Signature Based Safeguards Alternative to Nuclear Material Accountancy

In a facility that handles fissile material, the traditional safeguards approach requires establishment of input accountancy as the first step of material control and accountability. At various key measurement points, the mass balance is verified. If a significant quantity of fissile material is missing, a potential diversion is reported. Input accountancy is especially important for a nuclear fuel reprocessing facility. In the PUREX process, this is accomplished with an input accountability tank (IAT) in which spent fuel is dissolved. Based on the form of the fuel and the requirements of the process, such dissolution and sampling is not always possible. Incomplete dissolution, thermal treatment, electrochemical treatment, and mechanical sheering are examples of processes in which there exists no IAT. Operator declarations from reactor calculations and non-destructive assay (NDA) are often cited as promising IAT alternatives. However calculations can be spoofed, and current NDA methods have yet to be demonstrated for accurate assessment of spent fuel. Biases inherent to this current approach can lead to the “shipper receiver difference problem” which can have a negative impact on public opinion. Another alternative is to employ an approach called Signature Based Safeguards (SBS)—a concept built around the interpretation of input from various sensors in a declared facility with complementary access. The nature of the sensors depends on the type of processing being performed. Potential examples include motion trackers, off-gas analyzers, current/voltage measurements, thermocouples, and even NDA measurements. For each facility put under SBS surveillance, it is necessary to first bracket normal operations and then identify abnormal operations that could lead to material diversion or potentially disruptive false alarms. Then a dynamic model must be created for that facility. Sensors are built into the model. Then the model is used to simulate both normal and abnormal operations. Assessment of the responses to abnormal operations can then be used to trigger alarms and potentially shut down the facility until physical verification of materials can be confirmed. This approach to safeguarding nuclear facilities is extremely flexible and powerful—especially in the absence of ability to precisely determine the input quantities. Additionally, it may be applied as a supplemental safeguards method to enhance traditional nuclear material accountancy. Hypothetical examples of application of SBS will be presented.