PuO₂ Canister Verification System

Features

• Designed for simultaneous passive neutron coincidence measurement and gamma isotopic measurement of canisters of PuO₂.
• Load cell platform for weight verification.
• Computer and manual control of Germanium detector position.
• Lead shield with variable collimator lengths.
• Fast Amptek electronics.
• 12 ³He proportional detectors.
• Flat axial neutron response.

Description

The PuO₂ Canister Verification System is an integrated neutron/gamma safeguards system designed to measure, with a high standard of reliability, accuracy, and precision, the total plutonium mass for PuO₂ samples received at the Siemens facility in Germany. The system counts coincidence neutrons from the spontaneous fission of the even numbered isotopes of plutonium. Because high burnup plutonium generates significant heat, the neutron counter is designed to provide cooling air to be drawn through the detector and around the sample for temperature stability.

The system is made up of the following components:

• Neutron Coincidence Counter with ³He detectors and Gamma System with lead shield/collimator.
• JSR-12 Neutron Coincidence Analyzer.
• "Telescope" Germanium detector with NIM electronics.
• Computer system with MGA isotopics software and Neutron Coincidence Software.

In use, one to five cans of PuO₂ stacked inside a stainless steel canister are positioned in the neutron counter on the load cell platform by the robotics mechanism. The weight of the canister is automatically displayed and verified against the shipper's declared weight by the operator.

The polyethylene moderator of the neutron counter is designed to flatten the axial response, eliminating the effects of the fill height of the cans of PuO₂ and the number of cans in the canister. Twelve ³He tubes are surrounded by high-density polyethylene and a cadmium sleeve in the central region to flatten the axial response and decrease the counter die-away time. The tubes are arranged in a single ring and divided into six groups of two. Each group is wired together and coupled to a JAB-01 Amplifier/Discriminator circuit board. The six JAB-01s are mounted inside the high voltage junction box. LED indicator lights are placed remotely to the junction box to indicate proper operation of each JAB-01 channel. Dual outputs from the JAB-01 Amptek circuitry allow simultaneous data collection and analysis by the IAEA and EURATOM during inspections. Electrical connections between the neutron counter and JSR-12s include +5 V, HV, and dual ORed output signals.

The cadmium liner inside the sample cavity prevents re-entry of thermal neutrons which could induce fission in the sample and adversely affect the measurement result. The exterior of the neutron counter is covered with 1 mm (0.039 in.) thick cadmium and 1 mm (0.039 in.) thick stainless steel.

The gamma portion of the system includes a "Telescope" Germanium (Ge) detector with shield/collimator mounted on a platform that moves vertically. The Telescope detector consists of a 300 mm² x 10 mm thick Low Energy Germanium detector (LEGe) and a 12% coaxial type Ge detector with absorber material placed between them so the detectors have approximately the same count rate. This construction allows simultaneous data acquisition from low energies to well over 1 MeV.

The collimator is fabricated in three pieces to allow the detector to be moved closer to or further from the neutron counter depending upon the gamma count rate. The detector can be positioned ± 1 mm vertically next to gamma windows in the neutron counter. The five gamma windows correspond to the position of the axial centerline of each of the five cans. Positioning of the Ge detector is automatic because the computer is interfaced to a microprocessor-based NIM-configured controller via RS-232C. The operator selects a position from a menu in the program and the controller commands movement of the platform by a stepping motor. Accurate positioning of the platform is provided through closed-loop feedback from an encoder. In case of power failure, a fail-safe brake prevents the platform from falling.

The plutonium gamma ray spectrum is very complex and has several multiplet structures. Computer codes like the MGA code¹, which was developed at Lawrence Livermore National Laboratory, use the integrals of these close lying peaks to calculate the isotopic composition of the plutonium. MGA can analyze a second spectrum of high-energy (300-600 keV) plutonium gamma-rays that significantly improve the ²⁴¹Pu/²³⁹Pu, particularly in high-burnup plutonium. The high energy region can also be used by MGA to detect the presence of residual fission products down to less than 10⁵ Bq/gm plutonium. The isotopics from the MGA code or the shipper's declared isotopics can be inputted into the neutron code to convert ²⁴⁰Pu-effective into total plutonium.

Safety Features

• Fail-safe brake that prevents the detector platform from falling after loss of power.
• Tamper-resistant design including covered connectors, red-anodized high voltage junction box, and a locked equipment rack. Screws are fitted with tamper-indicating hardware to eliminate access to the JAB-01s.
• Stainless steel skin complies with fire requirements concerning exposed polyethylene.