Under the Microscope Prof William RevilleThe enhanced greenhouse effect is dangerously warming our world and we must therefore be very careful about how we generate energy from now on. Since nuclear energy does not emit greenhouse gases it must be seriously considered as a player in any future mix of power generation technologies. The role that nuclear fission energy might play in the future is described by John Deutch and Ernest Moniz in Scientific American, September 2006.
There are, of course, potential problems associated with nuclear power, including safety issues, the high level waste problem, dangers of nuclear weapons proliferation and the high capital cost of new nuclear installations. Some of these problems are well on the way to being solved, which is good news because it is estimated that a tripling of worldwide nuclear power to reach one million megawatts (MW) by 2050 would save up to 1.8 billion tons of carbon emissions (as carbon dioxide) per year to the atmosphere. This would significantly contribute to saving the annual seven billion tons of carbon emissions estimated to be necessary by 2050 to stabilise greenhouse gas emissions.
Nuclear fission power plants were first built in the 1950s. Generation Two nuclear power plants were built from the 1960s to the 1990s. Generation Three reactors are now being built. These are more efficient than type Two plants and incorporate passive safety, ie in the event of an accident, the reactor closes down automatically without need of operator intervention. Generation Four reactors are under development, but, with the exception of the pebble bed reactor, are a few decades from commercial viability.
Nuclear fission power generation is based on fission of uranium 235 (U235) atoms each into two fragments. U235 represents 0.7 per cent of natural uranium, the remaining 99.3 per cent being non-fissile U238. The U235 of natural uranium is enriched to about 4 per cent for the cylindrical nuclear fuel rods used in conventional nuclear power plants. A U235 atom splits in two when it captures a slowly moving subatomic particle called a neutron, releasing a lot of energy as heat in the process. Every fission of U235 also releases several fast neutrons. If these neutrons are slowed down they can initiate more fission events that in turn release more neutrons . . . and this is the basis of the nuclear chain reaction that steadily releases huge amounts of energy.
In a nuclear power plant, a substance (eg graphite) is used to slow down (moderate) the speed of the fast fission neutrons, hence its description as the moderator. The enormous heat, removed by coolant, is used to boil water to produce high pressure steam which turns a turbogenerator which generates electricity.
One of the risks in a conventional nuclear power station is that the fuel core may overheat, melt and subsequently explode releasing a lot of radioactivity to the environment.
The pebble bed reactor (PBR) now under development by the South African company Eksom is meltdown-proof. It is cooled by helium gas which directly powers the turbines. The fuel, tiny particles of uranium dioxide coated with impenetrable layers of ceramics and silicon carbide, is mixed with graphite and moulded into tennis ball sized pebbles. About 310,000 fuel pebbles plus 120,000 graphite balls, acting as moderator, fill the reactor vessel.
The primary advantage of PBR design is that it is inherently self-controlling. As the core temperature rises above desirable levels, more and more neutrons are captured by U238 and become unavailable to cause fission in U235. This natural negative feedback places an inherent upper limit on the temperature of the fuel without any need for operator intervention. All supporting machinery to a PBR can fail and the reactor will not crack, melt, explode or spew out hazardous waste.
PBR is of modular design, with each module generating around 100MW. Conventional nuclear power plants generate 1,000MW and more. The modular approach will be attractive to developing countries and modules can be added later if more power is needed. South Africa will begin building a demonstration PBR in 2007 and commercial modules of 165MW are planned for 2013.
Global use of electricity is projected to increase 160 per cent by 2050 and thousands of new power plants must be built to service that demand. Nuclear plants will not be built in large numbers if they are not economically competitive with coal and gas. Deutch and Moniz expect that nuclear will be competitive because of taxes on carbon emissions.
Burial in stable geological repositories is envisaged as being adequate for long term storage of high level waste. Such an underground facility is under construction in Finland where spent fuel rods will be placed in iron canisters, sealed in copper shells to resist corrosion and stored in holes 500m below the surface. Waste must be securely stored for more than 100,000 years. The spent fuel from PBRs is particularly refractory - the integrity of a pebble should last for one million years before it disintegrates - long enough for the slowest to decay radionuclide to disappear.
In order to eliminate the risk of nuclear weapons proliferation, Deutch and Moniz envisage an international system of fuel-supplier countries (eg US, Russia, France, UK ) and user countries. Fuel would be supplied to user countries and spent fuel taken away. User countries would have no need for fuel fabrication and enrichment facilities, the site of the risk of weapons manufacture.
William Reville is associate professor of biochemistry and public awareness of science officer at UCC - http://understandingscience.ucc.ie.