Molten Salt Thermal Generators

In a three-year trial, in Oak Ridge, USA, a 3 MegaWatt (MW) generator produced a continuous supply of electricity, and ultimately contained 0.05% more active fuel than had been put into it at the outset. It ran safely.

The design:

The design is called the Molten Salt generator. This runs at a very high temperature, but low/normal pressure, unlike many industrial plants which run at high pressures and have a tendency to blow up. The “fuel”, mostly thorium, which keeps it hot, is dissolved in a eutectic mixture of molten salts (much like table salt/sodium chloride), specifically, lithium fluoride (LiF) and beryllium fluoride (BeF₂). The mixture of these salts melts at about 460^oC, but does not boil until 1430^oC. The reactions only stay hot if the fuel in the salt is all close together in a lump – like a football.

Variable Electricity Production:

Because the fuel which produces heat is liquid, it can be circulated around heat-exchangers. In modern designs, the Molten salt containing the thorium and the Molten salt carrying the heat away to other processes would be two separate bodies of molten salt running past each other in heat-exchangers. The heat could be used directly for some chemical processes, eg Aluminium smelting, making concrete or steel, making ammonia (for nitrogenous fertilisers), or catalytic conversion of water to hydrogen and oxygen. You could have heat extracted at different temperature ranges, with one of the lower temperature ranges boiling water to steam to run steam turbines producing electricity and possibly the lowest, waste-heat cycle after that, being used directly to supply domestic hot water to buildings in the vicinity. By directing the circulating molten salt to different places, you could vary the amount of electricity produced, simultaneously varying the heat going into whatever other processes are being run by the system. 

Fail-safe:

To keep it safe, the vessel design has an actively cooled “plug” at the bottom. If the temperature becomes too hot, or if there is a power loss or any problem, the plug will melt and all the liquid salt containing the fuel will run out into a wide flat area so that rather than being “football” shaped it would resemble a sheet of aluminium foil. In this form, the salt cools quickly and is too spread out for any reaction to continue; this will happen long before there is any possibility of the salt “boiling” and producing any gas.

Relatively small:

The 3 MW Oak Ridge reactor was only a few metres in all dimensions.

The neutrons, if thorium is used as a fuel, are not very high-energy, so the system would not need massive amounts of concrete shielding. After all, the original design was for an aircraft engine. The thorium versions are basically a chemical plant.

Catalytic production of Hydrogen:

The high temperature at which the reaction continues is high enough to run the Sulphur-Iodine chemical catalytic separation of water into oxygen & hydrogen. Hydrogen could be reacted with CO₂ from the atmosphere to produce di-methyl-ether (DME) which could be used a liquid fuel or just stored for carbon capture. Di-methyl-ether has similar properties to ethanol for fuel. Certainly, if the Molten salt plant was being used as a heat source in the production of cement, (a process which produces large quantities of CO₂ in high concentrations) then capturing that CO₂ immediately by reacting it with hydrogen would be sensible.

Using up nuclear waste:

It is possible to use the same design to use up the nuclear waste we have from the various old nuclear plants. Indeed, these designs of Molten Salt Generators are likely to be the ones which are built first because of the current regulatory tangles over thorium. The nuclear waste burning ones would need the amount of concrete shielding one would expect for a plant handling nuclear waste, but at least they would use up that waste.

In existing nuclear plants the fuel remains solid and, because of this, chemicals which stop the cycle from progressing, build up in the fuel until it no longer generates enough heat to be useful in a power station. It is estimated at that point only about 3% of the fuel in those solid fuel lumps or rods has been used. We could also use the same design to burn up any dismantled nuclear weapons – which would be the modern day equivalent of turning swords into ploughshares and spears into pruning hooks.

Problems:

Political will – the original Molten Salt Generator was call a Molten Salt Reactor, and the word “reactor” is not popular, as people associate it with things nuclear, and anything nuclear with nuclear bombs and accidents from the old water-cooled reactors. However, it is worth remembering that even fire was used as a weapon when it was first discovered. Now we have learned how to control it, everybody is happy with using it in their kitchens! The problem with water-cooled nuclear reactors has always been that they are high-pressure systems. They had to maintain incredibly high pressures to stop the water from boiling. The danger was overheating, or failures in the seals causing the water to boil, or pumps failing, and thus causing things to rupture.

Ultimately, the fuel to be used in Molten Salt Generators would be thorium, which is cheap, and has. historically, been regarded as a waste-product in mining. Thorium is not radioactive itself, but will ultimately fission if hit by a neutron. There are not yet regulatory rules about using it and the risk-averse nature of government drags its feet on authorising any trials, so there are no “safety certificates” for this design of plant.

Research has been carried out on the effect of prolonged exposure to Molten salts and neutrons for the materials which will be used to make the reaction vessels.

We are in the bizarre position that it is easier to get certification for a molten salt generator which uses up the waste products from existing nuclear plants to provide the self-heating for the molten salt, than to get certification for the intrinsically better thorium-based design.

There is enough energy in thorium and existing waste from old nuclear plants to continue to power the planet for several thousands of years.

There are several groups looking at different designs of these plants, but all have struggled to persuade their governments to allow them to build one. It would appear that this political blockage might be starting to relax a little, and some should be built in the next few years, but it is taking far too long.