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A safer nuclear path?

First published in Cleantech magazine 2011 Issue 3. Copyright Cleantech Investor Ltd

Denis Gross reviews a more ‘holistic approach’ to nuclear energy

In late March, after the Fukushima disaster, Sir David Anthony King, FRS, the former chief scientific adviser to the UK Government, defended nuclear power in a report recommending Britain should recycle uranium and plutonium stockpiles, and calling for a "holistic approach" to nuclear policy. He argues that nuclear power is the safest form of electricity generation. With the exception of renewables, his is borne out statistically. However, Fukushima has joined the list of nuclear crises;  Three Mile Island, Chernobyl and, arguably, the Stuxnet computer virus, which highlighted the potential of cyberwarfare to attack nuclear power facilities (in this case an attack on Iran’s uranium enrichment site).

Nuclear power is already a key generation technology in a number of countries (France greater than 75% of electricity generation; East European countries with over 50%, and Japan, UK and USA with 29%, 18% and 20% respectively). However, China, which was seeking a cleaner method of meeting its burgeoning energy requirements than coal, has reduced its nuclear capacity target for 2020 of 80GW by an unspecified amount, and announced after the disaster that it will increase its target for solar power from 20GW. Judging from the reaction around the world, the prospects of nuclear meltdown cannot be dismissed from the public’s mind; neither can the deep concern felt over the safe storage of toxic, radioactive materials, of which 2,000 metric tons per year are produced by the 104 reactors in the US alone, and which remain active for millennia. Can geologic repositories be safe for that long?The ongoing crisis at the Fukushima Dai-ichi Nuclear Plant has caused – at the very least - an interruption in the rehabilitation process of nuclear power now under way. Given the current lack of alternatives for baseload energy under current timescales, the question is whether nuclear power, specifically fission-based, be made sufficiently safe?

At the core of current fission-based technology are the various isotopes (atomic variants) of uranium. Uranium ore consists primarily of the weakly radioactive and non-fissile (i.e. incapable of sustaining a chain reaction of nuclear fission)  isotope U-238, and a small percentage of the fissile U-235. Enrichment of the ore provides a 3 to 5 per cent U-235 content, which is sufficient for a chain reaction for nuclear reactors (in contrast, over 85% is required for a nuclear weapon). During a nuclear reaction, the U-238 absorbs neutrons from the splitting U-235 and goes through a rapid sequence of transmutation and decay to end up as plutonium – Pu-239, which in turn is a fissile material. Because most reactors are not 'tuned' to Pu-239 fuel, a large proportion of this becomes a waste by-product within spent fuel rods, although it can be reprocessed from spent fuel rods as MOX (mixed oxide fuel) and used in reactors. The reprocessing is expensive, and also is politically sensitive as plutonium potentially may be used in nuclear weapons proliferation. Furthermore, a slew of other radioactive isotopes appear amongst the fission by-products, including iodine.

An alternative to uranium has been under investigation for a number of years – the fellow actinide (element) thorium. Unlike U-235 and Pu-239, Th-232 is not fissile, and cannot sustain a chain-reaction without a beam of neutrons targeted on it. However, Th-232 is fertile (like U-238), and after absorbing a neutron will decay into the uranium isotope U-233, which is similar to U-235 in that it is fissile. From a nuclear power point of view, thorium is attractive in that, being a lighter element than uranium, it does not generate plutonium – in fact the by-products and waste are far less radioactive than uranium’s and with half-lives measured in the hundreds of years rather than thousands. Another attraction of thorium is that if it is combined with plutonium the consequent reactions will effectively incinerate the plutonium and a range of other toxic products, thus providing a useful waste disposal mechanism. The energy output per tonne of thorium is attractive, as is its relative abundance: it is around 550 times more abundant than U-235 in nature and it is also far more readily mined than uranium.

However, precisely because it needs an external source of neutrons to start and sustain a reaction – the U-233 produced in the thorium cycle does not produce enough neutrons – thorium has been largely neglected for power generation.   A number of solutions are being developed that address this issue. India has the world’s largest thorium deposits (followed by the US an Australia) and in the early years of India’s nuclear programme, in 1954, work started on reactors using a thorium blanket wrapped round plutonium. A mixed fuels approach (thorium and uranium in a blanket around a plutonium seed) is also being pursued by Lightbridge Corporation (LTBR:NASDAQ), formerly Thorium Power, which has been working with a team in Russia to generate a commercial product. Elsewhere, an Accelerator Driven System has been proposed by Carlos Rubbia (ex-Director of CERN), in which protons from a particle accelerator are fired at a lead target, which releases neutrons for sustaining the reaction in the thorium reactor. If the beam is shut off, the reaction slows and then extinguishes itself – an intrinsic safety feature.

China has committed itself to a new nuclear energy programme using thorium as a fuel within 20 years. The Chinese plan to use TMSRs (Thorium Molten-Salt Reactors), in which the fuel is contained in a low pressure reactor vessel. The Molten-Salt Breeder concept dates back to 1950, when a prototype was built in Oak Ridge, Tennessee, using uranium and plutonium fuel. The technology allows the fuel to be dissolved in the molten sodium salt, leading to simplified design, a high temperature for greater thermodynamic efficiency, and a compact core which lends itself to thorium fuel.

While technologies are being investigated to improve uranium and plutonium based fission, fast neutron reactors and 'waste-eating' travelling wave reactors (by TerraPower, the group set up by Nathan Myhrvold’s Intellectual Ventures), there is a clear path to a safer, thorium-based nuclear future.

Related articles:

Company Profile - Terra Power

Company Profile - Lightbridge Corporation

Nuclear Fusion 


 

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