Moltex, a molten salt reactor for the U.K. by Dennis Peterson
We can't prevent catastrophic climate change unless we convert electric power to non-carbon sources. We need reliable, dispatchable, abundant power, at low cost.
One near-term option is molten salt reactors: nuclear reactors with liquid fuel.
While liquid thorium reactors have gotten a lot of attention, simpler molten salt reactors using uranium fuel have similar advantages and are easier to develop, and several startup companies are attempting it.
A recent review of six MSR startups chose Moltex, a U.K. company, as the most promising for the U.K.'s purposes.
Which proposals are included in your plan and how do they fit together?
Oak Ridge Nationa Laboratory ran a molten salt reactor for four years in the 1960s. The experiment was a success, but the project lost funding, primarily because light water reactors were already being successfully used in submarines. Safety concerns raised by MSR proponents were discounted. In retrospect, that wasn't the best decision.
A conventional "light water" nuclear reactor uses solid fuel. The coolant is water, under pressure of up to 160 atmospheres to keep it from turning to steam. Refueling happens only every 18 months, so at the beginning an excess of fuel is required. Gaseous radioactive fission products build up over time. Continuous cooling is required. In disaster scenarios, hydrogen from the water can build up and explode, as we saw at Fukushima.
Despite all this, reactors have built up an impressive safety record, and even accounting for Chernobyl and Fukushima, nuclear is one of our safest energy sources. But it's taken a lot of complicated engineering and safety mechanisms to keep them safe, and that gets expensive.
A molten salt reactor is inherently safe, due to the physics of the fuel and coolant. It has a strong negative feedback: when the fuel heats up, the nuclear reaction slows down. Noble gas fission products are continuously removed. The fission products of most concern in conventional reactors (cesium, strontium, and iodine) are strongly bound into fluoride salts. There's nothing to drive a chemical explosion, and everything's at atmospheric pressure. Small amount of fuel can be added as often as desired, so there's no need for excess reactivity in the reactor. If there's a leak, everything radioactive will drip out and cool into rock, rather than venting to the atmosphere.
Moltex is a U.K. company with a unique approach to molten salt reactors.
Most MSRs keep their fuel in continuous circulation. Moltex keeps its liquid fuel contained in narrow fuel rods. These rods are immersed in a pool of molten salt coolant, which relies on simple convection for circulation. Above the pool there's a volume of inert gas.
It's a very simple design with easy construction, without the complex piping and pumps of some MSR designs. The components are simply lowered into place from above, which also allows easy replacement. Its likely that the reactor that can use materials which are already certified for nuclear construction. Independent costing put the mostly likely overnight capital cost lower than a modern coal plant, and less than half that of an AP-1000 nuclear plant.
The reactor can be refueled continuously, so there's never a large excess of fuel which could cause a runaway reaction. In a severe overheating scenario, the coolant salt boils, condenses at the top of the chamber, and drips back down, maintaining sufficient heat exchange to keep the fuel salt in liquid form. In the unlikely event that the fuel rods are damaged, the molten fuel would disperse into the surrounding salt, diluting enough to stop the reaction.
The reactor is initially envisioned as a fast reactor fueled by nuclear waste. Yhe U.K. is looking for a way to reduce its substantial stockpile of these materiala. Waste from the Moltex reactor would only require containment for several centuries, then would be back to the radioactivity of the original ore.
The design could be adjusted for use with low-enriched uranium, which would be more appropriate for countries without established nuclear industries.