Nuclear Physics Tutorial 8 - Nuclear Power
saw in the last tutorial that fission rarely occurs spontaneously.
|What kind of nucleus undergoes fission?|
also saw that fission occurs if we “tickle” large nuclei with slow, or
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 13 km/s, 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 ×
m/s, while particles in high energy physics experiments travel at nearly the
speed of light.
|Why do the neutrons need to be slow?|
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
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.
is a minimum mass of uranium (or other
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
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.
uranium is fed to the reactor inside
fuel rods. These are canisters of stainless steel which have fins to
transfer the heat
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.
rods made of boron or cadmium. These absorb neutrons.
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.
reactor is housed in a large steel vessel surrounded by several metres of
concrete to stop the radiation from getting out.
table below shows the different sorts of materials used in different kinds of
encased in a magnesium alloy can
at 400 oC
AGR (advanced gas cooled reactor)
dioxide in a stainless steel can
at 650 oC
(pressurised water reactor)
dioxide pellets in a zirconium can
under pressure at 320 oC
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
Power stations have the advantage that:
very clean, producing no polluting gases;
need a lot less fuel. 1 kg uranium
can give as much energy as 25 tonnes of coal.
to build and run
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.
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.
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.
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.
less serious but high profile case happened at
Three Mile Island
in the United States of America, blamed on incompetence and corporate failure.
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.
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.
disposal of waste has to be done with considerable care, and remains a truly
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.
|Why did alchemists have no chance of producing Gold?|
the work of alchemists did give rise to the discipline of chemistry.
alter elements at the nuclear level, we need to carry out a process of
whereby one element can be turned into another:
natural processes, i.e. radioactive decay;
bombarding the element with particles that are fast enough to penetrate the
|Why do the particles have to travel at a certain speed?|
will occur in the particle bear-garden of a reactor. The first artificial
was carried out by Rutherford in 1919, converting nitrogen to oxygen with alpha
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.
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.
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:
radiation could damage the patient;
patient would pose a risk of exposing others to radiation.
typical transmutation is:
|Where does the energy for this transmutation come from?|
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
a controlled chain reaction.
needs a moderator, control rods and a coolant.
off no polluting exhaust gases;
very dangerous waste materials.
is of paramount importance.
accidents involving radiation leaks have occurred.
of waste must be carried out carefully.
the nuclei of elements by exposing them to particles.
have to slow enough to be captured by the nucleus.