Briefly reviewed were recent R & D activities and achievements in Japan in the area of neutronics, core design and shielding studies on fast breeder reactor.

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This paper proposes a new concept for efficient verification of software for the protection systems of nuclear power plants. In this idea, all possible input combinations are applied to each module comprising the software and the module responses are compared with their correct responses which are provided beforehand. To realize the concept, algorithms have been developed for observing module responses through an external tester and for generating a quasi minimal test pattern set capable of testing all modules.
This approach remarkably reduces the number of required test input combinations in comparison with that of the combinations without observation of module outputs. In addition, most software errors are expected to be uncovered more efficiently with less effort and time.

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The oxidation of iron was studied by means of Rutherford backscattering spectroscopy in the temperature range from 523 to 673 K under the oxygen partial pressures of 1, 10^4.3and 10⁵ Pa for up to 7 d. It was found that the ionic diffusion in oxides is the rate-determining step of oxidation, since the oxidation of iron obeyed the parabolic rate law in the temperature and oxygen partial pressure ranges studied. As was observed at 573 K⁽¹⁾, two-stage oxidations were observed at temperatures lower than around 650 K, and one-stage oxidations at higher temperatures. The activation energies for the oxidation in the second stage were obtained to be 133±10, 149 ± 17 and 144 ± 6 kJ•mol^-1-1 at 1, 10^4.3 and 10⁵ Pa, respectively. The activation energies for oxidation obtained are nearly constant regardless of the oxygen partial pressure, but the absolute values of the parabolic rate constants are determined by the fraction of the coverage on magnetite with hematite. The changes in the temperature dependence of the parabolic rate constants were found around 670 K at 10^4.3 Pa and around 523 K at 10⁵ Pa, where the oxidation mechanism may change from the simultaneous growth of magnetite and hematite at higher temperatures to the dense hematite formation at lower temperatures. Comparing the temperature dependence of the corrosion rate in water with that of the oxidation rate in gas phase, the corrosion rate in water becomes considerably larger than the oxidation rate in gas phase below 523 K, where the metal dissolution may be dominant in the corrosion process in water.

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Dibutyl phosphate (DBP) was added to 30% TBP-70% dodecane solvent (DBP concentration, 301, 150 mg/l). These samples were used to investigate distribution behavior of Zr from a 3 NI nitric acid solution (Zr concentration, 604, 000 mg/l). The experimental results show that the Zr distribution depended mainly on the total DBP/Zr mole ratio. Below DBP/Zr=2, the Zr distribution to the organic phase became larger as the DBP/Zr mole ratio increased. Above DBP/Zr=2, the Zr distribution factor was saturated. Moreover, colloidal precipitation at the interface between organic and aqueous phases was observed above DBP/Zr=2.
Then in order to characterize the precipitation, 3 _M nitric acid solution containing Zr ion was directly reacted with DBP and the obtained products were analyzed by ¹³C, ³¹P and ¹⁴N NMR spectroscopy. The measured NMR spectra indicated that the products were polymers having four kinds of bonding structures. One was a hydrogen bonded structure and the others were Zr bridged structures with two to four DBP molecules coordinated per Zr atom. The Zr bridged structure with four DBP molecules was thought to make the biggest contribution to precipitate formation.

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The decontamination technique is required to have a high decontamination rate and decontamination factor, to apply to irregularly formed metallic waste, and minimize secondary waste volume for component replacement and decommissioning. The authors were developed a new decontamination technique using Ce⁴⁺ for decommissioning, which was named "REDOX decontamination technique". This paper has investigated decontamination conditions and regeneration conditions in a non-radioactive (namely cold) fundamental test. The decontamination conditions, namely dissolution conditions for stainless steel and carbon steel were selected using simulated samples in nitric acid solutions in varying Ce⁴⁺ concentrations, HNO3 concentration, temperatures, and linear flow rate of decontamination solutions. The regeneration conditions for decontamination reagent were selected in various electrode materials and varying electrode separation distance, anode area/cathode area ratio, current density, Ce³⁺ concentration, and HNO₃ concentration. The REDOX decontamination technique was ascertained to have a high dissolution rate and the decontamination reagent was ascertained to be regenerated effectively by electrolysis.

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Accumulation of the actinides in the extraction stage of the Purex process was investigated with mathematical models in terms of the U(VI)-loading effect, i.e. decrease of distribution ratios with increase of concentration of U(VI) in the organic phase. Time excursion of the accumulation profiles of U, Pu, Np and HNO₂ under several flowsheet conditions was estimated for a mixer settler process with a simulation code. Process experiments using U(VI) and U(IV) were carried out with mixer settlers to verify the simulation results. The concentration profiles of U(IV) and U(VI) agreed fairly well between the experimental and the calculated. Behavior of the peaking position of U(IV) build-up, however, did not always agree well. The causes of the discrepancy were discussed in terms of mass balance of U(VI). Fluctuation of the volume ratio of the two phases in the extractor during operation, particularly during the transient, affects significantly the total mass of U(VI) present in the extractor and hence the concentration profile of the element accumulated. The role of the third phase of Pu (IV) to criticality safety was mentioned.

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Ruthenium in nitric acid solution is oxidized by the addition of ceric nitrate to ruthenium tetroxide, which is extracted with n-paraffin oil.
Ruthenium tetroxide in paraffin is immediately reduced by the solvent to black ruthenium dioxide as suspension, which can be readily filtered off through the ordinary cellulose filter paper.
Through the combined process of extraction and filtration, the ruthenium in a nitric acid solution can be eliminated.

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The performance of a three-dimensional atmospheric transport and diffusion model to simulate an accidental release of radioactivity was evaluated by data sets from field tracer experiments over a mountain and valley area. The model consists of a mass-consistent model for wind fields and a particle random-walk model for atmospheric dispersion.
Model simulations of tracer concentrations were compared with observed values from eleven experiments ; four were conducted in down-valley winds ; two were in up-valley winds and five were in cross-valley winds. The calculated results almost agreed reasonably with observed concentration patterns. However, it became apparent that the model is subject to err from real dispersions in down-valley winds induced by nocturnal drainage flows.

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A three-dimensional diffusion calculation method has been proposed to rapidly and accurately calculate reactivity changes of LMFBRs caused by assembly displacements in accidental events. The method requires shorter computation times and provides almost the same accuracy as a conventional direct eigenvalue calculation method. In this method, changes in macroscopic neutron cross-sections and diffusion coefficient are defined so that changes in both region volume and material composition can be treated in a mesh-centered finite-difference program under the same coarse mesh division as used for the normal, non-deformed core. Reactivity changes are calculated from the above-mentioned changes by the first-order perturbation method using normal and adjoint neutron fluxes calculated beforehand for the normal core.
The method was applied to deformations of a 1, 000-MWe LMFBR core. Reactivity changes calculated by the method agreed within 0.4% with those by a conventional direct eigenvalue calculation method, while computation time was less than 1/35.

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High heat flux components in fusion reactors are examined from a viewpoint of structural design. Maximum admissible steady heat flux, which can be absorbed and removed by coated structures, was determined by one-dimensional numerical analysis basing on ASME code Sec. III Comparison of the current candidate materials is made in order to make heat flux as high as possible. The following conclusions were obtained. (1) Be-Cu structure can be used in order to remove high heat flux beyond 10 MW/m², however, the strong chemical activity of Be gives rise to a problem. (2) SiC-HT-9 and SiC-V alloy structures are promising in case of high heat flux less than 2 MW/m². (3) From thermal stress analysis tungsten and graphite are excellent coating materials and are available for many structural materials. The study of thermal shock resistance and thermal fatigue of these materials is now a problem to be solved.

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