A New Partial Defect Measuring Approach with Improved Sensitivity for Spent Fuel Assemblies

We have developed a new measurement approach for partial defects inspection of irradiated Light Water Reactor (LWR) fuel assemblies that is more sensitive and faster than present methods. The approach combines a new LWR Reactor Gate Monitor with the Fork Detector (FDET). Alternatively, an exit gate monitor to the shipping casks could be used. Presently, the FDET measures the neutron and gamma emissions from the mid section of a fuel assembly, and the neutron emission rate varies with the fourth power of the burn up (BU). Thus, the uncertainty in the declared BU limits the measurement accuracy to an ∼50% pin removal scenario by using the n/gamma ratio in the FDET. The new approach would be an order of magnitude more sensitive by doing a relative neutron change verification rather than the present absolute neutron emission approach. No change is required in the FDET. The new LWR gate monitor contains fission chambers and He-3 tubes to provide a quantitative neutron yield during irradiated fuel discharge through the water-filled gate that separates the reactor core from the spent fuel pool. The gate monitor provides a quantitative neutron yield from every discharged fuel assembly prior to the time period where pin removal is possible. A subsequent neutron measurement using the FDET (or an exit gate monitor during cask loading) can verify that the neutron emission rate has not decreased (after decay correction) during the storage period in the pool. The decay of the neutrons would be dominated (>90%) by Cm-244 (18.1y half-life) after the 2nd, 3rd, and 4th cycles, so the neutron decay prior to the FDET measurement could be calculated based on the measured discharge date. The statistical precision for both the FDET and the gate monitor is better than 1% for Pressurized Water Reactor (PWR) and VVER-1000 fuel, and a 2-5% accuracy in the neutron balance should be possible. It will be necessary to dedicate the FDET to a particular facility or to standardize the FDET setup for multiple facility use. The LWR gate monitor measures the fuel assembly yield and speed from both sides of the gate without altering the normal discharge procedure. The FDET measurement takes about 5 minutes per assembly plus the fuel and crane movements. The LWR gate monitor was designed primarily for monitoring the fuel movement into and out of the reactor core, and by integrating the data with subsequent FDET measurements, the partial defects measurement is possible. The verification results are independent of the declared BU. The paper will describe the two detector systems, the measured and simulated counting rates for the spent fuel discharge, and the accuracy that is possible for the neutron emission measurement. The calibration of the relative neutron counting efficiency of the gate monitor and the FDET is obtained by measuring any spent fuel assembly in both detectors while maintaining continuity-of- knowledge for the assembly. The integration of the containment and surveillance function with the NDA measurement will be described