Criticality detector exclusion zone in a spent-fuel hot cell
The main purpose of a criticality alarm system (CAS) is to protect personnel by detecting a criticality event (neutron radiation) and actuating an alarm system to initiate emergency response. Inadvertent criticality alarms from noncritical events, such as spurious voltage spikes or intense gamma radiation fields, can produce work cessation and time-consuming and costly event assessments and may result in harm to personnel during an evacuation. It therefore becomes a major concern to ensure that inadvertent or false criticality alarms do not occur or at least are minimized. Minimization of inadvertent criticality alarms due to intense gamma radiation emitted from spent-nuclear-fuel (SNF) elements as opposed to neutron radiation from an actual criticality event is the primary focus of this calculational and experimental study. The Irradiated Fuel Storage Facility (IFSF) located at the Idaho National Engineering and Environmental Laboratory is a government-owned, contractor-operated facility whose mission is to provide safe handling and dry storage for various types of SNFs. Although other fuel types (lower burnup) are stored in the IFSF, it is the high-burnup elements with the associated intense gamma radiation fields that have the potential to inadvertently set off the criticality alarms in the fuel-handling area adjacent to the storage vault. Typically, in the fuel-handling cave or hot cell of the IFSF, the cask lid is removed, and individual fuel elements are extracted from the cask and placed in special storage canisters. It is during the time period when fuel elements are extracted from their casks or when fully loaded canisters are moved in the hot cell that the CAS detectors are exposed to the intense gamma radiation fields from the spent fuel. The neutron detectors positioned in one of the manipulator ports are designed such that fast neutrons from a criticality event are thermalized by a polyethylene moderator, strike the scintillator detector material, and generate a light pulse. The cluster is composed of three scintillator tubes bound tightly together in a lead sheath. The lead plug and sheath provide gamma radiation shielding, but unfortunately, the sheath design does not fully shield the tube axial length circumferentially. The top of the sheath is basically open and can allow SNF scatter gamma rays that penetrate the concrete wall to encounter and strike the scintillator material without attenuation. Despite the fact that the detector cluster is at the 13 ft 1 in. elevation above the IFSF floor 0 ft 0 in. elevation, the potential for this detector cluster to inadvertently alarm is real. The CAS detector has been designed with a 10,000:1 gamma rejection ratio and zero response above background in gamma radiation fields {le}10 rads/h. The authors solution to prevent inadvertent criticality alarms involves setting up an exclusion zone around the detectors. Individual elements or loaded canisters would be prohibited from entering the exclusion zone. Centered about the CAS and extending from the north wall into the hot cell and from the hot-cell ceiling to an elevation below the detector elevation, the exclusion zone boundaries and dimensions were determined analytically.
- Research Organization:
- INEEL/LMIT, Idaho Falls, ID (US)
- OSTI ID:
- 20005817
- Journal Information:
- Transactions of the American Nuclear Society, Vol. 81; Conference: American Nuclear Society 1999 Winter Meeting, Long Beach, CA (US), 11/14/1999--11/18/1999; Other Information: PBD: 1999; ISSN 0003-018X
- Country of Publication:
- United States
- Language:
- English
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Fuel Handling Exclusion Zone Established to Prevent Spurious Alarms to CAS Neutron Detectors in the IFSF
Fuel Handling Exclusion Zone Established to Prevent Spurious Alarms to CAS Neutron Detectors in the IFSF