Is nuclear power safe?

After the Three Mile Island meltdown in 1979 and the Chernobyl disaster in 1986 nuclear power shot to the top of the environmental villains list, and now the situation in Japan has seen it again rise to the top of many political agendas. Since nuclear power produces no greenhouse gasses, proponents are touting it as a greener alternative to fossil fuels. They argue that one pound of enriched uranium (the chief nuclear fuel) can provide the same energy as 3 million pounds of coal or 1 million gallons of gasoline.

But there’s quite a catch. Nuclear fuel produces radioactive waste, which can cause cancer, trigger birth defects, and spawn mutants. The technology is both fascinating and ominous and we’ll be explaining just how it works in this article.

Nuclear power plants are complexes that span many square miles, but the real action happens on a subatomic level. The sole purpose of a plant is to harness the energy of nuclear fission – a reaction where an atom’s nucleus splits into two smaller nuclei.

Specifically, nuclear plants typically derive power from inducing nuclear fission in enriched uranium oxide, comprising 96-97 per cent uranium-238 and three-to-four per cent uranium-235. Uranium is the heaviest of all natural elements and one of the easiest to break apart. When a relatively slow-moving free neutron runs into a uranium-235 atom, the atom will absorb the neutron, and the extra energy will make the atom unstable. The atom immediately splits apart, into two smaller atoms and two-to-three free neutrons. A fraction of the atom’s original mass becomes energy, in the form of heat and high-energy photons called gamma rays.

With the right mix or uranium-235, you get a chain reaction. Some of the free neutrons generated in the fission reaction encounter other uranium-235 atoms, causing those atoms to split apart, producing more free neutrons. Collectively, the splitting atoms generate a substantial heat. All the equipment in a nuclear plant has one core function: safely harnessing this heat to generate electricity.

When water stopped circulating at the Boiling Water Reactor (BWR) station in Fukushima, Japan, a build up of hydrogen gas blew the roof of the building. The fear of a radiation leak occurred when the coolant water (which immerses the fuel rods) failed and exposed the fuel elements in the reactor vessel to air. In case of an emergency the control rods slide in between the fuel elements to halt the nuclear reaction process, but if the fuel elements are not cooled there can still be a risk of radiation leaking out. Technicians instead used seawater to attempt to cool the fuel rods. Here’s a look at a typical BWR.

The heart of a nuclear power plant is the reactor, which contains the uranium fuel and the equipment that controls the nuclear fission reaction. The central elements in the reactor are 150-200 bundles of 12-foot-long fuel rods. Each bundle includes 200-300 individual rods, which are made from small uranium oxide pellets. The rods are immersed in a coolant and housed in a steel pressure vessel.

The fission reaction continues indefinitely when, on average, more than one neutron from each fission reaction encounters another uranium atom. This state is called supercriticality. In order to safely heat the water, the reactor must keep the fuel slightly supercritical, without allowing a runaway fission reaction.

The key mechanism for controlling the reaction rate are a series of control rods, made from neutron-absorbing material such as cadmium. Operators can move the control rods in and out of the bundles of uranium rods. To slow down the fission reaction, operators lower the rods into the bundles. The rods absorb neutrons from the fission reactions, preventing them from splitting additional nuclei. Operators can stop the fission reaction by lowering the control rods all the way into the uranium rod bundle. To accelerate the fission reactions, operators partially raise the rods out of the bundle. This increases the rate of free neutrons colliding with uranium atoms to keep the fission reaction going.

Apart from the fission reaction, a nuclear plant works the same basic way as a coal-burning plant: the fuel generates heat, which boils water, which produces steam, which turns a turbine, which drives an electric generator.

In a pressurised water reactor, the heat from fission doesn’t produce steam directly. The fission reaction heats the water inside the pressure vessel to about 325 degrees Celsius, but the water is kept under high pressure to keep it from boiling. A pumping system drives this hot water through a pipe that runs to a separate water reservoir, in the steam generator. The pipe heats the water in the steam generator to the boiling point, and it produces steam. The rushing steam turns a turbine and then reaches a cooling system. As the steam cools, it condenses back into a liquid. The liquid water returns to the reservoir, and boils again, repeating the cycle. As the turbine spins, it powers a generator, which produces an electric current. And voilà: usable electric power.

The principles of nuclear power are remarkably simple. Here’s how a Boiling Water Reactor station such as Fukushima in Japan turns subatomic particle activity into usable power:

Nuclear fission produces high levels of gamma and beta radiation, which can mutate cells, causing cancer and birth defects, among other things. Naturally, the most important concern when designing a nuclear power plant is containing this dangerous radiation. A modern nuclear power plant has many layers of protection. The pressure vessel that contains the uranium rods is encased in a thick concrete liner, which blocks gamma radiation. The entire reactor and the steam generator system are housed in a giant steel liner, providing additional radioactive shielding. The steel liner is surrounded by an outer concrete structure, designed to contain the radiation, even in the event of an earthquake. Modern nuclear power plants also include advanced automatic cooling systems, which kick into action in the event of the reactor or other equipment overheating.

The spent uranium rods are also highly radioactive, which means power plants can’t just throw them away. The best solutions anyone has come up with so far is to encase the nuclear waste in massive concrete and steel structures or bury it underground.

Pros and cons of nuclear power

The remarkable advantage of nuclear power plants is they generate electricity without emitting any air pollution. The clouds billowing from cooling towers are nothing but harmless steam.

Nuclear power does take a toll on the environment, however. Mining uranium destroys natural habitats, and the activity involved in both mining and processing uranium produces greenhouse gases.

The bigger problem is fuel radioactivity. As Chernobyl demonstrated, accidents can cause widespread disease. Nuclear waste remains highly radioactive for thousands of years, and there’s already more than 60,000 metric tons of it to deal with. Nobody wants it in their backyard. Another concern is waste falling into the wrong hands, giving terrorists material for weapons.

In recent years, dozens of nations have decided the benefits are worth the risks and are forging ahead. They’re touting nuclear power as the way of the future – just as it was 60 years ago.

When nuclear reactors fail

For 23 years, Chernobyl has been a grim reminder of nuclear power’s risks. On 26 April 1986, reactor four at the Chernobyl Nuclear Power Plant exploded, after a safety test went wrong. The reactors at Chernobyl, in what is now Ukraine, had little shielding unlike those in Japan to protect against radioactive contamination. The blasted reactor burned for ten days, spewing 400 times the radioactive fallout that fell on Hiroshima in the World War II bombing. The pollution spread across 80,000 square miles, with radioactive rain reaching as far as Ireland. Authorities evacuated surrounding areas, including the nearby town of Pripyat.

In all, more than 300,000 people lost their homes. They couldn’t return to an 800-square-mile exclusion zone around the reactor. The explosion and radiation exposure killed 56 people soon after the blast. The total death toll is impossible to calculate, because of the contamination’s far reach and long-term effects. In 2006, the United Nations estimated that cancer cases stemming from the disaster will eventually claim 4,000 lives. A report commissioned by Greenpeace estimated the death toll at 200,000.

For more information about the Chernobyl disaster, head to the World Nuclear Association where you can read an in-depth analysis of the events and impact relating to the atrocities in Ukraine.