Pebble Bed Reactor - Safety Features

Safety Features

When the nuclear fuel increases in temperature, the rapid motion of the atoms in the fuel causes an effect known as Doppler broadening. The fuel then sees a wider range of relative neutron speeds. Uranium-238, which forms the bulk of the uranium in the reactor, is much more likely to absorb fast or epithermal neutrons at higher temperatures. This reduces the number of neutrons available to cause fission, and reduces the power of the reactor. Doppler broadening therefore creates a negative feedback because as fuel temperature increases, reactor power decreases. All reactors have reactivity feedback mechanisms, but the pebble bed reactor is designed so that this effect is very strong and does not depend on any kind of machinery or moving parts. Because of this, its passive cooling, and because the pebble bed reactor is designed for higher temperatures, the pebble bed reactor can passively reduce to a safe power level in an accident scenario. This is the main passive safety feature of the pebble bed reactor, and it makes the pebble bed design (as well as other very high temperature reactors) unique from conventional light water reactors which require active safety controls.

The reactor is cooled by an inert, fireproof gas, so it cannot have a steam explosion as a light-water reactor can. The coolant has no phase transitions—it starts as a gas and remains a gas. Similarly, the moderator is solid carbon; it does not act as a coolant, move, or have phase transitions (i.e., between liquid and gas) as the light water in conventional reactors does.

A pebble-bed reactor thus can have all of its supporting machinery fail, and the reactor will not crack, melt, explode or spew hazardous wastes. It simply goes up to a designed "idle" temperature, and stays there. In that state, the reactor vessel radiates heat, but the vessel and fuel spheres remain intact and undamaged. The machinery can be repaired or the fuel can be removed. These safety features were tested (and filmed) with the German AVR reactor. All the control rods were removed, and the coolant flow was halted. Afterward, the fuel balls were sampled and examined for damage and there was none.

PBRs are intentionally operated above the 250 °C annealing temperature of graphite, so that Wigner energy is not accumulated. This solves a problem discovered in an infamous accident, the Windscale fire. One of the reactors at the Windscale site in England (not a PBR) caught fire because of the release of energy stored as crystalline dislocations (Wigner energy) in the graphite. The dislocations are caused by neutron passage through the graphite. At Windscale, a program of regular annealing was put in place to release accumulated Wigner energy, but since the effect was not anticipated during the construction of the reactor, and since the reactor was cooled by ordinary air in an open cycle, the process could not be reliably controlled, and led to a fire. The 2nd generation of UK gas-cooled reactors, the AGRs, also operate above the annealing temperature of graphite. The continuous refueling means that there is no excess reactivity in the core. Continuous refueling also permits continuous inspection of the fuel elements. The design and reliability of the pebbles is crucial to the reactor's simplicity and safety, because they contain the nuclear fuel. The pebbles are the size of tennis balls. Each has a mass of 210 g, 9 g of which is uranium. It takes 380,000 to fuel a reactor of 120 MW_e. The pebbles are mostly high density graphite which keeps its structural stability at the maximum equilibrium temperature of the reactor. The graphite is the moderator for the reactor, and are strong containment vessels. In fact, most waste disposal plans for pebble-bed reactors plan to store the waste within the spent pebbles..

Berkeley professor Richard A. Muller has called pebble bed reactors "in every way... safer than the present nuclear reactors, and arguably safer than the global-warming danger posed by fossil fuels".