E.T. Gitau, W.S. Charlton, “Safeguards System Development for a Pebble-Fueled HTGR”, 52nd Annual Meeting of the Institute for Nuclear Materials Management (INMM), Palm Desert, CA, July 17-21, 2011.
The pebble bed modular reactor was first developed by Germany in the 1950s. More recently, the design has been embraced by the People’s Republic of China and the Republic of South Africa. Unlike light water reactors that generate heat from fuel assemblies comprised of fuel rods, pebble-fueled HTGRs utilize thousands of 60-mm diameter fuel spheres (pebbles) comprised of TRISO particle fuel microspheres. As this reactor type is deployed across the world, adequate methods for accounting for the nuclear material within each fuel pebble must be found. Current safeguards methods for the reactor focus on redundant containment and surveillance (C/S) systems to deter the proliferation of fuel pebbles. The disadvantage to this approach is the loss of continuity of knowledge (CoK) in normal and off-normal conditions should both C/S systems fail. One solution to maintaining CoK is to develop a system to identify each fuel pebble that is inserted and removed from the reactor. Work was performed to develop and evaluate the use of inert microspheres placed in each fuel pebble, whose random placement when imaged, could be used as a fingerprint for that fuel pebble. Multiple materials for the inert microspheres and imaging methods were considered to develop the chosen system. To prove the concept, work was performed to determine the optimum number of inert microspheres for a high match confidence. Reactor models were also developed to determine the reactivity impact of inert microsphere inclusion in either the fueled or non-fueled regions of the fuel pebble. Finally, a qualitative assessment of the chosen imaging system was performed using surrogate materials.