The experimental device for Doppler effect measurement was developed for the Fast Critical Assembly of Japan Atomic Energy Research Institute. The Doppler effect up to 1, 500°C can be measured with this device. The experimental data is to be used for evaluating and improving the calculation method of the Doppler effect in higher temperature range. The measurement technique is based on a reactivity worth measurement with a heated sample. The device is made of tungsten for the high temperature parts and stainless steel for the other structural parts. The measurements were carried out in a mock-up core of an oxide-fueled fast reactor.
In the analysis of the experimental data, the Doppler reactivity worth was calculated by a first order perturbation theory and JENDL-3.2 library. A collision probability cell code with ultra-fine group structure, PEACO-X, was used to obtain the precise effective cross sections in a pin cell model of the Doppler sample. The calculation underestimates Doppler effect by about 5% in high temperature range.
The Doppler effect above 1, 500°C was estimated by the extrapolation of the measured data below this temperature. When the calculated and extrapolated Doppler reactivity worths at 2, 000°C were compared, a good agreement between them was obtained. The significant improvement of calculation reliability of the Doppler effect in the higher temperature range was achieved by this experiment.

View full abstract

An analysis was performed for the temperature coefficient of reactivity measured in the six types of light-water-moderated and heavy-water-reflected cylindrical cores containing highly-enriched-uranium (HEU) or medium-enriched-uranium (MEU) fuel, which was constructed in the Kyoto University Critical Assembly (KUCA). The purpose of the present analysis was to reveal a mechanism why a light-water region existed in the core contributes to a large positive temperature effect on reactivity. Therefore, based on the assessment of the computational method to calculate the temperature coefficient of reactivity in a multi-region type core, studies were carried out to examine each effect of three physical processes (Doppler broadening, thermal expansion and thermal neutron spectral shift) on the temperature coefficient and to separate each contribution of the multi-regions to this physical quantity. The measured temperature coefficients were approximately simulated by the calculations using the SRAC code system. The Doppler broadening caused a slightly negative effect in the MEU cores and the thermal expansion a negative effect in all the cores, whereas the thermal neutron spectral shift caused a large positive effect in all the cores. The temperature effect on reactivity in the fuel region was negative, while that in the light-water region existed in the core was positive because of the decrease in neutron absorption due to the thermal expansion and the spectral shift effects, and it became positive in the present core where large light-water regions existed in the core.

View full abstract

Iodine mass transfer in a xenon-iodine mixed gas bubble rising through a liquid sodium pool is analyzed on the basis of a diffusion model applied to the first short stage just after the bubble generation and a convection model applied to the successive stage. In the diffusion model, production of sodium iodide aerosols and generation of the heat caused by the chemical reaction of iodine vapor and sodium vapor are taken into account in addition to the diffusion of vapor and aerosols and the heat conduction in a static spherical bubble. The diffusion of aerosols is composed of Brownian motion, thermophoresis and diffusiophoresis. In the convection model, the analysis is made for aerosol settling caused by inertial deposition, sedimentation and Brownian motion in an internal flow induced by a spherical cap bubble rising.
Increase in the initial iodine concentration in the bubble is shown to enlarge the temperature difference across a region between the reaction front and the bubble surface and to enhance a contribution of thermophoresis to the aerosol diffusion through the region. The decontamination factor obtained from the calculation describes well a rapid increase at the first stage and a slow increase in the successive period, which are seen in measured decontamination factors, and suggests the breakup of the original bubble during rising through the pool.

View full abstract

In-pile experiments of fresh fuel rods under reactivity initiated accident (RIA) conditions have been performed in the Nuclear Safety Research Reactor at the Japan Atomic Energy Research Institute in order to understand the basic pellet cladding mechanical interaction (PCMI) behavior. Rapid fuel pellet expansion due to a power excursion would cause radial and longitudinal deformation of the cladding. This PCMI could be one of the possible incipient failure modes of an embrittled cladding of a high burnup fuel under the RIA conditions.
Basic PCMI behavior was studied by measuring cladding deformation of a fresh fuel rod without complicated irradiation effects. The transient elongation measurements of the fuel with two kinds of gap width indicated not only PCMI-induced cladding elongation, but also reduction of the pellet stack displacement by the cladding constraint. In the tests under a high-pressure and high-temperature condition simulating an operation condition of BWRs, additional ridge-type cladding deformation was generated due to the axial collapse of the cladding. A preliminary analysis for interpretation of the tests was made using a computer code for the transient analysis of fuel rods, FRAP-T6.

View full abstract

Studies have been made on the void swelling of solution-annealed Fe-15Cr-20Ni alloy irradiated with 180 keV protons at temperatures from 723K to 923K. The displacement damage levels were 5, 10 and 20dpa for samples preinjected with 10 appm He at room temperature, and 5dpa for samples without He. In comparison with previously reported results of nickel-ion bombardment, void nucleation seemed to finish at earlier stage of irradiation in samples injected with He. Subsequent void growth was rather fast and, as a consequence, relatively large swelling was observed. Void microstructure and swelling were similar at 723 and 773K between the samples with and without preinjected He, but they were quite different at 823K probably due to helium effect which persists even at high temperatures. The results suggest that hydrogen introduced into the material by proton irradiation may play a significant role, which is similar and comparable to that of helium, in the microstructure evolution during irradiation at temperatures up to 773K.

View full abstract

It is sought to describe the interphase transfer kinetics governing the electrolytic reduction of chloride salt mixtures in the process performed for the purpose of recovering alkali, alkaline earth and rare earth metals from the salt mixture containing fission products generated during pyrometallurgical processing of highly radioactive waste from the Purex process of spent fuel. Data for analyzing the two-phase transfer kinetics of the metals of interest are acquired on simulated chloride mixtures submitted to experimental electrolysis in a system comprising anode immersed in molten salt mixture and cathode of liquid lead. Approximating the concentrations of individual metals remaining in the mixture as exponential functions of the duration of electrolysis, the apparent overall transfer coefficients of the metals in reductive reaction are estimated to range from 1.5×10^-6 to 3.8×10^-5m/s. The metal ions prove to rank in descending order of transfer coefficient: Eu³⁺>Ba²⁺>Sr²⁺>Na⁺>Li⁺>K⁺>Cs⁺, thus indicating the likely effect of ionic valence influencing the interphanse transfer of metals.

View full abstract

The purpose of this report is to publish results on a comprehensive review of research subjects on safety of the dry cask storage and to publish a part of the research results. (1) A method to calculate the maximum allowable temperature of fuel cladding was proposed, based on experiments using irradiated and non-irradiated Zircaloy tube specimens, in order to prevent excessive creep deformation of fuel cladding in dry cask. (2) Under an accidental test of "Cask drop onto concrete floor", the cask integrity was demonstrated using full-scale casks (100-t class). (3) Under an accidental test of "Building collapse and drop of heavy objects onto cask", the cask integrity was demonstrated using a full-scale cask (100-t class). (4) As to possibility of cask toppling by earthquake, analytical and experimental results using a scale model cask showed that the cask will not be toppled by earthquake.

View full abstract

The Japan Atomic Energy Research Institute (JAERI) and the United States Nuclear Regulatory Commission (USNRC) are jointly conducting confirmatory, integral testing on the Westinghouse AP600 reactor transient responses by using the ROSA-V Large Scale Test Facility of JAERI. This facility, built originally to simulate conventional 4-loop pressurized water reactors (PWRs), has been modified by adding components specific to the AP600 design. The modified LSTF now provides a full-pressure, full-height, 1/30.5 volumetrically-scaled simulation of AP600. Five loss-of-coolant accident (LOCA) experiments were performed by August 1994, simulating transients initiated by cold leg breaks, a Pressure Balance Line (PBL) break, and inadvertently open Automatic Depressurization System (ADS) valves. These experiments indicated adequate core cooling and decay heat removal performance of the AP600 passive safety components.

View full abstract