Assessment of on-line burnup monitoring of pebble bed reactor fuel by passive neutron counting

Due to the multi-pass fuel circulation design for pebble bed reactors, an on-line measurement system is needed to accurately assess whether a given pebble has reached its End-of-Life burnup limit and thereby provide an on-line, automated go/no-go decision on fuel disposition on a pebble-by-pebble basis. Gamma spectrometry has been used for burnup determination of pebble bed reactor fuels. In this work a preliminary investigation was conducted to assess the feasibility of using passive neutron counting techniques to analyze fuel pebbles in real time to provide the speed, accuracy, and burnup range required for burnup determination. Specifically, numerical simulations were performed to study the correlation between passive neutron emission rate of an irradiated pebble and its burnup level, the detectability of passive neutron emission from an irradiated fuel pebble by commonly used neutron detectors, and these detectors' abilities to discriminate gamma interference. The overall conclusion is that there is an acceptable correlation between burnup and passive neutron emission rate of an irradiated pebble at high-burnup levels; and if the neutron detection system is well designed, passive neutron counting can be used to provide an on-line, go/no-go decision on fuel disposition on a pebble-by-pebble basis for pebble bed reactors.