The picture is crystal clear. Human activity will soon drive the climate crisis all across our planet to the tipping point unless we rapidly transform the ways in which we produce and consume energy. While renewable energy technologies and energy efficiency measures can help dramatically cut emissions of greenhouse gases, they are not the panacea for the climate change related problems that we have created.

The scope and impacts of climate change, therefore, demand that we consider other possible low or zero greenhouse-gas-emitting sources of energy, including nuclear power. Indeed, nearly every major authority on climate change, including the International Energy Agency and the UN’s Intergovernmental Panel on Climate Change (Fourth Assessment Report), has said that to achieve deep decarbonisation, nuclear energy must be part of the solution.

All nuclear power plants in operation today rely on controlled fission, which involves neutron-induced splitting of one of the isotopes of uranium into two lighter fragments and two or three neutrons. Despite being a clean source of energy, there exists bitter controversy surrounding the risks of harnessing energy released during fission. Some of the risks are core meltdown (as seen in the 2011 Fukushima disaster), hazards of disposing of radioactive waste, harmful effects of radiation and nuclear proliferation. These risks have made nuclear power a contentious topic bordering between our greatest hopes and deepest fears for the future.

If fission-based nuclear power plants are to play a major role in combating global warming, then we want them to be free from fears of a catastrophic, runaway chain reaction.

If fission-based nuclear power plants are to play a major role in combating global warming, then we want them to be free from fears of a catastrophic, runaway chain reaction. Even more, we want a nuclear fuel that would produce manageable amounts of radioactive waste. We also want a fuel that does not possess the threat of falling into the wrong hands and becoming a deadly weapon of mass destruction.

Many countries are addressing the worrisome problems associated with uranium-fuelled reactors and exploring the possibilities of other forms of safe, clean and incontrovertible nuclear fuel. An alternative that is receiving serious attention from the nuclear stakeholders is using thorium, instead of uranium, as nuclear fuel.

Thorium is a non-fissile, “fertile”, slightly radioactive element. Being non-fissile, it cannot be split to create a nuclear chain reaction, so it must be bred through nuclear reactors to produce fissile uranium.

Thorium enjoys several advantages over uranium. First, the risk of nuclear proliferation of thorium is less than that of uranium. This comes mostly from the fact that plutonium, an essential ingredient of nuclear weapons, is not produced in thorium reactors. Thorium fuel cycle would also minimise toxicity and decay heat problems associated with current reactors.

Secondly, in the event of a runaway chain reaction, uranium-based reactors have the potential to become supercritical and get out of control, thereby causing a catastrophic accident. Since thorium reactors would operate sub-critically, runaway chain reactions that cause nuclear meltdowns would not occur.

Thorium has other advantages too. The inventory of radioactive waste produced by thorium would be much less than uranium. A thorium reactor burns nearly all of its fuel. As a result, it will produce less waste. While some trace elements in spent uranium fuels remain radioactive for many thousands of years, levels in spent thorium fuels drop off much faster. Moreover, unlike conventional reactors that run at potentially explosive, pressurised environments at much higher temperatures, thorium-fuelled reactors can be operated at atmospheric pressure.

Thorium reactors use a combination of thorium and liquid fluoride salts to power the reactor. Fluoride salts have very high boiling points, meaning even a large spike in heat will not cause a massive increase in pressure. This feature greatly limits the chance of a containment explosion. Besides, the reactors don’t require massive cooling, meaning they can be placed anywhere and can be air-cooled.

Thorium is roughly three-four times more abundant in nature than uranium. The most common source is a mineral called monazite, which contains about 12 percent thorium phosphate. Large known deposits are in India, Australia and Norway. Some of the largest reserves are found in Idaho in the USA.

With large, easily accessible reserves of thorium and relatively little uranium, India has made utilisation of thorium for large-scale energy production a major goal in its nuclear power programme. The country has successfully developed a thorium fuel cycle at the nuclear power plant in Kalpakkam, Tamil Nadu. China hopes to build a fully functional thorium-fuelled reactor within the next 10-15 years. Norway is currently in the midst of testing thorium as a fuel in existing nuclear reactors. Other countries with active thorium research programmes include the United Kingdom, Canada, Japan, Germany, Russia and Israel.

If thorium is a safe and versatile nuclear fuel, then why do we use unsafe uranium? The real reason we use uranium over thorium is a result of the Cold World-era politics. Nuclear superpowers backed uranium-based reactors because they produce plutonium—handy for making nuclear weapons. The fact that thorium reactors fail the weapon-making test meant the better reactor fuel got the short shrift.

Nevertheless, if the choice is between keeping nuclear power facilities running or shutting them down and replacing them with coal-fired power plants, the nuclear option with thorium as fuel is ideal for the climate. It is the best supplement to sustainable green energy, filling the gap until nuclear fusion reactors are built. (In an op-ed piece published in this newspaper on May 26, 2019, I discussed fusion energy as the safest form of nuclear energy.)

Finally, regardless of the fear among the public and many activists about nuclear power, thorium reactors are a safer, realistic solution to humanity’s greatest problem. Without nuclear power, we would foreclose our ability to avert the environmental disaster that we brought upon us.

Quamrul Haider is a professor of physics at Fordham University, New York. He is one of the authors of the book Nuclear Fusion—One Noble Goal and a Variety of Scientific and Technological Challenges (IntechOpen, 2019, UK).

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