P2bL14 Nuclear Power Station

Key Words

Control rods - rods that are lowered or raised to control the rate of the nuclear reaction by absorbing neutrons.

Core- the central part of the reactor, where the fuel rods are.

Heat Exchanger - a system of pipes where the reactor cooling fluid transfers heat to water,

Moderator - graphite is a material that slows neutrons down.

Plutonium - a heavy fissile metal, used as a reactor fuel.

Radioactive Waste - the highly radioactive waste material taken from a reactor, e.g. spent fuel rods.

Uranium - a heavy fissile metal, used as a reactor fuel.

Grade E

In a nuclear power station, the turbines and generators are exactly the same as an ordinary (coal or gas fired) power station.  The difference is in the boiler that generates steam.  A coal fired power station has a boiler that is basically a large box with a heat exchanger.  A fire is lit at the bottom, and chemical energy is turned into heat to boil the water.

In a nuclear power station, the reactor uses plutonium or uranium fuel rods to heat a cooling fluid, which is circulated to a heat exchanger to boil the water.  The reactor is the boiler.

Here is a diagram (schematic):

The key features are:

• The fuel rods which contain the fissile fuel (uranium and plutonium)
• The moderator which is graphite.  This reduces the speed of neutrons given off in fission reactions.  Otherwise they would go straight through other nuclei.  They need to be slow enough to be absorbed (or "tickle" the nucleus).  Water can act as a moderator.
• Control rods are made of boron or cadmium which absorb neutrons.  On average, 3 neutrons are given out by the fission reactions; 2 of these are absorbed.  The control rods can be dropped into the reactor in an emergency, and will stop the reactor completely.
• The primary loop carries cooling fluid to the steam generator (heat exchanger).  This is where the water boils.  The cooling fluid can be:
  • carbon dioxide;
  • water;
  • liquid sodium.

Grade C

People tend to be afraid of nuclear power.  They associate it with atomic bombs and various high-profile accidents.

The worst of these was, undoubtedly,  at Chernobyl in the Republic of Ukraine, on Saturday, 26th April 1986.  An unauthorised experiment took place in which scientists wanted to see what would happen in a worst case failure. 

They found out. 

The reactor overheated to the point where water split into hydrogen and oxygen.  The two gases rapidly accumulated at the top of the reactor vessel and recombined in a thunderclap explosion which blew the lid off the reactor and threw the vessel on its side.  All sorts of nasty muck spilled out of the reactor.  The building it was in was wrecked and as a result of the graphite moderator catching fire, a plume of radioactive material drifted over much of Europe, including the UK.   Although 9 tonnes of material spread over much of Europe suggests it was spread pretty thinly, a significant increase in radioactivity was detected.

It must be emphasised that the disaster was due to a chemical explosion, not a nuclear explosion.

The Soviet authorities initially claimed that the explosion was at a cement works, but nobody was convinced.  In the end, they called for international assistance, which led to the USSR opening up much more.  Eventually the Soviet Union fell apart, and communism was abandoned.  It can all be traced back to this disaster.

Despite this, the nuclear industry is remarkably safe, because it is governed by very strict procedures.  And a nuclear power station gives off no greenhouse gases or acid rain.  The lists below sum up the arguments for and against nuclear power.

FOR:

• Fossil fuels will run out.  We need something to give us electricity.  Nuclear power fits the bill
• No pollution is given out; nuclear power stations are clean.
• Alternative energy sources like wind are not reliable.
• Very little background radiation (0.1 %) is due to the nuclear industry.
• The technology is safe.
• Risks are much less than other risks in normal life, e.g. road accidents.
• Amount of waste is very small.

AGAINST:

• Renewable energy has less impact on the environment.
• No level of radiation is acceptable.  People can contract leukaemia (a blood cancer).
• Radioactive waste is the nastiest muck known to man.  It takes thousands of years for the radioactivity to reduce.
• 57 people died as the result of the Chernobyl Accident, and a large area was abandoned.

That said, 3000 people died as a result of Osama bin Laden's attack on New York's World Trade Center on Tuesday 9th September 2001.

Grade A

The fuel for reactors is Uranium 235, a fissile isotope of Uranium.  Both U 235 and the more common U-238 decay by alpha decay, but remember that radioactive decay and nuclear fission have nothing to do with each other.

The waste from nuclear power stations is mainly in the form of spent fuel rods, which contain plutonium.  This is formed when uranium 238 absorbs neutrons.  Plutonium is a radioactive metal that decays by alpha decay.  It is fissile.  It is also chemically very reactive.  There are also the products of fission that can be also highly reactive.

Waste is processed at Sellafield in Cumbria.  High level waste is incorporated into glass (vitrification) before being stored deep underground.  Also the fuel is reprocessed to isolate plutonium, and the uranium 238 that is put back into fuel rods.