"Development and Evaluation of a Safeguards System Concept for a Pebble-fueled High Temperature Gas-cooled Reactor,"
M.S. Thesis, Nuclear Engineering, Texas A&M University, College Station, TX (2011).
Pebble-fueled high temperature gas-cooled reactor (HTGR)
technology was first developed by the Federal Republic of 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 thousands of
TRISO particles. As this reactor type is deployed across the world,
adequate methods for safeguarding the reactor must be developed.
Current safeguards methods for the pebble-fueled HTGR focus on
extensive, redundant containment and surveillance (C/S) measures or
a combination of item-type and bulk-type material safeguards
measures to deter and detect the diversion of fuel pebbles. The
disadvantages to these approaches are the loss of continuity of
knowledge (CoK) when C/S systems fail, or are compromised, and the
introduction of material unaccounted for (MUF). Either
vulnerability can be exploited by an adversary to divert fuel
pebbles from the reactor system. It was determined that a 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 could be used as
a fingerprint to identify the fuel pebble. Ultrasound imaging of 1
mm zirconium oxide microspheres was identified as a possible
imaging system and microsphere material for the new safeguards
system concept. The system concept was evaluated, and it was found
that a minimum of three microspheres are necessary to create enough
random fingerprints for 10,000,000 pebbles. It was also found that,
over the lifetime of the reactor, less than 0.01% of fuel pebbles
can be expected to have randomly the same microsphere fingerprint.
From an MCNP 5.1 model, it was determined that less than fifty
microspheres in each pebble will have no impact on the reactivity
or temperature coefficient of reactivity of the reactor system.
Finally, using an ultrasound system it was found that ultrasound
waves can penetrate thin layers of graphite to image the
Associated Project(s):Safeguards System Concept for Pebble-fueled High Temperature Gas-Cooled Reactor