Jinichi Nakamura

Effects of oxygen partial pressure on oxidation of Zircaloy

Abstract The effects of oxygen partial pressure on the oxidation of Zircaloy-4 were examined at 773 K under the oxygen partial pressure between 0.1 and 2 × 10−5MPa to study the oxidation behavior of spent fuel during dry storage. The oxidation rate at first obeyed a cubic rate law, and changed to obey a linear rate law at almost the same weight gains even at different oxygen partial pressures. The oxidation rate during posttransition period showed pO20.15 dependence. The dependency of the oxidation rate in the posttransition region agreed with the results of the oxygen diffusion coefficient in zirconium oxide measured by Keneshea and Douglass. Degradation of the protective oxide film of Zircaloy-4 under low oxygen partial pressures was not observed within the oxygen partial pressure range examined. The results of this experiment showed that the oxidation rate in air can be adopted to estimate the cladding oxidation rate during dry fuel storage in air and under low oxygen partial pressures.

Behavior of Irradiated ATR/MOX Fuel under Reactivity Initiated Accident Conditions

Pulse irradiation experiments with irradiated ATR/MOX fuel rods of 20MWd/kgHM were conducted at the NSRR in Japan Atomic Energy Research Institute to study the transient behavior of MOX fuel rod under reactivity initiated accident conditions. Four pulse irradiation experiments were performed with peak fuel enthalpy ranging from 335 J/g to 586 J/g, resulted in no failure of fuel rods. Relatively large radial deformation of the fuel rods due to pellet-cladding mechanical interaction occurred in the experiments with peak fuel enthalpy above 500 J/g. Significant fission gas release up to 20% was measured by rod puncture measurement. The generation of fine radial cracks in pellet periphery, micro-cracks and boundary separation over the entire region of pellet were observed. These microstructure changes might contribute to the swelling of fuel pellets during the pulse irradiation. This could cause the large radial deformation of fuel rod and high fission gas release when the pulse irradiation conducted at relatively high peak fuel enthalpy. In addition, fine grain structures around the plutonium spot and cauliflower structure in cavity of the plutonium spot were observed in the outer region of the fuel pellet.

The heat capacity of tantalum from 80 to 1000 K

Abstract The heat capacity of metallic tantalum has been measured in the temperature range 80–1000 K by laser-flash calorimetry. The results are compared with available low- and high-temperature heat capacities, and the thermodynamic values are given.

Matrix calculation of radiant heat transfer in LWR fuel bundles under accident conditions

Abstract A computer code was developed for calculating the radiant heat transfer in a LWR fuel bundle under accident conditions. The calculation method is a modular one: a fuel bundle or its part is divided into unit cells, each of which is composed of a coolant subchannel surrounded by several segments of solid or imaginary faces. The view factor matrix in each cell is expanded over the whole bundle using the concept of ‘boundary face’ between cells, and the resultant heat transfer equations are simultaneously solved for solid wall temperatures. The geometrical flexibility of this method is suitable for treating various simulation experiments for accidents. The method is also effective for repeated calculations of the radiant heat transfer reflecting state or material property changes when analyzing fuel rod behaviour under accident conditions.

Irradiated Fuel Behavior under Power Oscillation Conditions

In order to examine high burnup fuel performance under power oscillation conditions, two tests of irradiated fuels under simulated power oscillation conditions were conducted in the Nuclear Safety Research Reactor (NSRR). Irradiated fuels at burnups of 56 and 25 GWd/tU were subjected to four to seven power oscillations, which peaked at 50 to 95 kW/m with intervals of 2 s. The power oscillations were caused by quick withdrawal and insertion of six regulating rods of the NSRR with a computerized control. Impacts of cyclic loads on the fuel performance under hypothetical unstable power oscillations arising during an anticipated transient without scram (ATWS) in boiling water reactors (BWRs) were examined in the tests. Deformation of the fuel cladding of the test rods was comparable to those observed in shorter transient tests, which simulated reactivity-initiated accidents (RIAs), at the same fuel enthalpy level up to 368 J/g. The fuel deformation was mainly caused by pellet-cladding mechanical interaction (PCMI) and was roughly proportional to the fuel enthalpy. Enhanced cladding deformation due to ratcheting by the cyclic load was not observed. Fission gas release, on the other hand, was considerably smaller than in the RIA tests, suggesting different release mechanisms in the two types of transients.

Laser-flash calorimetry. V: Effect of oxygen contamination on the heat capacity of vanadium from 80 to 850 K

Abstract The heat capacity of vanadium with 0.33 mass per cent of O and with 1.11 mass per cent of O in solid solution has been measured by laser-flash calorimetry through the temperature region 80 to 850 K. Comparison of the results with earlier values for vanadium at and above ambient temperature and for vanadium and for a terminal solution alloy at very low temperatures shows that below about 450 K oxygen in solution depresses the heat capacity of vanadium by amounts which increase with increasing oxygen content. Above about 450 K, the heat capacities of vanadium and of the terminal solution alloys are comparable. This heat-capacity behavior results in differences between the entropies of vanadium and of terminal solution alloys which increase up to about 450 K and then remain relatively constant. These entropy differences are sufficiently large that, when multiplied by temperature to determine a Gibbs-energy contribution, the product is such as to indicate that very minor amounts of oxygen in vanadium may appreciably modify its alloying behavior.