The Application of the Delayed Neutron Measurements to the Fuel Failure Detection System in HANARO

In HANARO, the fuel failure detection system (FFDS) based on the delayed neutron measurement is suggested, and the field application test is performed. In order to investigate the origins of the neutrons detected at the primary cooling circuit during normal operation of the reactor, the changes of neutron count rate before and after reactor shutdown are measured by a BF₃ detector, and predicted by assuming the neutrons as the photoneutrons emitted by N-16 gamma rays and delayed neutrons by the fuel surface contamination, From the comparison of predictions and measurements, it is confirmed that about 70 % of the measured neutrons are delayed neutrons, and 30 % are photoneutrons. Also, the neutron generation and transportation are simulated by the Monte Carlo method, and the neutron count rates at detector position are calculated. The count rate of photoneutron is calculated to be 11 % of the whole neutron count rate. The amount of uranium exposed to the coolant in the core is also estimated by the delayed neutron transport calculation, and it is lower than the allowable level of surface contamination of the fuel rods. Since a small amount of the delayed neutrons generated from the fuel surface contamination are sensitively detected, the delayed neutron measurement is very useful for monitoring the fuel failure. After the step-wise field application test, the new delayed neutron FFDS system has started its formal service as a trip parameter for the reactor protection system, and it has been carrying out its intended role of fuel failure detection effectively up to now. (author)