Nuclear power: how fission and fusion work

Worldwide, nuclear power supplies about 10 percent of generated electricity. In certain nations, including France, its share approaches 70 percent.

Major technology firms, including Google, are likewise looking to nuclear power to satisfy the enormous electricity requirements of their data centres.

All nuclear power originates in an atom’s binding energy. There are two principal methods for releasing the energy held within an atom: fission and fusion. Fission splits large, heavy atoms into smaller, lighter atoms, whereas fusion joins small atoms to create larger ones.

Each process can release vast quantities of energy. A single fission event involving U235, a uranium isotope commonly used as fuel in most power stations, for instance generates more than 6 million times as much energy per individual chemical reaction as the purest coal.

That makes both processes highly effective means of producing electricity.

What is fission?

Every nuclear power station currently operating relies on fission. The process begins when a minute subatomic particle, known as a neutron, strikes a uranium atom and causes it to split.

Further neutrons are then released. They collide with additional atoms and trigger a nuclear chain reaction, which produces an enormous quantity of energy.

A heat exchanger converts this energy into electricity by turning water into steam, which drives a turbine to generate power.

The fission reaction is controllable by limiting the available neutrons. This is done using "control rods", which absorb neutrons.

Nuclear incidents including Chernobyl have historically happened when control rods did not deploy to cut off the neutron supply, and/or when coolant circulation failed.

So-called "third generation" reactor designs build on earlier systems by using passive or inherent safety measures that need neither active controls nor human action to prevent accidents when a malfunction occurs. Such measures can depend on pressure differences, gravity, natural convection, or how materials naturally respond at high temperatures.

Japan’s Kashiwazaki 6 and 7 advanced boiling water reactors were the first third generation reactors.

One outstanding issue with fission is that its reaction products remain radioactive for a very long period, on the scale of thousands of years. When reprocessed, both the fuel source and the waste may also be used to produce a nuclear weapon.

Fission power is a proven technology. It can be deployed at large scale - the largest example is Japan’s 7.97 gigawatt Kashiwazaki-Kariwa Nuclear Power Plant - as well as in small-to-medium reactors producing roughly 150 megawatts of electricity, such as those used aboard ships or nuclear submarines.

These reactors will provide power for Australia’s eight nuclear submarines pledged under a trilateral security partnership with the United Kingdom and the United States.

What is fusion?

Fusion powers the Sun and the stars, and works in the reverse way to fission. It takes place when atoms are joined together.

In laboratories, the most readily initiated reaction is fusion between the hydrogen isotopes deuterium and tritium. For each unit of mass, it generates 4 times more energy than U235 fission.

Deuterium, the fuel ion, is exceptionally plentiful on Earth and across the universe. Tritium, by contrast, is radioactive and has a 12-year half-life, making it extremely scarce on Earth.

The universe is 13.8 billion years old, and the only light-nuclei isotopes - hydrogen, helium and lithium - that occur naturally are those stable over such timescales.

In a fusion power plant, tritium would be produced with a "lithium blanket". This would be a solid wall of lithium, where fusion neutrons slow down and eventually react to create tritium.

Scientists currently find it very difficult to produce a fusion reaction beyond the laboratory because fusion needs extraordinarily high temperatures: the optimum conditions are 150 million degrees Celsius.

At such temperatures, fuel ions are in a plasma state, in which electrons and nuclear ions separate. Rather than radioactive waste, this process produces helium, an inert gas.

The foremost route towards demonstrating sustained fusion is known as "toroidal magnetic confinement". Under this approach, plasma at extreme temperatures is held inside a very large, doughnut-shaped magnetic bottle.

In contrast with fission, this approach needs constant external heating to achieve fusion conditions, along with a strong confining field. If either is stopped, the reaction ends.

The difficulty is therefore not an uncontrolled meltdown, but making the reaction happen in the first place.

A key unresolved problem for toroidal magnetic confinement fusion, the approach receiving most research attention, is proving a burning, self-heated plasma. This means the heating energy generated by the reaction itself becomes the main source. It is the goal of the publicly funded, multinational ITER project, the world’s largest fusion experiment, and the privately funded SPARC experiment at Massachusetts Institute of Technology.

Nevertheless, much of the scientific community agrees that fusion is unlikely to become commercially viable before at least 2050.

Can nuclear power address climate change?

I am frequently asked whether nuclear power can save Earth from climate change. Many of my colleagues work in climate science, and my late wife was also a prominent climate scientist.

The evidence is unequivocal: climate change can no longer be stopped. The world must do all it can to cut carbon dioxide emissions and limit catastrophic harm - actions it needed to take decades ago.

For the planet, fission forms one part of a worldwide solution alongside the broad deployment and uptake of renewable power sources, including wind and solar.

Over a longer timescale, fusion may hopefully take the place of fission. Its fuel is far more abundant and widely distributed, its waste challenge is orders of magnitude smaller in both volume and duration, and the technology cannot be weaponised.

Matthew Hole, Professor, Mathematical Sciences Institute and School of Computing, Australian National University

This article is republished from The Conversation under a Creative Commons licence. Read the original article.