The SARNET objectives are to work on common research programmes in the field of severe accidents and to develop common computer tools and methodologies for safety assessment in this area. Ensuring that the research conducted on severe accidents is efficient and well-focused, the SARNET Severe Accident Research Priority (SARP) group is updating the priorities that were defined in SARNET FP6 on the basis of the EURSAFE FP5 project. This update is done by evaluating the most recent experimental results, e.g. from OECD/CSNI projects, and taking into account findings of SARNET2 WPs and of Level 2 PSA studies (including the results of the ASAMPSA2 FP7 project that just finished end of 2011).
Although all details to understand the Fukushima accident are still not known, this accident will affect the prioritization of the research topics. E.g. some topics related to core melt accidents and induced phenomena are likely to receive other priorities. Simulation models may be improved further to achieve an increased confidence in the predictive capabilities needed for assessing the success potential of countermeasures and/or mitigation tools planned.
Up to now four meetings took place at GRS/Cologne, discussing the procedure to propose new research topics and the possible reorientation of the issues. An essential point is a strong feed-back between the SARP group and other SARNET FP7 partners. Some examples of the current discussions are presented below.
Integrity of Reactor Cooling System (RCS) during severe accident: increased to medium priority. A failure of the RCS before the vessel failure has a strong influence on the accident progression, especially to the direct containment heating (DCH). Furthermore a steam generator (SG) tube rupture in an empty SG has an influence on the source term, caused by the containment by-pass. More detailed validation of computational tools (Computational Fluid Dynamics or CFD, Lumped-Parameter or LP) of the counter-current flow conditions with precise experimental data are needed.
Corium release following vessel failure: remains at low priority. The break location of the vessel could be estimated quite well for PWR. The situation for BWR type plants is much more complex, showing that priorities have to be estimated plant type specific. A still open point is the simulation of break size. This size may have an influence on other phenomena such as steam explosion and DCH.
Ex-vessel corium coolability: remains at high priority. Concerning “dry” Molten-Core-Concrete-Interaction (MCCI), the issue of the interaction of oxidic corium with a limestone-rich concrete has been closed. The issue for siliceous concrete is still open and the uncertainties remain high, but there are prospects for a closure at medium-term. Reactor scale calculations indicate that the major sources of uncertainties lie now with the oxide-metal corium interactions. This topic should be considered with a high priority, although at the borders of current experimental capabilities. Further on, knowledge on the cooling mechanisms by top flooding of the ex-vessel corium pool needs to be increased, because it influences significantly the calculated results, if the basemat erosion stops or not, which affects the containment integrity.
Chemistry impact on source term: remains at high priority. Significant experimental activity has been undertaken in order to solve open issues in iodine/ruthenium chemistry (such as Phébus FPT3, ISTP CHIP & EPICUR, OECD BIP, THAI) leading to improved models. These activities are further on classified with high priority, because they are still relevant for the source term and risk significant. In the recent severe accidents at Fukushima iodine has contributed significantly to the activity released from the plants. The release of volatile fission products has continued weeks after the initiation of the accident. Moreover, radioactive particles deposited within the plants have hindered repair crews from entering the buildings.
New topics ? A discussion started on new/renewed topics based on Fukushima accident, like recriticality in core by flooding with water, influence of sea water injection, pool scrubbing in wetwell water pool, spent fuel pool accidents and others. This discussion will continue. In addition the SARNET FP7 partners are invited to propose new topics. A template can be found on the SARNET website.
Contact: Walter Klein-Hessling