Neutron spectrum unfolding is a widely used technique for characterizing neutron fields for various types of reactor dosimetry, where the neutron spectrum is derived from integral measured data such as multiple-foil activation rates and moderated neutron detector counts. Many spectrum unfolding codes have been developed so far and their performances have been compared. However, a standardized metrology for neutron spectrum unfolding has not been satisfactorily established yet from the viewpoints of adequate selection and usage of unfolding codes, response function database and input data set preparation. This article reviews the present status of the neutron spectrum unfolding technique that is mainly related to reactor dosimetry with activation foils and discusses the validity of the solution spectra obtained from different unfolding codes under a typical fast reactor neutron field. The results show that the solution spectrum strongly depends on a priori (i.e., guess) spectrum required for the input data as well as the theoretical assumption in each unfolding code. The issues that must be resolved to improve the accuracy of reactor dosimetry are summarized for the a priori input spectrum, the nuclear database, and the standardization of the unfolding procedure.
This paper describes the objectives and specific applicational issues of the document authorized by the Nuclear Safety Commission entitled “Policy of the safety assessment of subsurface disposal after the period for active control”, focusing on the policy of assessment for the appropriateness of burial depth. Concerning the burial depth of subsurface disposal, the proposal of 50-100 m from the surface has been made by the Atomic Energy Commission as a sufficient depth to avoid general usage of the underground environment, furthermore, taking into account these circumstances, the above document stipulates the new policy of assessment for the appropriateness of burial depth considering uplift/erosion noted in the safety assessment for long-lived radioactive materials. In the case of considering uplift/erosion, the isolation from the living environment planned in the original scheme will be gradually lost, the disposal facility will come close to the surface and become near-surface disposal in the long term. This paper describes the required roles of burial depth for subsurface disposal considering the long-term evolutional changes, introduces the physical protection capability to human intrusion as one of the protective functions of engineered barriers, and discusses the specific important issues related to the actual application.
The interaction between compacted bentonite and hardened cement was evaluated. Cement-bentonite-cement-layered test specimens, after infiltrated with water, were subjected to various analyses. The EPMA analysis of the cross sections of the specimens indicated a homogeneous distribution of Ca leached from the cement paste in the bentonite. The quantity of Ca in the bentonite was apparently larger than the cation exchange capacity of the bentonite, suggesting the formation of secondary minerals involving Ca.
The integrated safety analysis (ISA) procedure has been developed to apply risk-informed regulation to uranium fuel fabrication and enrichment facilities. The major development efforts are as follows: (a) preparing the risk level matrix as an index for items-relied-on-for-safety (IROFS) identification, (b) defining requirements of IROFS, and (c) determining methods of IROFS importance based on the results of risk- and scenario-based analyses. For the risk level matrix, the consequence and likelihood categories have been defined by taking into account the Japanese regulatory laws, rules, and safety standards. The trial analyses using the developed procedure have been performed for several representative processes of the reference uranium fuel fabrication and enrichment facilities. This paper presents the results of the ISA for the sintering process of the reference fabrication facility. The results of the trial analyses have demonstrated the applicability of the procedure to the risk-informed regulation of these facilities.
In this paper, we made a comparison of application between nuclear safety agreements and pollution control agreements, and considered how to improve the way of applying the former. In order to comprehend the actual status of pollution control agreements, we conducted some hearings with the Chiba Prefecture and the Chiba City because there have been few empirical studies on the application of pollution control agreements. Based on these hearings, we found it a characteristic point of pollution control agreement that quantitative standards for emission function are the main criteria of application which local governments and utilities establish through negotiations. In contrast, the ways of applying nuclear safety agreements are often ambiguous because there are no clear criteria. This comparison suggests that the stakeholders around nuclear safety agreements discuss and agree to set out concrete standards of application of nuclear safety agreements as well as pollution control agreements. However, such a difference between the two could be an indication of the difficulty to create a consensus among stakeholders in the case of nuclear safety agreement given the historical context and the differences of risk characteristics. In that case, it can be found useful for risk governance that one introduce the idea of participatory risk management to the application of nuclear safety agreements.
Media coverage plays an important role in delivering information to the public in a rapid and easy-to-understand manner in terms of the subjects of nuclear energy. The mass media has so far covered nuclear accidents that occurred in nuclear facilities. The media coverage usually gains the attention of the public through the news media, such as TV and newspapers. In this study, three main cases of nuclear accidents were quantitatively examined by using the database of a newspaper. In addition, various comments of journalists whom the author interviewed were added for the evaluation of the three cases. As a result, it was revealed that the amount of media reporting commonly reached a maximum just after the nuclear accidents occurred. It became also clear that the smoothness of the information flow from the nuclear industry to the mass media affected the trend of the media coverage from the viewpoints of the duration and number of news reports. Most of the journalists considered that it was significant for the nuclear industry to strengthen the initial reaction on the occasion of nuclear accidents. The nuclear industry should understand the characteristics that are typical of the media coverage on nuclear accidents in the future.