[en] This document is to explain the severe accident issues. Severe Accidents are defined as accidents which are far beyond the design basis and result in severe damage of the core. Accidents at Three Mild Island in USA and at Chernobyl in former Soviet Union are examples of severe accidents. The causes and progressions of the accidents as well as the actions taken are described. Probabilistic Safety Assessment (PSA) is a method to estimate the risk of severe accidents at nuclear reactors. The methodology for PSA is briefly described and current status on its application to safety related issues is introduced. The acceptability of the risks which inherently accompany every technology is then discussed. Finally, provision of accident management in Japan is introduced, including the description of accident management measures proposed for BWRs and PWRs. (author)

[en] Secondary system pipe rapture at Mihama NPP Unit No.3 resulted in deaths of five workers and injuries of six workers who were working in a vicinity of the broken part. The Accident Investigation Committee was established. Under the Committee's direction NISA (Nuclear and Industrial Safety Agency) investigated the cause of the accident and the measures for preventing the recurrence of the accident. Based on the lessons learned from the accident, NISA makes continuous improvements of inspection methods and strongly requires all licensees to conduct maintenance management and quality assurance activities in a strict, through manner. New safety regulations effective in October 2003 and actions taken by NISA against the accident were described. (T. Tanaka)

No abstract available

[en] A comparison study of source term evaluation codes was carried out in order to understand the degree of uncertainty in the evaluated source terms and to identify the phenomena whose uncertainty contribute to the source term uncertainty. The codes compared are THALES/ART developed by the Japan Atomic Energy Research Institute, source term code package (STCP) by the US Nuclear Regulatory Commission and MAAP by the IDCOR program. The comparison was carried out by two steps: comparison of analytical models and comparison of calculated results for standard problems. Through the comparison of the analytical models, the uncertainty in the models for eight phenomena was identified as the candidate contributors to bring about significant uncertainty in the source terms. The comparison of the calculated source terms showed a large difference among the results as expected and suggested that there still exists a large uncertainty in the source terms. The findings obtained by this study will be utilized in correctly understanding the results of level 2 PSAs

[en] The unit conversion program library UCL1 has been developed for dynamics and thermodynamics. With this library, any unit can be converted into the unit belonging to the absolute unit system specified. When the UCL1 is applied to a computer code, the input data in given units are converted into the unit system employed in this code. Similarly, the output data are converted from the code's unit system into desired units. When the conversion factor between two arbitrary units is required, it is given as the ratio of two conversion factors to a specified unit system. Unit conversion factor tables for various physical quantities were produced from a sample run and are listed in the appendix. (auth.)

No abstract available

[en] The Japan Atomic Energy Research Institute has been engaged in the research on PTA of nuclear power plants since F Y 1980. It consists of: Development of PTA methodologies, including reliability analysis, core melt accident analysis and external event risk analysis, and Application of these methodologies, including PTAs of Japanese light water reactors. The past progress and future schedule is illustrated in Figure 1. This paper summarizes the important progress of the recent research as well as the topics in the current research plan. Currently a reliability benchmark test on fault tree analysis of the core spray system of a BWR is progressing under the cooperation of four contractors. As generally known, there is a high degree of freedom left to the analysis in the process of fault tree development. This characteristics, on the other hand, can be a source of uncertainty in the results. The objectives of the benchmark test are therefore to identify the causes of uncertainties and to quantify their effects. The project was systematically organized in order to distinguish the effects of various contributors to the uncertainty. At present, all the analyses by the contractors were completed and the results are reviewed at JAERI. Figure 2 shows the overall spread of the TOPO event probabilities calculated by the four contractors who established the boundary conditions, developed the fault trees and selected the reliability database independently in this analysis

No abstract available

[en] Fuel rod temperature increase during coolant boiloff accident due to unavailable ECCS was analyzed using a simple time dependent model. A standard case was first selected and its results clarified how is the fuel temperature behavior during the boiloff accident. Then the sensitivity studies for various parameters were performed to know what parameters have important roles. As a result of analyses, it was shown that the coolant mixture level in the core has a dominant effect on the core heatup and that fuel rod claddings will probably slump or melt before its full oxidation. (author)

[en] The unit conversion program library UCL2 has been developed. This library is the expanded and modefied version of UCL1, which was developed for the unit conversion for dynamics and thermodynamics. Using this library, the user can obtain the accurate unit conversion factors between arbitrary units in consistent way. UCL2 has two major functions. The first one is to register the information of dimensions, units and unit systems. This work can be done automatically by the program and/or individually according to the user's selection. The second one is to obtain the unit conversion factor from a certain unit to the unit with same dimension in a certain unit system. Besides, UCL2 has many utility subprograms including character handling programs and unit conversion factor printing-out programs. The application of UCL2 to computer codes will not only improve the accuracy of the codes but prevent careless mistakes in programming about unit conversion. Especially, various correlation programs, the development of which takes a large part of programming work for a large scale computer code, can be developed so that they may be used for any unit system, which will result in the decrease of programming work significantly. Unit conversion factor tables for various physical quantities in dynamics and thermodynamics field were produced from a sample run and are listed in the appendix. (author)