This study focuses on neutronic analysis to examine the criticality conditions for uranium depositions in geological formations resulting from geological disposal of damaged fuels from Fukushima Daiichi reactors. MCNP models are used to evaluate the neutron multiplication factor (k eff ) and critical mass for various combinations of host rock and geometries. It has been observed 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. This study has revealed that the planar fracture geometry applied in the previous criticality safety assessment for geological disposal would not necessarily yield conservative results against the homogeneous uranium deposition.

Neutron-deficient Z ≈ N nuclei 84,86 Mo have been investigated using pairing-deformation self-consistent cranked shell model calculations up to spin I > 20 . Our calculations are in good agreement with the experimental data, indicating γ-soft triaxial shapes at low rotational frequency and well-deformed triaxial-oblate shapes at high rotational frequency for both nuclei. The shape change is due to the alignments of the g 9/2 protons and g 9/2 neutrons.shape change, cranked shell model, molybdenum

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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. In order to evaluate the criticality safety for certain repository conditions and engineered barriers designs, neutronics models have been defined for different stages, and numerical results have been calculated by a Monte-Carlo code MCNP. For stages when fissile nuclides in the damaged fuels remains in the vicinity of the engineered barriers, the neutron multiplicity (k eff ) for a canister containing fuel debris surrounded by buffer was calculated over the leaching time. 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 methodology presented in the present paper could be further improved and utilized to assist the repository system design and criticality safety assessment in the future.