Molten Salt Reactors - The next generation of Nuclear Power
- Vaishnvi Tiwari
When it comes to the safety of Gen IV fission reactors, Molten Salt Reactors, or MSRs, stand a class apart. Though there are many reasons behind this, one of them is that in MSRs, the fuel is generally dissolved in the coolant to form a molten salt mixture with fissile material dissolved in it. If this raise concerns in your mind, I'd suggest you read further…
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As opposed to Gen III Reactors, MSRs offer High Salt Temperature + Low Operating Pressure (close to atmospheric pressure), thus reducing the large, expensive containment structures used for LWRs (Light Water Reactors) and eliminating a source of explosion risk, which implies greater Efficiency + Enhanced Safety. Also, the MSR concept is flexible in designs and purpose, they can be used to help close the nuclear fuel cycle, and they allow for effective utilization of our vast uranium and thorium resources.
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A Brief History: Though the MSR concept gained attention in the early 21st century following the efforts of the Generation IV International Forum (GIF) to develop new reactor technologies, they can be traced back to as early as the 1950s. The ARE, Aircraft Reactor Experiment (1950s) and MSRE, Molten-Salt Reactor Experiment (1960s) projects at Oak Ridge National Laboratory (ORNL) in the United States were the first to demonstrate MSR technology. While ARE was the first reactor to use circulating molten salt fuel, MSRE proposed a “practical” molten salt-fueled reactor that could be operated safely and reliably with easy maintenance.
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So how does an MSR work? : In a basic molten salt reactor, fissile material is dissolved in a molten salt solution. The core consisting of a neutron moderator allows the salt solution to flow at high temperatures - 700°C or higher - while remaining at low pressures. The heat generated by the nuclear reactions in the primary salt is transferred to the 1° heat exchangers that have an intermediate salt circuit, which would heat up water in the 2° heat exchangers to produce steam, and from there produce electricity.
This provides an inherent advantage that the fluid can be both the fuel (producing the heat) and the coolant (transporting the heat to the power plant). Since the fuel is dissolved in the coolant, it allows fuel reprocessing while the reactor operates, saving extra time and effort. This aspect of 'online fuel reprocessing' can also be easily applied to "breed" more fuel for future energy needs. The higher operating temperatures of MSRs imply greater electricity generation efficiency. Furthermore, MSRs can use processed spent fuel (produced from Pressurized Water Reactors (PWRs)) and the possibility of transmutation has a significant impact on reducing radioactive waste, while improving energy efficiency.
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A concept with many possibilities: MSR design concepts can vary, based on the choice of Neutron spectrum, salts, size, modularity and uses. They can operate as fast as thermal reactors or be designed to function as a “burner” of actinides or a “breeder” for fissile material. They can accept a variety of fuels (low-enriched uranium, thorium, depleted uranium, mix of spent fuel and waste products) and the salt mixture used for dissolving the fuel and for coolant can be based on fluorides, chlorides, lithium, beryllium, or even mixed! Furthermore, the Fuel cycle utilized can be either closed or once-through. The energy generation capacity of the reactors can be adjusted as per need-base and the reactor can adopt a loop, modular or integral configuration.
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Challenges of the MSR technology: Though these benefits are impressive, nothing in this world comes for free. MSR designs do have certain challenges, but none are unsolvable. One of the primary concerns with MSRs is prevention of corrosion, including suitable surveillance measures. Where some of these fission products and actinides are radioactive, others have chemical effects that can cause material deterioration. This can be resolved by ensuring regular replacement and thorough maintenance of the reactor containment equipment (such as heat exchangers, control rods etc.). There is need for more in-depth knowledge on the types of risks for all operational states, possible events. Moreover, some of the fission products can be easily filtered out through regular separation techniques. But to do the more serious fission product (or actinide) separation, complex processes need to be set up. Though these processes have been studied in detail, they are complex enough to be a disadvantage. Furthermore, the environmental impacts associated with decontamination and waste packaging, the techniques to be adopted for limiting radiological and non-radiological impact of off-site installations (salt manufacturing, extraction and conditioning of fissile and fertile matters, waste processing, etc.) need to be studied. A thorough evaluation is also needed for the materials so that they can withstand high temperatures while ensuring integrity of the highly corrosive and radioactive primary loop with liquid fuel.
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There is still a lot to be learnt. Fortunately, there is ongoing work in MSRs around the world. The European projects MOSART, MIMOSA, SAMOSAFER, ENDURANCE, the Japanese FUJI Molten Salt Reactor and the American FHR project are some of them. Other than that, countries like India, the United Kingdom, Denmark, and Indonesia are also developing and proposing their own MSR designs. Some of the major players currently developing reactor designs using the MSR concept include Stellaria, Naarea, Thorizon, Seaborg and Copenhagen Atomics (in Europe), MOLTEX Energy (the UK), Terra Power, Thorcon, Flibe, Southern Company, Transatomic Power Corporation (in the USA) and Terrestrial Energy (Canada).
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In a nutshell, though MSR technology requires a lot of research before it is commercially available, it sure presents a promising future for the sustainable clean energy transition.
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Breed - The process of breeding refers to the addition of non-fuel radioactive material in the reactor and allowing it to react with neutrons inside the reactor to convert it into fuel material. For example, Thorium-232 is a radioactive element that cannot produce fission energy directly. Instead, if it captures a neutron, Thorium converts into Uranium-233, which is a fuel material for nuclear reactors.
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