The Development of a Rugged Weather-Resistant Continuous Holdup Monitor
for Ductwork at Nuclear Facilities

Researchers at Oak Ridge National Laboratory (ORNL) are developing a low-cost, distributed detector system for real-time, in situ monitoring of nuclear material holdup. This system will improve the accuracy of holdup measurements and decrease facility burden and risk to personnel by enabling longer count times and automated data analysis. Ductwork surveys for nuclear material holdup in nuclear facilities are typically conducted manually on a periodic basis and represent the highest level of effort for holdup monitoring. There can be hundreds of monitoring points. Often, monitoring points are difficult to access or pose safety hazards to the personnel recording the measurements. Additionally, count times are typically short (e.g., 6–10 seconds) due to the high number of monitoring points resulting in undesirably high measurement errors. Needs related to ductwork monitors vary across nuclear facilities, depending on the expected measurement location and facility layout. Two distinct frameworks for power and data transmission have been developed to accommodate indoor and outdoor monitoring points. The first is designed for readily reachable points and uses power-over-ethernet (PoE) for device power and communications. The second is designed for monitoring points to which running wired connections is not feasible. In this framework, the device is battery-powered, supports solar charging and wireless communication, and counts only periodically throughout the day. The prototype holdup monitors are collimated self-contained units combining a scintillator, photomultiplier tube (PMT) or silicon photomultiplier (SiPM), and custom control and counting electronics. Different types of scintillators may be used depending on facility sensitivity and cost requirements. Benchtop prototypes were built using sodium iodide (NaI) and plastic scintillator–based detector assemblies; these were validated with a range of radioactive sources to test the efficacy, accuracy, and detection limits of the counting electronics. The count rates detected by the electronics were compared to standard Nuclear Instrumentation Modules (NIM) bin acquisition and showed excellent agreement, within 20 counts. This paper describes the approach being pursued, discussing the efforts to design, construct, and test a prototype detector and associated counting electronics. Such a holdup monitor will aid greatly in material accountancy efforts and reduce risk to personnel.