Fusion Engineering and Design 58–59 (2001) 997–1000

Advanced waste management techniques for fusion reactorsWith SiC/SiC structures

M. Zucchetti a,*, P. Rocco a, R.A.H. Edwards b

a Dipartimento di Energetica, Politecnico di Torino, Corso Duca degli Abruzzi 24, 10129 Turin, Italyb En�ironment Institute, JRC Ispra, 21020 Ispra (VA), Italy

Abstract

This paper concerns the management of activated material from operation and decommissioning of three SEAFP99Fusion Plant Models with in-vessel structures made with SiC/SiC composites. The management strategy aiming atreducing the amount of permanent radioactive waste is based on: (a) Recycling within the nuclear industry of in-vesselmaterials other than SiC/SiC. (b) Clearance (declassification to non-radioactive status). (c) SiC/SiC reprocessing forextraction of noxious radionuclides, and achieving clearance conditions for the bulk of the waste. Withoutreprocessing, SiC/SiC becomes a Permanent Disposal Waste (PDW). In the light of this result, two methods ofreprocessing activated SiC/SiC are described. © 2001 Elsevier Science B.V. All rights reserved.

Keywords: Waste management; PDW; SiC; SEAFP

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1. Introduction

The Safety and Environmental Assessment ofFusion Power has extended, in the SEAFP99phase, the analysis to three additional Plant Mod-els (PM-4, PM-5, PM-6) with in-vessel structuresmade with SiC/SiC composites [1]. SiC/SiCamounts in PM-4, PM-5, PM-6 are9730, 1650, 3160 tons, respectively.

This paper concerns the assessment of radwastearising from operation and decommissioning of

these Plant Models, applying advanced manage-ment strategies aimed at reducing the amount ofradioactive waste. The strategies are based on:� Recycling within the nuclear industry of in-ves-

sel materials.� Clearance (declassification to non-radioactive

status to allow simple disposal or recyclingoutside the nuclear industry) of the otherstructures.

� SiC/SiC reprocessing for extraction of noxiousradionuclides, concentrating them in small sec-ondary waste streams, and achieving clearanceconditions for the bulk of the waste.Details on recycling–clearance are described in

[2]. Activation levels are from [3].

* Corresponding author. Tel.: +39-011-564-4464; fax: +39-011-564-4499.

E-mail address: zucchetti@polito.it (M. Zucchetti).

0920-3796/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved.

PII: S0920 -3796 (01 )00529 -4

M. Zucchetti et al. / Fusion Engineering and Design 58–59 (2001) 997–1000998

2. Conditions for recycling and clearance

Table 1 shows the proposed categories of acti-vated materials, adopted in the presentevaluations.

It shows how clearance depends on the specificactivities of the relevant radionuclides containedin the material, weighted according to their ra-diotoxicity. Clearance levels adopted here derivefrom those proposed in [4], modified by reductionfactors [2].

3. Sorting of activated material from PM-4,PM-5, PM-6

The materials arisings from SEAFP99 PlantModels 4, 5, and 6 have been sorted using thedata in [3]. Data were taken from [5,6]. Table 2gives the percentage of materials in the variousclasses for the three Plant Models, for 50 and 100years decay, while Table 3 shows some details forPM-4.

Comments to these results:� All plant models:Unlike SEAFP-2, SEAFP-99

Plant models have significant fractions of acti-vated materials which must be disposed of.This difference is due to:(a) The SiC/SiC resid-ual activity is very low, so that according to therating described in Section 2, this materialcould be SRM. Recycling however is not con-sidered feasible, due to its composite structure.Hence SiC/SiC is rated as PDW.(b) SiC/SiChas poor shielding properties, so that the SS316 shield also becomes PDW. Note that the

shield material adopted in SEAFP2 was OPT-STAB (low-nickel, high-manganese austeniticstainless steel). The choice of SS 316 in place ofOPTSTAB is due to considerations on short-term safety. OPTSTAB has low long-term ac-tivity but high decay heat levels at shut-down.

� Results specific to PM-4 are:After 50 years ofdecay, about 17% of the total waste is PDW,consisting of all SiC/SiC, the SS 316 shield andthe inner layer of the inboard SS 316 VV, beingabout 17% of the total.If decay is extended to100 years, PDW is reduced to 13.5%, as SS 316of the outboard shield and inboard VV becomerecyclable. Most of recyclable material be-comes SRM.

4. Reprocessing of SiC–SiC composite

4.1. SiCl4 route

SiC reacts with dry HCl gas at 500 °C accord-ing to the formula:

SiC+4HCl(g)=SiCl4(g)+CH4(g)

Fig. 1 shows the calculated thermodynamically-stable reaction products (in the presence of 25%excess HCl and 30% hydrogen, needed to keep allcarbon in the form of methane) as a function oftemperature: at 500 °C the reaction should occurstoichiometrically.

The kinetics should be investigated, but it isknown that silicon reacts rapidly with chlorine atthis temperature, so the prospects are good.

Table 1Categories of fusion activated material adopted in SEAFP-2 and SEAFP-99

Ic (c)Activated material classifications D (a) mSv/h H (b) W/m3

PDW, Permanent Disposal Waste (Not Recyclable) �20 �101–10CRM, Complex Recycle Material (Complex RH procedures) 2–20

�2 �1SRM, Simple Recycle Material (Simple RH procedures; HOR for D �10 �Sv/h)�1NAW, Non-Active Waste (to be cleared)

a Contact dose rate at 50a.b Decay heat per unit volume at 50a.c Clearance index at 50 a. It is: Ic =�i=1

Z Ai/Li ; Ai, Li is the specific activity and clearance level of the ith nuclide contained inthe activated material.

M. Zucchetti et al. / Fusion Engineering and Design 58–59 (2001) 997–1000 999

Table 2Management options for the activated materials of theSEAFP-99 Plant Models, % of total weight

PM-5Plant model PM-6PM-4

100aDecay 50a50a 100a 50a 100a

PDW 17.2 13.5 11.3 3.5 28.1 7.1– 40.7 7.927.8 –CRM 8.659.0 23.9 64.5SRM 36.427.3 47.627.5 24.1 24.127.5 36.7NAW 36.7

90 10078 850 59 200Weight (tons)

Fig. 1. Thermodynamically stable products of the reactionbetween HCl and SiC as a function of temperature, predictedby a free-energy minimization program. Note that: (a) 500 °Cis close to the maximum temperature at which only methaneand silicon tetrachloride are the sole products; (b) liquid SiCl4only condenses below room temperature. The input reactantsare: (1) SiC 100 mol; (2) HCl(g) 500 mol (i.e. 400 molreactant+100 mol excess HCl). Excess H2: 200 mol (neededto suppress methane decomposition).

The main radioactive species are C-14, Be-10and Al-26. The Be and Al impurities are con-verted to vapours of BeCl2 and AlCl3. Theseproducts can be selectively removed by fractiona-tion: BeCl2 boils at 532 °C, and AlCl3 at 193 °C.They can be added to a neutral buffer solution toprecipitate the oxides, and set in cement.

To ensure efficient collection of the low partialpressure of radioactive BeCl2 and AlCl3 vapoursproduced, one should add maybe 1% Al and Beto the SiC (as metal pellets) as isotopic swamping,to increase their partial pressures.

Silicon tetrachloride vapour can be removedfrom the gas by bubbling through limewater toprecipitate silica. The methane and hydrogen re-main; the methane is easily removed by pressure-swing adsorption (PSA). The C-14 fraction couldthen be concentrated by PSA or distillation (cryo-genic or pressurized). To fix the C-14 in theradioactive methane, one could use thermal de-

composition at 1000 °C over Fe or Ni catalyst toproduce carbon. Alternatively, one can burn theradioactive methane to make C14O2, then fix thisin alkaline solution or on a commercial CO2

sorbant and set in cement.

4.2. Dissol�ing in steel

Molten iron dissolves about 6% SiC by weight,or about 22% by volume, at 1600 °C. Bubblingair or oxygen through the melt (steel-making pro-

Table 3Rating of activated material from PM-4

Weight, tons ComponentsClass %

PDW 13.5 Inboard SS316 shield10 441Inboard and outboard: SiC/SiC blanket and FW

–CRM – –46 673SRM 59.0 Outboard SS316 shield

Inboard and outboard: Li17Pb83, SS 316 vesselSteel divertor structure

NAW 21 720 27.5 Inboard and outboard toroidal field coils (SS316, insulator, wind. pack)Inboard B4C and Pb

Total, tons 78 850 100

100 years of decay.

M. Zucchetti et al. / Fusion Engineering and Design 58–59 (2001) 997–10001000

cess) oxidises the Al and Be impurities to form anoxide slag, and burn off the carbon (includingC-14) to produce radioactive CO. This removesnearly all the radioactive components from themelt. The radioactive slag is collected by dilutingwith commercial steelmaking fluxes (silicates etc.).The radioactive CO could be converted to meth-ane by a reaction with excess hydrogen, over a Nicatalyst at 500 °C and separated by PSA. Themethane could then be distilled and the C-14 fixedas described above.

Only the silicon, which is not radioactive, re-mains dissolved in the steel. The cast steel wouldbe NAW, containing the the silicon (not radioac-tive) and only tiny traces of radioactive elements.

5. Conclusions

After a 50 years decay the activated materialsfrom SEAFP-99 Plant Models which require dis-posal include the SiC/SiC which is not consideredrecyclable due to its structure, while the innerlayers of SS 316, the W armour in PM-5, the

Li4SiO4 breeder in PM-6 have contact dose ratesexceeding the recyling limit. An increase of thedecay time to 100 years allows to recycle addi-tional materials, but some SS-316 and all SiC/SiCremain PDW.

Two possible methods are described for repro-cessing SiC/SiC, concentrating noxious nuclides insmall secondary waste streams.

References

[1] N.P. Taylor, J.-Ch. Sublet, Description of additional plantmodels with silicon carbide structure, UKAEA Fusion,SEAFP99/S2.1/UKAEA/1 (Rev. 1), April 1999.

[2] P. Rocco, M. Zucchetti, this conference.[3] R.A. Forrest, Activation calculations for SEAFP-99 Plant

Models, SEAFP99/S2.1/UKAEA/3 (Rev. 0), May 1999.[4] Clearance levels for radionuclides in solid materials: appli-

cation of the exemption principles, Interim report forcomment, IAEA TECDOC-855, Vienna, January 1996.

[5] K. Broden, M. Lindberg, G. Olson, Repository analysisfor fusion reactor waste, DRAFT, Studsvik RadwasteReport RW-98/46, August 1998.

[6] P. Rocco, M. Zucchetti, R.A.H. Edwards, Waste manage-ment strategies for plant models with Sic/Sic blankets,SEAFP99/S2.4/JRC/1 (Rev. 2), October 1999.