Verification of the "ARKAS" code, using problems defined in the IWGFR Coordinated Research Programme (CRP) on the comparison between LMFBR Core Mechanics Codes, is discussed. The problems of verification were defined and calculated by 11 core mechanics codes from 9 countries. A comparison of the solutions obtained by these codes was carried out as Stage 1 of the CRP. This paper describes some typical ARKAS's results, together with comparisons with average values taken from the solutions obtained by the participants in the CRP. All the problems defined in the CRP have been completed and were solved with the core structural mechanics code ARKAS. Comparisons between the ARKAS's solutions and the average values of all available solutions suggest that displacements can be predicted to better than 1% and loads to better than 3% by ARKAS for the well-defined verification problems of Stage 1.

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Thermal diffusivities of samples of UO₂ and UO₂ doped with 3, 5, 7 and 10 ^w/₀ Gd₂O₃ were measured over the temperature range of 2982, 023K by a laser flash method. Then thermal conductivities were calculated from them. The thermal conductivity decreased with increasing Gd₂O₃ content at low temperatures, while it was independent of Gd₂O₃ content at high temperatures. An expression of the thermal conductivity was proposed for (U, Gd)O₂ solid solution as a function of Gd₂O₃ content and temperature by applying Klemens' model.

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Creep rupture tests were carried out on a γ"-Ni₃Nb precipitation strengthened alloy (Alloy 7818) after helium implantation (10 at. ppm) at high temperatures in order to clarify the effect of helium on mechanical properties and results were compared to those of SUS 316 stainless steel, a core component material of a presently operating fast breeder reactor. Significant degradation of creep rupture strength and elongation by helium occurred in both alloys. However, suppression of degradation was discerned in long-term tests on Alloy 7818 while the variation of the degradation with time to rupture in SUS 316 followed a usual manner, i.e. increasing with increasing rupture time. SEM fractography showed that helium extensively introduced intergranular brittle failure on the ruptured surfaces of both alloys. The microstructure examination by TEM revealed that in the case of Alloy 7818 the Laves phase in the matrix, which became predominant after long time creep as a result of transformation from γ", acted as a trapping site of helium. This observation implies that the abundant attendance of this phase may reduce the flux of helium from grain interiors to grain boundaries and consequently may cause suppression of mechanical property degradation by helium observed in this alloy.

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In order to meet up the requirement for the safe disposal of high level radioactive waste, immobilization of Ba, Cd and Sb over the freshly prepared hydrothermally stable and dense type zirconium oxide is studied. Investigation about the adsorption of Ba, Cd and Sb at tracer level was undertaken with the hydrous zirconia, dried at 70°C for 24 h, and very high uptakes of ¹⁴⁰Ba (95.69%), ¹¹⁵Cd (99.43%) and ¹²⁵Sb (95.18%) were observed. Weighable quantities of Ba, Cd and Sb cations were separately precipitated alongwith zirconium-hydroxide and 46.5^w/₀for Ba, 58^w/₀ for Cd and 53.0^w/₀ for Sb were adsorbed respectively. After calcining the mixed mass in pellet form at 1, 200°C for 20 h, soxhlet leach test at 97°C in every 24 h for 7 days showed the release of cations in the order of 10^-7 g•cm^-2•d^-1. Surface area of the immobilizer pellet was determined by N₂ gas adsorption BET method. X-ray diffraction patterns have been also obtained for each of the zirconia samples accommodated with Ba, Cd and Sb, and distinct structural changes have been indicated.

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The general expression of noise coherence function including the energy dependence of neutron detection cross section and the higher harmonic contributions in multi-dimensional model is developed on the basis of the modal expansion technique. Applicability of the method is demonstrated by numerical calculations carried out for a two-dimensional model of large fast reactor assemblies ZPPR-9 and -13C. The agreement between the theory and the measurement is satisfactory, which indicates the validity of the theory and the calculational model employed. In the assemblies, the coherence for a detector pair in a specific location can be approximately described by including the fundamental and the first harmonic modes.

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A numerical calculation method for two-phase flow behavior using a three-dimensional, two-fluid model has been developed. In this model, the control volume formulation and non-staggered mesh scheme were employed instead of the finite difference method and staggered mesh scheme which have been commonly used in numerical analyses for two-phase flow. Moreover, the porous medium approach was implemented for modeling of internal structures in the flow path in order to evaluate their effects with comparatively fewer meshes.
Since the control volume formulation is based on the space integrated amount of the conserved quantities, the present method can satisfy the accuracy requirements of conservation equations based on the two-fluid model. In the non-staggered mesh scheme, pressure, density and void fraction are defined at the same location as phase velocities are defined.
Calculation accuracy of this scheme can be higher than that of staggered mesh scheme because that does not require the interpolation of the void fraction between neighboring nodes at the location where phase velocities are defined.
The present method has been applied to the analyses of two-dimensional two-phase flow behavior in a piping elbow and in a three-dimensional sudden expanded flow area, in order to confirm the calculation capability of phase separation and void fraction distribution caused by the density difference.

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Maintenance Management Support Systems (MMSSs) for nuclear power plants have been developed using component aging estimation methods and decision tree analysis for maintenance planning. The former evaluates actual component reliability through statistical analysis on field maintenance data. The latter provides preventive maintenance (PM) planning guidance using heuristic expert knowledge and estimated reliability parameters.
The following aspects have been investigated :
(1) A systematic and effective method of managing components/parts design information and field maintenance data
(2) A method for estimating component aging based on a statistical analysis of field maintenance data
(3) A method for providing PM planning guidance using estimated component reliability/performance parameters and decision tree analysis.
Based on these investigations, two MMSSs were developed. One deals with "general maintenance data", which are common to all component types and are amenable to common data handling. The other system deals with "specific maintenance data", which are specific to an individual component type. Both systems provide PM planning guidance for PM cycles propriety and the PM work priority. The function of these systems were verified using simulated maintenance data.

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The residual defect in pulse height defect in a silicon surface barrier detector (SSB) has been a theme of extensive studies. The residual defect was taken as recombinations of electrons and holes under a weak electric field strength⁽¹⁾. Although many works reported models of recombination, none of them could explain the dependences of the residual defect on bias voltage and on resistivity of SSB⁽²⁾⁽⁴⁾.
Recently, the author reported a model of dielectric effect of a plasma column⁽⁵⁾. In this model, the residual defect was theoretically explained by a dielectric property of a plasma column, which was produced by an incident heavy ion. The residual defect is caused by an incomplete charge induction by electrons and holes inside the dielectric plasma column, which screens the movement of inner carriers. The model of dielectric effect included one parameter, called screening factor. The screening factor indicated the time averaged rate of screening carriers from positive and negative electrodes and were determined by experiments on the residual defect. With the experiments on the residual defect of many kind of charged particles, it was concluded that the screening factor was almost proportional to the density of electron-hole pairs of a plasma column⁽⁶⁾.
In this short note, a method of deducing heavy ion energy is proposed, first. Withthis method, the energy of heavy ion is estimated more accurately than before. Secondly, a feasibility of an SSB as a charged particle identifier is discussed. With one SSB, the atomic number and mass number of incident heavy ion are determined, provided that the screening factor is proportional to the electron-hole density of a plasma column.

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