Kenji Konashi

Radiation effect on partial pressure of fission product iodine

Abstract The knowledge of the partial pressure of fission product iodine is important for evaluating the FCCI (fuel cladding chemical interaction) in FBR. Thermodynamical analysis shows that iodine is believed to react with cesium so strongly that the partial pressure of iodine is kept too low to cause FCCI. A molecule of cesium iodide is, however, excited by radiation in the reactor and dissociates into radical and ion. In this work, the radiation effect on the partial pressure of iodine is theoretically studied for the gaseous cesium-iodine system. Fission fragment flux is considered as the radiation and the iodine partial pressure is evaluated based on the theory of gaseous reaction. The results of the calculation show that the partial pressure of iodine can be increased enough to cause FCCI in the radiation field. In the case of the FBR fuel pin, the partial pressure of iodine under radiation conditions can be calculated to be ~ 10−2 Pa (~10−7 atm), which is higher than that of the non-radiation condition ~10−9 Pa (~10−14 atm). These calculations were carried out under the assumption that the cladding inner surface temperature was 873 K (600°C) and the oxygen potential was −418 kJ/mol (−100 kcal/mol).

The melting temperature of irradiated oxide fuel

Abstract The melting temperature of oxide fuel is generally considered to decrease with increasing of burnup because of the buildup of fission products. Melting temperatures were measured for UO 2 and (U, Pu)O 2 fuels. The (U, Pu)O 2 specimens were irradiated to a burnup of 110 GWd/t in FBR. The solidus temperatures of (U, Pu)O 2 were almost independent of burnup up to 50 GWd/t. The melting temperatures were correlated with Pu content, oxygen-to-metal ratio of the fuel and burnup by ideal solution theory.

Chemical Vapor Transport of Iron by Iodine Liberated due to Radiolysis of Cesium Iodide

The cladding component chemical transport (CCCT) is one of the important modes of the fuel-cladding chemical interaction (FCCI) of LMFBR. In order to explain this phenomenon, a model based on the vapor phase chemical transport of cladding components by iodine was proposed by Johnson et al. and Calais et al. In this study, experimental work has been done to check whether such a mechanism can work due to the free iodine generated by the radiolysis of Csl vapor. As a result, it was confirmed that a significant amount of Fe can be transported via vapor phase from the Fe sheet heated at 430°C to the Mo plate heated at 720 or 800°C. Preliminary comparisons between this study and the in-pile irradiation tests have been made. This result qualitatively supports the appropriateness of the model for the CCCT mechanism based on the vapor phase transport of cladding components by radiation-induced iodine.

Transmutation of 99Tc with the use of an accelerator

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 [sup 99][Tc] 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 [sup 99][Tc] to meet the energy balance criterion.

CsI decomposition due to collision cascade initiated by fission fragments

Abstract Fission fragments generate energetic atoms in a gas region in an operating fuel pin. The flux of the energetic atoms was estimated based on a simplified collision cascade theory. Using the calculated flux, the decomposition rate of CsI vapor was calculated, and was compared with the value evaluated from the direct collision of fission fragments with CsI gas molecules. The calculated results revealed that the collision cascade initiated by fission fragments has a larger effect on the CsI decomposition than that by the direct collision with fission fragments.

Reactivity of cesium iodide vapor under irradiation

Abstract An out-of-pile experiment was performed to clarify the chemical forms of iodine and cesium under irradiation. Cesium iodide vapor was irradiated by an electron beam and the resulting radiolysis was examined by atomic absorption spectroscopy. The effective cross-section of CsI radiolysis, 2.8 × 10 −16 cm 2 , and the recombination rate constant between Cs and I, 1.45 × 10 13 (mol −1 cm 3 s −1 ), at 733 K, were determined. These values were confirmed using the results of previous transport experiments.

Design of a high flux reactor for the transmutation of fission products

We studied the transmutation of 137Cs into a stable nuclide in a fission reactor. The reactor we considered is a high-flux fast reactor with an inner thermal region for the transmutation. Neutronic calculations for the reactor were carried out. The average thermal neutron flux was 2.5 × 1015 n/cm2s in the Cs region under the constraints that the burnup reactivity and the peaking factor are within 3.0 and 1.7% Δk/kk′, respectively, which correspond to conditions in ordinary fast reactors. The reactor gives a transmutation rate of 2.8%/yr for 137Cs.

Incineration of 90Sr and 137Cs by an inertial fusion target

Abstract We discuss the inertial confinement fusion system as a transmutator of radioactive waste (90Sr and 137Cs). An analytic model of the implosion of the target, which is composed of DT fuel and radioactive waste, is used to evaluate its internal energy and the probability of neutron utilization. From the results of this calculation, we could evaluate the energy that is required to transmute radioactive waste.

Transmutation of 90Sr by inertial confinement fusion

Transmutation of [sup 90]Sr by inertial confinement fusion is discussed. A pellet composed of deuterium-tritium fuel surrounded by [sup 90]Sr is compressed by a laser or a particle beam. It is shown that a high transmutation rate and a small transmutation energy are obtained because of the highly compressed [sup 90]Sr, which has a large probability of a transmutation reaction. The number of cycles, including recovering and refabrication of the target, is also discussed. 16 refs., 8 figs., 2 tabs.