The 169th “Nuclear reactor physics” seminar took place

On April 14, 2017 National Research Center «Kurchatov Institute» hosted the 169-th “Nuclear reactor physics” seminar of the Kurchatov nuclear and technological complex “Kurchatov Institute”. Mr. Dmitry Yashnikov, the Head of laboratory on accuracy assessment of Codes Assessment Division of SEC NRS, made presentation on topic “Assessment of calculations accuracy performed in the course of nuclear facilities safety demonstration”.

Approach for calculations’ accuracy assessment was proposed. This approach is based on taking into consideration the parameters uncertainty of software calculation model and consideration of uncertainty of measurements used for software validation.

The speaker made an overview of the methods available for the analysis of parameters uncertainty in software calculation model used in the course of a nuclear facility safety demonstration in Russia and abroad. In particular, the application of uncertainties analysis method based on Wilkes ratio was presented.

This presentation addressed main problems regarding the justification of software calculation accuracy values:

• the number of experiments carried out at real nuclear facilities and the number of measurements conducted are strongly limited;
• lack of a unique industry-specific bank of the experimental data appropriately assessed and which are recommended for a software validation;
• it is difficult on practice to take into account the scale factor for integral experiments;
• the assessment of measurements’ uncertainty in experiments that are applied for a software, is not performed or it is performed nominally;
• lack of generally recognized approaches to uncertainties analysis in different topical areas of calculations (strength, building structures and constructions, radiation safety, physical and chemical processes).

Based on the outcomes of the presentation, the following proposals were introduced concerning the Rostechnadzors’ regulatory documentation improvement in the part of requirements to accuracy of calculations conducted in the course of a nuclear facility demonstration:

1. When assessing the software calculations’ accuracy:
   • to take into account the parameters uncertainty of a software calculation model and the measurements’ results uncertainty;
   • to present the mentioned uncertainties in the form of intervals with probable characteristics.
2. In the course of a nuclear facility safety demonstration:
   • to apply the accuracy values of software calculations that are only collected during a software validation on the basis of experiments carried out at real nuclear facilities or on integral experimental facilities (taking into account the scale factors);
   • to confirm the conservatism of an approach adopted in the course of a nuclear facility demonstration by applying one of the methods for uncertainty analysis of a software calculation model parameters.
3. To develop a safety guideline with recommendations regarding assessment of uncertainty of NPPs’ thermohydraulic calculations using a method based on Wilkes ratio.

The presentation made by D. Yashnikov has generated great interest among specialists, as evidenced by the fact that more than 200 persons from 30 organizations of the nuclear energy sector participated in this seminar.