Accuracy of transport calculations for the advanced neutron source

The advanced neutron source (ANS) is a new research facility, based on a steady-state reactor neutron source, that is designed to produce an unprecedented thermal flux peak level of ∼ 5 to 10 x 1019 n/m2 · s. This is achieved by the simultaneous reduction of the core volume and of the moderation within the core. Hence, all the candidate designs for the ANS reactor are characterized by their very compact core geometry, high fuel enrichments, and low D2O moderator/coolant content within the core. It follows that the thermal flux gradient in the core, as well as the epithermal and fast flux gradients in the core, as well as the epithermal and fast flux gradients in the large D2O pool reflector, are extremely large. This raises questions about the adequacy of diffusion calculations for this problem, and also about the accuracy of the DORT transport calculations performed to verify the diffusion calculations; in this paper the latter question is considered. In the most rigorous sense, the accuracy must be determined on a pointwise basis for all calculated quantities, i.e., the group angular flux. Practically, however, only a few integral quantities, whose accuracy is crucial for the proper design and operation of the ANS, can be identified, e.g., the effective multiplication factor, the fission source, the heating rate of the pressure vessel, etc. For an early design model of the ANS these quantities are calculated by DORT to a high degree of accuracy because the large group flux gradients occur at locations where the corresponding group flux level is extremely low