Helium hope

The world-wide concern over greenhouse gas emissions, highlighted by last month’s international climate conference in Kyoto, Japan, has given nuclear power a fresh glimmer of opportunity, if not a new lease of life. There is a growing consensus that unless more atomic capacity is brought on stream in the next decade, carbon dioxide (CO2) emissions […]

The world-wide concern over greenhouse gas emissions, highlighted by last month’s international climate conference in Kyoto, Japan, has given nuclear power a fresh glimmer of opportunity, if not a new lease of life.

There is a growing consensus that unless more atomic capacity is brought on stream in the next decade, carbon dioxide (CO2) emissions from power generation will rise. No practical increase in the use of renewable sources of energy will be able to match the growth in world demand for electricity over the coming decades let alone reduce dependence on power generated from fossil fuels. Energy conservation seems unlikely to achieve the goal without carbon taxes set at levels that would be unacceptable.

The British Nuclear Industry Forum said in November that, provided the same generating mix was maintained with a nuclear contribution of 17%, power generation around the world would be putting 2,900 million tonnes of carbon into the atmosphere in CO2 emissions by 2020, compared with 1,400 million tonnes in 1995. If nuclear plants were phased out, the figure would rise to 3,200 million tonnes.

Nevertheless, the prospects for a substantial programme of new nuclear construction are not good. The high capital cost of nuclear plants, and the long-term decommissioning liabilities, make them too much of a commercial risk in the competition-driven privatised electricity markets which are becoming the norm in developed countries.

In markets where the length of sales contracts are likely to shorten, it would be a brave company that would commit £2bn £3bn to build 1,200MW of new capacity over 10 years when it could erect a combined-cycle gas turbine (CCGT) plant in one third of the time at a seventh of the cost.

Robin Jeffrey, deputy chairman of the main UK nuclear generator British Energy, has conceded that ‘it is not economic to build nuclear plants in the present climate’.

But an international team of nuclear companies General Atomics of the US, Framatome of France, Russia’s Minatom and Japan’s Fuji Electric believe they have the answer: a small-scale helium-cooled reactor combined with recent advances in gas turbine technology.

Walt Simon, senior vice-president of General Atomics, says the proposed gas turbine modular helium reactor (GT-MHR) would have a capacity of 285MW, a design life of 60 years, and an installed cost of between $1,500 and $1,600 per kilowatt, suggesting a figure of around £250m for the unit. ‘The construction period is going to be about three years,’ he adds.

General Atomics and Minatom began work on the GT-MHR in 1993 with the aim of developing an export commodity for the US and Russia, with the design and construction of the prototype to be carried out in Russia. The reactor’s potential to dispose of weapons-grade plutonium was recognised at the outset, and this became a priority of the programme in 1994 once Russia agreed to cease plutonium production.

Tomsk-7 (now know as Seversk) in Siberia, where much of Russia’s weapons plutonium was produced, was selected as the site for the first prototypes. Framatome joined the programme in January 1996 and Fuji a year later.

Russia’s OBKM Institute completed the conceptual design at the end of 1997, and Simon says there will be about another five years’ work on the detail design and materials testing before construction starts.

The potential advantages of using helium as the coolant for nuclear reactors were explored on five prototypes, including one in the UK, in the 1970s and 1980s.

The combination of the inert gas with graphite to moderate the nuclear reaction offered inherently attractive safety characteristics: a coolant which is unaffected by radioactivity and has a negative temperature coefficient (so the reaction is shut down if normal operating temperatures are exceeded) could provide a reactor which was meltdown-proof. However, the designs used the helium circuit to raise steam to drive the turbines in the same way as conventional thermal reactors and proved too expensive.

By contrast, the GT-MHR design includes the turbine in the helium circuit. Gas heated in the reactor expands through the turbine to generate electricity and returns via a recuperator unit and two compressors.

As with CCGTs, the elimination of the steam raising plant improves the thermal conversion efficiency 47% versus about 32% for conventional nuclear plants, such as pressurised water reactors.

Its promoters say the helium reactor, which at around 900 C operates at more than double the temperature of a PWR, achieves 2.5 times the fuel burn-up of the latter and produces 60% fewer radioactive elements per unit of electricity produced.

John Large, a nuclear consultant who worked on the design of UK helium reactors in the 1970s, says the main challenge with the concept is to develop a fuel matrix which can maintain its integrity at temperatures of 900 C and pressures of 50 bar because any fuel failure will contaminate the turbines with radioactivity and make maintenance of the turbines extremely difficult.

‘The problem is that turbines have to be maintained. They will become contaminated if you have fuel failures and you will have fuel failures,’ he warns.

Simon says the GT-MHR’s fuel configuration is designed to minimise this risk. The basic fuel units are coated particles of either a uranium oxide/uranium carbide mix or plutonium oxide of 200 400 microns in diameter. There are four coatings on each particle to contain the fission products.

The particles are then formed into fuel compacts, two inches long and half an inch in diameter, which are inserted into channels in hexagonal graphite blocks. Each block is 30 inches tall, 14 inches across and contains 200 fuel channels. The reactor will contain 1,020 of the blocks, a third of which will be replaced in each year of operation.

Simon concedes, however, that a risk of fuel failure remains, although it is considered to be as small as one in 100,000.

The turbines on the GT-MHR are also designed to be as maintenance-free as possible, with magnetic rather than mechanical bearings to avoid the most common source of breakdown, and Simon says the characteristics of the coolant should prevent some other types of intervention that prove necessary on gas or steam driven turbines.

‘Hopefully we don’t have to re-blade, because helium is a non-corrosive material.’

Despite these precautions, Simon acknowledges there is still a possibility that work would have to be carried out on a contaminated turbine. He says the detail design of the plant will include facilities to decontaminate the turbine and for remote maintenance.

Large says a further challenge with the intended prototype will be the all-plutonium fuel, which experiences far more neutron decay after reactor shutdown than uranium fuel. Simon says this problem has been countered on the GT-MHR design by inserting erbium oxide, a burnable ‘poison’ that absorbs neutrons, into designated channels in the reactor.

The development team is trying to secure wider support from governments and industries as it enters the detail design stage for the prototype, with the view to forming a broader international partnership to take the project through this phase.

Following the completion of the prototype and subject to an adequate performance demonstration, the GT-MHR will be marketed to the world-wide electricity industry. And its potential market apparently extends beyond the developed industrialised countries. ‘I know the Chinese are interested in this thing,’ says Simon.