In this study, we propose rational and simple safety requirements that can be applied to new light water nuclear power reactors on the basis of the concept of current regulatory requirements. By introducing a “new design-basis accident response system” with diversity and “division completeness” from the design stage, it will be possible to integrate the conventional “Design-basis accident response system,” “Severe accident prevention system,” and “Severe accident mitigation system.” This significantly reduces the risk of common-cause failures. The functional objectives of “Specified Severe Accident Response Facilities” are also incorporated into the new system, with the diversity division responsible for these functions. Severe accidents, defined as extremely rare events that exceed design conditions, require flexible countermeasures. These include a combination of static equipment with safety margins and mobile equipment. The safety requirements discussed in this study are primarily applied to large light water nuclear power reactors, but they can also be applied to future nuclear power reactor developments such as small modular reactors (SMRs), contributing to improved design efficiency, shorter construction periods, faster reviews, and cost reductions. The proposed safety requirement framework supports the development of next-generation reactors that can achieve both decarbonization and energy security.

Targeted radionuclide therapy using Lu-177 is mainly used to treat prostate cancer. Although Lu-177 is being produced by research reactors around the world, Lu-177 production has not yet been attempted in Japan. There are two methods of producing Lu-177 in reactors: direct route by the Lu-176(n,γ) reaction and indirect route by the Yb-176(n,γβ) reaction. The direct method using enriched Lu-176 yields a large amount of Lu-177, but its specific activity is approximately as small as 0.5 TBq/mg-Lu at the neutron flux level of commercial light water reactors. On the other hand, the indirect method using enriched Yb-176 yields a smaller amount of Lu-177 than the direct method, but it has a high specific activity of about 4 TBq/mg-Lu that enables the production of no-carrier-added (nca) Lu-177 without the waste problem of Lu-177m (half-life: 160 days). The objective of this study is to produce Lu-177 in PWR while generating electricity. Assuming that the annual demand for Lu-177 in Japan is equivalent to 15,000 castration-resistant prostate cancer patients, one pressurized water reactor could produce the required amount of nca Lu-177 by irradiating 200 g of enriched Yb-176 for 2 weeks × 17 times a year.

In this study, the regulatory decision-making process concerning faults within the site of the Tsuruga Nuclear Power Plant was examined to identify major issues essential for achieving rational regulation. An analysis of the regulatory rules applied in this process, together with meeting records, revealed the following deficiencies:

1. The hazard of fault displacement events has not been evaluated in terms of both occurrence frequency and potential consequences.

2. The latest scientific knowledge on secondary faults has not been adequately incorporated.

3. Regulatory rules have been formulated without clearly defining the classification of events, such as design-basis events.

4. The specificity required of legal and regulatory standards is insufficient.

5. Reasonable mitigation measures have not been recommended for other nuclear power plants facing similar concerns.

6. Expert knowledge has not been utilized appropriately.

On the basis of these findings, the current regulatory decision-making process can be evaluated as lacking the rationality required of conformity assessments to regulatory standards. It is therefore concluded that a systematic re-examination of the validity of this process and the implementation of appropriate corrective measures where deficiencies are identified constitute urgent tasks.

Nuclear operators have generally improved their procedures to eliminate human errors that cause troubles during maintenance work. However, it is difficult to anticipate every possible situation and establish appropriate procedures in advance. A new perspective on safety is required for preventing troubles. Although the Safety-II concept has been developed, applying it to maintenance is challenging. This is because the troubles the nuclear operators face in performing maintenance work vary in scale and the difficulty level of solutions. The appropriate problem-solving approach should be based on the complexity. In this study, we aim to propose a systematic and effective analysis method based on the Safety-II concept, focusing on human actions in troubles primarily related to maintenance experienced at nuclear power plants. By combining the Cynefin framework and resilience engineering, a methodology focusing on the complexity is proposed. The analysis flow for the proposed methodology consists of the following steps: (1) organize the timeline by focusing on human actions and changing problems, (2) classify the problem status on the basis of the Cynefin framework, (3) confirm the achievement status of the problem resolution phase, and (4) develop countermeasures of monitoring and responding. Through the analysis of 65 troubles that occurred at nuclear power plants, the effectiveness of the proposed methodology was confirmed.