Speaker
Description
Recently, a second Target Accuracy Requirements (TAR) exercise has been launched in the framework of the WPEC/S46 on “Efficient and Effective Use of Integral Experiments for Nuclear Data Validation”. The TAR exercise aims at quantifying nuclear data needs (in terms of uncertainty reduction) to meet target accuracies on specific integral parameters. These target accuracies are driven by reactor and fuel cycle design needs.
The first TAR exercise was performed at NEA in the period 2005-2008 within the framework of WPEC/SG26 on “Uncertainty and target accuracy assessment for innovative systems using recent covariance data evaluations”. The WPEC/SG26 exercise did provided both crucial entries to the NEA High Priority Request List (HPRL) and guidance for new experiments and data evaluations via the NEA/HPRL. Moreover, it was the triggering factor for a worldwide revival of covariance data evaluations in all major cross section data evaluation projects. A large expert participation contributed to WPEC/SG26 (2005-2008). As a main outcome of this project, a first list of 14 data priorities (i. e. uncertainty reductions) for GEN-IV reactors was established and implemented in the HPRL at NEA.
Thus, the aim of second TAR exercise is intended mainly for updating and enlarging the scope of the previous WPEC/SG26 exercise, and for providing updated target accuracies for nuclear data uncertainty reduction. This updated TAR exercise will take advantage of:
- The progress of covariance data assessment in different international projects (e. g. WPEC/SG44 on “Investigation of Covariance Data in General Purpose Nuclear Data Libraries”) and evaluation projects of nuclear data (e. g. JEFF, ENDF, etc…)
- The potential nuclear data issues and requirements for systems (reactor and associated fuel cycles) recently emerging as new and innovative concepts of interest for the international community
- The expansion of Sensitivity/Uncertainty methodologies
- The assessment of “correlations” between reactions in the “inverse method” optimization. Neglecting of the correlations in WPEC/SG26 may be one of the major concerns. Current studies have shown that taking into account correlations may require tighter uncertainties on the considered nuclear data.
- The credit of the uncertainty reduction of nuclear data for design optimization, margins reduction, optimized reactor operation and innovative fuel cycles feasibility assessment
Target Accuracy Requirements (TAR) represent typical industrial targets to be met. For nuclear reactors and nuclear fuel facilities these targets may represent safety acceptance limits on different integral parameters for a safe operation both in normal (e. g. nominal operation, manoeuvres, …) and accidental transients (e .g. Loss-Of-Coolant Accident (LOCA) …). Different safety design limits may be defined to assess: i) the source term with limits in radiotoxicity, dose, etc… ii) the criticality with safety limits in keff, iii) the damage of fuel in accidents with safety limits peak power distributions. However, the target accuracies are difficult to specify and are often hidden behind/included in an engineering margin. The values of TAR should include only the impact of nuclear data uncertainty, assuming and recognizing that this is only one component of the total uncertainty. These TARs should result as far as possible from reactor and associate fuel cycle designers, reactor physicists involved in data validation using integral experiments, and nuclear data physicists. Consequently, design code system developers should also be included in this work. It is also recognized that for this second TAR exercise it will be essential to update and verify the status of WPEC/SG26 design target accuracies and their potential evolution (reactor operation and fuel cycle parameters) since then.
The updated TAR exercise will be applied for nuclear systems (reactor and associated fuel cycles) including: i) WPEC/SG26 (2005-2008) systems: Gen IV, MA burners, and ADS systems, ii) WPEC/SG33 (2014-2018) systems: ABR type and JOYO, iii) new reactors concepts (SMR, MSR, HTFR,…), sodium-cooled fast reactors and lead-cooled fast reactors, ADS systems (e.g. MYRRHA), back-end facilities and concepts, iv) review of SG26 systems: LWRs.
The specifications for the updated TAR exercise were delivered in May 2021. It is worth noting that a new energy structure for sensitivities and uncertainties for the exercise, consists of only seven energy groups.
This exercise will be also useful to test different methodologies with the “traditional” methodology used in WPEC/SG26 via sensitivity analysis. Particularly, this work will assess the importance of cross-correlations of covariance data.
The results of this TAR exercise will be presented in this paper. These results will serve to assess the effect of nuclear data uncertainty reduction on output relevant for reactor design and operation; with the goal to provide the NEA/HPRL with new requirements of “nuclear data uncertainty reduction”. This work will motivate and focus new experiments needed for meeting the new requirements.