Year
2006
Abstract
A new compact spent fuel counter has been designed to determine the fissile content of irradiated research reactor fuel stored underwater. The counter consists of a polyethylene moderator containing boron-lined 3He tubes with a lead shield surrounding the sample hole to keep the gamma dose rate at the 3He locations down to acceptable levels. The counter uses an americium-lithium (AmLi) neutron source to generate fissions in the fuel item. The 3He tubes are divided into two sets. One set of tubes distinguishes fission neutrons from background by measuring neutron coincidences (the same principle that is used in the active well coincidence counter—AWCC). This set of tubes could also be used for the measurement of passive neutron emissions from items other than research reactor fuel. The second set of tubes uses the differential transmission principle; that is, the tubes are set farther back into the polyethylene moderator and are therefore much less sensitive to AmLi neutrons than to fission neutrons, so that the signal is approximately proportional to the fission rate in the fuel item. The detector also contains a collimated ionization chamber to record the gamma emission profile of the measured item. Monte Carlo calculations have been carried out to optimize the location of the AmLi source, 3He tubes, and the thickness of the lead shield. A prototype unit has been built. The effects of known gamma fields on the boron-lined tubes have been measured. Initial measurements have been made of an unirradiated 19-plate materials test reactor (MTR) assembly. This detector provides a tool for verification of research reactor spent fuel, and it can be easily deployed for on-site measurements.