STANDARD PROBLEM EXERCISE TO VALIDATE CRITICALITY CODES FOR LARGE ARRAYS OF PACKAGES OF FISSILE MATERIALS

A study has been conducted by a Working Group under the auspices of the Organisation for Economic Co-operation and Development, Committee on the Safety of Nuclear Installations that examined computational methods used to compute keff for large > 53 arrays of fissile material (in which each unit is a substantial fraction of a critical mass). Five fissile materials that might typically be transported were used in the study. The 'packages' used for this exercise were simplified to allow studies unperturbed by the variety of structural materials which would exist in an actual package. The only material present other than the fissile material was a variation in the moderator (water) surrounding the flssile material. Consistent results were obtained from calculations using several computational methods. That is, when the bias demonstrated by each method for actual critical experiments was used to 'correct' the results obtained for systems for which there were no experimental data, there was good agreement between the methods. Two major areas of concern were raised by this exercise. First, the lack of experimental data for arrays with size greater than 53 limits validation for large systems. Second, there is a distinct possibility that the commingling of two shipments of unlike units could result in a reduction in safety margins. Additional experiments and calculations will be required to satisfactorily resolve the remaining questions regarding the safe transport of large arrays of fissile materials.