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OP-ED: Molten salt reactors: The future of power?

  • Published at 10:50 pm August 4th, 2020
nuclear reactor
Representational photo Bigstock

The MSR design is far superior to the conventional reactor in terms of safety


Bangladesh will need more power as we industrialize. As we are vulnerable to climate change, we must find an alternative to fossil fuels. Solar and wind are intermittent sources which can’t supply the continuous power which modern cities demand. Utilities in countries which have invested heavily in wind and solar are burning fossil fuels (as backup power) 70% of the time.

Nuclear power is the best option to replace fossil fuels. The Rooppur nuclear plant will use the conventional light water reactor (LWR) design. However, a much safer and more economical design is now available. China is building its first molten salt reactor (MSR) at Wuwei in Gansu Province. The MSR design is safer than the conventional (LWR) nuclear reactor design; the accidents which occurred at Fukushima and Chernobyl simply cannot happen in a MSR.

The root cause of both the Chernobyl and Fukushima accidents was failure of cooling, leading to overheating of the core. Both of these nuclear plants were light water reactor (LWR) plants. In the LWR design, continuous coolant circulation is needed to prevent the reactor core from overheating. Coolant circulation is needed even after the reactor is shut down because of “residual radioactivity”; after the chain reaction has been stopped, unstable fission products in the fuel continue to decay and generate heat.

At Fukushima, the reactors were automatically shut down when the earthquake hit; however, coolant circulation was still needed to remove heat generated by residual radioactivity. Coolant circulation was interrupted because of a power failure (caused by earthquake damage). The backup generators could not be used because the switches were located in a basement, which had been flooded by the tsunami. Without coolant circulation, the core overheated, causing a reaction between high-temperature zirconium (in the fuel cladding) and water (coolant) which produced explosive hydrogen gas. The root cause of the hydrogen gas explosion was the overheating of the core.

At Chernobyl, the coolant circulating pumps had been shut down to conduct a test of the safety systems. Without coolant circulation, the core overheated; water in the core became high-pressure steam, causing an explosion. The root cause of the steam explosion was the overheating of the core.

In a MSR, if the coolant circulation is interrupted, the molten salt in the core starts to become hotter, and expands; expansion increases the space between uranium atoms, making it less likely that they will be hit by neutrons (which are produced by the fission of other uranium atoms); this stops the chain reaction and powers down the reactor, all without any human (or automated) intervention.

If for some reason the high temperature persists (which is extremely unlikely), a plug of salt at the bottom of the reactor will melt, and the molten salt (containing the fuel) will drain into many tanks below the core. The chain reaction will stop, as the drain tanks do not have any moderator (moderators are substances like graphite which slow down neutrons to make it more likely that they will trigger uranium fission). Also, the drain tanks have a large surface area, allowing neutrons to escape from the fuel; this also causes the chain reaction to stop.

The MSR design incorporates what nuclear engineers refer to as “passive safety” (safety which does not depend on human operators or automated safety systems working as they should).

There are several reasons why MSRs will be very economical. MSR reactors use fuel more economically than conventional (LWR) reactors. In a conventional (LWR) reactor, solid uranium oxide fuel rods are clad in zirconium. Uranium fission products accumulate in these rods (as they can’t escape the cladding).

Some of these fission products are “neutron absorbers.” As neutron absorbers accumulate, they compete with uranium to absorb neutrons; the chain reaction slows down because uranium is deprived of neutrons. In the LWR, fuel rods become waste when only a small percentage (less than 5%) of the uranium has been fissioned. In a MSR, the fission products (which are produced as gases) simply bubble to the top of the reactor vessel, where they can be removed and processed for storage. So a MSR can consume 90% of its uranium fuel, and produces far less radioactive waste.

Molten salt reactors can use cheaper fuel than conventional nuclear reactors. In a conventional (LWR) reactor, the fuel is (expensive) uranium. In a MSR, the fuel is uranium mixed with (very cheap) thorium. Thorium becomes Uranium-233 when irradiated with neutrons in the reactor core; so the MSR converts cheap thorium into valuable uranium-233 fuel. LWR uranium fuel must be fabricated into fuel rods. MSR fuel will not require expensive fabrication; in a MSR, fuel is simply dissolved in the molten salt.

Molten salt reactors will produce far less long-lived radioactive waste than conventional (LWR) reactors. LWRs produce spent fuel which is intensely radioactive for 10,000-50,000 years.

MSRs will produce waste which is intensely radioactive for about 300 years; a very short time compared to the LWR waste.

Molten salt reactors are also much cheaper to build than conventional (LWR) reactors.

LWR reactors require a huge containment building to contain any accidental release of high pressure radioactive steam from the core. The MSR reactor does not have steam (or water) in its core, so it does not need an expensive containment building. It also does not need an expensive reactor vessel built to operate at high pressure.

Proponents of molten salt reactors expect that electric power from MSR nuclear plants will actually be cheaper than electric power from coal-burning power plants. In other words, once the technology is proven on a commercial scale (by the project underway in China), it will be more economical to build MSR nuclear plants than to build new coal-burning power plants.

In conclusion, the MSR reactor design is far superior to the conventional (LWR) reactor design in terms of safety. The primary consideration when choosing a nuclear plant design must be safety; Bangladesh is a small, densely populated country, and we must avoid a nuclear accident.

Bangladesh is now building coal-burning power plants because industrialization requires cheap electrical power; however, MSR technology is likely to produce electrical power cheaper than coal. If MSR power really does prove to be cheaper than coal power, all of our future power plants should be MSR nuclear plants.

Sources:

1. “Molten salt and traveling wave nuclear reactors” (Asia Times, 4 February 2020) https://asiatimes.com/2020/02/molten-salt-and-traveling-wave-nuclear-reactors/

2. “Chernobyl Accident 1986” (World Nuclear Association) https://www.world-nuclear.org/information-library/safety-and-security/safety-of-plants/chernobyl-accident.aspx

3. “Fukushima Daiichi accident” (World Nuclear Association): https://www.world-nuclear.org/information-library/safety-and-security/safety-of-plants/fukushima-daiichi-accident.aspx

4. “Thorium: Energy Cheaper than Coal,” book by Robert Hargraves, 2012.

5. “The Resurrection of the Molten Salt Nuclear Reactor,” a presentation by nuclear scientist Dr Alan Rice https://www.msr-rice.com/5d0a1faee238c/the-resurrection-of-the-molten-salt-nuclear-reactor

Wikipedia page on molten salt reactors: https://en.wikipedia.org/wiki/Molten_salt_reactor

Kazi Zahin Hasan is a businessman and an avid reader.

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