Passive safety in nuclear design

Long term readers of this blog would know that I used to follow with much interest the development of pebble bed reactors - in particular in the development program South Africa had going until they ran out of money. (Use the "search this blog" bar at the side for "pebble bed reactors" and you'll find lots of past posts.)

The big attraction of the pebble bed was its (claimed) passive safety features. As the Wikipedia article on pebble bed reactors notes (although it does sound like it has been written by a strong proponent of pebble beds):

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. U²³⁸, which forms the bulk of the uranium in the reactor, is much more likely to absorb fast or epithermal neutrons at higher temperatures. [2] 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.^[6]. 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.

Wikipedia has a short article on passive safety design in nuclear reactors generally, noting that there are some designs using liquid metals as a heat sink for passive safety. Somehow, I can't help but think that using liquid metal just doesn't sound as passively safe as a gas cooled pebble bed.

In any event, surely recent events show that the goal of passive safety deserves to be a major component of future nuclear design.