Additional Physics Topic 14 - Nuclear Fission

Nuclear Energy

Nearly seventy years ago the power of nuclear energy was demonstrated to the world.  Little Boy, a free-fall atomic bomb was dropped on Hiroshima in August 1945.  A few days later an aerial mine, Fat Man, was detonated over Nagasaki.  The destruction and carnage caused by these bombs is well known.  The energy released was due to the conversion of 20 grams of nuclear material to heat.

The conversion of nuclear energy to heat is at the heart of nuclear energy.  Scientists have learned to control the process so that instead of an explosion, a steady heat source is achieved.  A nuclear reactor can boil water to steam to turn a steam turbine.  In the early days, nuclear energy was greeted with unbounded optimism.  All sorts of nuclear powered devices were conceived, including railway engines and aeroplanes.  It was hoped that nuclear power would be so cheap that it would not be necessary to meter electricity.  However this proved not to be the case.

We will look at the processes that release nuclear energy.  It is strange to think that matter (material) can be turned into energy and back again, but in nuclear physics, this happens. The amount of energy that can be released from materials is huge. It is described by Einstein's simple equation:

E = mc2

The Physics code are:

• E - energy in joules;

• m - the mass in kg

• c - speed of light (300 million m/s)

So the term c2 gives us 9 × 1016, a huge number.

Fission

Very large nuclei tend to be rather unstable.  This means that they are radioactive.  Some nuclei, for example, Uranium-235 and Plutonium-239, can be made so unstable that they split into two or more nuclei of more stable elements.  This is called fission.  The nuclei are called fissile.

These fissile nuclei are isotopes of more stable elements (e.g. Uranium-238).  If left alone, they decay radioactively by emitting alpha particles.

Fission is not a spontaneous process.  It has to be started by injecting a neutron into the nucleus.

The neutron has to be injected at the right speed:

• too fast, the neutron will pass right through, or knock out another neutron.

• too slow, the neutron will bounce off the nucleus.

Many pictures show the neutron smashing the nucleus like a bullet.  This is wrong.  It's more like that the neutron "tickles" the nucleus.

The nucleus is not a neat array of protons and neutrons.  It is very active , changing shape all the time.  It's like a "wobbly drop".  When the extra neutron is taken into the nucleus, the wobbly drop goes dumbbell-shaped like this:

The weak spot at the neck makes the nucleus fly apart to form two or more new nuclei.  A lot of energy is released.  Nuclear energy gives off far more heat energy than chemical reactions

Also two or three (or more) neutrons are released.  These can go on to be absorbed by other nuclei to cause a chain reaction, which is shown in the picture below.

For the reaction to occur, there has to be a critical mass. For uranium, this is about the size of a tennis ball. The critical mass has a mass of about 15 kg (uranium has a very high density, 19 g/cm3). Anything less, the neutrons escape without setting off a chain reaction.  If the chain reaction is not controlled, a nuclear explosion will occur.  In a nuclear reactor, only one neutron is allowed to pass on to be incorporated into one nucleus.

 Question 1 What sort of nuclei are needed for fission?

This is a typical nuclear fission equation:

Note that:

• The mass numbers balance (235 + 1 = 90 + 143 + 3).

• The atomic (proton numbers) balance (92 = 36 + 56).

• Three neutrons on average are released, which go on to be captured by other nuclei.

Note that there are about 40 other products of fission.

Fission has NOTHING whatever to do with radioactivity.  Alpha and beta particles are NOT emitted during fission.  However many of the new daughter nuclei are radioactive.

Nuclear Power

Reactors in a nuclear power station do the same job as the boiler; they boil water to steam.  They also can be used to make radioactive isotopes for medical purposes.

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):

Source not known

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.

 Question 2 Complete the interactive matching exercise on parts of the nuclear power station.

Nuclear Accidents

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.

Wikimedia Commons

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.

On Friday 11th March 2011, Japan was rocked by a severe earthquake.  In the subsequent tsunami that killed many thousands of people, the emergency power supply to Fukushima Daiichi (Fukushima No 1) power station was wrecked, resulting in the overheating of all four reactors.  All four consequently exploded.

Nuclear Arguments

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.

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.

 Summary: Fission has nothing to do with radioactivity. Fission occurs with certain large fissile nuclei. A neutron is absorbed by a fissile nucleus which splits into smaller nuclei, releasing energy and neutrons. More neutrons are absorbed by other fissile nuclei to form a chain reaction.