In-Situ Verification of Spent Fuel at the Atucha-I Nuclear Power Plant

The problem of gross defect detection at the Atucha-I spent fuel pool is a particularly challenging one since the fuel assemblies are stored in two layers in the pool. Atucha-I is a one-of-a-kind pressurized heavy water reactor in Argentina that uses natural or, more recently, slightly enriched uranium as fuel. A tool based on in-situ neutron detection using a fission chamber was developed and successfully tested at the site for the verification of the presence of spent fuel that had specific burnups in the range of 5000-8000 MWd/t, a range that was consistent with the use of natural uranium fuel. However, since then the plant has started using slightly enriched uranium at the level of 0.85 w% that allows burnups of beyond 8000 MWd/t and ranging up to 11,000 MWd/t. Beyond the 8000 MWd/t burnup range the calibration curve that was originally used in conjunction with the instrument cannot be used since the neutron source strength versus the burnup becomes non-linear. A new fast and accurate methodology was developed based on a pre-calculated database of parameters including intrinsic sources, fission matrix coefficients, and detector importance functions that can produce an expected detector response in any part of the spent fuel pool in tenth of a second. An algorithm based on the methodology is being developed that can be deployed in the field and this will be coupled with a tool that will characterize the neutron field when inserted in the space between four assemblies. The new tool will fulfill a need in the safeguards portfolio in the context of gross defect verification at the Atucha-I spent fuel pool.