FUEL & FUEL CYCLE | STABLE SALT REACTOR


The waste-burning Stable Salt Reactor


Moltex says its Stable Salt Reactor can solve two major issues with current nuclear: the upfront costs and the growing of waste. Ian Scott and Simon Newton explain


Ian Scott


Founder and chief scientist, Moltex Energy


THERE ARE SOME SIGNS THAT public and political acceptance of nuclear power is turning in the right direction. Moltex believes there is no net zero without nuclear power, but in most places nuclear is seeing more retirements than new builds. Even some of the worst carbon polluting countries, like Australia and Germany, entirely exclude nuclear. One reason for the moribund state of the industry is its high upfront capital costs. Another is near or actual disasters like Three Mile Island and Chernobyl. And although Fukushima — hit by an earthquake and tidal wave without loss of life — was, in many ways, a demonstration of the safety of nuclear, the natural disaster shook confidence was the catalyst for backward steps in many countries. The industry has responded with new layers of engineered safety systems and administrative controls — improving safety, but at mounting cost. Moltex believes its approach (see diagram) is more effective and less expensive, first seeking to eliminate or reduce the fundamental hazard and secondarily managing the remaining hazards through engineered safety systems or administrative controls. The Stable Salt Reactor (SSR) is a new nuclear reactor


concept, based on ideas conceived and tested but not commercialised in the 1960s and 1970s. It addresses the


fundamental hazards in two main ways: ● The fuel is a molten salt in which the most hazardous fission products are not gases but non-volatile salts.


● The coolant is another molten salt, which operates at atmospheric pressure and will never boil due to decay heat if the reactor were to fail.


Simon Newton


Chief commercial officer, Moltex Energy


SSRs vs molten salt reactors Nuclear reactors using molten salts as fuel and coolant are not new (a prototype molten salt fuelled reactor was built and operated in the USA in the 1960s). There is an essential difference between a molten salt reactor and the SSR. In every reactor design using molten salt fuel, the molten


salt fuel is pumped into a reaction chamber, where it achieves critical mass and generates heat. A heat exchanger transfers the heat out of the reactor to generate electricity. This design has some real attractions — particularly that the fuel can be chemically processed continuously as it passes around the circulating system. But it creates a major new hazard. If any pumps, seals or valves leak, highly radioactive fuel spills into the reactor space, where decay heat could cause even the salt to boil, releasing radioactive fission products as vapours. In the SSR, the molten salt fuel is held in conventional fuel tubes (in place of uranium oxide pellets). The fuel tubes are cooled by a separate molten salt, which transfers the


30 | February 2022 | www.neimagazine.com


heat to turbines to generate power. The safety advantages of molten salts are still achieved. However, by separating the fuel salt from the coolant salt, the reactor avoids the leakage hazard posed by pumped molten salt fuel. Simple sealed tubes are far less likely to leak than


complex plumbing — leaks are almost unheard of in present-day nuclear fuel tubes. Even if a fuel tube did leak, the leaked fuel would be massively diluted in the large pool of coolant salt, ensuring that it can neither achieve critical mass nor be heated to the point where the fission products become volatile. The reactor could continue operating despite several such leakages, because the radioactivity remains securely within the containment.


The key science The idea of putting molten salt fuel into conventional fuel tubes is simple, yet novel. The reason for the novelty is a lesson in the dangers of projects that achieve momentum. In the years immediately after the Manhattan Project, a


team in the USA sought to design a nuclear reactor to power an aeroplane. The team considered putting molten salt fuel into tubes but rejected the idea. Their reasoning was sound: heat moves in fluids primarily by convection, which requires gravity, but an aircraft’s gravity changes under acceleration and disappears in freefall. The decision to reject fuel in tubes was never revisited.


But after reviewing the original project, Moltex’s founder performed the fluid dynamic calculations to show that fuel in tubes works perfectly well at normal gravity, and patented the result. That discovery led to the formation of Moltex Energy.


Moltex has since acquired a broad patent covering this invention in most major jurisdictions worldwide.


The SSR as a ‘waste burner’ Moltex decided that Canada has a positive attitude to new thinking about nuclear energy. In 2016, it established Moltex Energy Canada Inc. and transferred some of its intellectual property rights to that company. Moltex also found in NB Power a supportive nuclear operator with a licensed nuclear site perfectly suited to advanced reactors. The New Brunswick provincial government and the Canadian federal government have both invested directly in Moltex. Financial support has also come from the US Department


of Energy through its ARPA-E programme. Moltex launched its first ever crowdfunding campaign in 2019, which was extremely successful. In 2021, Moltex completed the first stage of the Canadian Nuclear Safety Commission’s nuclear regulatory process. There are many possible variants of the SSR. The ‘waste burner’ is the first of what may become a family of reactors.


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