David Shapshak
Now that Pakistan has shown the world that it could quietly purify enough plutonium for the five nuclear devices it detonated recently – and probably enough for many more – attention is turning to how to divorce nuclear weapons from nuclear power plants.
And while conventional nuclear power stations have had a bad press since the Chernobyl meltdown about 12 years ago, the attractiveness of nuclear energy means alternatives to energy sources are still being investigated.
One way of countering meltdown is to build a reactor small enough so that the heat it loses is more than the heat it generates.
This is what South Africa’s electricity utility, Eskom, has done. It unveiled plans this week for a “pocket-sized” nuclear reactor, roughly the size of a suburban house, which produces less power (100MW) than current larger nuclear power stations (usually 900MW) but has significant other features.
Firstly, the fuel is configured differently: particles of uranium are embedded in carbon balls, the size of tennis balls, as a ceramic material. Then the fuel heats up helium, an inert and therefore unreactive gas, which simultaneously cools the reactor (by extracting its heat) and drives the turbines, which will generate electricity.
Conventional reactors use water for this and the helium produces a more efficient energy conversion, 45% as opposed to 35% in current coal-burning energy-generation plants, says Tony Stott, Eskom’s representative on nuclear matters.
The smaller reactor also does not need to be next to a water source for the coolant.
“This particular concept provides flexibility in where the reactor can be located, obviously after an environment impact study. We are able to locate a power station close to where the demand is, such as next to a big factory or a large town,” he says.
This also reduces the amount of transmission lines, losses of power, and the increased risk of lines being broken.
The reactor’s size means it is relatively easy to switch on and off, and can therefore can be utilised for specific or additional electricity demands, such as early morning and late evening.
The core is a gas-cooled pebble bed molecular reactor, which would produce less toxic waste.
Should Eskom build a prototype of the power station – which, in this early conceptual phase, is estimated to cost between R380-million and R450-million – it could do so next to South Africa’s only nuclear power plant, Koeberg, in the Western Cape.
But as it uses conventional fission, albeit on a smaller scale, it would still require standard nuclear fuel, containing uranium, to produce electricity.
n The New York Times reports that a small Washington-based company says it has found an answer to this problem, by substituting thorium for some of the uranium.
The company, Radkowsky Thorium Power, headed by physicist Alvin Radkowsky, formerly the chief scientist of the United States Naval Propulsion Program, says its fuel could be used in existing reactors in place of the ordinary uranium fuel, and would produce very little plutonium, a normal by-product of the reaction when uranium is used.
Proponents say it could also be offered to countries like North Korea and Cuba, which say they want reactors to generate electric power. But some fear they want reactors mostly to generate plutonium for weapons.
The thorium reactor, however, could also be useful for burning up surplus weapons plutonium – the US has at least 50 tons of it – without producing nearly as much new plutonium as existing reactors do.
And the plutonium it does produce would be a mix of types that makes the weapon prone to “fizzle,” or sustain a chain reaction for only a brief period, cutting its explosive yield by 95% to 97%.
“It’s a lot better than what we’ve got,” said Robert H Williams, a physicist at the Princeton University Centre for Energy and Environmental Studies.
He pointed out, though, that even if production is cut by 80%, a large nuclear reactor would still produce enough plutonium each year for several bombs.
The US Energy Department has contributed R5,5-million to help with development.
Radkowsky designed the fuel, but some people question whether it can compete with existing ones, because the operating characteristics of the existing fuels are so well known.
Developers refer to Radkowsky’s system as a reactor, but it is essentially a new core, replacing the fuel in an existing reactor. Its benefit is to change a characteristic of nuclear reactors that was long considered a virtue but is now a problem: that as they consume the kind of uranium that can be readily split, they make a new kind of atom that can also be split easily – plutonium.
The fuel for nearly all reactors today is uranium-235, which is called “fissile,” because it can be fissioned, or split. It is mixed with uranium-238, which is very hard to split. When the uranium-235 is split, it gives off neutrons that go on to fission other atoms, sustaining a chain reaction.
But often a neutron hits an atom of uranium-238, a material that is called “fertile,” because instead of being split by the neutron, it absorbs the neutron and changes into a new material, in this case, plutonium-239.
Plutonium is a different element, and thus is fairly easy to chemically separate from the rest of the fuel when the fuel bundle is removed from the reactor.
The Radkowsky fuel avoids plutonium production by minimising the use of fertile uranium. Instead it uses thorium, a material that was tried with uranium in the first civilian reactors in the 1950s and 1960s but later dropped in favour of straight uranium fuel. Thorium, cheap and plentiful, cannot be fissioned to produce energy, but it is also fertile, and as it absorbs stray neutrons it is converted to something fissile, in this case, uranium-233. But uranium-233 is hard to separate from other uranium in the core, and thus is hard to purify for bombs.