Sketch of the reactor core rotated by 90 degrees. The reflector top is at 100 cm, the position in which criticality is achieved. The Emergency shut-down rod is not inserted, but the hafnium absorber is inserted. 

Contexts in source publication

Context 1

... the basis of Bathke’s approach the nuclear material always shows a high (FOM 1 > 2) or medium-high (1<FOM 2 <2) level of proliferation attractiveness, depending on whether pre-ignition is a main concern for weapon development. It is clear that both criteria, each one in a different way, identify a significant proliferation risk for the fuel loaded in the UO core. In this section the proliferation risk associated with the use of MOX fuels is analyzed. The starting point is the reference UO 2 core design (section III.A), with the fuel assemblies replaced by MOX containing a mix of plutonium, 235 U, and 238 U in different wt% depending on their location in the core (see Table IX). The plutonium vector considered at BOL is reported in Table X; it is representative of the plutonium produced in traditional LWR fuels after an exposure of 45 GWd/t and 7 years of cooling/storage (Ref. 9). It should be noted that the 240 tot current fissile material has a Pu/Pu ratio greater than 30% i.e. the fuel can be categorized as MOX-grade according to Pellaud’s criteria. The maximum amount of plutonium has been limited to 14 wt% for safety reasons in order to avoid any risk of positive reactivity void coefficient in case of generation of coolant voids. This value has been determined on the base of the following correlation that gives the maximum allowable plutonium content T(wt%) 239 as a function of mass percentage C i of 4 isotopes Pu, 240 241 242 Pu, Pu and Pu (Ref. 9): The MOX assemblies loaded in the core have the same mechanical and geometrical characteristics as those in the UO 2 core. Minor modifications only concern the contents of Gd 2 O 3 (reduced from 8 wt% to 2 wt%). Thermal-hydraulic operating conditions, rated power and reactivity control systems are those already defined for the UO 2 core. Results of core simulations in steady state conditions up to the equilibrium cycle are reported in Table XI. As can be seen, the use of MOX fuels has an impact mainly on the effectiveness of boron and reactor kinetics (lower values for boron coefficient and β eff ). The cycle length is slightly shorter (burnup reduced by 1.82 GWd/t for the MOX core) while no significant alterations are observed for the values of the Doppler and the moderator temperature coefficient. Main results for the isotopic evolution of the plutonium content in the core during the first burnup cycle are presented in Table XII. Both Pellaud’s and Bathke’s criteria are used for the categorization of the nuclear fuels with respect to proliferation risks. According to Pellaud’s approach the abundance of 240 Pu in the MOX core is always higher than 30%, i.e. the plutonium is always of MOX-grade type; this would induce the conclusion that the fuel is not attractive for the development of nuclear weapons at each exposure level (this condition is preserved up to equilibrium cycle). Different conclusions can be deduced if Bathke’s criteria are used. As it can be seen from Table XII, MOX fuel shows a high attractiveness to proliferation (FOM 1 >2) in those countries that are technically advanced and able to deal with pre-ignition issues. The attractiveness level is reduced to medium-low (FOM 2 ~ 1) if the fuel is managed by a technically relatively unsophisticated proliferator. However, according to Bathke most states dedicated to develop a nuclear weapon would be able to develop the necessary technology to overcome pre-initiation due to spontaneous neutron emission over time. Therefore, in most cases, FOM 1 is the only Figure of Merit to be considered. In this sense, although the use of a MOX core in SMRs reduces the proliferation risk compared to UO 2 fuel, such a core would still be attractive for diversion or undeclared production of nuclear material or misuse of technology to acquire nuclear weapons. For this analysis a generic sodium-cooled reactor model based on the publicly available design information of the Toshiba 4S was established using (Ref. 10) and (Ref. 11). The reactor core has 10 MWe output and an expected lifetime of 30 years without refuelling or reshuffling of fuel elements. Figure 5 shows the vertical section of the core modelled by using the Monte Carlo Neutron Transport Code MCNPX (Ref. 12). The long core lifetime of 30 years is possible due to the implementation of an adjustable annular reflector. The reflector consists of a reflecting steel region (yellow) with helium filled gas tanks above (red) to replace the sodium coolant (green) which is also a strong neutron reflector. Over the lifetime the whole annular structure moves progressively upwards (in Fig. 5 to the right due to rotation of the sketch) and hence covers new regions of the core. Criticality is reached by covering a part of the core large enough to reflect sufficient neutrons back into the core to obtain a self-sustaining chain reaction. Only half of each fuel rod is filled with actual fuel (dark blue), the other half is a gas plenum for resulting fission gases. The assembly of fuel rods can be better seen in Figure 6. The core consists of 19 subassemblies with the inner one filled with a hafnium absorber and space for the emergency shut-down rod. The fuel is a uranium- zirconium alloy with 10 wt% zirconium and an enrichment of 17 wt% and 19.9 wt% 235 U with the six outermost subassemblies having the higher enrichment. Each subassembly contains 169 fuel rods that are densely packed. The main core and assembly data for the SMR core are summarized in Table XIII. With this model of the reactor core different calculations, especially criticality calculations were executed to validate the geometry. The criticality in dependence of the reflector position and the neutron flux distributions show the expected behaviour. Fig. 6. Cross section of the reactor core. Each subassembly consists of 169 fuel rods with an enrichment 235 235 of 17 wt% U in the inner and 19.9 wt% U in the outer burning ...

Context 2

... the basis of Bathke’s approach the nuclear material always shows a high (FOM 1 > 2) or medium-high (1<FOM 2 <2) level of proliferation attractiveness, depending on whether pre-ignition is a main concern for weapon development. It is clear that both criteria, each one in a different way, identify a significant proliferation risk for the fuel loaded in the UO core. In this section the proliferation risk associated with the use of MOX fuels is analyzed. The starting point is the reference UO 2 core design (section III.A), with the fuel assemblies replaced by MOX containing a mix of plutonium, 235 U, and 238 U in different wt% depending on their location in the core (see Table IX). The plutonium vector considered at BOL is reported in Table X; it is representative of the plutonium produced in traditional LWR fuels after an exposure of 45 GWd/t and 7 years of cooling/storage (Ref. 9). It should be noted that the 240 tot current fissile material has a Pu/Pu ratio greater than 30% i.e. the fuel can be categorized as MOX-grade according to Pellaud’s criteria. The maximum amount of plutonium has been limited to 14 wt% for safety reasons in order to avoid any risk of positive reactivity void coefficient in case of generation of coolant voids. This value has been determined on the base of the following correlation that gives the maximum allowable plutonium content T(wt%) 239 as a function of mass percentage C i of 4 isotopes Pu, 240 241 242 Pu, Pu and Pu (Ref. 9): The MOX assemblies loaded in the core have the same mechanical and geometrical characteristics as those in the UO 2 core. Minor modifications only concern the contents of Gd 2 O 3 (reduced from 8 wt% to 2 wt%). Thermal-hydraulic operating conditions, rated power and reactivity control systems are those already defined for the UO 2 core. Results of core simulations in steady state conditions up to the equilibrium cycle are reported in Table XI. As can be seen, the use of MOX fuels has an impact mainly on the effectiveness of boron and reactor kinetics (lower values for boron coefficient and β eff ). The cycle length is slightly shorter (burnup reduced by 1.82 GWd/t for the MOX core) while no significant alterations are observed for the values of the Doppler and the moderator temperature coefficient. Main results for the isotopic evolution of the plutonium content in the core during the first burnup cycle are presented in Table XII. Both Pellaud’s and Bathke’s criteria are used for the categorization of the nuclear fuels with respect to proliferation risks. According to Pellaud’s approach the abundance of 240 Pu in the MOX core is always higher than 30%, i.e. the plutonium is always of MOX-grade type; this would induce the conclusion that the fuel is not attractive for the development of nuclear weapons at each exposure level (this condition is preserved up to equilibrium cycle). Different conclusions can be deduced if Bathke’s criteria are used. As it can be seen from Table XII, MOX fuel shows a high attractiveness to proliferation (FOM 1 >2) in those countries that are technically advanced and able to deal with pre-ignition issues. The attractiveness level is reduced to medium-low (FOM 2 ~ 1) if the fuel is managed by a technically relatively unsophisticated proliferator. However, according to Bathke most states dedicated to develop a nuclear weapon would be able to develop the necessary technology to overcome pre-initiation due to spontaneous neutron emission over time. Therefore, in most cases, FOM 1 is the only Figure of Merit to be considered. In this sense, although the use of a MOX core in SMRs reduces the proliferation risk compared to UO 2 fuel, such a core would still be attractive for diversion or undeclared production of nuclear material or misuse of technology to acquire nuclear weapons. For this analysis a generic sodium-cooled reactor model based on the publicly available design information of the Toshiba 4S was established using (Ref. 10) and (Ref. 11). The reactor core has 10 MWe output and an expected lifetime of 30 years without refuelling or reshuffling of fuel elements. Figure 5 shows the vertical section of the core modelled by using the Monte Carlo Neutron Transport Code MCNPX (Ref. 12). The long core lifetime of 30 years is possible due to the implementation of an adjustable annular reflector. The reflector consists of a reflecting steel region (yellow) with helium filled gas tanks above (red) to replace the sodium coolant (green) which is also a strong neutron reflector. Over the lifetime the whole annular structure moves progressively upwards (in Fig. 5 to the right due to rotation of the sketch) and hence covers new regions of the core. Criticality is reached by covering a part of the core large enough to reflect sufficient neutrons back into the core to obtain a self-sustaining chain reaction. Only half of each fuel rod is filled with actual fuel (dark blue), the other half is a gas plenum for resulting fission gases. The assembly of fuel rods can be better seen in Figure 6. The core consists of 19 subassemblies with the inner one filled with a hafnium absorber and space for the emergency shut-down rod. The fuel is a uranium- zirconium alloy with 10 wt% zirconium and an enrichment of 17 wt% and 19.9 wt% 235 U with the six outermost subassemblies having the higher enrichment. Each subassembly contains 169 fuel rods that are densely packed. The main core and assembly data for the SMR core are summarized in Table XIII. With this model of the reactor core different calculations, especially criticality calculations were executed to validate the geometry. The criticality in dependence of the reflector position and the neutron flux distributions show the expected behaviour. Fig. 6. Cross section of the reactor core. Each subassembly consists of 169 fuel rods with an enrichment 235 235 of 17 wt% U in the inner and 19.9 wt% U in the outer burning ...