Nuclear energy is one of the cleanest forms of energy, but it still has a very bad reputation — which is somewhat understandable. Throughout the history of nuclear energy, there have been few (but very severe) problems. But a new generation of reactors could virtually eliminate the risk of nuclear accidents.
What comes to mind when you think of “nuclear power?” I carried out a small survey with my friends and family, and the results were telling. People thought of reactors, bombs, radiation, or even Homer Simpson. It was generally a pretty bleak picture, but a pattern stood out: people are associating nuclear energy with the old-fashioned light water reactors (LWRs) that caused the Three Mile Island, Chernobyl, and Fukushima disasters. However, since then, the safety of nuclear energy production has been improved with the creation of new reactors.
I’m talking about molten salt reactors. To see why this sort of reactor is much safer, let’s first see how nuclear energy is generated.
In modern nuclear reactors, power is the result of nuclear fission. Fission is the process in which the nucleus of an atom splits into two or more smaller nuclei. In reactors, it’s a chain reaction that starts when a uranium atom is struck by a neutron. The impact releases energy in the form of heat and radiation, which results in more neutrons flying off the uranium atom, restarting the cycle.
“Classic” nuclear reactors have their uranium stored in solid fuel rods. To moderate the heat and speed of the reaction, sea or lake water is typically used to keep the rods cool. Without a coolant, the fuel rods begin to melt. If they melt the reactor core and containment storage area, they can release radiation into the environment – this is what a nuclear meltdown is in a nutshell. But there’s another type of nuclear reactor that isn’t prone to melting: Molten Salt Reactors (MSRs).
While a lot of people associate the word molten with lava, this molten salt looks more like green-tinted water. The molten salt is not table salt, but rather a mixture of lithium and beryllium fluoride with the nuclear fuel melted into it. This fuel can be uranium like the older generation of reactors, or it can be plutonium and thorium from the old nuclear waste from the spent fuel rods that we already have from our LWRs, thus eliminating our need to deal with the long term safety issues regarding storage.
The reason why MSRs can’t have meltdowns is surprisingly simple: You can’t melt what has already been melted. However, this can be difficult to understand without the proper context. Therefore, to understand just how safe MSRs are, we must contrast them with the LWR disasters of the past.
When old reactors fail
When people think about nuclear disasters, they generally refer to the three following events.
Three Mile Island
On March 28th, 1979, there was a partial meltdown in a nuclear reactor near Middletown, Pennsylvania. The root cause of the accident was that (either for mechanical or electrical reasons) the main feedwater pumps failed to send water to the steam generators. Since the water was meant to be the coolant, the reactor automatically shut down.
The pressure in the primary system began to increase, so an emergency valve opened. Once the pressure had returned to normal, instruments in the control room indicated that it had closed, when in fact, it was still open. As a result, the cooling water was escaping through the valve as steam, which made the pressurizer vibrate, so the crew turned it off. Without the pumps operating, the emergency cooling water threatened to flood the pressurizer, so crews decreased the water supply, and this caused the reactor to overheat.
In this case, there was a mechanical failure (in the valve) and without adequate system diagnostics, the crew couldn’t stop the meltdown.
On April 26th, 1986, in what is now Ukraine and was then a part of the USSR, there was an explosion in the Chernobyl nuclear power plant. The day before, the crew was preparing to test how long the turbines would spin and supply power to the circulating pumps in the case of an electrical outage. The operators disabled the automatic shutdown mechanisms and then attempted the test the next day. By the time they started the test, the reactor was already unstable.
The design peculiarities combined with incorrect operating procedures resulted in an uncontrollable power surge. This power surge resulted in a rapid increase in heat, which ruptured the pressure tubes. The fuel inside the tubes spilled into the water, and the reaction resulted in two steam explosions releasing radioactive material into the environment.
On March 11th, 2011, there was a magnitude 9.0 earthquake off the east coast of Japan. The reactors in the Fukushima Daiichi Nuclear Power Plant were automatically shut down, but the earthquake took out all six external power supply sources. To make up for the loss of power, emergency power generators in the basements started up. Two tsunamis hit less than an hour later, and took out the generators and the seawater pumps. After the power outage, the reactors continued to produce some heat from fission product decay. Without the pumps, the heat wasn’t being removed, so this resulted in a buildup of steam in the reactors.
There were six reactor units at the time, but only units 1-3 were active.
In unit 1, there were attempts made to vent the steam, but without the power, steam built up on the service floor, resulting in a hydrogen explosion. This nuclear meltdown occurred as a result of the loss of the cooling system. Units 2 and 3 also experienced their own meltdowns, but to a lesser extent than unit 1. Unit 2 had a leak and released the most radioactive material. Unit 3 had a similar build-up in steam as unit 1, which resulted in a hydrogen explosion. Some of the steam from unit 3 made its way into the defueled unit 4 through the shared ventilation system. So, this caused another explosion in unit 4 despite the fact that it wasn’t even active at the time.
Unlike the Three Mile Island and Chernobyl disasters, Fukushima’s disaster involved multiple reactors. But, every explosion and leakage in Fukushima was caused by a power outage and sea pumps.
Why Molten Salt Reactors won’t fail
The key shared issue between these three disasters was the failure of the cooling systems. In every case, the coolant (water) wasn’t adequately cooling the fuel rods. Safety features either malfunctioned or were tampered with, and it often ended with an abundance of steam that would either leak out of a valve or blow up its container.
The biggest difference between LWRs and MSRs is that, while LWRs use water as a coolant, MSRs don’t need water to moderate the temperature of the reaction. The fuel acts as its own coolant. Essentially, MSRs can’t meltdown. Furthermore, in an emergency situation, the fuel can be quickly drained out of a reactor and passively dumped.
First, it’s important to know that thorium, which is present in the fuel, absorbs neutrons as well. But, unlike uranium, it doesn’t release more neutrons to perpetuate the chain reaction. The hotter it gets, the more neutrons the thorium will absorb. By reducing the quantity of neutrons in the fuel, the thorium limits how fast the reaction can be.
Thermal expansion also plays a role in the natural cooling of the fuel. When molecules in any substance are heated up, they move faster and expand. In the case of the molten salt, this process pushes out the active core region, making it so that the neutrons have to travel farther to continue the cycle. Just like the thorium, thermal expansion also limits the speed of the nuclear reaction.
Because the fuel passively moderates its own temperature with both thorium and thermal expansion, the odds of it overheating are low. But in the case of an emergency, there is a backup plan: A plug of frozen salt at the bottom of the reactor. It’s kept cool by a fan, but if the fuel surpasses a critical temperature, it melts the salt plug. Once the plug has melted, the fuel is drained out of the reactor and into a catch basin, where it can cool down and solidify, thereby reducing the pressure. The process is similar to that of a sink with a drain: Removing the plug will allow for the fluid to drain out of the sink.
Unlike LWRs, MSRs operate under low pressure conditions. Because of these conditions, and because there isn’t any water in the system, there can’t be any buildup of steam or hydrogen. Furthermore, as a result of the natural temperature limitations of the fuel, the salt can’t boil. There is a chemical system within the MSR that is continuously removing the vapors produced by nuclear fission. Therefore, there is no possibility of a buildup of any form of vapor, which eliminates the possibility of an explosion.
Why should we care?
All nuclear energy is zero-carbon green energy. However, molten salt nuclear reactors aren’t just limited to new uranium: we can use our old nuclear waste to power them. By reusing nuclear waste, we will be using it to its maximum potential, and will not have to store it for thousands of years.
While they are both classified as nuclear reactors, LWRs and MSRs are very different. They operate under different conditions, have different cooling systems, and use different fuels. MSRs are safer because, since the fuel is liquid, they can’t have devastating meltdowns similar to those in Chernobyl, Fukushima, and Three Mile Island. The safety concerns of LWRs simply do not apply to MSRs. We can’t let our fear of nuclear meltdowns get in the way of the development of meltdown-proof reactors.
Now more than ever, it’s important to invest in nuclear energy. Our dependence on fossil fuels has already plunged the planet into a sixth mass extinction, and on August 9th 2021, the IPCC released a code red for humanity. We need to act fast if we want to preserve our planet.
Molten salt reactors can play a huge role in this effort.