Nuclear power is one of the most divisive topics you can encounter today: it is labelled by some as the future of low-carbon electricity and a necessary complement to solar and wind, yet others believe it to be a grave danger to humanity, and envision a future without nuclear reactors. As many of you may know, nuclear reactors generate electricity through the fission process, where uranium-235 absorbs a neutron, becoming uranium-236, which is unstable and therefore splits into daughter nuclei. Rather importantly, this repeated process releases energy which is used to generate electricity. The carbon intensity of electricity generation from nuclear is the lowest of all common energy sources tied with wind, at 12 gCO2eq/kWh compared to 48 for solar 820 for coal. Additionally, nuclear is a stable source unlike variable sources wind and energy. When the weather is cloudy and there is a lack of wind, nuclear is able to cover the deficit.
So, despite all of these advantages, why are some so strongly against nuclear power?
When discussing nuclear, there is, one might say, an elephant in the room: safety and some high-profile accidents. In 1986, the Chernobyl disaster in Ukraine led to deaths of 30 employees and negative health effects on thousands in the region. In 2011 in a power plant in Fukushima, Japan, there were 3 nuclear meltdowns releasing radioactive materials into the area, while radiation-related deaths are to this day still being evaluated.
You may be wondering: why are nuclear reactors so dangerous and how can we improve safety to securely access this vital energy. Currently, many technologies are being engineered to address this problem, and I will highlight one which I believe has the capacity to tackle this: the Compact Molten Salt Reactor (CMSR).
It should be noted that safety is not the only concern with nuclear energy; nuclear proliferation, storage, and scalability are other issues which are also being addressed, but which deserve articles of their own.
In order to fully grasp how the CMSR is trailblazing with safety we must get to grips with a few concepts. Firstly, criticality is the regular operating condition of nuclear reactors, in which the fission chain reaction can be continually sustained. A criticality accident is when an accidental uncontrolled fission chain reaction occurs, and as such the unintended accumulation of critical mass of fissile material occurs. This releases potentially fatal doses of radiation, and it is obvious that it must be avoided. The process of fission creates on average 2.5 neutrons per event. Thus, to maintain a an exactly critical (and hence stable) chain reaction, 1.5 neutrons per fission event have to be absorbed without causing further fission reactions or leak from the system. Otherwise, a criticality accident will ensue.
In a CMSR, the fuel salt is a mixture lithium and beryllium fluoride salts (FLiBe) with low-enriched uranium (U-233/235) fluorides dissolved in it. This fissile mixture acts as the reactor fuel, as well as the coolant, instead of solid fuel rods in conventional nuclear reactors that need constant cooling by water under high pressure of around 150 atm. Unlike conventional reactors, the CMSR operates at near-atmospheric pressures, eliminating a wide range of accident scenarios such as salt leakage or system destruction due to high pressures. Moreover, the absence of water as a coolant eliminates the possibility of a steam or hydrogen explosion. These inherent safety features, coupled with the fact that gaseous fission products such as Xe, Kr and T are very encouraging for the future of nuclear energy.
The fuel salt can only become critical when it is in contact with the graphite moderator that makes up the core of the CMSR. This means criticality accidents can be avoided with leaked fuel salt, as there is no graphite moderator. If fuel salt comes into contact with the atmosphere (which it shouldn’t) it will cool down and become solid rock, containing all radioactive material within itself. Thus it cannot introduce a meltdown or explosion. The fuel and coolant salts are chemically inert and not subject to concerns of mechanically violent reactions or explosions, which is what happened in the Fukushima disaster.
It is the traditional light-water reactors that have developed the somewhat infamous reputation for nuclear. The LWR follows the traditional defence in depth approach. The reactors structure provided 3 key barriers, preventing the release of fissile products to the surrounding environment. The first of which is the clad that is on the fuel element, where the fission products are generated. Secondly is the reactor vessel, which contains all the fuel elements and forming the core. The leak-tight containment is the last physical line of defence, which should keep any fission products from escaping to the environment. It is truly key to assure the integrity of each of these physical barriers and in any accident scenario becomes the defence in depth approach against the release of radioactivity to the public environment. This simple mechanism is susceptible to corrosion due to the chromium contained in the clad structure, as well as not defending against the high pressures necessary to maintain the coolant. This basic defence cannot prevent explosion for a range of reasons which the CMSR can as I have outlined.
Ultimately, its role will be to act as a stable partner to other variable and inconsistent energy sources, such as wind and solar. Hopefully, the CSMR and other emerging nuclear technologies will be able to limit the amount of disasters we see in years to come.