Risk information obtained from probabilistic risk assessment (PRA) can be used to evaluate the effectiveness of measures against severe accidents in nuclear facilities. The PRA methods used for reprocessing facilities are considered immature compared with those for nuclear power plants, and to make the methods mature, reducing the uncertainty of accident scenarios is crucial. In this review, we summarized the results of a literature survey on the event progression of evaporation to dryness caused by boiling of high-level liquid waste (HLLW), which is a severe accident in reprocessing facilities. In addition, migration behavior of associated radioactive materials was also summarized. Since one of the important characteristics of ruthenium is its tendency to form volatile compounds over the course of the event progression, the migration behavior of ruthenium is categorized into four stages (early boiling stage, late boiling stage, drying stage and after drying stage) on the basis of temperature. Although no ruthenium is released from the waste in the after drying stage, other volatile elements such as cesium can be released. Sufficient experimental data, however, have not yet been obtained. Thus, it is, necessary to further clarify the migration behavior of radioactive materials, which predominantly depends on temperature in this region.

Primary Containment Vessel (PCV) venting is one of the measures for mitigating severe accidents in a nuclear power plant. To prevent PCV failure, the PCV venting is conducted as judged by an authorized person in a plant when the plant reaches relevant severe accident conditions. From the viewpoint of public safety, the judgement should be performed with cooperation among relevant organizations such as the plant site, cabinet, regulator, and local government, considering the evacuation of residents. In this study, a system risk analysis is qualitatively performed to identify the hazard causal factors of PCV venting, focusing on interactions among systems including organizations using the System Theoretic Accident Model and Processes/System-Theoretic Process Analysis (STAMP/STPA), which assumes the accidents involve a complex and dynamic “process” and focuses on the interactions among humans, machines, and the environment. It is identified that STPA helps to find the broad interactions among organizations responsible for PCV venting, which has the potential to lead to an accident.

A method called the Initiating Event (IE) Matrix Method (IEMM) is proposed to resolve several issues regarding the hierarchization of IEs applied in the seismic Probabilistic Risk Assessments (PRAs) of nuclear facilities. In the hierarchization of IEs, the influence of low-order IEs is assumed to be smaller than that of high-order IEs. Consequently, the accident progressions of low-order IEs are basically omitted, which makes the risk assessment models simple and makes it feasible to assess the seismic risks associated with complex systems such as nuclear facilities. However, in the hierarchization, since the accident progressions of the simultaneous occurrence of multiple IEs are omitted, there is a possibility that the combinations of multiple IEs having small occurrence frequencies but large impacts on the environment might also be omitted from the risk assessment models. In addition, since there are cases where the combinations of multiple IEs may be considered only for some specific accident progressions depending on the assessor, the assessment results may depend on the assessor. Therefore, the authors propose the IEMM that assesses all the IEs with significant occurrence frequencies and all the combinations of such IEs. By comparing the IEMM and the hierarchization method in a case analysis of a simple system with a few IEs, the assessment results show less dependence on the assessor and fewer overlooks of accident progressions of significant IEs and combinations of such IEs.

We have developed a SiC operational amplifier (op amp) and applied it to a pressure transmitter with high radiation resistance. The op amp was prototyped on the basis of SiC complementary MOS (CMOS) technology. Generally, in CMOS, electric charges are accumulated in the oxide film by irradiation. In op amps using CMOS, the offset voltage increases owing to the induction of defects in the semiconductor layer, which result from the accumulation of electric charges. On the other hand, SiC is a wide-bandgap semiconductor and has the characteristic that defects are less likely to be induced. The prototype SiC op amp showed radiation resistance of 2 MGy or more. We mounted this op amp on a pressure transmitter and evaluated the pressure transmitter in terms of input/output characteristics, temperature characteristics, step response, and radiation resistance. We confirmed that the measurement accuracy of the input/output characteristics and temperature characteristics was within ±0.5%. The step response was 450–455 ms. Regarding use in radiation environments, we confirmed that the transmitter can be operated at doses up to 580 kGy with a measurement error of 10% or less. These results show that the device effectively improves the reliability of nuclear power plants.