One of the purposes of the Corrective Action Program (CAP) in nuclear power plants (NPPs) is to suppress any future increase in Core Damage Frequency (CDF), which represents a large part of safety affected by the issues associated with cause correction. The application of the graded approach in choosing the issues to be corrected could enhance the CAP effectiveness in suppressing CDF increase. In Japan, the significance scale used in the CAP is poorly correlated with CDF, and the CAP is not focusing on the significant risk issues. First the gap between the significance used in traditional Japanese CAP and risk-informed significance is clarified in this paper. Second, a CAP process model with two conditions of potential safety issues depending on whether the cause is removed or not is prepared. This model can explain how the CAP works to suppress the recurrence of safety issues. Third, using this model, the trend that the CDF is increased by the safety issues is parametrically simulated. Simulation results show that the CAP using risk-informed significance can reduce the CDF increase by 40％ over 20 years. For the NPPs that have long been using an inappropriate significance scale, the application of risk-informed significance in CAP is one of the most effective actions for suppressing future CDF.

　A new method of evaluating colloid diffusion and filtration in compacted bentonites using dendrimers was developed. Polyamidoamine (PAMAM) dendrimers with sizes of 5.7 and 7.2 nm were chosen as diffusion probes owing to their high monodispersity and well-defined molecular structure. The diffusion and filtration behaviors were investigated by a through-diffusion experiment in bentonite compacted to 0.8 Mg/m³ and saturated with 0.005–0.5 mol/L NaCl. The breakthrough curves were observed under all conditions, demonstrating that dendrimers could diffuse through the pore network in compacted bentonite. The effective diffusivity (D_e) and filtration ratio (R_f) of dendrimers were determined from the breakthrough curves and the depth profiles in compacted bentonite, respectively. The D_e values of the negatively charged dendrimers increased when porewater salinity increased and dendrimer size decreased owing to the anion exclusion effect on negatively charged clay surfaces. The R_f values increased when porewater salinity decreased and dendrimer size increased, demonstrating that significant fractions of dendrimers were filtered by the narrow pores in the complex pore networks. The results of this study confirm the validity of the evaluation method using dendrimers and the importance of the need for further investigation under various conditions to understand the factors controlling colloid diffusion and filtration and their relationships with the microstructure in compacted bentonite.

　It is necessary to consider the effect of photoneutrons produced by photonuclear reactions in the shielding calculation of a medical linac room with incident electron greater than 10 MeV. For copper and tungsten used as the target materials of linac, we compared the experimental data with the evaluated data in four photonuclear reaction files of photoneutron production cross sections. We also compared the calculated results using four photonuclear files with experimental results on the angular distributions of photoneutrons emitted from targets bombarded by 18 and 28 MeV electrons. Consequently, we found that the shielding calculations by LA150 and JENDL/PD-2016.1 gave the dose rates within a factor of 1.5 of accuracy in the electron energy of less than 28 MeV.

　We measured count rates and air dose rates at 11 measurement points little affected by the influence of the Fukushima Dai-ichi nuclear power plant accident to obtain parameters for a background equation applying KURAMA-II loaded with the high sensitivity CsI(Tl) detector, C12137-01. It was found that the sensitivity of KURAMA-II loaded with C12137-01 was about 10 times or more for background measurement, compared with KURAMA-II loaded with the standard CsI(Tl) detector, C12137. The background equation for the energy range of 1400-2000 keV was determined to be y (µSv/h)=0.062 x (cps). We evaluated background air dose rates using KURAMA-II loaded with C12137-01 for 71 municipalities and compared them with the results of a previous study of using KURAMA-II loaded with C12137. Evaluated background air dose rates in this study were almost equal to those in the previous study. We confirmed that the background equation evaluated in this study was applicable for KURAMA-II loaded with C12137-01.