Overview of the international reactor physics experiments evaluation project (IRPhEP) guide to the expression of uncertainty

The methods used by the International Reactor Physics Experiment Evaluation Project (IRPhEP) to treat uncertainties encountered in experimental data and in the derivation of benchmark models have followed guidance provided by the International Criticality Safety Benchmark Evaluation Project (ICSBEP). Those methods have evolved significantly since initiation of the ICSBEP in 1992 and formal initiation of the IRPhEP in 2003. While the methods developed by the ICSBEP have been very helpful, the IRPhEP Technical Review Group has recognized the need for specific guidance on the treatment of uncertainties associated with reactor physics methods since its inception. An effort to develop such guidance was initiated by the Organization for Economic Cooperation and Development / Nuclear Energy Agency in late 2016. Development of the IRPhEP Guide to the Expression of Uncertainty is expected to be an evolutionary process as is the ICSBEP Guide and may take several years to provide guidance for all types of reactor physics measurements, but the first edition of the Guide will be available in the 2018 Edition of the IRPhEP Handbook (Spring 2018). Uncertainties addressed by the IRPhEP are divided into 3 categories: 1) Uncertainties in measurement methods, 2) Uncertainties in all physical and chemical parameters of the experiments, and 3) uncertainties in biases introduced by the use of parameters derived from calculations or from other experiments, desired simplification, or by modelling limitations. Guidance on how to address those uncertainties is provided in the Guide. An overview of the Guide is provided in this paper.