Chernobyl Conditioning Facility Detector Design

Unattended monitoring systems (UNARM) are a cost effective way of safeguarding nuclear material. A fundamental component of such a monitoring system is the radiation (neutron and gamma) detectors since they respond to a direct attribute of the nuclear material. With the Chernobyl Conditioning Facility (CCF), detecting the motion of spent fuel, as it moves from storage pools - through the conditioning facility - and then to longer-term dry, is the goal. This paper summarizes the Monte Carlo modeling results (gamma ray and neutron transport) for the various detectors needed in the Chernobyl Unattended Monitoring System. In order to simplify detector maintenance in the acidic environment of the CCF, a standard, chemically sealed detector was designed: two 3 He tubes surrounded by lead that was then surrounded by polyethylene; furthermore, two ion chambers are located inside the polyethylene. The lead thickness was determined to keep gamma ray pulse pileup to an acceptable level when the detector was exposed to the maximum gamma dose rate possible. Variation in the polyethylene thickness was then modeled to optimize this thickness for the energy spectrum expected in the facility given the source fuel and the effect of reflection from within the rooms. This standard detector was positioned at various locations in the facility to determine what neutron sensitivity to expect at each location. The sensitivity of this detector to absorber rods and extension rods was modeled as well. Deviations from this standard were made for a few special monitoring situations: on the railroad car that takes the fuel from the reactor area to the CCF, in the room where the railroad car cannon attaches to the building, and for detectors embedded in concrete. The results of experimental benchmarking are presented as well