A Safeguards Perspective on Pebble Bed Modular Reactors (PBMR) – Considerations,
Approaches and Challenges

Many States are expressing interest in commercialization of small modular reactor (SMR) technology. One of the SMR designs under consideration is the Pebble Bed Modular Reactor (PBMR). The distinctive fuel of the PBMR consists of small-diameter spheres, or “pebbles,” that are roughly the size of billiard balls. Fresh pebbles containing uranium fuel are fed into the reactor from a fresh fuel storage bin, creating a sea of unidentifiable pebbles in the core. Each pebble is recirculated through the reactor several times to achieve optimal burn-up, at which point the irradiated pebbles, now also containing plutonium, flow out of the reactor and are kept in spent fuel (SF) storage bins. Traditional safeguards consider currently operating power reactors as “item facilities” because the fuel is in the form of discrete and integral items that can be individually identified and verified. The PBMR design, and its variants, are more akin to “bulk handling facilities” in which the fuel is transferred in bulk form from the fresh fuel drums to the core, and then to the SF storage containers and silos in a large number of small units (i.e. pebbles) that are not each individually identified for nuclear material accountancy purposes. Maintaining continuity of knowledge (CoK) and safeguarding thousands of continuously moving pebbles is a daunting task. This paper identifies some of the challenges associated with safeguarding PBMRs and discusses the safeguards considerations and approaches needed to preserve CoK. Dialogue among stakeholders during the early design and construction process, in accordance with the principles of safeguards-by-design, is discussed as a strategy to facilitate the efficient and effective implementation of appropriate safeguards measures and minimize the impact on the operator.