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How do nuclear reactors work?


Nuclear fuel rods

Nuclear fuel rods inside a power station. (Source: Michaela Rehle/Reuters)

I understand with nuclear fission that simply a molecule is crashed in to an atom, this splits the atom, releasing more molecules which go on to crash into other atoms and also releases a large amount of heat. Does this chain reaction only continue as long as there are atoms there to split? And also if you super cooled the atoms would this stop the chain reaction?

Nuclear power stations are a major source of energy for many countries around the world. But the recent events at Fukushima nuclear plant in Japan demonstrate that when things do go wrong, they can have a serious impact.

Nuclear power stations use the principle of nuclear fission to generate energy, says Professor Stephen Lincoln, lecturer in nuclear chemistry at the University of Adelaide.

This involves smashing neutrons into very large atoms, particularly uranium 235, releasing more neutrons.

"Hit the nuclei at just the right speed with a neutron and it will split, in the process producing more neutrons which slam into other atomic nuclei of uranium 235 causing them to split as well, releasing even more neutrons and so on.

"That's called a chain reaction", says Lincoln.

Water is used in the power plant to slow down the neutrons to the right speed to cause the chain reaction.

"If the collision is too fast, the neutron is absorbed by another isotope called uranium 238 and converted into plutonium 239."

According to Lincoln, some types of nuclear reactors are specifically designed to do this.

"Because the neutron is travelling faster and because they make plutonium, these reactors are called fast breeders."

Lincoln says some reactors use plutonium as their fuel source and others, called mixed oxide or MOX reactors, use a mixture of uranium and plutonium.

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The nuclear fuel cycle

Uranium starts off as an ore, which is mined and purified into a fluorescent yellow powder called yellow cake. The yellowcake is then processed to give uranium oxide enriched in uranium-235, which is used in fuel rods.

When enough fuel rods are loaded into a reactor core to reach critical mass, Lincoln says neutrons start flowing and the chain reaction begins.

Control rods, made of boron or other neutron-absorbing material, are used to control the number of neutrons in the nuclear reaction.

"These can be raised and lowered to adjust the chain reaction rate as needed so it doesn't go too fast, or so slow nothing happens", says Lincoln.

In most modern light water reactors the fuel rods are inserted into a water container making the water boil and turn into superheated (300°C) high pressure steam.

Lincoln says the steam is radioactive as a consequence of being bombarded by neutrons from the fuel rod chain reaction.

"They convert oxygen-16 into nitrogen-16 which is radioactive but only has a half-life [the time it takes a substance to lose half its radioactivity] of about seven seconds, before decaying back to oxygen-16 and emitting gamma- and beta-radiation, which is very dangerous".

To keep the radioactive steam and water isolated from the outside world, it's fed through a closed circuit loop to a heat exchanger, which transfers the heat to non-radioactive water in a separate loop to make steam which spins generator turbines producing electricity.

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Nuclear meltdowns occur when the fuel rods overheat, leaving a molten mass of fuel and fission products at the bottom of the reactor vessel — the casing that contains the fuel rods and coolant. Melt-throughs occur when the molten fuel and fission products leak through the casing into the environment.

Reactors are cooled by the same water used to moderate the fuel cycle.

Fukushima's water-cooling system failed when the motors powering the pumps were destroyed by the tsunami, leading to a meltdown, and now possibly a melt-through.

Super-cooling — cooling something to absolute zero (-273.15°C) — could in theory stop the chain reaction, says Lincoln.

"But it's never been done. Even if we could, it would be both very difficult and very expensive on a large scale like a nuclear reactor."

"So far it's not been possible to extract all of the heat to get to absolute zero."

"The lowest temperatures commonly achieved involve cooling liquid helium for use as a cryogenic refrigerant for super-cinducting magnets such as those used in MRI machines," he says.

Professor Stephen Lincoln a lecturer in nuclear chemistry at the school of chemistry and physics at Adelaide University was interviewed by Stuart Gary.

Coming up: how does nuclear radiation affect humans?

Tags: nuclear-accident, chemistry, energy, nuclear-energy

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Published 16 June 2011