This Report summarizes the results of the studies performed by the Joint Research Centre / Institute for Energy (JRC/IE) on a specific task dedicated to Thermal-hydraulics within the SONIS (Safety Of Nuclear InstallationS) 2008 program.
The aim of task 4 of the SONIS programme is to analyse European practice in verification and optimization of plant operational procedures for normal, abnormal and emergency conditions. More specifically task 4.1 analyses the effect of using new Best Estimate plus Uncertainty Methods (BEPU) in the re-licensing processes on plant operational procedures directly affecting the Thermal-Hydraulic (T-H) behaviour of the nuclear facilities.
Current trends in the industry to increase power production challenge the initial safety design limits of the plant which were performed generally using conservative tools and hypothesis. Advance numerical tools and methods allow demonstrating that safety margins are still respected. These tools are modern fully validated thermal-hydraulic codes, coupled thermal-hydraulic / neutron-kinetic (N-K) codes and methodologies that use realistic hypotheses rather than conservative ones and estimate also the uncertainty
Their effect on operational procedures for normal and emergency conditions and for Operating Limits and Conditions is investigated by asking directly the stakeholders of the European Union.
A questionnaire was sent to several stakeholders in the Nuclear Safety domain in the European Union and information was gathered on the new T-H tools for the re-licensing processes (for power uprates, SG replacements etc), their effect on the Operational Limits and Conditions (OLC) and on Operating Instructions and Procedures (OIP) of the Nuclear Power Plants (NPPs), the need of performing specific investigations in operational modes and of exchanging of information on new T-H tools/methodologies.
It was seen that almost all the interviewed countries confirmed the uprate of their Nuclear Power Plants (NPPs) with a maximum value of 10%. The major modifications were generally associated with the replacement of Steam Generators (SG) and introduction of new type of fuel.
All the participants also confirmed the impact on the original OLC (for example, changes were introduced in setpoints of reactor protection) but there no major effects on the original OIP were noticed (except some optimization of procedures).
In general, no additional operating or accidental modes were identified for development or improvement although two participants mentioned the current development of SAMG (Severe Accident Management Guidelines)
The full set of T-H safety analyses were done only at those NPPs where major modifications were performed and detailed simulations were executed manly for evaluation of set-points and modifications of accident management procedures.
The participants agreed that an exchange of experience by a forum/workshop with a qualitative comparison on of OLC and OIP of plants of different countries would be beneficial. They also agreed on different kinds of research activities mainly focussed on use of three dimensional and coupled T-H and N-K codes.
Finally, a large number of organizations participate directly in current research and international projects but the findings aren't systematically applied to their plants.JRC.F.5 - Safety of present nuclear reactor
