Nuclear Physics Tutorial 8 - Nuclear Power

 Contents

We saw in the last tutorial that fission rarely occurs spontaneously.

 Question 1 What kind of nucleus undergoes fission?

We also saw that fission occurs if we “tickle” large nuclei with slow, or thermal neutrons.  A thermal neutron means that the kinetic energy is equivalent to the photon energy of infra red radiation.  A simple kinetic energy calculation shows the speed of the neutrons as being about 14 km s-1, pretty fast for us, but a snail’s pace for particles.  The electrons in a cathode ray tube of a TV set travel at 5 × 106 m s-1, while particles in high energy physics experiments travel at nearly the speed of light.

 Why do the neutrons need to be slow?

We also saw in the last topic that the fission of Uranium nuclei results in a chain reaction.  Although the fission products are not easily predictable, the key point to remember is that three more neutrons are produced.  These go on to tickle three other uranium nuclei, which each produce three thermal neutrons.  As we saw, the energy released in an uncontrolled chain reaction results in a violent explosion.

There is a minimum mass of uranium (or other fissile material) before a chain reaction can happen, called the critical mass.  This is because neutrons can escape before they interact with nuclei.  The size of the lump of uranium is about the size of a grapefruit, with a mass of 13 kilograms.

The nuclear power station is identical in most respects to a normal power station in that steam is used to turn the turbines, which drive the generators.  The difference is in the boiler that produces the steam, the reactor.

The uranium is fed to the reactor inside fuel rods.  These are canisters of stainless steel which have fins to transfer the heat

The reactor harnesses the heat energy produced when the uranium nuclei split.  It also controls the reaction so that two out of the three neutrons produced are absorbed.  Only one neutron out of the three goes on to tickle another nucleus.  If any more neutrons are produced, the reaction would start to go out of control.  If fewer are produced, the reaction stops.  This is achieved by:

• Moderator, which slows fast neutrons from the fission to slow thermal neutrons by repeated collisions with the nuclei of the moderator material.  Graphite or water are commonly used as moderators.

• Control rods made of boron or cadmium.  These absorb neutrons.  If the control rods are fully in, the neutrons are absorbed completely.  At a certain level, the ideal is reached and the reactor is balanced.  If the control rods are too far out, then more neutrons than needed can cause the chain reaction to go out of control.

The coolant gas (carbon dioxide, helium) is at high temperature, up to 650 oC and transfers the energy as heat to the heat exchanger.  This in turn boils the water to turn the turbines. In  a pressurised water reactor, liquid water at 320 oC is taken to the heat exchanger.

The reactor is housed in a large steel vessel surrounded by several metres of concrete to stop the radiation from getting out.

The table below shows the different sorts of materials used in different kinds of reactor.

 Reactor Type Fuel Coolant Moderator Magnox (gas cooled) Uranium encased in a magnesium alloy can CO2 at 400 oC Graphite AGR (advanced gas cooled reactor) Uranium dioxide in a stainless steel can Helium at 650 oC Graphite PWR (pressurised water reactor) Uranium dioxide pellets in a zirconium can Water under pressure at 320 oC Water

The type of reactor built depends on many factors, not least the cost.  Nuclear power stations have to have many built-in safety systems, as a result of which they are very expensive to build and run.  They also have a limited lifespan.  The intense radiation produced can weaken the reactor vessel.  To replace the vessel requires decommissioning, a long and highly expensive process.

Nuclear Power stations have the advantage that:

• They are very clean, producing no polluting gases;

• They need a lot less fuel.  1 kg uranium can give as much energy as 25 tonnes of coal.

• Expensive to build and run

• Produce dangerous waste.

France generates 80 % of its power with nuclear power stations.  Its last coal mine closed in 2004.   Britain generates 20 % using nuclear.  They remain extremely controversial and inextricably linked with the production of nuclear weapons.

The hazards associated with the nuclear power generation industry are well known and were shown in sharp focus on Saturday 26th April 1986.  An unauthorised experiment was carried out at the nuclear power station at Chernobyl in which the operators overrode safety systems to enact a worst case scenario failure.  They found out.  The reactor became unbalanced, and went out of control.  The overheating caused decomposition of water into hydrogen and oxygen and these gases collected at the top of the vessel.  Mixed with carbon monoxide from the graphite core, the mixture ignited in a thunderclap explosion, which blew the lid off the reactor and turned the vessel on its side.

The damage was done by a chemical explosion, not nuclear.  However many tonnes of radioactive muck was hurled into the air, and nine tonnes of caesium-137 floated across Europe.  Catastrophic environmental damage was done in the local environment and 135 000 people were evacuated permanently.

The then Soviet authorities tried desperately to cover up the accident, claiming that the accident was a fire in a limestone works.  Eventually they had to come clean, and ask for international help to clear up the mess.

The area around the power station is heavily contaminated, and has been abandoned.  Ironically, as it has become wild again, the eco-system had thrived and the area has become a haven for a number of rare species that have adopted it as their new home.  (They clearly cannot read the notices that say "Radioactive area.  Keep out".)

A more recent accident that was just as severe took place on Friday 11th March 2011.  The North East of Japan was rocked by a severe earthquake, which was accompanied by a catastrophic tsunami.  The nuclear power station at Fukushima Dai-ichi (Fukushima Number 1) had its electricity supply interrupted.  Its reactors shut down as they were supposed to, and diesel generators cut in to keep power to the plant.  Unfortunately these were on the shore-line and were swamped by the waves from the tsunami, and wrecked.  The plant went on to battery-power, but then the batteries went flat, leaving the reactors to overheat.  Despite the heroic efforts of the staff, each one of the four reactors in turn blew up.  The resulting mess will take many years to clear up.

Another less serious but high profile case happened at Three Mile Island in the United States of America, blamed on incompetence and corporate failure.

The safety of nuclear facilities has to be of paramount importance, and many systems are built in to prevent failure.  The last resort is to drop the control rods into the reactor.

In normal operation, nuclear power generation is very safe; there have been few accidents involving radiation to personnel, although there are the "normal" industrial accidents that happen from time to time.  It is right and proper that there are strict controls, for the waste from nuclear reactors is some of the nastiest muck known to man, with radioactive isotopes with long half-lives.  Britain processes nuclear waste, a valuable economic business which has to be monitored very carefully.  However the reputation of the industry was dealt a major blow some years ago when there was a serious breach of trust by employees at Sellafield who falsified documentation about batches of waste.

The disposal of waste has to be done with considerable care, and remains a truly controversial issue.

For many centuries alchemists tried very hard to make gold by mixing various substances together.  They did not have a snowball’s chance in Hell of doing so.

 Question 3 Why did alchemists have no chance of producing Gold?

However the work of alchemists did give rise to the discipline of chemistry.

To alter elements at the nuclear level, we need to carry out a process of transmutation, whereby one element can be turned into another:

• by natural processes, i.e. radioactive decay;

• by bombarding the element with particles that are fast enough to penetrate the nucleus.

 Question 4 Why do the particles have to travel at a certain speed?

Transmutation will occur in the particle bear-garden of a reactor.  The first artificial transmutation was carried out by Rutherford in 1919, converting nitrogen to oxygen with alpha particles:

If we measure the mass off the products carefully, we see that it is greater than the combined masses of the nitrogen nucleus and the alpha particle.  Kinetic energy from the alpha particle has been converted to this mass.  This is not as strange as it may seem; at this level mass and energy are interchangeable.

Transmutation is put to good use in a modern form of alchemy, which is the production of radioisotopes that are used in medicine.  These need to be of short half-life because:

• the radiation could damage the patient;

• the patient would pose a risk of exposing others to radiation.

A typical transmutation is that of Tellurium-130 being bombarded with neutrons to form Tellurium-131 which decays by beta minus decay to Iodine-131:

 Question 5 Where does the energy for this transmutation come from?

Radioactive nuclides such as iodine-131 are used as tracers.  A camera sensitive to gamma rays is placed next to the thyroid of a patient and can monitor the uptake of iodine in the patient's thyroid gland.

A further discussion on fission and fusion can be found at Physics 6 Tutorial 12.