Uranium and plutonium mixed oxide (MOX) pellets used as fast reactor fuels have been produced from several raw materials by mechanical blending through various processes, such as ball milling, additive blending, granulation, pressing, and sintering. It is essential to control the pellet density, which is one of the important fuel specifications, but it is difficult to understand the relationships among many parameters in the production of MOX pellets. The database for the production of MOX pellets was prepared from production results in Japan, and input data of eighteen types were chosen from the production process to form a data set. A machine learning model for predicting the sintered density of MOX pellets was derived using a gradient boosting regressor and could represent the sintered density of MOX pellets with R² = 0.996 as a parameter that affects production conditions, such as the type of raw material used and sintering temperature.

In this report, we describe the results of a survey on the concentrations of radioactive Cs (R-Cs) in forest floor soils and leaves of trees in the forests, from 7 to 9 years after the accident at the Fukushima Daiichi Nuclear Power Plant (FDNPP). The survey areas were three natural forests mainly composed of deciduous broad-leaved trees in the Iitate village, Fukushima Prefecture. More than 7 years after the FDNPP accident, relatively higher concentrations of ¹³⁷Cs have been detected in leaves of deciduous broad-leaved trees, despite the concentrations of ¹³⁷Cs becoming established in the soil surface layer, particularly immobilized in frayed edge sites within clay minerals. From 2018 to 2020, the annual flux of ¹³⁷Cs in the litterfall in the Shimohiso survey area was estimated to be about 5.2 kBq·m⁻²·year⁻¹. In this survey area, the circulation of R-Cs is still active at a level comparable to and observed in the litterfall produced in the coniferous forest after the FDNPP accident.

A siltfence is installed to prevent the spread of suspended sediments generated by engineering construction works in rivers and on the coast. Thus, the siltfence is regarded to be a tool to prevent radioactive materials adsorbed on suspended sediments from diffusing into the sea in ports of nuclear power plants during nuclear accidents. However, the prevention effects of the siltfence as sediment control structures have not yet been fully evaluated. Therefore, in this paper, we simulate the behaviors of suspended sediment particles using a coupled simulation scheme composed of water flow and deformation of the siltfence in a rectangular open channel under uniform steady flow, and evaluate the promotive effects of the installed siltfence on suspended sediment deposition. The results demonstrate that the installation of the siltfence significantly promotes deposition, especially of particles with low settling velocities. However, it should be noted that the opposite case is also possible, i.e., the siltfence promotes the diffusion of suspended sediments under particular conditions. These results suggest that a careful installation strategy is necessary. The present simulation scheme is useful in assessing the prevention effects of siltfences prior to their installation.

Since the accident at the Fukushima Daiichi Nuclear Power Plant, many decontamination works have been carried out. Although the exposure status of decontamination workers has been appropriately managed by the government, there are few examples of detailed analysis of external dose exposure conditions. In this study, on the basis of the GPS location information obtained from GPS devices carried by the workers together with their personal dosimeters, the air dose rate in the work area and the characteristics of each type of work were analyzed. The results showed that more than 50% of the measured actual exposure doses were more than twice the median planned exposure dose calculated from the air dose rate and the number of actual working hours. Furthermore, the analysis by work type showed that demolition workers tended to have higher exposure doses. Such information on workers' external exposure doses linked to location information is useful for a detailed analysis of external exposure factors. Such an analysis also contributes to the optimization of future radiation protection programs for workers.