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.
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.
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.
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.