Scope

In the past few decades, there has been an increasing interest in the use of Best Estimate Plus Uncertainty (BEPU) methodologies for the safety analyses of Nuclear Power Plants (NPPs). The behavior of the reactor cooling system during operational and accidental scenarios is therefore simulated with a best-estimate thermal-hydraulic system code (such as ATHLET, CATHARE, RELAP, SPACE, TRACE) using realistic assumptions, but the impact of the uncertainties needs to be quantified. One of the crucial issues is to quantify the input uncertainties associated to the physical models in the code. Such quantification is preferably performed by comparison with experimental data. In such cases, it is usually referred to as Inverse Uncertainty Quantification (IUQ). In this framework, the NEA SAPIUM project led to the definition of a guideline containing several recommendations and best-practices to perform IUQ. It highlighted that IUQ should be performed as a global process involving a clear specification of the problem, an efficient strategy to construct adequate experimental database and an assessment of the simulation models. The uncertainty ranges and distributions can then be obtained with adequate IUQ methods and finally validated.

The scope of the proposed project is therefore to perform practical exercises of demonstration of the SAPIUM approach with the objective to:

• Verify the applicability of the best-practices;
• Identify possible new issues;
• Summarise the lessons learnt from the different participants and possibly update the recommendations based on the results of the activity.

In particular, it is proposed to quantify the uncertainties associated to some physical phenomena during a Loss Of Coolant Accident (LOCA). Two main IUQ exercises with increasing complexity are planned:

1. In the first exercise, the experimental database is composed of Separate Effect Tests (SETs) which allows isolating the physical phenomenon of interest, simplifying the IUQ process. The selected phenomenon is the critical flow at the break, which is of significant relevance during a LOCA.
2. In the second exercise, several influential phenomena are involved at the same time in the experiments, which may then be called Combined Effect Tests (CETs). In this category, the post-CHF heat transfer phenomena will be studied.

This progressive approach could help to treat the key issues identified and to gradually apply the best-practices developed in SAPIUM. Particular attention will be dedicated to the evaluation of the adequacy of the experimental databases for extrapolation to the study of a LOCA.

Finally, the obtained input model uncertainties will be propagated on a suitable Integral Effect Test (IET), such as LSTF-ROSA IB-HL-01, in order to validate their application in experiments at a larger scale and possibly justify the extrapolation to reactor scale.

References

Task leaders: Lucia Sargentini, Alberto Ghione, Guillaume Damblin, Philippe Fillion (CEA, France)
Member countries: Belgium, China, France, Germany, Italy, Japan, the Netherlands, Korea, Spain, Switzerland, United States

Members' area