Citizens Electoral Council - Election 2016 - Enact Glass-Steagall


Super-Safe Nuclear Power:
the Meltdown-Proof Pebble Bed Reactor

(published in The New Citizen, February 2002, reprinted in April 2006 edition.)

This diagram, a cutaway of Fig. 1, illustrates the breathtakingly simple function of a Pebble Bed Modular Reactor (PBMR), as designed by South Africa’s Eskom company. The steel pressure reactor vessel of the PBMR is six metres in diameter and 20 metres high, inside a building that is 21 metres below ground. The walls of the reactor vessel are lined with one- metre thick graphite bricks. Inside the reactor vessel are 310,000 fuel “pebbles” which are the size of tennis balls, plus 130,000 graphite balls, which moderate the reaction. The fuel pebbles contain uranium, which releases the neutrons that cause fissioning in other uranium, thereby releasing even more neutrons that expand the process in what is known as a chain reaction, while the moderator pebbles slow the neutrons down enough to ensure a controlled chain reaction.

The fuel pebbles consist of about 15,000 tiny particles of uranium oxide, each coated with layers of ceramics and silicon carbide, forming an impenetrable barrier which contains the fuel. These particles are then mixed with graphite and moulded into pebbles. These pebbles can operate even at very high temperatures of nearly 900 degrees Celsius, and in fact can withstand temperatures at which normal fuel rods in conventional reactors would fail. A further safety aspect of these pebbles is that the radioactive fission products of the spent fuel are locked inside the fuel particles, thanks to their silicon carbide coating. Therefore, even in the worst conceivable emergency, the radiation is safely contained, and after use, the pebbles can be safely stored, very cheaply.

The interior of the fuel pebbles that contain the nuclear reaction and by-products, and make PBMRs “meltdown proof”.

To produce electricity in a PBMR, helium gas at 500 degrees Celsius is inserted at the top of the reactor, and passes among the fissioning fuel pebbles, leaving the reactor core at 900 degrees Celsius. From there it passes through three turbines, the first two driving compressors, and the third the generator. There the natural thermal expansion of the helium is transformed into the rotational motion to generate electricity. The expanded helium is then recycled into the reactor core by two turbo-compressors. The helium leaves the recuperator at about 140 degrees Celsius, and its temperature is lowered further to about 30 degrees Celsius in a water-cooled pre-cooler. The helium is then repressurised, and moves back to the heat exchanger to pick up heat before going back to the reactor core. This direct cycle helium turbine simplifies the normal reactor operations, and makes many standard aspects of conventional reactors unnecessary. The outlet temperature of 900 degrees Celsius is also far higher than the 280–330 degrees of conventional reactors, and gives this type of reactor its name: high temperature reactor.

The inherent and passive safety systems of the PBMR make it “meltdown proof”. In any imaginable accident scenario, the reactor shuts itself down, without any additional safety systems. Further, there is a self-stabilising temperature effect in the reactor core: if the temperature rises, it slows down the neutron production that is central to the chain reaction fissioning process, and fission decreases, because of the large amount of unfissionable uranium- 238 in the fuel particles which capture the neutrons.

Continue onto Thorium: The Preferred Nuclear Fuel of the Future

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