Why Molten Salt Reactors are so cheap

Terje Peterson of the Atomic Australia Facebook Group wrote this brilliant summary:-

This post examines one class of advanced reactor and the ways in which it can radically reduce the cost of nuclear power. That is the class of reactors know as “molten salt reactors”. These come in many different designs. Some seek to use thorium as fuel and some would use uranium fuel or some sort of mix of the two. Some designs use thermal spectrum neutrons and some operate in the fast spectrum. Some designs would use already existing nuclear waste and some would use fresh fuel. This post will try and address what is common to the category in general and not dwell on the specifics of these different design choices. This post will focus on the cost reduction benefits of these designs over conventional reactors. As such it is focused more on the technical factors driving cost rather than the policy factors. Though both are of course important.

Water freezes at 0°C and boils at 100°C. If you put water under pressure the boiling point can be raised to a much higher temperature such that water can get very hot whilst remaining a liquid. That is precisely what happens in a conventional light water reactor where water in liquid form inside the reactor typically operates at around 300°C. But to keep water liquid at these temperatures requires pressures around 150 times atmospheric pressure. That causes some complications that get expensive.

Molten salt reactors do away with water as the primary reactor coolant. Heat is still typically transferred to water for the turbine part of the power plant but inside the reactor itself the coolant is molten salt. Depending on the mix of salt selected the molten salt typically freezes at about 300°C and boils at about 1400°C. In normal operation the salt would generally operate around 650°C. The salt is liquid across a very large temperature range and remains liquid in that range even at low atmospheric pressure. This offers several enormous advantages. Many of which drive down costs.

In short molten salt in the reactor will act as water does in a conventional reactor. It will absorb heat and carry it to a heat exchange where that heat will be transferred to another fluid (typically water) that drives a turbine to make electricity. But the molten salt inside the reactor will operate at low pressure and yet also it will operate much hotter than in a conventional reactor. The following list outlines the many ways that a molten salt reactor is expected to make nuclear power radically cheaper.

1. Molten salt is less corrosive than water. This may seem counter intuitive because we all know salt water will rust your car. However Molten salt is nothing like salt water. This is not a major issue but it warrants mentioning.

2. Molten salt reactors operate at low pressure like the kettle that makes your morning coffee. As such the reactor vessel and the reactor plumbing don’t need to be designed to withstand high internal pressures. This massively reduces the thickness of steel required. Not only does this make the reactor vessel and the associated plumbing much cheaper but it massively expands the number of steel foundries that can make such reactors. It does away with the need for many rare and highly specialised foundry processes and skills. A significant cost saving.

3. If water in a conventional nuclear reactor leaks it expands rapidly around 1000 fold in volume as it turns to gas and expands. For safety this necessitates a huge concrete containment building roughly 1000 times bigger than the reactor. All this concrete is very expensive and the precision requirements often cause much delay in power plant construction. A molten salt reactor does not require such a building. Unlike water that leaks from a reactor molten salt that leaks from a reactor does not turn to a gas. And it does not race outward under pressure that would act as a dispersal mechanism for radioactive material. If the salt in a molten salt reactor leaks it drips to the floor and turns solid as it cools. The main benefit from a cost point of view is the removal of that massive expensive concrete structure.

4. This means molten salt reactor power plants can be much smaller. Many companies are looking to build entire plants in factories that will be shipped to site. In fact some propose to build them like ships and float them to site. Factory construction is known to make many things much cheaper due to the learning it enables.

5. Molten salt reactors do not require fuel fabrication. Fuel is about 30% of the operation cost of a conventional nuclear power plant. The cost of the raw uranium is negligible and nearly all of that fuel cost relates to fabrication. Fabrication is where the solid fuel is made into ceramic pellets which are put inside metal tubes and formed into bundles. This fuel fabrication process is often very profitable for the companies that sell nuclear power plants but it is expensive for those that operate them. In molten salt reactors we can expect the fuel cost to be 90% cheaper due to the removal of the fabrication process. This shaves off another chunk of cost. But as outlined below that fuel will also do a lot more.

6. Operating at 650°C instead of 300°C means that the turbine system used to turn heat to electricity can be off the shelf turbine systems from the coal sector. These turbines are much cheaper due to their smaller size and a larger existing rate of production. The heat from the molten salt does need to be transferred to a secondary loop using water to drive the turbine but this secondary loop is already the case for most nuclear reactors so it is nothing particularly new. The key point is the high temperature means a smaller, cheaper more readily available turbine set. Operating at 650°C instead of 300°C also means that the heat to electricity conversion process is substantially more efficient. Not quite double the efficiency but close. This means a given amount of nuclear plant and nuclear fuel is producing much more electricity for sale.

7. Nuclear reactors have an emergency planning zone (EPZ). In the USA for conventional light water nuclear reactors this entails a region of 10 miles (16km) in radius around the power plant. This is to manage things like evacuation if the worst of the worst should occur in terms of an accident. Molten salt reactors need an EPZ no bigger than the perimeter fence of the power plant. This is because the physics of a molten salt reactor is so radically different. There is no dispersal mechanism. A leak lands on the floor where the fission products are chemically bound and then physically bound also as the salt cools and turns solid. This hugely simplifies accident planning and dealing with neighbours and regulators. At least in principle following education.

The future of nuclear power is advanced designs like molten salt reactors. Nobody is building molten salt reactors in 2021 but they will be doing so during this decade. Many are already going through the long extensive licensing process in the USA, Canada and elsewhere. Australia should be opening up the licensing process right now to such advanced designs so we too can deploy them when the time comes. To do this we should adjust our laws now. And also begin to include such possibilities in analysis of the future NEM.

A blanket ban on nuclear power in Australia is short sighted. We should move away from brute force prohibition and towards a mature, future oriented, system of regulation. And allow advanced nuclear reactor designs to start on the licensing process in Australia.

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