The reactor has been designed so that loss-of-coolant accidents are prevented and decay heat is removed without automatic or operator actions. Prism is also capable of processing impure plutonium, with minimal need for chemical pre-clean-up or discharge of contaminants.
Loewen says: “From a passive safety design standpoint, I could have this reactor running at 100% power, 100% flow and hold the control rods out and then turn off the reactor coolant pump. The reactor would reduce power on its own and have no fuel damage.
“The other thing that is interesting is that with all reactors when you turn them off they still generate heat. If you don’t remove that heat then you will cause the fuel to melt. We saw that at Fukushima.
“What’s unique about the Prism design is that it features metal fuel, inside of metal tubing, inside metal coolant, inside a metal vessel. If we pass air beside that vessel, that removes the heat. That means we can remove the heat without opening any valves or turning on any pumps or having any operator doing anything, and that will last not for hours or days or months, but forever. So those are probably two of the more significant passive safety aspects of the design.”
The ability to process impure plutonium is also important, he says. “Because we are making a metal alloy, that means the process is very robust. We don’t care if there is any iron content in it. We don’t care about chlorine content either, because we are doing our electro-reduction in a chloride bath.
“As far as isotopics go, we don’t care about the in-growth of americium 241, because in a high-energy Prism reactor it will fission – it becomes fuel. In fact we prefer to have mixed isotopes, because that means you are not left with a bag of americium and the need to have to find a place to put it into the ground. We would prefer to turn it into electricity.”
Some industry experts have expressed doubts that the Prism technology hasn’t been proved on the scale that is being proposed for the Sellafield installation. The sodium-cooled Experimental Breeder Reactor II (EBR II) in the US, on which Prism is based, had a thermal power rating of 62.5MW and produced 19MW of electricity through a conventional turbine generator. Prism will have to be significantly bigger, which presents engineering challenges. But Loewen thinks these can be managed.
“I’d argue that the technology has been proven,” he says. “The US built a large-scale prototype – the EBR II. For the commercial option for the UK, we are looking at a scaling-up factor of 10. GE Hitachi, as a reactor vendor, put its first private-funded boiling-water reactor in the US with a scaling factor of 280, and we did that successfully.
“Most recently, we built a first-of-a-kind reactor called the Advanced Boiling Water Reactor for Japan and Taiwan. We take proven technologies to market.”
Now that GE Hitachi has submitted its feasibility study to the Nuclear Decommissioning Authority (NDA), the watchdog’s experts are expected to spend the rest of the year assessing the reactor’s viability. The four-volume document includes an independent licensing assessment carried out by private consulting firm DBD, which concluded that there were “no fundamental impediments” to Prism becoming licensed in the UK.
In terms of the licensing process, GE Hitachi says it is keen to avoid putting Prism through the generic design assessment, preferring instead to carry out a site-specific approach focusing on safety questions related to Sellafield. Loewen says that locating the reactor at Sellafield would make sense because it would eliminate the need to transport plutonium oxide to a fuel fabrication plant.
As far as the NDA is concerned, it has stated that the government’s preferred policy for managing plutonium stocks is to convert the material into Mox for use in light-water reactors. But, in addition to evaluating the Prism design, the authority has confirmed that it is also assessing a feasibility study put forward by Canadian nuclear technology company Candu. This proposes that the UK should use the enhanced Candu 6 reactor to dispose of plutonium.
One alternative to these solutions would be to convert the plutonium into fuel that could be used for a thorium-based plant. Thorium was explored several decades ago as an alternative to the then-current reactors until the research was discontinued. Some experts believe it could provide a cleaner and more environmentally friendly alternative to today’s nuclear designs.
Two dozen members of parliament, led by Labour peer Baroness Bryony Worthington, have formed an informal committee to study the potential of thorium.
According to GE Hitachi, if the NDA assessment process does result in the progression of the company’s Prism technology, the licensing process could begin.
The company claims that adoption of Prism would mean risk being transferred away from government, with industry taking the lead in managing the plutonium stockpile. The taxpayer would only pay as plutonium is dispositioned, it says, and the government would neither own nor operate the plant.