Characterization of detection limits using mock waste matrices in a 3He passive drum counter for plutonium waste verification

Waste streams of fuel cycle, research facilities or decommissioning activities may contain measurable quantities of plutonium that must be accounted for under international safeguards agreements. Inspectors must perform measurements to verify operator declared plutonium masses within waste matrices. In some cases, inspectors might be asked to verify the presence of only a few grams of plutonium distributed in a waste drum. Neutron coincidence measurements are a standard tool to measure ²⁴⁰Pu content due to its high spontaneous fission rate. Given prior knowledge of isotopics, total plutonium mass can be estimated. For such measurements, the European Commission´s Joint Research Centre (JRC) maintains, tests and upgrades a ³He drum monitor for deployment to European nuclear facilities at the request of EURATOM. This drum monitor consists of 148 standard ³He tubes in a 4π geometry for passive neutron coincidence measurements using a shift register and standard INCC software. The neutron drum monitor is designed to handle standard 208 L (55 gallon) waste drums. The drum counter´s design is optimized to handle conditioned waste drums of low plutonium content. This nondestructive measurement tool recently underwent extensive electronic and structural refurbishments resulting in CE certification (compliance with relevant European Union legislation). Before redeployment, the passive neutron detection system must undergo an extensive measurement campaign to ascertain its limits of detection of plutonium in a variety of waste matrices. For this purpose, mock waste matrices were produced. These consist of concrete filled standard waste drums. A number of cavities at different distances from the drum center allow for a variety of source locations to be tested. Plutonium metal and oxide samples ranging from milligram to gram quantities were measured during this campaign.