Neutronics/Thermal-hydraulics Coupling in LWR Technology – CRISSUE-S WP2: State-of-the-Art Report

Controlled fission power has been utilised 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 as a reliable and durable energy source. The foundation pillar in the peaceful utilisation of fission nuclear power has always been the 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 at the design and in the permissible operation conditions of the NPPs in order to always ensure adequate margins against critical system conditions, thus preventing the occurrence of accidents. It is realised that as new findings and analysis capabilities become available safety will be increased, and it is further possible that the safety margins presently employed will eventually be relieved (decreased) without compromising the actual safety. Prevention and mitigation measures, however, must be properly balanced with cost-reduction needs. A thorough knowledge of fundamental issues – in the present case the interaction between neutronics and thermal-hydraulics – allows pursuing the goal of ensuring safety at reasonable costs. Consistent with this goal, the CRISSUE-S project was created with the aim of re-evaluating fundamental technical issues for LWR technology. Specifically, the project seeks to address the interactions between neutron kinetics and thermal-hydraulics that affect neutron moderation and influence the accident performance of the NPPs. This is undertaken in the light of the advanced computational tools that are readily available to the scientific community today. The CRISSUE-S activity deals with the control of fission power and the use of high burn-up fuel; these topics are part of the EC 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 must be addressed and resolved. RIAs constitute one of the most important of the “less-resolved” safety issues, and treating this problem may have significantly positive financial, social and environmental impacts. Public acceptance of nuclear technology implies that problems such as these be satisfactorily resolved. Cross-disciplinary interaction (regulators, industry, utilities and research bodies) and co-operation within CRISSUE-S provides results which can directly and immediately be beneficial to EU industry. Concerning co-operation at an international level, the participation of the EU, former Eastern European countries, the USA, and observers from Japan testify to the wide interest these problems engender. Competencies in broad areas such as thermal-hydraulics, neutronics and fuel, overall system design and reactor surveillance are needed to address the problems that are posed here. Excellent expertise is available in specific areas, while limited knowledge exists in the interface zones of those areas, e.g. in the coupling between thermal-hydraulics and neutronics. In general terms, the activities carried out and described here aim at exploiting available expertise and findings and gathering together expert scientists from various areas relevant to the issues addressed. Added value for the CRISSUE-S activity consists of proposing and making available 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 is specific to the different NPP types such as PWR, BWR and VVER. The acceptability thresholds for calculation precision are general in nature and are applicable to all LWRs. The creation of a database including the main results from coupled 3-D neutron kinetics/thermal-hydraulic calculations and their analysis should also be noted. The CRISSUE-S project is organised into three work packages (WPs). The first WP includes activities related to obtaining and documenting relevant data. The second WP is responsible for the state-of-the-art report (SOAR), while the third WP concerns the evaluation of the findings from the SOAR and includes outcomes of the entire project formulated as recommendations, mainly to the nuclear power industry and to regulatory authorities. The present state-of-the-art report is the result of the second WP. It discusses such aspects as: thermal-hydraulics, neutron kinetics, fuel, coupled analyses, operator training, regulatory position and a database of results. It also summarises findings from recent international projects and activities. A comprehensive report such as the present one, composed of contributions from the different CRISSUE-S participating organisations, unavoidably implies non-homogeneous treatment of the various topics, although an effort was made to provide consistency between the various sections. However, it is realised that the adopted level of detail is not commensurate with the safety relevance or the technological importance of the issues discussed. The report has been written to accomplish the objectives established in the contract between the EU and its partners. Expected beneficiaries include institutions and organisations involved with nuclear technology (e.g. utilities, regulators, research, fuel industry). In addition, specific expected beneficiaries are junior- or senior-level researchers and technologists working in the considered field of research and development and application of coupled neutron kinetics/thermal-hydraulics. Six plenary CRISSUE-S meetings took place over the course of the project implementation period. The meetings were held at: • University of Pisa, Pisa, Italy, 25-26 February 2002 (kick-off meeting). • OECD/NEA, Issy-les-Moulineaux, Paris, France, 5-6 September 2002. • Technical University of Catalonia (UPC), Barcelona, Spain, 23-24 January 2003. • SKI, Stockholm, Sweden, 26-27 June 2003. • European Commission, Luxembourg, 12 November 2003 (status information meeting). • University of Pisa, Pisa, Italy, 11-12 December 2003 (final meeting). An Internet site has been established at the University of Pisa and has been kept alive during the project lifetime (2001-2003). The address is www.ing.unipi.it/crissue_s. The site also contains the discussion records of the six meetings. The importance of the CRISSUE-S project has been expressed by the OECD/NEA Nuclear Science Committee. This interest has also been emphasised by the OECD/NEA Committee on the Safety of Nuclear Installations, as the project discusses many of their activities. It was agreed that the CRISSUE-S reports be published by the OECD/NEA as its contribution to the project. This report was produced by the members of the CRISSUE-S project for use within their organisations. The present version is being made widely available for the greater benefit of organisations and experts working in the nuclear power area. Several of the graphics in the report are in colour; interested readers can request a colour version of the report on CD-ROM from the NEA.