Passive Neutron Monitor

Strong economic and technical advantages can be gained from mobile, reconfigurable plutonium assay instrumentation which can be moved to non routine situations and geometries, eg gloveboxes, filters and wastes from the decommissioning of active facilities. The N93 Versatile Passive Neutron Monitor is a series of identical neutron counting modules, which can be used individually or in any reconfigurable combination of up to eight modules. However, even the standard eight module system can be expanded by the addition of further modules to suit user requirements. The flexible positioning of the counting modules allows the Pu content of almost any waste container to be assayed, provided that a calibration can be made under similar geometrical conditions.

A small electronics package provides all necessary neutron counting functions and is interfaced to a portable computer for on-line analysis.

Description

The principle of operation of the N93 monitor is the thermalisation of fast neutron emission from the active waste by a polyethylene moderator containing thermal neutron detectors. Each neutron counting module consists of a polyethylene slab, of dimensions 1050 mm high, 270 mm wide and 100 mm deep, and houses three 51 mm diameter 3He proportional counters. The counters within each module are connected via super screened cables to a composite charge amplifier/discriminator unit which is mounted on the rear face of the slab in a stainless steel weatherproof enclosure. The Low and High voltage power supplies are applied to each module via short inter-connecting cable assemblies which allow easy reconfiguration of the counting modules. The digital outputs from each module are transmitted to the electronics package for processing.

Each neutron counting module is mounted on a wheeled base to facilitate movement around the measurement sample, whilst a complete system of 8 modules can readily be transported on site by a small truck or light goods vehicle. Alternatively, the N93 can be supplied with fixing plates to allow for permanent site installation.

The electronics package can be tailored to suit individual plant requirements; the N97 Neutron Coincidence Analyser is the preferred interface used to provide a portable capability. It will record total neutron counts for low level wastes, eg land burial, while the coincidence mode of operation is used for the quantitative measurement of the plutonium content of higher level wastes. Alternatively, the processing electronics can be supplied in Harwell NIM format incorporating the 953021-1 Neutron Coincidence Unit, 950007-1 High Voltage and Power Link and the 950008-1 RS422 Mixer Interface. The data from both types of processing electronics are transmitted to the system control computer via an RS232 Serial communications link.

Applications

Some typical applications for the Versatile Passive Neutron Monitor are:

• Measurements on a large range of material types and geometries such as packets, drums, filters, glove boxes and decommissioning waste.
• Monitoring of Pu movement in doorways and other access control locations
• Discrimination between Intermediate and Low-level waste
• Options available for permanent plant installation

This system has been designed to meet the current recommendations on Electromagnetic Compatibility and has been successfully assessed as compliant with EMC directive EC89-336-EEC.