Resolution of open core anomalies at LWRs

Full text: Safeguards anomalies caused by surveillance failures or inconclusive surveillance during the open core period at LWRs are the most difficult anomalies to resolve as they are usually detected after refuelling and core closure and the re-start operation of these reactors. The absence of diversion of irradiated fuel, whether in the core or in the pond, with the use of undeclared fresh fuel, dummies or even without replacement - which is less likely - needs to be confirmed. Safeguards has no authority to request immediate stoppage of reactor operation to verify the reactor cores, as such action would cause a high interruption of power production and cost. Therefore, resolution of these types of anomalies is of high safeguards value, particularly before the broader safeguards conclusion of the absence of undeclared nuclear material and activities in the State, in particular the absence of undeclared reprocessing activities or spent fuel storage, has been drawn. In this paper, we are going to present the resolution of three different types of open core anomalies at LWRs where different methods, mainly based on analysis and advanced non-destructive assay (NDA) methods, are successfully used. The first type of open core anomaly is related to late detection of the failure of open core surveillance after refuelling, core closure and restart operation. The core fuel under surveillance is not verified by NDA but only item counted and identified in the core. This is the most common anomaly type, where the removal of either irradiated core fuel or spent fuel could be replaced by fresh fuel assemblies in the core. The resolution of this type of anomaly is done with the use of an advanced NDA method, mainly a specially designed fork detector irradiated fuel measurement system (FDET), to assess the irradiation period of different core fuel assemblies upon discharge at the end of the cycle. This solution is more likely to be applied to PWRs where normally the entire core fuel assemblies are discharged temporarily to the spent fuel pond before refuelling. The second type of anomaly is related to the failure of surveillance during the core opening period until the discharge of the entire core fuel assemblies and the verification of the entire irradiated fuel (core fuel and spent fuel) in the spent fuel pond - called 100% verification - using an improved Cerenkov viewing device (ICVD). The resolution in this case is based on detailed analysis of the possibility to discharge and reload the entire core, introduce a number of undeclared fresh fuel assemblies in the core, close the reactor, perform undeclared operation of the reactor, shutdown, open the core again, and discharge the entire core. In addition to this analysis, the ICVD is used to confirm the absence of unirradiated dummies or very low irradiated spent fuel assemblies. The third type of anomaly is related to an undeclared emergency core opening and closure with declaration that no core fuel assemblies have been discharged, while the surveillance was inconclusive shortly before this undeclared core opening. The resolution of the anomaly in this case is based on the prompt verification of all the entire spent fuel assemblies in the pond using the advanced ICVD, namely the DCVD, to confirm mainly the cooling time of the assemblies and consequently the absence of undeclared core fuel assemblies discharged during the undeclared emergency core opening. (author)