the 35 cm high bentonite block B 1 instead of the 5 metres in the TILA-99 analyses.

15

4 BEHAVIOUR OF THE MODEL: STABLE ELEMENTS

The behaviour of the near-field transport system is illustrated by means of four stable elements (Table 4-1) representing a non-sorbing (N-S) and sorbing (S) neutral species, a non-sorbing anion (A) and a sorbing cation (C), which were used also in Appendix I of TILA-99. The analyses are made for the Olkiluoto canister in the cases of a small hole (5 mm2

) through the canister wall (SH) and a severely damaged canister (DC) in four scenarios representing non-saline and saline groundwater conditions, and the minimum and maximum values of the near-field flow rates and transfer coefficients of TILA-99 (Table 4-2).

Table 4-1. Stable elements used to illustrate the behaviour of the near-field transport system.

N-S s A c

Speciation Neutral Neutral Anion Cation

Bentonite I non-saline • ~ (m3/kg) 0 0.3 0 0.1 • £ 0.43 0.43 0.05 0.43

• De (m21s) 1·1 o-10 1·10"10 5·10"12 5·10"9

Backfill I non-saline • ~ (m3/kg) 0 0.3 0 0.01 • £ 0.20 0.20 0.20 0.20

• De (m21s) 2·10"10 2·10"10 2·10"10 1·10"9

Bentonite I saline • ~ (m3/kg) 0 0.3 0 0.002 • £ 0.43 0.43 0.05 0.43

• De (m21s) 1·10"10 1·10"10 1·10"11 1·10"9

Backfill I saline • ~ (m3/kg) 0 0.3 0 0 • £ 0.20 0.20 0.20 0.20 • De (m21s) 2·10"10 2·1 o-10 2·10"10 4·10"10

Table 4-2. Near-field transfer coefficients.

Case From bentonite into rock From top of the From tunnel fissures intersecting deposition hole into EDZ deposition hole into EDZ

OF (litreslyr) Ooz (litreslyr) OToz1 (litreslyr)

sal50: median flow 0.2 2 100 of saline gw

ns50: median flow 0.2 2 100 of non-saline gw

vhflowsal: very high 3.0 30 1500 flow of saline gw

vhflowns: very high 5.0 50 2000 flow of non-saline gw

16

Table 4-3 and Figure 4-1 show the behaviour of a solubility-limited input: a constant concentration of one unit per litre is assumed to prevail in the canister interior. The maximum release rate from the near-field then gives the equivalent flow rate from the canister interior into the geosphere. In the SH cases, the maximum release rates are governed by the mass transport resistance provided by the hole and, therefore, the maxima are only marginally increased by the two new escape routes from the near-field in the new model. The releases are delayed due to the larger amount of bentonite taken into account. In the DC cases, the maximum release rates are somewhat lower than in TILA-99 because of the diffusive mass transport resistance between the bentonite around the canister (B 1 in Figure 2-1) and the bentonite above the canister (B2), whereas in TILA-99 the canister interior was in a contact with the bentonite above the canister. The new model shows a two-step release curve in the SH as well as DC cases: first the rapid release into the rock around the deposition hole and a later steady state phase where a significant part of the release takes place from the top of the deposition hole and via the tunnel.

Table 4-3. Behaviour of solubility-limited inputs of stable elements: the maximum release rates from the near-field ( = Qekv) and time points when the output reaches 90% of the maximum.

N-S s A c tsomax Oekv tsomax Oekv tsomax Oekv tsomax Oekv

{yr} {1/yr} {yr} {1/yr} {yr} {1/yr} {yr} {1/yr}

SH-ns50

• TILA-99 4.6·102 0.0049 4.9·10s 0.0049 6.5·101 0.0010 1.5·1 os 0.014

• New 3.7·103 0.0050 4.2·106 0.0050 3.7·103 0.0010 1.5·1 as 0.016

SH-sal50

• TILA-99 4.6·102 0.0049 4.9·10s 0.0049 6.5·101 0.0018 3.7·103 0.0060

• New 3.7·103 0.0050 4.2·106 0.0050 2.7·103 0.0018 7.1·103 0.0062

SH-vhflowns

• TILA-99 2.4·101 0.0050 2.6·104 0.0050 1.2·101 0.0010 7.1·103 0.016

• New 1.7·103 0.0050 1.6·106 0.0050 3.9·102 0.0010 6.8·104 0.016

SH-vhflowsal

• TILA-99 3.4·101 0.0050 4.2·104 0.0050 8.7·10° 0.0018 2.8·102 0.0061

• New 2.0·103 0.0050 2.0·106 0.0050 3.9·102 0.0018 4.4·103 0.0062

DC-ns50

• TILA-99 5.4·102 3.9 1.5·106 3.9 5.4·102 0.44 3.6·104 38 • New 7.4·102 3.2 1.7·1 06 3.2 7.4·102 0.32 3.6·104 36

DC-sal50

• TILA-99 5.4·102 3.9 1.5·1 06 3.9 5.4·102 0.69 5.4·102 17 • New 7.4·102 3.2 1.7·1 06 3.2 6.3·102 0.52 7.4·102 15

DC-vhflowns

• TILA-99 1.3·102 9.0 1.5·1 as 9.0 5.5·101 3.5 3.7-103 68 • New 2.4·102 6.5 2.7·10s 6.5 2.4·102 0.83 5.1·103 64

DC-vhflowsal

• TILA-99 1.5·102 7.0 1.9·10s 7.0 6.5·101 3.1 2.0·102 1) 24

• New 2.8·102 5.3 3.1·1 as 5.3 1.7-102 1.3 3.9·102 21

1) There is a misprint in Table 1-2 on page 243 of TILA-99, t9omax of C in DC-vhflowsal should

be 2.0·1 02 yr instead of 2.0·1 03 yr.

1/yr 10°

10-1

10-2

A 10-3 , , , ,

10-4 , ~,,'

, , , , , 10-5 I

I

I I

10-8

10° ld Hf leT Time (yr)

104

Hf

a) SH-sal50 (small hole, median flow, saline groundwater)

1/yr leT

Hf

ld

10°

10-1

10-2

10-3

10° ld Hf leT 104

Time (yr)

c) DC-ns50 (damaged canister,

c

lcT

c

median flow, non-saline groundwater)

108

108

17

1/yr 10°

10-1

10-2

10-3

10-4

10-5

10-8

10° ld

.c

Hf leT 104

Time (yr) Hf

b) SH-vhflowsal (small hole, very high flow, saline groundwater)

1/yr leT

Hf

ld

10°

10-1

10-2

10-3

10° ld Hf leT 104

Time (yr)

d) DC-vhflowns (damaged canister,

lcT

very high flow, non-saline groundwater)

Figure 4-1. Release rates of solubility-limited inputs of stable elements from the near-field into the geosphere. The steady state values present Qekv in litres/year. N-S, S, A, and C represent non-sorbing neutral species, sorbing neutral species, non-sorbing anion, and sorbing cation, respectively. The solid lines represent the new model, and the dashed lines the TILA-99 model.

18

Table 4-4 and Figure 4-2 show the maximum release fractions per year from the near­field for delta pulse inputs of one unit into the canister interior at t = 0. When the new results are compared to those obtained with the TILA-99 models, the following can be observed: • In SH scenarios, the maximum release rates are obtained later (the release curves

of the non-sorbing species N-S and A are, however, fairly flat in shape). The maximum release rate of the sorbing species S is reduced. These effects are caused by the larger amount of bentonite included in the model.

• In DC scenarios, the more concentrated release from the canister into the 35 cm high section of bentonite and from there into the rock fissure results in earlier and higher maxima for the N -S, A and C species. In the case of the anion A with the low diffusivity in the bentonite, the effect of the more concentrated release is suppressed by the lower diffusion area in this main release route.

• In the DC cases, both the new and old model produce a two-peak release curve: The first peak is due to the release into rock fissures intersecting the deposition hole and the later maximum present the release from the top of the deposition hole and via the tunnel.

• One of the shortcomings of the TILA-99 models was that for the sorbing species S, the small hole (SH) model results in higher release rates than the disappearing canister (DC) model as can be seen in Table 4-4. The new unified model erases this artefact.

Table 4-4. Maximum release fractions from the near-field for delta pulse inputs of stable elements.

N-S s A c tmax (yr) max (yr"1

) tmax (yr) max (yr"1) tmax (yr) max (yr"1

) tmax (yr) max (yr"1)

SH-ns5a

• TILA-99 1.2·1a3 7.a.1a"6 1.7·1 as 2.1·1a"6 2.4·1a2 1.5·1a"6 5.8·1a4 6.4·1a"6

• New 7.1·1a3 6.8·1a"6 6.5·1as 4.9·1a"7 1.4·1a4 1.5·1a"6 5.8·1a4 6.9·1a"6

SH-sal5a

• TILA-99 1.2·1a3 7.a.1a"6 1.7·1as 2.1·1a"6 2.4·1a2 2.5·1a"6 7.1·1a3 8.1·1a"6

• New 7.1·1a3 6.8·1a"6 6.5·1as 4.9·1a"7 8.3·1a3 2.5·1a"6 9.8·1a3 8.a.1a"6

SH-vhflowns

• TILA-99 9.a.1a1 7.2·1a·6 3.a.1a4 5.9·1a"6 5.5·1a1 1.5·1a·6 8.3·1a3 1.9·1a·S

• New 5.1·1a3 6.9·1a"6 1.9·1as 1.8·1a"6 7.1·1a3 1.5·1 a·6 3.a.1a4 1.3·1a-s SH-vhflowsal

• TILA-99 1.3·1a2 7.2·1a"6 4.2·1a4 5.5·1a"6 3.4·1a1 2.6·1a"6 7.4·1a2 8.7·1a"6

• New 5.1·1a3 6.9·1a"6 2.3·1as 1.5·1 a·6 5.1·1a3 2.5·1a"6 8.3·1a3 8.2·1a"6

DC-ns5a

• TILA-99 4.6·1a2 5.8·1a"4 8.5·1as 4.3·1a"7 5.4·1a2 3.1·1a·4 2.2·1a4 1.2·1a·S

• New 3.3·1a2 7.9·1a·4 4.7·1as 5.1·1a"7 6.3·1a2 2.9·1a"4 1.6·1a4 1.2·1a·S DC-sal5a

• TILA-99 4.6·1a2 5.8·1a"4 8.5·1as 4.3·1a"7 4.6·1a2 4.5·1a·4 5.4·1a2 3.6·1a·4

• New 3.3·1a2 7.9·1a·4 4.7·1as 5.1·1a"7 4.6·1a2 4.3·1a"4 3.3·1a2 5.1·1a"4

DC-vhflowns

• TILA-99 1.1·1a2 1.6·1a"3 1.3·1as 1.6·1a"6 4.a.1a1 2.9·1a·3 4.4·1a3 3.6·1a-s

• New 9.a-1a1 2.3·1a"3 4.4·1a3 9.a.1a·6 1.5·1a2 8.4·1a"4 2.7·1a3 5.4·1a·S DC-vhflowsal

• TILA-99 1.5·1a2 1.2·1a·3 1.9·1 as 1.2·1a"6 3.4-1a1 2.3·1a"3 2.a.1a2 5.9·1a·4

• New 1.3·1a2 1.9·1a"3 4.4·1a3 5.6·1a·6 1.3·1 a2 1.3·1 a·3 1.3·1 a2 1.1·1 a·3

1/yr 10-2

10-3

10-4

10-5

10-6

10-7

10-8

10° ld Hf leT 104

Time (yr) Hf

a) SH-ns50 (small hole, median flow, non-saline groundwater)

1/yr 10-2

10-3

10-4

10-5 _____ ....

10-6

10-7

10-B 10° ld Hf leT

Time (yr) 10

4 Hf

c) DC-sa/50 (damaged canister, median flow, saline groundwater)

19

106

106

1/yr 10-2

10-3

10-4

10-5

10-6

10-7

10-8

10° ld Hf leT Time (yr)

104

Hf 106

b) SH-vhflowns (small hole, very high flow, non-saline groundwater)

1/yr 10-2

10-3

10-4

10-5

10-6

10-7

10-8

10° ld Hf leT Time (yr)

104 leT

d) DC-vhflowsal (damaged canister, very high flow, saline groundwater)

106

Figure 4-2. Release rates from the near-field into the geosphere for delta pulse inputs of stable elements. N-S, S, A, and C represent non-sorbing neutral species, sorbing neutral species, non-sorbing anion, and sorbing cation, respectively. The solid lines represent the new model, and the dashed lines the TILA-99 model.

20

The proportions of the three release routes at the time point of the maximum release (Table 4-5) highlight the impact of the assumed effective flushing in the tunnel section above the deposition hole. In the small hole (SH) as well as in the damaged canister (DC) cases, a major part of the releases from the deposition hole into the geosphere takes place via this route if the flow rate in the rock around the deposition hole is low. With a high flow rate in the rock, the short route through the bentonite around the canister becomes a major release route especially for the non-sorbing anion with the low diffusivity in the bentonite. Cations with the high diffusivity still take the upward route. In the DC-vhflowns and DC-vhflowsal scenarios, the maximum release rate of the sorbing species S is totally dominated by the short release route through the bentonite. However, there is a later, somewhat lower maximum (Figure 4-2d) where a major part of the release takes place from the top of the deposition hole and via the tunnel.

Table 4-5. Maximum release rates from the near-field for delta pulse inputs of stable elements and the proportions of the three release routes: from the bentonite in the deposition hole into rock fissures (QF in Figure 2-1), from the backfill in the top of the deposition hole into damaged rock zone ( Qvz), and from the tunnel section above the deposition hole into geosphere ( QrvzJ ).

N-S s A c

SH-sal50 (QF=0.2 1/yr, Ooz=2 1/yr, Oroz1=1 00 1/yr) • tmax (yr) 7.1·103 6.5·105 8.3·103 9.8·103

• maximum (yr"1) 6.8·10"6 4.9·10"7 2.5·10"6 8.0·10"6

bentonite (0/o) 6 5 31 1 backfill in the deposition hole (0/o) 7 9 6 5 tunnel (o/o) 87 87 64 94

DC-ns50 (QF=0.2 1/yr, 0 0 z=2 1/yr, Oroz1=1 00 1/yr) • tmax (yr) 3.3·102 4.7·105 6.3·102 1.6·104

• maximum (yr"1) 7.9·10"4 5.1·10"7 2.9·10"4 1.2·10"5

bentonite (o/o) 5 5 46 1 backfill in the deposition hole (0/o) 8 9 5 3 tunnel (0/o) 87 87 50 96

SH-vhflowsal (QF=3.0 1/yr, Ooz=30 1/yr, Oroz1=1500 1/yr) • tmax (yr) 5.1·103 2.3·105 5.1·103 8.3·103

• maximum (yr"1) 6.9·10"6 1.5·10"6 2.5·10"6 8.2·10"6

bentonite (o/o) 44 43 81 13 backfill in the deposition hole (0/o) 28 31 10 30 tunnel (0/o) 28 27 10 57

DC-vhflowns (QF=5.0 1/yr, Ooz=50 1/yr, Oroz1=2000 1/yr) • tmax (yr) 9.1·101 4.4·103 1.5·102 2.7·103

• maximum (yr"1) 2.3·10"3 9.0·10"6 8.4·10"2 5.4·10"5

bentonite (o/o) 59 100 93 6 backfill in the deposition hole (0/o) 27 0 5 25 tunnel (0/o) 14 0 3 69

21

5 RADIONUCLIDE TRANSPORT

Eight TILA-99 scenarios, which were introduced in Chapter 4 and which represent the cases of a small hole (5 mm2

) through the canister wall (SH) and a severely damaged canister (DC), non-saline and saline groundwater conditions, and the minimum and maximum values of the near-field flow rates and transfer coefficients of TILA-99, are recalculated with the new near-field model. The analyses are made for a canister containing 2.14 tU of spent fuel from the Olkiluoto reactors. Except for the new near-field compartment data, presented in Chapter 2, all data are identical with TILA-99 (Vieno & Nordman 1999).

In the scenarios with median flow and transport in the near-field and geosphere the overall effects of the new near-field model are fairly small as will be seen in Tables 5-2 and 5-3. The differences between the new and old near-field model are more obvious in the scenarios with very high flow and transport. Table 5-1 presents the maximum release rates from the near-field into the geosphere in these scenarios. In the small hole (SH) scenarios the new model results in equal or lower maximum release rates. The effect is most significant for nuclides with a relatively short half-life (Ni-63, Sr-90, Cs-137, Pu-240, Am-241, Cm-246). It is caused by the delaying and diluting effects of the larger amount of bentonite taken into account in the new model in the small hole case.

In the DC scenarios the maximum release rates are reduced for solubility-limited nuclides and for anions with the low diffusivity in the bentonite. The reason for this is the smaller diffusion area in the short release route through the bentonite into the rock. The maximum release rates of non-solubility-limited nuclides with a moderate or high sorption in the bentonite (Nb, Cs, Cm, Am, Th; and Sn which is solubility-limited in TILA-99 only for a short time) are, however, increased due to the smaller amount of bentonite in the short release route. The increases in the maximum release rates of U-233 and Pu-242 are related to decay chain effects. The maximum release rate of Ra-226 is slightly increased in DC­vhflowns, but slightly decreased in DC-vhflowsal. In the latter case bentonite block B4 in the lower part of the deposition hole acts as a diluting store for radium with the high diffusivity in saline conditions.

22

Table 5-1. Maximum release rates from the near-field in the very high flow scenarios. Green and red colours indicate that the new model results, respectively, in a lower or higher release rate than the T/IA-99 model.

Nuclide SH-vhflowns SH-vhflowsal DC-vhflowns DC-vhflowsal TILA-99 New TILA-99 New TILA-99 New TILA-99 New

C-14

Cl-36 2.7·103

Ni-59 5.8·1 02

Ni-63 2.9·1 02

Se-79 2.2·1 01

Sr-90 3.1·1 05

Zr-93f

Nb-94 1.3·105

Tc-99 1.6·101

Pd-107 -Sn-126 9.6·102

1-129 2.0·103

Cs-135 1.7·1 05

Cs-137 1.4·1 07

Sm-151 2.8

Pu-240 3.4·102

U-236

Cm-245 1.9·1 02

Th-229 2.0·1 03

Cm-246 1.1·1 02

Pu-242 8.5·101

U-238

U-234

Th-230 3.9·1 02

Ra-226 6.2·1 03

Am-243 2.0·1 03

Pu-239 8.0·1 02

Pa-231 1.9·101

s same as in TILA-99

23

The overall effects are illustrated in Tables 5-2 and 5-3 where the end-point results -ratios of release rates and the nuclide-specific constraints for the release rate from the geosphere into the biosphere, and indicative dose rates obtained by means of the WELL-97 dose conversion factors - are shown.

In the scenarios with median flow and transport of groundwater (ns50, sal50), the overall results are only marginally affected by the changes in the near-field compartment model. In the SH-vhflowsal scenario with the initial canister defect and very high flow of saline groundwater, the maximum dose rate is decreased by almost one order of magnitude thanks to the reduced release of Sr-90. On the other hand, in the damaged canister scenarios with very high flow of non-saline or saline groundwater the maximum release rate ratios and dose rates are increased by a factor from 1.6 to 2.0. The dominant nuclide is Ra-226 and the main reason for the increase in its release rate into the biosphere is the increased release of the Th-230 parent from the near-field (see Table 5-1).

Table 5-2. Maximum ratios of the release rates (moving average over 1rf years) and the nuclide-specific constraints for the release rate into the biosphere. The constraint is 1 rf Bq/yr for the double-underlined nuclides (alpha emitters), 1 rf Bq/yr for the under­lined nuclides, and 1010 Bq/yr for others. Green and red colours indicate that the new model results, respectively, in a lower or higher maximum ratio for the dominant nuclide than the T/IA-99 model.

1st tmax max 2nd tmax max 3rd tmax max nuclide ratio nuclide ratio nuclide ratio

SH-ns50 • TILA-99 Cs-135 9.2·1 05 2.2·10-5 Cl-36 1.6·104 2.7·10-6 C-14 6.6·1 03 2.6·10-6

• New Cs-135 s s Cl-36 2.0·104 s C-14 s 2.3·10-6

SH-sal50 • TILA-99 Cs-135 3.2·105 3.6·1 o-5 Cl-36 1.6·1 04 4.5·1 o-6 C-14 6.6·103 3.9·1 o-6

• New Cs-135 3.6·105 s Cl-36 s s C-14 s 3.5·1 o-6

SH-vhflowns • TILA-99 Cs-135 4.9·104 Nb-94 1.4·1 04 5.9·1 o-5 Am-243 5.8·1 04 5.5·10-6

• New Cs-135 8.4·1 04 Nb-94 2.3·104 2.8·10-5 C-14 5.5·103 3.0·10-6

SH-vhflowsal • TILA-99 Sr-90 5.0·1 03 Cs-135 5.8·1 04 6.3·10-5 Nb-94 2.3·104 3.3·10-5

• New Cs-135 1.0·105 Nb-94 3.4·1 04 1.4·1 o-5 Sr-90 5.0·103 1.0·10-6

DC-ns50 • TILA-99 Sn-126 2.1·1 04 2.1·1 o-4 Cl-36 1.7·104 1.3·1 o-4 C-14 1.5·1 04 7.3·10-5

• New Sn-126 s s Cl-36 s s C-14 s 6.8·10-5

DC-sal50 • TILA-99 Sn-126 1.8·104 2.5·10-4 Cl-36 1.5·104 1.5·1 o-4 C-14 1.5·1 04 7.1·10-5

• New Sn-126 s s Cl-36 s 1.4·1 o-4 C-14 s 6.8·1 o-5

DC-vhflowns • TILA-99 Sn-126 1.5·104 Ba-226 5.9·1 05 3.1·1 o-4 Ib-230 5.9·1 05 3.0·10-4

• New Ba-226 5.3·1 05 Ib-230 5.3·1 05 6.3·10-4 ~u-242 4.7·105 4.2·10-4

DC-vhflowsal • TILA-99 Ba-226 5.9·105 Sn-126 1.5·1 04 3.9·10-4 Cl-36 1.5·104 1.6·1 o-4

• New Ba-226 5.7·105 Sn-126 s 3.8·10-4 Ib-230 6.1·1 05 2.3·10-4

s same as in TILA-99

24

Table 5-3. Maximum dose rates (obtained with the WELL-97 dose conversion factors) and the most important nuclides. Green and red colours indicate that the new model results, respectively, in a lower or higher total dose rate than the TIIA-99 model.

tmax max 1st max 2nd 3rd max nuclide nuclide nuclide

SH-ns50 • TILA-99 8.4·1 05 1.3·1 o-9 1-129 1.1·1 o-9 Cs-135 2.2·10-10 C-14 8.9·10-11

•New s s 1-129 s Cs-135 s C-14 7.3·10-11

SH-sal50 • TILA-99 8.4·105 1.7·1 o-9 1-129 1.3·1 o-9 Cs-135 3.6·10-10 C-14 1.3·1 o-10

• New s s 1-129 s Cs-135 s C-14 1.1·1 o-10

SH-vhflowns • TILA-99 8.4·1 05 Ra-226 2.0·10-9 Cs-135 1.2·10-9 1-129 1.1·1 o-9

• New s 1-129 1.1·1 o-9 Cs-135 7.4·10-10 Ra-226 7.1·10-10

SH-vhflowsal • TILA-99 9.8·101 Sr-90 7.9·1 o-7 Ra-226 1.7·1 o-8 1-129 1.3·1 o-9

• New 1.4·1 02 Sr-90 8.4·10-8 Ra-226 8.1·1 o-9 1-129 s

DC-ns50 • TILA-99 1.1·1 04 1-129 2.9·1 o-8 Sn-126 5.7·10-9 C-14 4.4·10-9

• New s 1-129 2.7·1 o-8 Sn-126 s C-14 4.1·1 o-9

DC-sal50 • TILA-99 1.1·1 04 1-129 4.2·10-8 Sn-126 6.9·1 o-9 C-14 4.5·10-9

• New s 1-129 4.0·10-8 Sn-126 7.3·10-9 C-14 4.2·10-9

DC-vhflowns • TILA-99 6.7-105 Ra-226 3.4·10-7 1-129 2.7·10-7 Pa-231 9.2·1 o-8

• New 5.9·1 05 Ra-226 7.1·1 o-7 Pa-231 9.7·10-8 1-129 7.8·10-8

DC-vhflowsal • TILA-99 6.1·1 05 Ra-226 2.7·10-6 1-129 2.1·1 o-7 Pa-231 4.2·10-8

• New 5.7·105 Ra-226 4.1·10-6 1-129 1.1·1 o-7 Pa-231 4.3·10-8

s same as in TILA-99

25

6 DISCUSSION

The new compartment model leads to a more realistic treatment of the transport situation in the case of a small hole in the canister as well as in the case of a severely damaged canister. In TILA-99 the latter case was actually called a "disappearing" canister, partially because of the unrealistic modelling assumptions. Now the same compartment model is used in both cases. The modelling artefacts caused by the overly conservative presentation of the near-field in the small hole case as compared with the disappearing canister case in TILA -99 are erased.

Also the new model is not free of pitfalls. Such are related, for example, to that: • Releases from the small hole are distributed into a compartment, which goes all

around the canister. • In the small hole case most of the transport resistance is located close to the hole

and thus depends crucially on the local properties of the bentonite at the mouth of the hole. This is, however, rather an issue or a problem of data than a problem of modelling.

• Lumped parameters are used to estimate the transport at the bentonite-rock inter­face.

Simplifications are unavoidable when the 3-dimensional transport situation in the KBS-3 type deposition hole is modelled with a !-dimensional, or at best quasi-2-dimensional, compartment model. The most significant uncertainties are, however, not related to the transport within the buffer and backfill, but to the boundary conditions of the near-field transport model: • behaviour of the water- gas- corrosion products system in the two-layer copper­

iron canister • rate and characteristics of groundwater flow and fracturing in the rock around the

deposition hole • the high flow rates through the backfill in the upper part of the deposition hole and

tunnel assumed in TILA-99. Therefore, resource-intensive efforts to develop a truly 3-dimensional, transient near­field transport model were, in our opinion, not warranted at the present phase.

26

REFERENCES

Andersson, J. 1999. SR 97- Data and data uncertainties. Stockholm, Swedish Nuclear Fuel and Waste Management Co (SKB), Technical Report TR-99-09.

Hautojarvi, A. 1990. Simplified modelling of mass transport in a fissured rock-clay buffer system. Engineering Geology, 28 (1990), p. 353-358.

Hillebrand, K. 1985. Diffusion of radionuclides from the bentonite buffer into the groundwater flowing in rock fractures. Helsinki, Technical Research Centre of Finland, Nuclear Engineering Laboratory, Technical Report TUMA-2/85. (In Finnish).

Lindgren, M. & Lindstrom, F. 1999. SR 97 - Radionuclide transport calculations. Stockholm, Swedish Nuclear Fuel and Waste Management Co (SKB), Technical Report TR-99-23.

Moreno, L. 2000. Impact of the water flow rate in the tunnel on the release of radionuclides. Stockholm, Swedish Nuclear Fuel and Waste Management Co (SKB), Technical Report TR-00-03.

Neretnieks, I. 1982. Leach rates of high level waste and spent fuel - Limiting rates as determined by backfill and bedrock conditions. New York, Elsevier Science Publishing Company Inc., Scientific Basis for Nuclear Waste Management V, p. 557- 568.

Neretnieks, I. 1986. Stationary transport of dissolved species in the backfill surrounding a waste canister in fissured rock: Some simple analytical solutions. Nuclear Technology, Vol. 72, p. 194-200.

Nilsson, L., Moreno, L., Neretnieks, I. & Romero, L. 1991. A resistance network model for radionuclide transport into the near field surrounding a repository for nuclear waste (SKB, Near Field Model 91). Stockholm, Swedish Nuclear Fuel and Waste Management Co (SKB), Technical Report 91-30.

Nordman, H. & Vieno, T. 1994. Near-field model REPCOM. Helsinki, Nuclear Waste Commission of Finnish Power Companies, Report YJT -94-12.

Romero, L., Moreno, L. & Neretnieks, I. 1995. Fast multiple-path model to calculate radionuclide release from the near field of a repository. Nuclear Technology, Vol. 112, October 1995, p. 89-98.

Romero, L., Thompson, A., Moreno, L., Neretnieks, 1., Widen, H. & Boghammar, A. 1999. Comp23/Nuctran user's guide - Proper version 1.1.6. Stockholm, Swedish Nuclear Fuel and Waste Management Co (SKB), Report R-99-64.

Ruokola, E. (ed.) 2000. Posiva's application for a Decision in Principle concerning a disposal facility for spent nuclear fuel - STUK's statement and preliminary safety appraisal. Helsinki, Radiation and Nuclear Safety Authority (STUK), Report STUK-B­YTO 198.

27

Vieno, T. & Nordman, H. 1996. Interim report on safety assessment of spent fuel disposal TILA-96. Helsinki, POSIV A 96-17.

Vieno, T. & Nordman, H. 1999. Safety assessment of spent fuel disposal 1n Hastholmen, Kivetty, Olkiluoto and Romuvaara TILA-99. Helsinki, POSIV A 99-07.