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Materials optimisation for fusion power plants waste management from neutronics and activation assessment
Fusion Engineering and Design 69 (2003) 705 /709 www.elsevier.com/locate/fusengdes
Materials optimisation for fusion power plants waste management from neutronics and activation assessment G. Cambi a,*, L. Di Pace b, D.G. Cepraga c, M. Frisoni d, A. Chiasera e, M. Zucchetti f, R. Forrest g a
Physics Department, Bologna University, Via Irnerio 46, I-40126 Bologna, Italy b Associazione Euratom-ENEA sulla Fusione, Frascati, Italy c ENEA, FIS-MET, Bologna, Italy d Athena s.a.s., Bologna, Italy e ENEA, UDA, Bologna, Italy f Energetics Department, Polytechnic of Turin, Turin, Italy g Euratom/UKAEA Fusion Association, Culham Science Centre, Abingdon, UK
Abstract The paper describes a study dedicated to the decommissioning and waste management issues of a commercial fusion power station. It is focused on the optimisation of structural materials for ex-vessel components of two safety and environmental assessment of fusion power (SEAFP) plant models (PM2 water-cooled and PM3 helium-cooled). The results of this study are: for PM2, the inboard and outboard coil case, the outboard vacuum vessel (VV), the outboard insulator and winding pack, the neutron and gamma shield and the concrete cryostat can be cleared; the same for PM3, excluding the outboard VV. Possible optimisations proposed are related to the outboard vessel both for PM2 and PM3. They deal with impurity level control and/or use OPTSTAB, a reduced-activation austenitic high-manganese steel. # 2003 Elsevier Science B.V. All rights reserved. Keywords: Waste management; Fusion power plant; Vacuum vessel
1. Introduction and theoretical background The minimisation of active waste from the operation and decommissioning of a fusion power plant is one the main scopes for fusion waste management studies. Clearance of ex-vessel components is one of the ways to achieve this goal.
* Corresponding author. Tel.: /39-51-6098-297; fax: /3951-6098-062. E-mail address: [email protected] (G. Cambi).
In the frame of the European fusion development agreement technology work-programme 2000, as a part of the waste management study (WMS) activities, an assessment was performed to study the optimisation of fusion power reactors wastes. This paper focuses on the structural materials composition for ex-vessel components in order to make possible their clearance, considering both clearance for non-active disposal or free-release recycling. Application to SEAFP plant models 2 and 3 is shown and discussed. Ex-vessel components in a fusion power reactor generate most of the activated material volume in
0920-3796/03/$ - see front matter # 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0920-3796(03)00102-9
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G. Cambi et al. / Fusion Engineering and Design 69 (2003) 705 /709
the decommissioning phase and have a relatively low activation due to low neutron fluxes to which they are exposed. Therefore, they are exactly those components for which an optimisation of a ‘clearance’ procedure should have positive impact on the total amount of radioactive waste produced by the reactor. By the application of waste management strategies, it is possible to limit the volume of material, which ultimately becomes classified as active waste requiring long-term storage or disposal in a repository. Together with recycling within the nuclear industry, clearance is the other viable option for reducing the active waste from fusion reactors. In fact, if the level of activity in a material falls to a sufficiently low level, there is the possibility of freeing it from regulatory control as an active material. This will be defined as ‘clearance’ in the rest of this paper. A clearance strategy including the definition of clearance limits has been proposed in Ref. [1] and it will be adopted here. This strategy defines clearance options for activated fusion materials, with two possible paths: (a) disposal as non-active waste and (b) recycling outside the nuclear industry (or unconditional recycling). For the former option, clearance levels may be derived from those proposed in Ref. [2]. Clearance levels taken from the IAEA-TECDOC-855 are derived from categorisation of safety analyses of waste repositories producing a maximum individual dose of 10 mSv/a. Additional safety factors, up to a reduction of one order of magnitude, have been applied to those clearance levels, when they are higher than 1000 Bq/kg. They are illustrated in detail in Ref. [1]. For the latter option, recycling outside the nuclear industry, different clearance limits defined in Ref. [3] are adopted. Clearance limits for all relevant radio-nuclides have been computed in Ref. [1], for both the NAW (clearance with disposal, or clearance D) and NARM (clearance with recycling, or clearance R) routes.
2. Clearance of SEAFP plant models 2 and 3 exvessel components The main features of the SEAFP plant model 2 and 3 (PM2 and PM3) are as follows [4]: liquid
Li17Pb83 as tritium-generating and neutron multiplier material and water as coolant, for PM2; ceramic pebble bed Li4SiO4 for tritium generation, beryllium as neutron multiplier and helium as coolant for PM3. Low activation martensitic steel as FW/blanket structural material, both for PM2 and PM3. New activation data have been computed for SEAFP PM2 and PM3 based on new neutronic calculations [5]. One-dimensional Sn radiation transport calculations have been performed; neutron and gamma flux distributions have been determined by means of the Sn coupled n /g onedimensional discrete ordinates transport calculation sequence (Bonami-Xsdrnpm) from Scale 4.4a computer code system [6]. The Vitamin-ENEA Master Library (174n /38g groups), based on ENDF/B-VI data, is used for transport calculation. The activation calculations have been carried out using the EASY-99 (FISPACT-99 code [7]). The irradiation conditions are based on a plant life of 25 years. Clearance indices Ic(D) and Ic(R), respectively, for NAW and NARM, have been evaluated for the activated material, taking into account the contribution of each radio-nuclide contained. Table 1 summarises the results related to the overall waste management for PM2 and PM3. Data for in-vessel components have been derived from Ref. [8]. It turns out that, for many materials, the clearance goal can be reached without any optimisation of the material composition or of reactor design. In particular, for PM2, about 26 200 tons could be cleared (27.5% of the total), while for PM3, about 20 000 tons could be cleared (30.8% of the total). In all the percentage evaluations, cryostat components are not included. At any rate they can be disposed of as NAW (clearance with disposal).
3. Materials optimisation for clearance of PM2 and PM3 From the results analysis, the following materials could be considered for optimisation of their composition, since they do not reach the goal of
G. Cambi et al. / Fusion Engineering and Design 69 (2003) 705 /709
707
Table 1 Management of activated materials of SEAFP plant models 2 and 3 (cryostat components are not included) Management option
Permanent disposal waste Recycling within the nuclear industry Clearance Total weight a b
PM2
PM3
Weighta
%
Weighta
%
22 69 047 26 219 95 288
0.02 72.46 27.52 100.00
22 45 101b 20 036b 65 159
0.03 69.22b 30.75b 100.00
Weight, of activated material, including decommissioning, in tonnes. These figures could be changed by a slight optimisation of SS316 composition in the outboard vessel.
clearance: inboard winding pack (magnets) for PM2 and PM3, inboard insulator outer zone for PM2 and PM3, inboard vessel (SS316) for PM2 and PM3, outboard vessel (SS316) for PM3. Concerning inboard insulator, this component will not be further examined, because its weight is negligible (12.4 tons), compared with the other ones (1655 tons for inboard winding pack, 6183 tons for outboard vessel, 1295 tons for inboard vessel). Moreover, it is very likely that, being so close to inboard winding pack, insulator will not be separated from it in case of disposal. Concerning inboard vessel, its clearance indices are too high to consider any possible optimisation on material composition. Contact dose rates after 50 /60 years of cooling fall, within the recycling limit within the nuclear industry; however, handson recycling is not possible. The former route is the most convenient [9], for the inboard vessel, both for PM2 and PM3. Concerning the inboard winding pack, i.e. the inboard toroidal magnets, clearance indices are relatively high, but optimisation may be at least explored as an option. The most significant nuclide (up to 99%) is Nb94, which has a very long half-life. Other nuclides giving contribution to clearance indices greater than or close to one are: Co60 (only at 50 years of cooling, after 100 y is negligible), Ni63, Mo93 and Nb93m. The Ni63 is produced by the two nuclear reactions Ni62(n, g)Ni63 and Cu63(n, p)Ni63. Concerning the Niobium isotopes, the Nb94 is a product of Nb93(n, g)Nb94 reaction and the Nb93m is a product of Nb93(n, n?)Nb93m reaction. Niobium is used in magnets super-conducting
cables. It seems questionable the adoption of a different super-conducting alloy just for clearance reasons. Furthermore, no isotopic tailoring is possible, since Nb93 is the only isotope of Niobium. We can therefore conclude that no materials optimisation is possible in this case. Concerning outboard vessel, its activation for PM3 is quite higher than for PM2, since PM3 blanket has, by far, lower shielding capabilities than PM2 blanket. Due to the relevant weight of this component (6183 tons, about 10% of all the decommissioning activated materials), some material optimisation could be recommendable, in order to make possible the clearance of this component. We will concentrate first on PM3 outboard vessel steel pipes, which has the lower clearance index values and most of the weight (5636 tons). Concerning clearance with recycling, it turns out that most of the clearance index is due to Ag108m. This is an activation product of Ag, which has been assumed to be present as an impurity in SS316, with a concentration of 400 ppm. This concentration is rather conservative, since estimates assess that Ag concentration could be reduced down to 0.5 /1 ppm. If Ag concentration could be reduced to 40 ppm, then the clearance index for recycling would be less than one. In the case of clearance with disposal, Ni63 is the most responsible nuclide. In this case, Ni63 is totally an activation product of Ni62(n, g)Ni63. The second relevant nuclide is Ag108m (see above for discussion). Other relevant nuclides are Ni59, Nb91, Nb94 and Mo93. All these nuclides, Ag108m excluded, are activation products of Ni and Mo,
708
G. Cambi et al. / Fusion Engineering and Design 69 (2003) 705 /709
two essential alloying elements in SS316. Therefore, no optimisation is possible in this case. In previous stages of the SEAFP study, the use of OPTSTAB, a reduced-activation austenitic high-manganese steel, was considered for the vessel of SEAFP plant models. OPTSTAB has a reduced content of Ni and Mo: it is likely that this material may meet the goal of a clearance with disposal index inferior to 1, as far as PM3 outboard vessel steel pipes is concerned. For OPTSTAB too, of course, impurity content (Ag especially) should be kept low. For the PM3 outboard vessel wall, that weighs 547 tons, the clearance with recycling index after 100 y of cooling is about 13.5, while the clearance with disposal index is about 47. The most significant nuclides for it are (at 100 y) Ag108m and Nb94. According to this result, we recommend the following: a) reduction of the Ag impurity level by 100 times, i.e. from 400 to 4 ppm; b) melting of SS316 coming from the outboard vessel wall with other less-activated SS316, (for example with the one from the outboard vessel pipes). Considering the respective weights of the two zones, an average clearance index of about 0.8 could be obtained. In this case also, another viable solution is the adoption of the OPTSTAB, however controlling the impurity content (Ag especially).
4. Conclusions Clearance of ex-vessel components is one of the ways to reduce radioactive waste production from fusion power reactors. As a part of the WMS activities, this paper has examined the optimisation of fusion power reactors ex-vessel components composition in order to make possible their clearance, considering both clearance with disposal and clearance with recycling. Application to SEAFP plant models 2 and 3 has been discussed, using new activation data. It turns out that, for many materials, the clearance goal
can be reached without any optimisation of the material composition or of reactor design. Some inboard ex-vessel components (winding pack, insulator and vacuum vessel), however, do not reach the clearance goal or reach it only if diluted with fresh material, both for PM2 and PM3. Activation of all PM3 vessel and ex-vessel components is higher than for PM2, due to the lower shielding capability of PM3 blanket, compared with PM2. Moreover, for the same reasons, PM3 outboard vessel (SS316) cannot be cleared without optimisation, while PM2 correspondent component can. As far as design optimisation is concerned, it may be concluded that the shielding performance of the PM3 blanket should be improved. Materials composition optimisation has also been considered for all these components. The results show that material composition optimisation could be worth only in the case of PM3 outboard vessel that also represents about 10% of the total decommissioning materials for this plant model. The results show that most of the outboard vessel (5636 tons over 6183 in total) could be recycled outside the nuclear industry (clearance R) if the Ag impurity content could be reduced from the considered 400 ppm level. For an Ag reduction down to 4 ppm, all the outboard vessel of PM3 would have an average clearance (R) index lower than 1, after about 100 y cooling. In that case, also for PM3, the amount of clearable material could be the same as for PM2 (26 200 tons), rising the percentage of clearable material from about 30% to about 40%. The adoption for the outboard vessel of the OPTSTAB steel, in place of SS316, would permit to obtain the same goals, making also feasible clearance with disposal option.
References [1] M. Zucchetti, Clearance of Activated Materials: The ‘De Minimis’ Problem WMS/TSW1D5/ENEA/1 (Rev. 3), ENEA FUS-TN-SA-SE-R-021, July 2001. [2] Clearance Levels for Radionuclides in Solid Materials: Application of the Exemption Principles, Interim Report for Comment, IAEA TECDOC-855, Vienna, January 1996. [3] Radiation Protection 89-Recommended Radiological Protection Criteria for the Recycling of Metals from the
G. Cambi et al. / Fusion Engineering and Design 69 (2003) 705 /709 Dismantling of Nuclear Installations, European Commission, Directorate General Environment, Nuclear Safety and Civil Protection, 1998. [4] N. Taylor, I. Cook, Description of SEAFP-2 Plant Models, blankets and structural materials, SEAFP2/5.1/UKAEA/1 (Rev. 0), March 1997. [5] D.G. Cepraga, G. Cambi, M. Frisoni, Neutronic Calculations for SEAFP-2 Plant Models 2 and 3, WMS TSW1D5/ ENEA/4 (Rev. 0), ENEA FUS-TN-SA-SE-R-024, July 2001. [6] SCALE: A Modular Code System for Performing Standardized Computer Analyses for Licensing Evaluation, NUREG/CR-0200, Rev. 6 (ORNL/NUREG/CSD-2R6), vols. I, II, and III (May 2000). Version 4.4a of the code
709
package is available from Radiation Safety Information Computational Center (RSICC) at Oak Ridge National Laboratory as CCC-545. [7] R.A. Forrest, J.-Ch. Sublet, FISPACT-99: User Manual, UKAEA FUS 407, 1998. [8] P. Rocco, M. Zucchetti, Integrated Approach to Recycling and Clearance. More Realistic Management Of Activated Materials, Final Report-Task S5.2, SEAFP99/S5.2/JRC/1 (Rev. 1)-November 1999. Presented to the IAEA Technical Committee Meeting on Fusion Safety, Cannes, June 2000. [9] M. Zucchetti, R. Forrest, L. Di Pace, Clearance of Activated Materials: Optimisation of ex-vessel Material Composition WMS/TSW1D5/ENEA/2 (Rev. 3), ENEA FUS-TN-SA-SE-R-022, December 2001.
Materials optimisation for fusion power plants waste management from neutronics and activation assessment G. Cambi a,*, L. Di Pace b, D.G. Cepraga c, M. Frisoni d, A. Chiasera e, M. Zucchetti f, R. Forrest g a
Physics Department, Bologna University, Via Irnerio 46, I-40126 Bologna, Italy b Associazione Euratom-ENEA sulla Fusione, Frascati, Italy c ENEA, FIS-MET, Bologna, Italy d Athena s.a.s., Bologna, Italy e ENEA, UDA, Bologna, Italy f Energetics Department, Polytechnic of Turin, Turin, Italy g Euratom/UKAEA Fusion Association, Culham Science Centre, Abingdon, UK
Abstract The paper describes a study dedicated to the decommissioning and waste management issues of a commercial fusion power station. It is focused on the optimisation of structural materials for ex-vessel components of two safety and environmental assessment of fusion power (SEAFP) plant models (PM2 water-cooled and PM3 helium-cooled). The results of this study are: for PM2, the inboard and outboard coil case, the outboard vacuum vessel (VV), the outboard insulator and winding pack, the neutron and gamma shield and the concrete cryostat can be cleared; the same for PM3, excluding the outboard VV. Possible optimisations proposed are related to the outboard vessel both for PM2 and PM3. They deal with impurity level control and/or use OPTSTAB, a reduced-activation austenitic high-manganese steel. # 2003 Elsevier Science B.V. All rights reserved. Keywords: Waste management; Fusion power plant; Vacuum vessel
1. Introduction and theoretical background The minimisation of active waste from the operation and decommissioning of a fusion power plant is one the main scopes for fusion waste management studies. Clearance of ex-vessel components is one of the ways to achieve this goal.
* Corresponding author. Tel.: /39-51-6098-297; fax: /3951-6098-062. E-mail address: [email protected] (G. Cambi).
In the frame of the European fusion development agreement technology work-programme 2000, as a part of the waste management study (WMS) activities, an assessment was performed to study the optimisation of fusion power reactors wastes. This paper focuses on the structural materials composition for ex-vessel components in order to make possible their clearance, considering both clearance for non-active disposal or free-release recycling. Application to SEAFP plant models 2 and 3 is shown and discussed. Ex-vessel components in a fusion power reactor generate most of the activated material volume in
0920-3796/03/$ - see front matter # 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0920-3796(03)00102-9
706
G. Cambi et al. / Fusion Engineering and Design 69 (2003) 705 /709
the decommissioning phase and have a relatively low activation due to low neutron fluxes to which they are exposed. Therefore, they are exactly those components for which an optimisation of a ‘clearance’ procedure should have positive impact on the total amount of radioactive waste produced by the reactor. By the application of waste management strategies, it is possible to limit the volume of material, which ultimately becomes classified as active waste requiring long-term storage or disposal in a repository. Together with recycling within the nuclear industry, clearance is the other viable option for reducing the active waste from fusion reactors. In fact, if the level of activity in a material falls to a sufficiently low level, there is the possibility of freeing it from regulatory control as an active material. This will be defined as ‘clearance’ in the rest of this paper. A clearance strategy including the definition of clearance limits has been proposed in Ref. [1] and it will be adopted here. This strategy defines clearance options for activated fusion materials, with two possible paths: (a) disposal as non-active waste and (b) recycling outside the nuclear industry (or unconditional recycling). For the former option, clearance levels may be derived from those proposed in Ref. [2]. Clearance levels taken from the IAEA-TECDOC-855 are derived from categorisation of safety analyses of waste repositories producing a maximum individual dose of 10 mSv/a. Additional safety factors, up to a reduction of one order of magnitude, have been applied to those clearance levels, when they are higher than 1000 Bq/kg. They are illustrated in detail in Ref. [1]. For the latter option, recycling outside the nuclear industry, different clearance limits defined in Ref. [3] are adopted. Clearance limits for all relevant radio-nuclides have been computed in Ref. [1], for both the NAW (clearance with disposal, or clearance D) and NARM (clearance with recycling, or clearance R) routes.
2. Clearance of SEAFP plant models 2 and 3 exvessel components The main features of the SEAFP plant model 2 and 3 (PM2 and PM3) are as follows [4]: liquid
Li17Pb83 as tritium-generating and neutron multiplier material and water as coolant, for PM2; ceramic pebble bed Li4SiO4 for tritium generation, beryllium as neutron multiplier and helium as coolant for PM3. Low activation martensitic steel as FW/blanket structural material, both for PM2 and PM3. New activation data have been computed for SEAFP PM2 and PM3 based on new neutronic calculations [5]. One-dimensional Sn radiation transport calculations have been performed; neutron and gamma flux distributions have been determined by means of the Sn coupled n /g onedimensional discrete ordinates transport calculation sequence (Bonami-Xsdrnpm) from Scale 4.4a computer code system [6]. The Vitamin-ENEA Master Library (174n /38g groups), based on ENDF/B-VI data, is used for transport calculation. The activation calculations have been carried out using the EASY-99 (FISPACT-99 code [7]). The irradiation conditions are based on a plant life of 25 years. Clearance indices Ic(D) and Ic(R), respectively, for NAW and NARM, have been evaluated for the activated material, taking into account the contribution of each radio-nuclide contained. Table 1 summarises the results related to the overall waste management for PM2 and PM3. Data for in-vessel components have been derived from Ref. [8]. It turns out that, for many materials, the clearance goal can be reached without any optimisation of the material composition or of reactor design. In particular, for PM2, about 26 200 tons could be cleared (27.5% of the total), while for PM3, about 20 000 tons could be cleared (30.8% of the total). In all the percentage evaluations, cryostat components are not included. At any rate they can be disposed of as NAW (clearance with disposal).
3. Materials optimisation for clearance of PM2 and PM3 From the results analysis, the following materials could be considered for optimisation of their composition, since they do not reach the goal of
G. Cambi et al. / Fusion Engineering and Design 69 (2003) 705 /709
707
Table 1 Management of activated materials of SEAFP plant models 2 and 3 (cryostat components are not included) Management option
Permanent disposal waste Recycling within the nuclear industry Clearance Total weight a b
PM2
PM3
Weighta
%
Weighta
%
22 69 047 26 219 95 288
0.02 72.46 27.52 100.00
22 45 101b 20 036b 65 159
0.03 69.22b 30.75b 100.00
Weight, of activated material, including decommissioning, in tonnes. These figures could be changed by a slight optimisation of SS316 composition in the outboard vessel.
clearance: inboard winding pack (magnets) for PM2 and PM3, inboard insulator outer zone for PM2 and PM3, inboard vessel (SS316) for PM2 and PM3, outboard vessel (SS316) for PM3. Concerning inboard insulator, this component will not be further examined, because its weight is negligible (12.4 tons), compared with the other ones (1655 tons for inboard winding pack, 6183 tons for outboard vessel, 1295 tons for inboard vessel). Moreover, it is very likely that, being so close to inboard winding pack, insulator will not be separated from it in case of disposal. Concerning inboard vessel, its clearance indices are too high to consider any possible optimisation on material composition. Contact dose rates after 50 /60 years of cooling fall, within the recycling limit within the nuclear industry; however, handson recycling is not possible. The former route is the most convenient [9], for the inboard vessel, both for PM2 and PM3. Concerning the inboard winding pack, i.e. the inboard toroidal magnets, clearance indices are relatively high, but optimisation may be at least explored as an option. The most significant nuclide (up to 99%) is Nb94, which has a very long half-life. Other nuclides giving contribution to clearance indices greater than or close to one are: Co60 (only at 50 years of cooling, after 100 y is negligible), Ni63, Mo93 and Nb93m. The Ni63 is produced by the two nuclear reactions Ni62(n, g)Ni63 and Cu63(n, p)Ni63. Concerning the Niobium isotopes, the Nb94 is a product of Nb93(n, g)Nb94 reaction and the Nb93m is a product of Nb93(n, n?)Nb93m reaction. Niobium is used in magnets super-conducting
cables. It seems questionable the adoption of a different super-conducting alloy just for clearance reasons. Furthermore, no isotopic tailoring is possible, since Nb93 is the only isotope of Niobium. We can therefore conclude that no materials optimisation is possible in this case. Concerning outboard vessel, its activation for PM3 is quite higher than for PM2, since PM3 blanket has, by far, lower shielding capabilities than PM2 blanket. Due to the relevant weight of this component (6183 tons, about 10% of all the decommissioning activated materials), some material optimisation could be recommendable, in order to make possible the clearance of this component. We will concentrate first on PM3 outboard vessel steel pipes, which has the lower clearance index values and most of the weight (5636 tons). Concerning clearance with recycling, it turns out that most of the clearance index is due to Ag108m. This is an activation product of Ag, which has been assumed to be present as an impurity in SS316, with a concentration of 400 ppm. This concentration is rather conservative, since estimates assess that Ag concentration could be reduced down to 0.5 /1 ppm. If Ag concentration could be reduced to 40 ppm, then the clearance index for recycling would be less than one. In the case of clearance with disposal, Ni63 is the most responsible nuclide. In this case, Ni63 is totally an activation product of Ni62(n, g)Ni63. The second relevant nuclide is Ag108m (see above for discussion). Other relevant nuclides are Ni59, Nb91, Nb94 and Mo93. All these nuclides, Ag108m excluded, are activation products of Ni and Mo,
708
G. Cambi et al. / Fusion Engineering and Design 69 (2003) 705 /709
two essential alloying elements in SS316. Therefore, no optimisation is possible in this case. In previous stages of the SEAFP study, the use of OPTSTAB, a reduced-activation austenitic high-manganese steel, was considered for the vessel of SEAFP plant models. OPTSTAB has a reduced content of Ni and Mo: it is likely that this material may meet the goal of a clearance with disposal index inferior to 1, as far as PM3 outboard vessel steel pipes is concerned. For OPTSTAB too, of course, impurity content (Ag especially) should be kept low. For the PM3 outboard vessel wall, that weighs 547 tons, the clearance with recycling index after 100 y of cooling is about 13.5, while the clearance with disposal index is about 47. The most significant nuclides for it are (at 100 y) Ag108m and Nb94. According to this result, we recommend the following: a) reduction of the Ag impurity level by 100 times, i.e. from 400 to 4 ppm; b) melting of SS316 coming from the outboard vessel wall with other less-activated SS316, (for example with the one from the outboard vessel pipes). Considering the respective weights of the two zones, an average clearance index of about 0.8 could be obtained. In this case also, another viable solution is the adoption of the OPTSTAB, however controlling the impurity content (Ag especially).
4. Conclusions Clearance of ex-vessel components is one of the ways to reduce radioactive waste production from fusion power reactors. As a part of the WMS activities, this paper has examined the optimisation of fusion power reactors ex-vessel components composition in order to make possible their clearance, considering both clearance with disposal and clearance with recycling. Application to SEAFP plant models 2 and 3 has been discussed, using new activation data. It turns out that, for many materials, the clearance goal
can be reached without any optimisation of the material composition or of reactor design. Some inboard ex-vessel components (winding pack, insulator and vacuum vessel), however, do not reach the clearance goal or reach it only if diluted with fresh material, both for PM2 and PM3. Activation of all PM3 vessel and ex-vessel components is higher than for PM2, due to the lower shielding capability of PM3 blanket, compared with PM2. Moreover, for the same reasons, PM3 outboard vessel (SS316) cannot be cleared without optimisation, while PM2 correspondent component can. As far as design optimisation is concerned, it may be concluded that the shielding performance of the PM3 blanket should be improved. Materials composition optimisation has also been considered for all these components. The results show that material composition optimisation could be worth only in the case of PM3 outboard vessel that also represents about 10% of the total decommissioning materials for this plant model. The results show that most of the outboard vessel (5636 tons over 6183 in total) could be recycled outside the nuclear industry (clearance R) if the Ag impurity content could be reduced from the considered 400 ppm level. For an Ag reduction down to 4 ppm, all the outboard vessel of PM3 would have an average clearance (R) index lower than 1, after about 100 y cooling. In that case, also for PM3, the amount of clearable material could be the same as for PM2 (26 200 tons), rising the percentage of clearable material from about 30% to about 40%. The adoption for the outboard vessel of the OPTSTAB steel, in place of SS316, would permit to obtain the same goals, making also feasible clearance with disposal option.
References [1] M. Zucchetti, Clearance of Activated Materials: The ‘De Minimis’ Problem WMS/TSW1D5/ENEA/1 (Rev. 3), ENEA FUS-TN-SA-SE-R-021, July 2001. [2] Clearance Levels for Radionuclides in Solid Materials: Application of the Exemption Principles, Interim Report for Comment, IAEA TECDOC-855, Vienna, January 1996. [3] Radiation Protection 89-Recommended Radiological Protection Criteria for the Recycling of Metals from the
G. Cambi et al. / Fusion Engineering and Design 69 (2003) 705 /709 Dismantling of Nuclear Installations, European Commission, Directorate General Environment, Nuclear Safety and Civil Protection, 1998. [4] N. Taylor, I. Cook, Description of SEAFP-2 Plant Models, blankets and structural materials, SEAFP2/5.1/UKAEA/1 (Rev. 0), March 1997. [5] D.G. Cepraga, G. Cambi, M. Frisoni, Neutronic Calculations for SEAFP-2 Plant Models 2 and 3, WMS TSW1D5/ ENEA/4 (Rev. 0), ENEA FUS-TN-SA-SE-R-024, July 2001. [6] SCALE: A Modular Code System for Performing Standardized Computer Analyses for Licensing Evaluation, NUREG/CR-0200, Rev. 6 (ORNL/NUREG/CSD-2R6), vols. I, II, and III (May 2000). Version 4.4a of the code
709
package is available from Radiation Safety Information Computational Center (RSICC) at Oak Ridge National Laboratory as CCC-545. [7] R.A. Forrest, J.-Ch. Sublet, FISPACT-99: User Manual, UKAEA FUS 407, 1998. [8] P. Rocco, M. Zucchetti, Integrated Approach to Recycling and Clearance. More Realistic Management Of Activated Materials, Final Report-Task S5.2, SEAFP99/S5.2/JRC/1 (Rev. 1)-November 1999. Presented to the IAEA Technical Committee Meeting on Fusion Safety, Cannes, June 2000. [9] M. Zucchetti, R. Forrest, L. Di Pace, Clearance of Activated Materials: Optimisation of ex-vessel Material Composition WMS/TSW1D5/ENEA/2 (Rev. 3), ENEA FUS-TN-SA-SE-R-022, December 2001.