Controlled fission power has been used for electricity production worldwide in nuclear power plants (NPPs) based on light water reactor (LWR) technology for several decades. It has proven its efficiency and safety during these years and has manifested itself to be a reliable and durable energy source. The foundation pillar in the peaceful utilisation of fission nuclear power has always been a strong emphasis on safety. Safety has been accomplished by continuously pursuing in-depth reviews and re-evaluation of safety-related issues incorporating findings from ongoing nuclear safety research activities worldwide. Specific requirements have been deployed for the design and the permissible operation conditions of NPPs to consistently ensure adequate margins against critical system conditions, thus preventing the occurrence of accidents. As new findings and analysis capabilities become available, it is ascertained that safety will be increased. It is further possible that the safety margins employed will eventually be decreased without compromising actual safety. However, prevention and mitigation measures must be properly balanced with cost-reduction needs.
Thorough knowledge of fundamental issues – the interaction between neutronics and thermal-hydraulics – allows the pursuit of the goal of ensuring safety at a reasonable cost.
The CRISSUE-S Project was created to re-evaluate fundamental technical issues with the technology of light water reactors (LWRs). The project specifically sought to address the interactions between neutron kinetics and thermal-hydraulics that affect neutron moderation and influence the accident performance of the nuclear power plants (NPPs). The project was undertaken in light of advanced computational tools that are readily available to the scientific community today.
CRISSUE-S dealt with the control of fission power and the use of high burnup fuel; these topics are part of the European Commission Work Programme as well as that of other international organisations such as the OECD/NEA and the IAEA. The problems of evaluating reactivity-induced accident (RIA) consequences and eventually deciding the possibility of NPP prolongation had to be addressed and resolved. RIA constitutes one of the most important of the “less-resolved” safety issues. Treating this problem may have huge positive financial, social and environmental impacts. Public acceptance of nuclear technology implies that issues like these are satisfactorily resolved.
Cross-disciplinary (regulators, industry, utilities and research bodies) interaction and co-operation within CRISSUE-S provide results which can directly and immediately be beneficial to EU industry. Co-operation at an international level – participation of the EU, former Eastern European countries, the United States and observers from Japan – testify to the broad interest these problems engender. Competencies in broad areas such as thermal-hydraulics, neutronics and fuel, the overall system design and reactor surveillance were needed to address the problems that were posed. Excellent expertise was available in specific areas, while limited knowledge existed in the interface zones of these areas (e.g. in the coupling between thermal-hydraulics and neutronics). In general terms, the activities carried out and described here were aimed at taking advantage of available expertise and findings, and gathering together expert scientists from various areas relevant to the issues addressed.
CRISSUE-S consisted of proposing and making a list of transients to be analysed by coupled neutron kinetics/thermal-hydraulic techniques and of defining “acceptability” (or required precision) thresholds for the results of the analyses. The list of transients was specific to different NPP types such as pressurised water reactors (PWRs), boiling water reactors (BWRs) and water-water energetic reactors (VVERs). The acceptability thresholds for calculation precision are general and apply to all LWRs. The creation of a database including the main results from coupled 3-D neutron kinetics/thermal-hydraulic calculations and their analysis was also noted.
The project was organised into three work packages (WPs). The first WP included activities related to obtaining and documenting relevant data. The second WP was responsible for a state-of-the-art report (SOAR) and the third WP concerned the evaluation of the findings from the SOAR and included the outcomes of the entire project formulated as recommendations, mainly to the nuclear power industry and regulatory authorities. The report was the result of the first WP and discusses the type of transients that are of interest to reactivity-initiated accidents in LWR NPPs and elaborates on the data required for coupled 3-D neutron kinetics/thermal-hydraulic analysis and also on data needed to perform associated validations.
Although an effort was made to provide consistency between the various sections, comprehensive reports composed of contributions from the different CRISSUE-S participating organisations unavoidably implied non-homogeneous treatment of the various topics. However, it was realised that the adopted level of detail was not commensurate with the safety relevance or the technological importance of the issues discussed.
The reports were written to accomplish the objectives established in the contract between the European Union and its partners. Expected beneficiaries included institutions and organisations involved with nuclear technology (e.g. utilities, regulators, research, fuel industry). Also, specific beneficiaries were junior- or senior-level researchers, and technologists working in the field of research and development, and the application of coupled neutron kinetics/thermal-hydraulics.
Six plenary CRISSUE-S meetings took place throughout the project implementation period:
Finally, a website was launched at the University of Pisa and remained online for the duration of the project (2001-2003).
The importance of the CRISSUE-S project had been expressed by the NEA Nuclear Science Committee. Interest had also been emphasised by the NEA Committee on Nuclear Safety, as the project discussed many of their activities. It was agreed that the CRISSUE-S reports be published by the NEA as their contribution to the project.
Members of the CRISSUE-S produced project reports for use within their organisations. The later versions were made widely available for the greater benefit of organisations and experts working in the field of nuclear energy.
