Experiments were conducted to investigete two-phase flow stability of a natural circulation BWR due to flashing at low pressure. The facility used in the experiment was designed to have non-dimensional values which are nearly equal to those of typical natural circulation BWR. The observed instability is suggested to be the flashing induced density wave oscillations, since the oscillation period was nearly one and a half to two times the passing time in the chimney section, and correlated well with a single line regardless of system pressure, heat flux, and inlet subcooling. Stability maps were obtained in reference to the core inlet subcooling and the heat flux at the system pressures of 0.1, 0.2, 0.35, and 0.5MPa. The flow became stable below a certain heat flux regardless of the channel inlet subcooling. The stable region enlarged with increasing system pressure. According to the stability map, the stability margin becomes larger in a startup process of a reactor by pressurizing the reactor sufficiently before withdrawing control rods.

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As an application of probabilistic fracture mechanics (PFM) and a financial method, risk-benefit analyses were performed for the purpose of optimizing maintenance activities of steam generator (SG) tubes used in pressurized water reactors (PWRs). Parameters such as in-service inspection (ISI) detection accuracy, ISI interval, sampling inspection, replacement of SGs and stress corrosion cracking (SCC) allowance operation were selected for sensitivity analyses. In the analysis of the operation introducing maintenance criteria, the effect of quantitative accuracy of the inspection was also taken into account. Although the analyses were mainly conducted for SG tubes made of Inconel 600 mill anneal (MA) materials, the analyses were also performed for SCC-resistant materials with making assumptions on their crack initiation probabilities and crack propagation laws. To justify whether or not it is worth while implementing the selected maintenance strategies in terms of an economic point of view, net present value (NPV) was calculated as an index which is one of the most fundamental financial indices for decision-making based on the discounted cash flow (DCF) method.

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In Japan, a preliminary safety assessment for geological isolation of TRU waste has reported that the rate of radioactive exposure can be controlled by radioactive iodine (I-129). In order to decrease the rate of radioactive exposure, the HIP (Hot Isostatic Pressing) solidified method has been developed for an iodine adsorbent waste (silver loaded silica gel).
The HIP method can manufacture a solidified waste, which don't separate and volatilize I-129 from iodine filter waste. For the solidified waste manufactured at 750°C (treatment temperature), 100MPa (treatment pressure), the base material changed from silica gel to quartz which is high density and high strength. The grain size grew up and became a few-10μm in diameter. Iodine was solidified between quartz grains as AgI on physically stable form.
The test and estimation result for iodine-solidified structure of the HIP "rock" solidified waste show on this report.

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In Japan, a preliminary safety assessment for geological isolation of TRU waste has reported that the rate of radioactive exposure can be controlled by radioactive iodine (I-129). In order to decrease the rate of radioactive exposure, the HIP (Hot Isostatic Pressing) solidified method has been developed for an iodine adsorbent waste (silver loaded silica gel) to produce a stable "HIP rock solidified waste form".
Static leaching tests for the HIP rock solidified waste form, which were performed in a reducing condition with 3×10^-3M Na₂S solution for 300 days, showed that the normalized leaching rates for I and Si were extremely low (10^-7-10^-8g/dm²/day). For initial 60 days of the tests the cumulative amount of leached iodine was proportional to square root of time, which indicates that the leaching can be controlled by diffusion.
Therefore, this phenomenon will be controlled by diffusion. By TEM analysis for the reacted waste form surface, formation of sulfides was observed at quartz grain boundary, which indicates that the diffusion path of iodine can be the grain boundary.
Consequently, it is possible to adapt "grain boundary diffusion model" as iodine-leaching mechanism of the HIP "rock" solidified waste.

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Previously, authors reported that the 14 MeV-neutron direct interrogation method has made possible to measure for the discrimination of clearance levels of concrete solidification uranium waste.
In this paper, applicability of the method to metal waste matrix is discussed based on the results of simulation experiments by the continuation energy Monte Carlo calculation code (MVP). The problem is that self-neutron moderation effect in a waste cannot be expected when a waste matrix is metal. To solve this, a moderator is adopted so as to surround a metal waste drum and to slow down suitably a 14 MeV neutrons. The simulation calculation showed that this effect is satisfactorily large. The detection limit of radioactivity concentration to 4.5% enriched uranium has been found to be 0.0973Bq/g in the metal waste model of 215.59kg gross weight, in which 61 pipes are stuffed into its drum. Moreover, the position-dependent sensitivity difference in a metal waste drum can be settled as small as to ±13.5%.
In conclusion, it can be said that 14MeV-neutron direct interrogation method can be applied to the waste of a metal system: the detection sensitivity is high enough and the position-dependent sensitivity difference is small admittedly. Hence the method can be applied also to discrimination measurement of the clearance level of metal uranium waste.

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To investigate the reproducibility of experimental approach for dose evaluation using a realistic phantom that faithfully reproduced the shape of a head, we considered the manufacture of a patient's realistic phantom and the reappearance of actual medical irradiation conditions. We selected the rapid prototyping technique to produce the realistic phantom from the Computed Tomography (CT) imaging. This phantom was irradiated under the same clinical irradiation condition of this patient, and the thermal neutron distribution on the brain surface was measured in detail. Several subjects on material and data conversion in the production of realistic phantom were mentioned. As a result of reproducing medical irradiation using the realistic phantom, the maximum thermal neutron flux became a value about 22% lower than the surface of the actual brain. lf the problems pointed out in this paper are solved, it may also be expected that it would become possible to check computational dosimetry system.

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Numerical tests were performed about an effect of neutron anisotropic scattering and treatment of angular dependency of neutron flux in effective cross section on criticality based on previous researches. Three approximations described on a previous report were compared to each other in both one-dimensional slab model and two-dimensional cylindrical model. As a result, it was found that the transport approximation, which has been conventionally used in fast reactor analyses in Japan, has a good accuracy in criticality analyses of typical fast reactors. However, the transport approximation is not enough to calculate accurately cores which have fuel-reflector boundary. Therefore it is desirable that the extended transport approximation with higher order of anisotropic scattering is used in the analyses for such cores.

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This paper describes the development of wall decontamination robot systems for nuclear power plants.
In nuclear power plants, it is required to reduce maintenance costs, including annual inspection, repairs and so on. Most of such maintenance activities are actually performed after decontamination processes are completed. In particular, the decontamination process of reactor wells is very important for reducing the radiation exposure of human workers.
In the past, decontamination of reactor wells used to be done by extra large machine and tools, which caused long working hours and tiresome works. It was one of the reasons maintenance costs couldn't have been easily reduced.
There are narrow spaces in the reactor wells that have to be decontaminated by human workers. In order to minimize the radiation exposure to humans, wall decontamination robot systems have been developed.
The decontamination robots have rolled brushes and suction mechanisms and are capable of removing contaminants attached to the wall surface of the reactor wells. By making the robots smaller, it is possible to work in narrower spaces.
In this paper, the effectiveness of decontamination by the developed robots is shown through experiments in the actual nuclear power plants.

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In fifty years metal binding to humic substances (HS) has been investigated and several binding models have been developed. Among them the Model VI developed by E. Tipping and the NICA-Donnan model developed by D.G. Kinniburgh and his coworkers are prominent in that they can successfully describe the binding of various metal ions to HS over a wide range of conditions: the equilibrium concentration of metal ion interest, pH, salt concentration and the concentration of competitive metal ions. These models are contrastive in their approach to the distribution of the affinity constant of the binding sites of HS (i.e. the "heterogeneity" of the binding sites). The Model VI is based on the discrete affinity distribution and the NICA-Donnan model on the continuous affinity distribution. In this review these two models will briefly be introduced, and subsequently the comparison between the NICA-Donnan model and the charge neutralization model, which is developed by J.I. Kim and frequently used in the binding of the actinide ions to HS, will be made using a data set of Cu²⁺ binding to the purified Aldrich humic acid.

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