[en] In the transmutation method of radioactive nuclides in radioactive wastes by nuclear transformation, since neutron energy obtained from a reactor is continuously distributed, it is difficult to efficiently obtain neutron fluxes of a specific energy which agrees with the resonance level energy of nuclei. Then, in the present invention, radioactive nuclides to be processed, which is accelerated to an energy corresponding to a composite nuclear resonance energy level are injected to the field of thermal neutrons to cause composite nuclear resonance reactions. As a result, even if the thermal neutrons have no resonance level energy, the composite nuclear resonance reaction can be taken place between the accelerated nuclides and the thermal neutrons. Then, long-life radioactive nuclides are converted to stable nuclides or short-life nuclides efficiently. For example, the natural half-decay time of 210,000 years of Tc-99 can be shortened to one-half in 13.8 hours, if it is accelerated to the composite nuclear resonance energy and injected to the field of thermal neutrons. (N.H.)

[en] Purpose: To convert long-life radioactive nucleides into short-life radioactive nucleides or stable nucleides by using gamma rays instead of neutrons. Method: Gamma-rays at 10 - 25 MeV are irradiated on the long-life radioactive nucleides to be processed to perform nuclear conversion. The gamma-rays are obtainable from an electron accelerator at about 30 MeV and an electron beam - gamma ray converter made of platinum. In the case of processing Cs-137, gamma-rays at 10 - 22 MeV are preferred. In the case of Sr-90, gamma rays at 14 - 22 MeV are preferred. (Ikeda, J.)

[en] An LMFBR type reactor is used as a target reactor, in which radioactive wastes and fuels are disposed respectively to the inner side and the outer side of the reactor and, further, moderators are added to the inside of the radioactive wastes or disposed between the radioactive wastes and the fuel material. Then, a γ -ray accelerator is disposed at a position of γ -ray irradiation to radioactive wastes. Radioactive wastes are eliminated by (γ, n) reactions under the γ -ray irradiation. Then, neutrons generated by the reaction are irradiated to the fuel material to maintain a high neutron flux and, at the same time, heat generated in the target reactor is converted into electric power and supplied to the electron ray accelerator. Further, the neutrons are irradiated to the radioactive waste to eliminate the radioactive wastes under (n, γ) reaction. It is possible to hybridize the radioactive wastes elimination device and the reactor and increase the amount of elimination and improve the energy efficiency. (N.H.)

[en] Hf hydride is proposed to be used as neutron control materials for fast reactors. The electronic, mechanical and thermal properties of its three phases: δ′-HfH_1.5, δ-HfH_1.75, ε-HfH₂, are investigated. Their relative stabilities at 0 K by our calculation are consistent with the explanation of Jahn–Teller mechanism. The mechanical properties like elastic constants are calculated and agree well with the experiments. At finite temperatures, in addition to the direct method for phonon calculation, electronic free energy is also calculated in order to investigate the thermal expansion and bulk moduli of three phases. Hf–H system has an increasing relationship in bulk moduli with respect to the H concentration before about 360 K, after which ε-HfH₂ seems to decrease more quickly in the softness of the structure than δ-HfH_1.75 as the temperature increases. The relation between heat capacity and Hf and H atoms vibration is discussed

[en] Phonon contribution to thermal conductivity of ZrH_1.6 is studied by nonequilibrium molecular dynamics (NEMD) method. Calculated thermal conductivities were compared with the values estimated from experiment results. The difference between simulated and experimental values is acceptable, considering no adjustment on potential function used in NEMD and uncertainty in estimation of the experimental value by Wiedemann-Franz relationship. The analysis of vibrational modes has been done by equilibrium molecular dynamics (EMD) method. It was pointed out that the high frequency vibration mode of hydrogen is important for heat conduction in ZrH_1.6

[en] It is considered that the strong radiation field during irradiation is one of important factors for considering the chemical reaction in nuclear fuel pins. The gaseous phase reaction of nuclear fission products, iodine and cesium, in fuel pins was taken up, and the effect of radiation was evaluated. As to the reason why this reaction was taken up, because iodine is one of the substances that cause FCCI in fast breeder reactors and stress corrosion cracking (SCC) in light water reactors, and it is important to understand its chemical behavior in fuel pins during irradiation for evaluating the soundness of fuel. As radiation, the heavy ions having large cross section for exciting atoms and molecules are considered. The compound CsI decomposes through the excitation process by the Coulomb interaction with this nuclear fission fragments or by the collision with high energy secondary gas molecules. The deviation of gaseous phase Cs-I reaction from the ordinary thermal equilibrium by these effects was evaluated. The thermodynamic examination, the radiation decomposition of CsI, the method of calculating partial pressure by gas velocity theory, the example of the calculation, and the Monte Carlo simulation calculation are reported. (K.I.)

[en] The chemical state of fission products (FPs) in U+ZrH_1.60 fuel was studied from the thermodynamic point of view. Twenty most abundant FP elements were taken into account in the system of U-Zr-H-O-FP in which oxygen is treated as an impurity. The Thermo-Calc computer code was used to calculate the equilibrium state of the multi-phase and multi-component system. This calculation shows that yttrium, the alkaline earth metals (Ba, Sr) and most of the lanthanides prefer to form corresponding binary hydrides. Oxygen impurities in the system are likely to form a mixture of Y₂O₃, Pr₂O₃, Sm₂O₃ as well as Ce₂O₃ depending on their fission yields. With increasing of burn-up, only a slight decrease of the hydrogen potential in the fuel pin can be expected because of the very little consumption of hydrogen in the hydrogenation process of the FP. In the gas phase, H₂, Cs, Rb, CsRb as well as CsI are the main vapor species apart from the noble gases Xe and Kr. Solid swelling of the fuel due to formation of condensed phases was calculated as a function of burn-up. (authors)

[en] Two transmutation methods, the spallation neutron and the muon-catalyzed fusion methods, both which use an accelerator, are employed for the transmutation of long-lived nuclides in high-level radioactive wastes. The transmutation energies and the effective half-lives of ⁹⁹ T_c for both transmutation methods are calculated by the Monte Carlo simulation codes for particle transport, the NMTC/JAERI code and the MCNP code. Both methods could obtain short effective half-lives, which are 17 times smaller than those of a fission reactor. The transmutation energies are calculated to be 25 to 55 MeV for both methods. These calculated transmutation energies reveal that it is possible for the foregoing two methods for transmutation of ⁹⁹ T_c to meet the energy balance criterion

No abstract available

[en] Two waste treatment methods, geological disposal and transmutation, have been studied. The transmutation method changes long-lived radioactive nuclides to short-lived one or stabilizes them by nuclear transformation. The transmutation by actinoid hydride is exactly alike that transformation method from actinoid disposal waste to Pu fuel. For this object, OMEGA project is processing now. The transmutation is difficult by two causes such as large amount of long-lived radioactive nuclides and not enough development of control technologies of nuclear reaction except atomic reactor. The transmutation using actinoid hydride has merits that the amount of actinoid charged in the target increases and the effect of thermal neutrons on fuel decreases depending on homogeneous transmutation velocity in the target. Development of stable actinoid hydride under the conditions of reactor temperature and irradiation environment is important. The experimental results of U-ZrH_1.6 are shown in this paper. The irradiation experiment using Th hydride has been proceeding. (S.Y.)