4 Conclusions
This paper summarizes our previous works on neutronics analysis for the disposal
of damaged fuels from Fukushima Daiichi reactors. Three major stages have been
identified for the criticality safety assessment after disposal.
For stages when fissile nuclides in the damaged fuels remains in the vicinity of
the engineered barriers, the k eff for a canister containing fuel debris surrounded by
buffer was considered over the leaching time. Based on literature review, the fuel
debris has been modeled as a hexagonal lattice of spherical fuel particles. Based on
the numerical results, the following key observations can be made: (a) the calculated neutron multiplication factor (k eff ) is sensitively dependent on assumptions
related to moderation, (b) the carbon steel canister plays an important role in
reducing the potential for criticality, (c) the maximum k eff of the canister-buffer
system could be achieved after a fraction of fissile nuclides been released from the
canister, and (d) under several assumptions, the maximum k eff of the canister-buffer
system could be principally determined by the dimension and composition of the
canister, not by the initial fuel loading. Future works in this area are planned to
apply the present approach for damaged fuels from Unit 2 and Unit 3, to consider
more modes for release from the canister, such as leaching of the damaged fuels by
reducing the radius of each fuel particle, to consider buffer swelling or collapsing
due to degradations, and to develop detailed models to connect the models for
single canister with models for the deposition from multiple canisters. The
dependence on model parameters, such as fuel particle radius, need to be further
examined. We will also investigate the option of using backfilling materials to
control criticality in the engineered barrier design.
For the stage when fissile nuclides originated from multiple packages deposit in
far-field host rocks, the critical masses for uranium depositions were studied for
various rock types and geometries. The analysis has been made for two kinds of
rocks by considering a finite system with three different geometries, containing
various masses of uranium. The three different geometries include heterogeneous
(fractured I and II) and homogeneous systems. The exploration was performed to
find optimized combinations of geometry, fracture aperture and the model parameter HMVF, to give the minimum rock porosity (VVF) for criticality. The numerical
results show that: the k eff for the deposition become greater with (1) smaller concentrations of neutron-absorbing materials in the host rock, (2) larger porosity of the
host rock, (3) heterogeneous geometry of the deposition, and (4) greater mass of
uranium in the deposition. After the present analysis, we conclude that various
far-field critical configurations are conceivable for given conditions of materials and
geological formations. Whether any of such critical configurations would occur in
actual geological conditions remains unanswered. To answer this question, we need
to extend the present study into the following directions. First, from the neutronics
point of view, a more “realistic” fractured system with both the fracture orientation
and size randomly distributed is suggested. Second, we need to perform the mass
transport analysis to explore whether such a configuration obtained by neutronics
analysis is likely to be occurred in geological formations.
276
X. Liu
This paper summarizes our previous works on neutronics analysis for the disposal
of damaged fuels from Fukushima Daiichi reactors. Three major stages have been
identified for the criticality safety assessment after disposal.
For stages when fissile nuclides in the damaged fuels remains in the vicinity of
the engineered barriers, the k eff for a canister containing fuel debris surrounded by
buffer was considered over the leaching time. Based on literature review, the fuel
debris has been modeled as a hexagonal lattice of spherical fuel particles. Based on
the numerical results, the following key observations can be made: (a) the calculated neutron multiplication factor (k eff ) is sensitively dependent on assumptions
related to moderation, (b) the carbon steel canister plays an important role in
reducing the potential for criticality, (c) the maximum k eff of the canister-buffer
system could be achieved after a fraction of fissile nuclides been released from the
canister, and (d) under several assumptions, the maximum k eff of the canister-buffer
system could be principally determined by the dimension and composition of the
canister, not by the initial fuel loading. Future works in this area are planned to
apply the present approach for damaged fuels from Unit 2 and Unit 3, to consider
more modes for release from the canister, such as leaching of the damaged fuels by
reducing the radius of each fuel particle, to consider buffer swelling or collapsing
due to degradations, and to develop detailed models to connect the models for
single canister with models for the deposition from multiple canisters. The
dependence on model parameters, such as fuel particle radius, need to be further
examined. We will also investigate the option of using backfilling materials to
control criticality in the engineered barrier design.
For the stage when fissile nuclides originated from multiple packages deposit in
far-field host rocks, the critical masses for uranium depositions were studied for
various rock types and geometries. The analysis has been made for two kinds of
rocks by considering a finite system with three different geometries, containing
various masses of uranium. The three different geometries include heterogeneous
(fractured I and II) and homogeneous systems. The exploration was performed to
find optimized combinations of geometry, fracture aperture and the model parameter HMVF, to give the minimum rock porosity (VVF) for criticality. The numerical
results show that: the k eff for the deposition become greater with (1) smaller concentrations of neutron-absorbing materials in the host rock, (2) larger porosity of the
host rock, (3) heterogeneous geometry of the deposition, and (4) greater mass of
uranium in the deposition. After the present analysis, we conclude that various
far-field critical configurations are conceivable for given conditions of materials and
geological formations. Whether any of such critical configurations would occur in
actual geological conditions remains unanswered. To answer this question, we need
to extend the present study into the following directions. First, from the neutronics
point of view, a more “realistic” fractured system with both the fracture orientation
and size randomly distributed is suggested. Second, we need to perform the mass
transport analysis to explore whether such a configuration obtained by neutronics
analysis is likely to be occurred in geological formations.
276
X. Liu