A Very High Efficiency Neutron Counter for the Measurement of Plutonium in Decommissioning Wastes

Abstract

The design of the ANTECH Model 2203 Very High Efficiency Neutron Counter (V-HENC) is a natural progression from the well-proven ANTECH Series 2200 Passive Neutron Drum Monitor used for measuring plutonium in intermediate and low level waste (LLW) 200 litre drums. ANTECH has had considerable experience in the implementation of the base design, originally licensed to ANTECH from the Joint Research Centre at Ispra, Italy. Three Series 2200 systems have been supplied by ANTECH: two are in operation at AWE Aldermaston for waste monitoring, and a third is implemented at the SMP facility at BNFL Sellafield. Some 15 years cumulative operating experience has been gained, and ANTECH provides technical support as part of continuing maintenance and support arrangements. The design has proven to be inherently reliable, safe, easy to operate and maintain. Recently, both AWE instruments have been fully characterised and calibrated to function in conventional coincidence counting mode with calibrations. ANTECH has used MCNP to optimise the design of the fast detector packages in order to achieve the lower detection levels required to measure Pu at USA TRU/LLW and UK Nirex LLW levels. With the aid of simulations a typical detection efficiency of 36% with Cd filters deployed and up to 45% with the internal Cd liner removed has been achieved. Statistical data filtering is used to decrease the cosmic ray induced neutron background, the latter also being minimised by the absence of steelwork within the drum measurement chamber. An outer shielding of 270 mm thickness of polyethylene is used to shield the system from external neutrons. A total of 16 fast detector modules are used, which consist of eight, 7.5 atmospheres 3He tubes (25.4mm diameter and 1033 mm active length) embedded in high density polyethylene and arranged in a double row. Tubes are connected using HN connectors to a junction box at the top of the vertical modules or on the end of the horizontal modules. The junction boxes are hermetically sealed and contain the high voltage distribution and AMPTEK model A-111 charge sensitive amplifiers. The operation of the V-HENC is based on passive neutron counting of the correlated neutrons from spontaneous fission of the even Pu nuclides, principally 240Pu and is coupled with an ANTECH/Ortec Advanced Multiplicity Shift Register employing the Los Alamos INCC code. Alternatively, the multiple gate ANTECH Time Correlation Analyser (TCA) may be used for enhanced data acquisition for multiplicity counting. The V-HENC can be operated in conventional shift register coincidence counting (Reals) mode (with a calibration function), the absolute multiplicity counting mode (histogram function) or totals counting mode. Plant measured isotopic ratios can be used by the software to convert 240Pueffective mass to total Pu mass.