Activation product transport in the helium cooling circuit of the SEAFP Plant Model 1

P.J. Karditsas and C.B.A. Forty

UKAEA Governnient Division, Fusion, Culham, Abingdon, Oxfordslure, OX14 3DB, UK.+ (UKAEAEuratom Fusion Association)

ABSTRACT

This paper presents results using the steady-state activation transport and deposition code TRAP. Activation of the helium coolant and pipe wall deposits are calculated at important locations of the primary circuit loop of the SEAFP Plant Model 1. The donunant sources of active material in the coolant comprise a nuclear sputtering mechanism and direct generation in the coolant due to the neutron-coolant interaction. Coolant activity behaviour is shown to be dictated by the volatile nuclides whereas surface activiF behaviour is dictated by the non-volatiles. Resulting hazards are estimated to be extremely small.

INTRODUCTION

The European SEAFP (Safety and Env i ronmen ta l Assessment of Eusion Power) program has identified tn o designs for detailed study. The blanket modules and divertor of Plant Model 1 (PM-1) are to be cooled by high pressure helium gas [ 11. With the proposed helium purity envisaged for the PM-1, pipe wall corrosion and activated corrosion product contamination of the circuit will not be significant.

However, activated blanket pipe material will still enter the gas stream in the form of atoms through nuclear sputtering processes [2]. Sputtered atoms are transported to all other parts of the circuit, thus activating the entire cooling loop, Volatile species like tritium, nitrogen and carbon are generated, not only in the structural material but also directly in the coolant. The distribution of tritium and other \.ohtile and non-volatile activated species in the cooling circuit of the power plant must be known because:

1 ) Pipework needs periodic inspection andor maintenance. therefore any active deposits may contribute to the occupational doses received by plant personnel.

2) Leakage from the coolant circuit may further contribute to the doses received by plant pcrsonnel. Also, the collection and treatment of such leak volumes may lead to controlled effluent discharges to the environment [3].

3) There may be potential for release of activated coolant to the environment through a loss of coolant incident.

In practice, these hazards tumed out to be small

Acti1.e atonis contained in the coolant may collide with pipe U all surfaces and deposit. Critical parts of the circuit include the steam generator SG where there is a very large surface area available for deposition. Build-up of activity on tube bundles, for example, may lead to the occupational radiation exposure ORE of plant personnel when they conduct periodic inspection or maintenance [4]. Doses for the SG are calculated using the results of this paper in [ 5 ] . The inclusion of an activation product AP trap is one means of reducing such exposures [6].

T h ~ s paper outlines the inass transfer and nuclear sputtering models and identifies the most important nuclides involved. The system behaviour is described in its steady state, with the loop structure segmented into a series of finite elements. The computer code TRAP (manspor t of Activation Products) is used to solve the coupled coolant and surface mass transfer equations for each element around the loop. The active nuclide coolant, surface and pipe concentrations are calculated at all locations in the primary circuit.

THERMAL HYDRAULICS

The SEAFP PM-1 consists of eight primary circuit loops PCL and hvo divertor loops DL. Approximately 3600 MW, \vi11 be shared by the PCLs and 500 MW,by the DLs. The acti\.ated blanket pipework is made from the vanadium alloy V-j%Ti, whle the rest of the loop (outside the neutron flus) is made from femtic steel and nickel-based alloy. Each PCL has h v o blanket segments, with each blanket segment constructed with two inboard and three outboard canisters. Each blanket canister contains thirty-five compartments arranged along the poloidal length, with twenty blanket tubes 8.7 ni in length and 72 nim in width. In addition, each PCL has its own SG and helium circulator HC.

The entire primaq circuit is 160 ni long, with the starting point arbitrarily set to the blanket inlet position. The helium circulator HC is adjacent to the 'cold-leg' CL which runs for 50 ni before splitting into a series of successively smaller inlet manifolds IM wlzich feed the blanket pipes. The outlet niamfolds OM collect the blanket pipe flow into a single

+ This work was jointly funded by the UK Department of Trade and Industq and Euratom 0-7803-2969-4195/$4.000 1995IEEE 1002

50 m ‘hot-leg’ HL pipe. The coolant then flows into the SG. The He gas passes down through the SG shell and 10% bypasses the helically wound tube bundles whch carry the cooling water/steam. The bypassed flow joins the remaining 90% of the flow at the exit of the tube bundles. The flow then enters the HC to begin the circuit again.

The analysis presented in this paper refers to the activation, transport and deposition in a single PCL as shown in Fig. 1. Other input data were obtained from the detailed blanket [7] and cooling circuit [6] designs.

SP’UTTERING

In a previous study, involving circuit activation of a helium cooled fusion power plant [2], direct daughter recoil sputtering DDRS was identified as the principle source of active contamination in the circuit. This has been confirmed for the SEAFP PM-1 despite the differences in materials and neutronics conditions [8]. DDRS occurs in the blanket and first wall coolant pipes where the neutron energies and fluxes are greatest. A nucleus in the coolant pipe captures a neutron to form a short-lived compound nucleus. This decays by emission of a particle(s) to form a further nucleus. The energy released in the reaction is shared between the particle

and the furtheir nucleus such that they both leave the disintegration site at h g h velocity. Each will slow in the pipe material ithrough interaction with lattice atoms. However, if eitiher crosses a coolantlpipe surface before sufficient energy is liost, they may escape into the coolant stream.

Neutron exposure of the blanket and first wall pipes results in the gradual accumulation of activation products over time. The range of active nuclides sputtered is consequently also large. However. cooling circuit activation is dominated by a few nuclides exhibiting h g h sputtering yields, the most important of which are summarised in Table I, and direct generation in the coolant, with generation rates given in Table 11.

MASS TRANSFER

As the helium flows through the blanket pipes, the DDRS and neutron-coolant interaction processes provide the major source of active material in the coolant volume. Additional processes considered include diffusion from the bulk material

Table I Dominant active nuclides in sputtering source term

I

T

secondary cool ant

b

steam generator

4

blanket pipework

I ’ i 3 l a 9 t cannisters 1

Fig. 1 Schematic of the primary cooling loop.

Table I1 The production rates of nuclides in the coolant

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to the surface layer and subsequent absorption and desorption of the volatile species. Very minor source mechanisms not included in the model are corrosion and erosion of the pipe surfaces.

Material loss mechanisms from the coolant volume include radioactive decay, deposition onto surfaces and convection downstream. Deposition is driven by a gradient between the bulk coolant and pipe surface concentrations, but also depends on local flow conditions and on the available pipe surface area.

The particle mass balance equations are formulated for the coolant volumes, surface layers and bulk solids at steady- state conditions. The bulk solid is sectioned into finite elements in the radial direction and the layers close to the coolant include the DDRS mechanism. A linear sorption rule is assumed to describe accurately the surface behaviour. The resulting set of equations is solved using the computer code TRAP at different locations around the circuit. The entire circuit is discretized into a number of elements. with each successive element linked to its nearest neighbour in an

Activity 108

10

106

1 0 5

10

10 3 , , , r 1 , r . . , I . . . , . . . I , I . I . , . * . . 0 25 50 75 100 125 150 175

Length (m)

Fig 2 A typical variation of surface and coolant activity around the loop for the non-volatile nuclide Ti5 1.

overall closed loop. Many of the compartments represent a single large diameter pipe section, but for the blanket core and the SG, an arrangement of manifolds and branches is constructed and used in the calculations. Thermo-fluid mechanics data in these branches are assumed to be identical.

RESULTS AND DISCUSSION

The total active atomic concentration in the coolant and deposition on pipe surfaces was calculated as a function of primae circuit position. In general, the results for the coolant and surface activities show that :

1) the volatiles stay in the coolant, thus a purification plant is needed to extract them,

2 ) the coolant activity behaviour follows closely, and is dominated by, the behaviour of the volatile nuclides, and

3) the surface activity behaviour is dominated by the behaviour of the non-volatile nuclides.

The atom concentration in the coolant at the HL and CL is -1 .OE+8 Bq/m3 and remains approximately constant over all the stages, except at the SG and blanket. In the SG there is a small decrease due to the 10% bypass. In the blanket, the sputtering and direct coolant activation source terms cause a rise in the coolant atom concentration, to a value of 1.4E+8 Bq/m3 over the 8.7 m of blanket pipe.

The pipe surface deposited atom activity shows a different behaviour around the circuit when compared with the coolant acti\,it\.. The CL activity is in the range of 5-10E+9 Bq/m2 and the HL activity in the range 4-6Et6 Bq/m2. There is a decrease in the blanket from a value of 7.4E+9 Bq/ni2 at the inlet to a value of 2.7E+9 Bq/m2 at the exit. Tlus is due to the niobilisation of the non-volatiles as temperature increases toi\mds the blanket exit. For example see Fig. 1 for the Ti5 1 behaviour; the non-volatiles dictate the surface activity behaviour.

The variation of total nuclide coolant activity is shown to v a p around the primary circuit by a factor of -1.25. It is loivest just prior to entering the blanket and highest at the hot and cold legs.

The IM and OM regions exhibit an increased deposition due principally to the change in mass transfer. Deposition in the blanket and SG varies in both by a factor of -2.5, decreasing arid increasing respectively. Deposition is enhanced in regions nhere flow turbulence increases.

The indi\.idual nuclide contributions to coolant and pipe surface activity are listed in Table 111 for four regions, namely the blanket and SG inlet and exit positions. The total coolant activity is dominated by the nuclide N13 with H3, Na24, N16. Ti51 and Se48 also contributing in that order,

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with a decreasing importance. The total surface activity is not dominated by a particular nuclide but comprises mainly contributions from N13, Na24, Sc46, Sc47, Sc48, Ti51 and Wl8l .

Since most of the sputtering reactions are threshold events, DDRS is unlikely to be as important in lower energy and flux positions of the power plant. Moreover, since the density of cooling pipes is a maximum in the high flux regions, the results presented should be indicative of the whole machine. Although on activation of the vanadium alloys there are hundreds of active nuclides available for sputtering, the source term is dominated by a small proportion. This is expected to hold true for other potential candidate materials, although the dominant nuclides will be different.

Table I11 Coolant and deposited activity of the important nuclides.

nuclide

H 3 C 14 N 13 N 16 Na 24 A1 28 Sc 46 s c 47 Sc 48 Ti 51 Cr 51 w 181

H 3 C 14 N 13 N 16 Na 24 A1 28 Sc 46 s c 47 Sc 48 Ti 51 Cr 51 W 181

coolant acti bla

inlet 6.09E+6 5.25E+3 1.05E+8 1.72E+5 5.53E+5 4.29E+2 6.10E+3 2.51E+3 1.32E+3 1.68E+4 2.54E-2 1.56E+2

surf 2.83E+ 1 3.14E+ 14 5.86E+8 9.9 1E+4 2.61E+9 9.19E+5 3.80E+9 6.52E+7 1,11E+8 1.39E+7 2.45E+4 1.38E+8

$et exit

6.82E+6 5.89E+3 1.29E+8 4.75E+5 1.16E+6 7.02E+3 1.15E+4 2.68E+4 1.73E+4 2.75 E+5 6.3 9E-1 4.22E+2

e activity 7.05E-1 4.87E+O 1.31E+5 3.57E+2 3.12E+8 1.05E+6 4.69E+8 1.37E+9 3.35Et8 2.70E+7 4.22E+6 1.15E+8

t~ (Bq/mj) :

inlet 6.82E+6 5.90E+3 1.29E+8 4.63E+5 1.12E+6 5.03E+3 1.16E+4 2.44E+4 1.49E+4 2.00E+5 4.83E-1 4.16E+2

Iq /n? ) 7.75E-1 6.22E+0 1.41E+5 4.06E+2 1.45E+8 5.45E+3 1.5 1E+8 4.97E+7 1.72E+7 5.44E+5 9.07E+3 1.89E+7

exit 6.76E+6 5.84E+3 1.27E+8 3.20E+5 9.29E+5 7.17E+1 1.02E+4 4.18E+3 1.36E+3 3.1 OE+3 9.22E-3 2.58E+2

CONCLUSIONS

The main sources of activation products in the PCLs are: sputtering from the first wall and blanket cooling pipes and direct generation in the coolant due to the neutron coolant interaction in the blanket and first wall.

At steady state, cooling helium enters the blanket and its activity increases at the exit by a factor of -1.25. Very little activity is deposited in the large diameter hot leg pipe and the activity is maintained at blanket exit levels. There is negligible activity reduction in the SG and the cold leg, therefore 1% of the flow is continually diverted to a punfication plant for clean-up purposes.

The total surface activity behaviour is dictated by the non- volatile nuclides. Total activity values change around the loop, and decrease across the blanket by a factor of -2.7, stay fairly constant in the hot leg, increase in the steam generator by a factor of -C!.4, and stay fairly constant at the cold leg. The largest values are observed at the cold leg and blanket inlet sections.

Preliminary estimates based on the activation results summarised in ,this paper show that, in terms of hazard, the effluents are extremely small, operator doses are siniilat to those achieved with the best modem optimised PWRs, and the maximum public doses from accidental releases are small.

REFERENCES

[ l ] A. Natalizic,., G. Vieider, P. Dinner and A. Angellini, Cooling System Design, SEAFP/R-M8/I1(93). [2] W.E. Bickfcird, Transport And Deposition Of Activation Products In A Helium Cooled Fusion Power Plant, PNL- 3478 UC-20e (1980). [3] C.B.A. Forty, Gaseous Effluent Source Term Calculations, SEd4FP/R-A7/2(94). [4] C.G. Kinniburgh and S.R. Herndlhofer, Operator Doses And Blanket/Coolant System Design, SEAFP/R-A10/I2(94). [5] C.B.A. Forty, Gamma radiation fields around the cooling circuit of the SEAPF Plant Model 1, sanie conference. [6] J. Wagner, Helium Cooling System Design, SEAFP/R- M8/2(94). [7] W. Danner, E. Salpietro and G. Sinibolotti, Helium Cooled Ceramic Blanket (HCCB) For SEAFP, Paper presented at the SEAFP WS workshop, April 19-20, 1994 Culham, UK. [8] C.B.A. Forty and P.J. Karditsas, Activation Product Transport And Deposition In The Helium Cooling Circuit Of The SEAFP Reference Plant Model, SEAFP/R-A 10/5(94).

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