Professional Engineering

Experts at the Nuclear Decommissioning Authority are busy poring over a 1,000-page report that contains details of what is claimed to be a safe, cost-effective method of dealing with the mounting stockpile of plutonium waste.

The feasibility study, submitted by GE Hitachi Nuclear Energy at the end of last month, supports the development of what would be the first Prism reactor in the UK. The company claims that the sodium-cooled reactor could dispose of 100 tonnes of plutonium through fuel manufacture within five years, while generating 600MW of low-carbon electricity long into the future.

The Prism design is based on a much smaller experimental breeder reactor built 30 years ago in the US. If it gets the go-ahead, it will be the first plant of its type in private operation in the world.

The decommissioning authority is assessing the feasibility of Prism because it provides a potential alternative to converting the UK’s plutonium stocks into mixed-oxide (Mox) fuel for conventional reactors. The construction of a Mox plant at Sellafield in the late 1990s was deemed an expensive mistake after a decade of failed operations.

Eric Loewen, chief engineer at GE Hitachi, says the use of a Prism reactor to dispose of plutonium would position the UK at the forefront of nuclear technology. “Prism would put the UK as a world leader as far as plutonium management is concerned,” he says. “The US and Russia have not disposed of one tonne – yet the UK could achieve disposition of 100 tonnes of plutonium within five years. That’s the really exciting thing about this technology.”

The Prism reactor employs liquid sodium as a coolant instead of water. That fundamental principle allows the neutrons to have a higher energy which drives fission of the transuranics. The Prism reactor consumes transuranics in used nuclear fuel from water-cooled reactors, essentially turning plutonium waste into energy. The reaction that occurs produces heat energy, which is then converted into 600MW of electricity by a conventional steam turbine.

At the start of the process, plutonium oxide powder in 7kg cans is mixed with an equal amount of uranium oxide and converted to metal in an electro-reduction unit. That metal is placed in an induction furnace to convert it into an alloy comprising 15% plutonium, 10% zirconium and 75% uranium. The alloy is stacked into a fuel bundle made up of 231 individual rods and weighing around a tonne. 

“So we’ve taken a 7kg can and turned that into a fuel bundle that’s 10ft tall,” says Loewen. “We’ve taken pure plutonium and downgraded it with zirconium and uranium and in doing so we’ve made the material significantly more difficult to take away.”

Around 150 of these fuel bundles are placed in the sodium-cooled reactor. “The sodium coolant allows fission to occur with higher-energy neutrons – all of the isotopes of plutonium are fissioned.”

After the fuel is spent, the waste product that remains is safer than plutonium in the form that the UK stores it today, says Loewen. It would be less liable to be used in weapons and would be more easily stored. “The very high radiation dose becomes self-protecting. What’s left would be put into a storage facility, and then in geological disposal when that facility is ready,” he says. 

GE Hitachi says that, by using Prism, the UK’s 100-tonne plutonium stockpile could be disposed of within five years, during which time the plant could also generate electricity. Once all the stockpile has been dealt with, the plant could start reusing the fuel solely for electricity generation.

GE Hitachi believes that the reactor has intrinsic benefits. It is based on a passive safety design approach, featuring passive reactor core cooling. The company says that the coolant pumps have no moving parts, which it claims leads to increased reliability, while the simplified design eliminates the need for valves and motors.