OECD-NEA Criticality Working Group- A Status Report and the Bumup Credit Challenge

The computational methods that are available to criticality safety specialists provide them with the ability to extend their knowledge beyond the data available from experimental data. Given the importance of the computational tool, it is vital that some evidence exists for the validity of the computational methods. One activity that has addressed this important issue is presented in this paper. At a 1979 meeting of the Fuel Cycle Safety Group, sponsored by the Organization for Economic Cooperation and Development's Nuclear Energy Agency (OECD-NEA), a proposal was submitted for an international intercomparison of criticality calculations relating to the criticality safety of spent light-water-reactor (LWR) fuel element transport casks. The attendees were challenged by the Chairman of the Fuel Cycle Safety Group to prepare a set of problems that would compose the intercomparison test. Based on the problem sets that were presented in response to the request, a Working Group of criticality safety calculation specialists was convened in May 1980. The following underlying logic for carrying out the work was established; it has proven to be successful in the project, and has been used, essentially unchanged, on all the succeeding efforts of the Working Group. The process was first to identify a set of actual critical experiments that contained the various material and geometric properties present in the spent L WR transport containers. The logic in choosing and applying the experimental data was to be able to establish, in a stepwise fashion, the validity of the method by introducing a new parameter with each new problem. In this way, the effect of the new parameter on the validity of the method can be observed. This procedure was designed to prevent the masking of errors by a combination of negative and positive biases in the results caused by the simultaneous introduction of various parameters, thereby leading to unwarranted confidence in the results. In all cases the primary focus was the adherence to the requirements of the ANS-8.11-1975 Standard, Validation of Calculational Methods for Nuclear Criticality Safety. This Standard has subsequently been incorporated into the body of the ANSI/ANS- 8.1-1983 Standard, Nuclear Criticality Safety in Operations With Fissionable Materials Outside Reactors. The final phase of the procedure, after a method was deemed to be validated, was to apply the method in the solution to several hypothetical problems for which the kerr was unknown. As these problems had no known answer, they, in effect, represented a \"blind\" test that could be used to judge the uniformity of results produced by methods that had been validated against experimental data.