The development of cost effective methods for long-term safeguarding of spent reactor fuel in temporary storage is necessary in order for the IAEA to meet all demands for safeguards resources. To this end, we have developed a measurement system to verify spent CANDU type fuel in sealed long-term storage casks. Given the large number of spent-fuel storage casks to be monitored, we have attempted to minimize the cost of the proposed system while still providing a useful detection signature from the entire storage cask. The detection system described consists of three small 3(sup)He neutron counters placed inside the storage cask in IAEA reverification tubes. The limiting factor on the size of each detector is a bend in the reverification tube around which the detector must pass. Each detector will be shielded with lead to reduce the gamma dose rate below 30 R/hr while still providing a significant neutron detection signal. Specially designed electronics will be designed at LANL to process the signals from the 3He neutron counters and store data. The electronics will be located outside the cask to minimize gamma dose, maximize long-term stability, and provide for ease of maintenance. Monte Carlo calculations of neutron transport through a CANDU spent fuel storage cask suggest that a single string of neutron detectors will provide visibility of better than 90% of the cask contents at a 5% detection level.
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