Expert elicitation has traditionally been accepted in some countries as a way to quantify the uncertainty of radionuclide migration parameters in the safety assessment of radioactive waste disposal. However, expert elicitation has not yet been explicitly performed in the field of radioactive waste disposal in Japan. To discuss the applicability of expert elicitation in Japan, here we broadly review the histories and methodologies of expert elicitation in some papers and review in more detail case studies on the utilization of expert elicitation in the safety assessment of radioactive waste disposal in the US, UK, and Sweden. From the literature review, we suggest that it is valuable to adopt expert elicitation to quantify the uncertainty of parameters in Japan. In particular, the documentation of each elicitation step is critical to ensuring the traceability and transparency of expert elicitation. The documentation enables the regulator to evaluate whether the expert judgment including the elicitation process is adequate. Furthermore, we recommend providing not only an aggregated expert judgment for safety assessment but also the distribution of individual expert judgements. Individual expert judgments will be used for related analyses (e.g., sensitivity or uncertainty analyses), leading to increased confidence in the safety assessment.
Japan Atomic Energy Agency (JAEA) has been developing an advanced head-end process based on a disassembly machine using cutting technology as well as a shearing machine with short-stroke shearing for reprocessing fast reactor (FR) fuel assemblies. The FR fuel pins should be removed from the fuel assembly wrapper tube by cutting without damage to reprocess them. In this paper, the applicability of fiber laser cutting as a promising cutting technology for disassembly machines is reported. Toward solving problems such as the welding of wrapper tubes and spacer wires, cutting tests (slit cut and crop cut) using simple test pieces and mock-up tests using a simulated fuel assembly were carried out to determine optimal cutting conditions (e.g., power, cutting speed, focus position, and cutting position (lower plug/knock bar)) and procedure. Under the selected cutting conditions and procedure, damage to the fuel pin and the welding of the spacer wires were reduced, demonstrating that cutting using a fiber laser can be applied to disassemble FR fuel assemblies.
We propose an extraction–electrodeposition process involving the solvent extraction of platinum group metals (PGMs) to an ionic liquid phase followed by the direct electrodeposition of a selected metal from the ionic liquid phase to separate and recover PGMs efficiently. To examine the compatibility of betainium bis(trifluoromethanesulfonyl)imide ionic liquid ([Hbet][Tf2N]) in this process, we investigated the electrochemical behavior of Ru(III), Rh(III), and Pd(II) actually extracted from HNO3(aq) to the [Hbet][Tf2N] phase. Through the potentiostatic electrolysis of Pd(II) in [Hbet][Tf2N], fine particles of Pd metal were deposited over time at applied potentials at －500, －600, and －750 mV vs Fc/Fc＋. In contrast, no electrodeposition of Ru(III) and Rh(III) from [Hbet][Tf2N] solutions was observed. We also examined the potentiostatic electrolysis of a mixture of Pd(II), Ru(III), and Rh(III) in [Hbet][Tf2N] at －600 mV vs Fc/Fc＋. As a result, Pd(II) was selectively electrodeposited in a nearly quantitative yield (＞99％) with high purity (＞97％), while little Ru and Rh were included in the deposit.
During the decommissioning of accident-damaged nuclear power plants, three fundamental safety functions, i.e., stopping the reactor, cooling the reactor, and confining radioactive materials, have to be maintained as well as normally operating plants. However, for severely accident-damaged nuclear power plants such as Fukushima Daiichi units 1 to 3, the stopping function is lost and the confinement function is degraded. Even in such cases, it is very important to suppress the additional release of radioactive materials to outside during the decommissioning. Because of the high radiation level, it is very difficult to access the inside of the primary containment vessel and reactor building to investigate and repair equipment or components. Because of this, a lot of time and effort may be needed to complete the project, and it may be necessary to take a strategic, efficient, and effective step by taking a risk-informed approach. In this paper, we focused on a negative pressure control system for a primary containment vessel and a reactor building during fuel debris retrieval, evaluated its reliability, and, on the basis of the evaluation results, considered appropriate safety management for nuclear decommissioning.
Integrity confirmation for buildings against collisions of projectiles has been conducted to evaluate their behavior upon collision between a projectile with a simple shape and a wall using empirical formulas. However, there is a possibility that structures with a complex shape such as a stack may collide with a reactor building. However, there have been few studies of collisions between structures with a complex shape and buildings. In this study, it is necessary to set the physical properties of reinforced concrete to carry out such an evaluation; however, the physical properties depend on the number of reinforcing bars. For this reason, the physical properties of reinforced concrete are set so as to fit conventional empirical formulas recommended. Impact evaluation was carried out using a reactor building and a stack with a realistic shape and physical properties.