'Organisation for Economic Co-Operation and Development (OECD)'
Abstract
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 report is the result of the third WP
and summarises the results, selects the most important findings and indicates the industry position on
related subjects. It emphasises achievements, highlights the most important conclusions reached in the
second WP and briefly refers to the first WP.
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