AVR, the German pebble bed reactor Image via Wikipedia
As the reactor failure at the Fukushima Plant in the aftermath of the Sendai earthquake increases public apprehension of nuclear technology, the future of this type of energy production might not be as bleak as it looks. There is one design that seems to be a marriage made in heaven between safety and efficiency. An experimental design, called pebble bed reactor, uses small, golf size balls of graphite instead of fuel rods to generate heat. There are about 360,000 balls in a reactor and 3,000 of them are changed daily. The advantage of this design is that the pebbles capture more neutrons as the reactor heats up. This means that the reactor cannot overheat or explode, since it has a negative feedback. As temperature raises the reactivity declines, reducing in turn the temperature. This allows the operators to remove part of the pebbles during the low portion of the heat cycle. This process is described in more technical terms by Popular Mechanics
A typical pebble-bed reactor would function somewhat like a giant gumball machine. The design calls for a core filled with about 360,000 of these fuel pebbles—”kernels” of uranium oxide wrapped in two layers of silicon carbide and one layer of pyrolytic carbon, and embedded in a graphite shell. Each day about 3000 pebbles are removed from the bottom as fuel becomes spent. Fresh pebbles are added to the top, eliminating the need to shut down the reactor for refueling. Helium gas flows through the spaces between the spheres, carrying away the heat of the reacting fuel. This hot gas—which is inert, so a leak wouldn’t be radioactive—can then be used to spin a turbine to generate electricity, or serve more exotic uses such as produce hydrogen, refine shale oil or desalinate water.
The pebbles are fireproof and almost impossible to use for weapons production. The spent fuel is easy to transport and store,
Graphite pebble Image via Wikipedia
though there still remains the long-term problem of where to store it. And the design of the nuclear reactor is inherently meltdown-proof. If the fuel gets too hot, it begins absorbing neutrons, shutting down the chain reaction. In 2004, the cooling gas and secondary safety controls were shut off at an experimental pebble-bed reactor in China—and no calamity followed, says MIT professor Andrew Kadak, who witnessed the test.
Pebble-bed reactors also could be far more cost-effective than Gen II plants, which had an average construction time of more than nine years. Even proposed Gen III designs have an estimated build time of more than five years. Kadak’s group at MIT has developed a pebble-bed design in which every part is small and light enough to be shipped by train and truck, so the components could be mass-produced off-site.
Fourth-generation nuclear power plants differ radically from current reactors by replacing water coolants and moderators, reaching higher temperatures, and gaining the potential to create hydrogen, as well as electricity.
One of the six Gen IV designs under consideration is the meltdown-proof pebble-bed reactor, which uses grains of uranium encased in balls of graphite as fuel. Helium gas is heated as it circulates through a vessel of these pebbles [1] and then powers a turbine [2] to generate electricity. A heat exchanger [3] can transfer heat from the helium to adjacent facilities [4] for the production of hydrogen. The plant relies on “passive safety”: If the cooling system fails, the nuclear reaction grinds to a halt on its own.
The Bulletin of Atomic Scientists, however, disagrees with the view that pebble bed reactors are intrinsically safe. The authors of a recent article that announced the South African government’s decision to shelve a pebble bed reactor believe that the temperatures in the reactor can go far higher than expected and that the constant rubbing of the pebbles against each other could create radioactive graphite dust. Furthermore, they believe that the graphite in the balls can ignite and burn with a lot of smoke, which would spread radioactivity far and wide. The Chernobyl disaster was particularly harmful because the graphite moderators in the reactors were ignited and the smoke generated traveled across Europe. Yet, again, the proponents of pebble bet reactors believe that such scenario cannot happen due to the negative feedback implicit in this power generation technology.
For now, the technology has only been tried in Germany and China, both countries putting it on the back burner due to prototyping and translation costs. Yet, if it will ever work, it could be a true revolution in nuclear power generation.
No it won’t – they have been proven to be even more unsafe. The German model had ONE THOUSAND times more damaged “pebbles” than prognosticated and a dismal record of safety failures and radiation leaks. This is a dead end. See here:
The only questions you need ask your governments to end the nightmare for good are here:
http://crisismaven.wordpress.com/2011/03/23/how-i-brought-down-the-nuclear-industry-in-my-country-and-how-you-can-do-it-in-yours/