LOW-COST, LOW-POWER SENSORS FOR MONITORING CANDU REACTOR WASTE IN DRY STORAGE SILOS

Canadian deuterium-uranium (CANDU) reactors are of significant interest because they offer the ability to use natural uranium as a fuel, greatly enhancing the accessibility of nuclear power. The spent fuel, which contains both uranium and plutonium, requires continuous accountability. As reactor storage pools reach their storage capacity limits, it is necessary to periodically transfer the spent fuel to external dry storage silos for longterm storage. Once the fuel is in the silos, it is sealed in place, and a scan of the radiation profile of the silo (via verification tubes running down the length of the silos) is recorded. Subsequent scans of the radiation profile are the primary means for verifying the presence and positions of the stored fuel containers. A means for cost-effective insitu monitoring would greatly enhance the ability to safeguard the materials. A sensor system for in-situ monitoring of CANDU dry storage silos has been developed and demonstrated. It relies on simple, low-power, inexpensive detectors and electronics. A three-detector system has been shown to be capable of monitoring the status of the fuel at the top and bottom of a storage silo while reliably indicating the direction of movement of fuel into or out of the silo. As an additional feature, the three sensors incorporate a highly sensitive motion-sensing feature that prevents any undetected tampering with the sensors themselves. Once installed in the silos, the sensors are extremely stable, responding only to changes in radiation level. However, any attempt to remove the sensors will result in a large anomalous signal fluctuation that cannot occur as a result of any activity other than tampering. This motion-sensing feature is, in fact, sufficiently sensitive to provide a small but easily recognized transient signal that occurs as the fuel transfer equipment contacts the top of the storage silo.