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Lesson 14: Nuclear power

Introduction

In this lesson we look at the operation of nuclear power plants.

Try Why Do Astronauts Float by Julian Hamm

We do look at some of the social, environmental and economic issues but much has already been covered in lesson 6: Science and Risk.  So this lesson is more about the science of nuclear power than the politics.

We’ll find out how uranium is processed and how nuclear waste can be handled.

How an electrical generator works

Let’s start by making sure we understand how electricity is generated.

All electric generators work by spinning a coil of wire very fast in a magnetic field.  The coil normally has thousands of turns.  A full-scale generator has a very complex design.  This is to make it as efficient as possible.  We’ll really simplify it.

The faster the coil spins the bigger the voltage.  The actual voltage depends on the design of the particular generator.  Our generator is designed to have an output of 20 volts when it spins at 2000 revs / min.

If we make our generator run three identical bulbs by connecting them in parallel then they are all equally bright because they all have the same voltage across them.  The faster the generator, the brighter the lights.

A turbine is like a windmill and spins a generator

How can we make our generator spin?  We attach a ‘turbine’ to it.

Turbines come in all sorts of shapes and sizes.  In a modern wind generator the turbine is made up of a few big windmill sails.  In a hydroelectric power plant the turbine looks like a big ship's propeller.  Water runs through it, making it turn.

In a power station the turbine consists or hundreds of tiny little blades.  Superheated high pressure steam is forced through them at very high speed making the turbine spin very fast.

Sometimes gears make the turbine spin the generator at the right speed

Generators that feed into the UK National Grid always need to spin at 50 revolutions per second so they produce 50 hertz a.c. electricity.  In the US and some other countries generators spin at 60 revoluations per second and so product 60 Hz a.c. electricity.

Wind and water turbines need gears to make the turbine spin the generator at the right speed.  A steam turbine is designed to spin at 50 (or 60) revolutions per second.

Burning fuel heats water into steam for a steam turbine

To boil water into high pressure steam for a steam turbine we need to burn fuel.  The most common fuels are finely powdered coal and natural gas.

We can also use a nuclear reactor to produce heat.  The heat from the nuclear reactor boils water into steam for the turbine.

It's harder to turn a generator as it has to run more appliances

Before we look at nuclear power stations in more detail let’s see how a generator responds to different loads.  The more things a generator has to run (for example lights, factory machines, heating, kettles, etc.) the harder it is to turn the generator.

You might expect this because you’re getting more out, so somehow you’re going to have to put more in.  In other words you have to supply energy very quickly to light all the bulbs brightly.

The reason why the generator gets harder to turn is that the coil acts as an electromagnet.  The more things that are connected to the generator, the bigger the current in it and the stronger it is.  The electromagnet’s poles constantly change so that the permanent magnet (in our simplified model) is always tending to slow it down.

Increasing the load can cause the lights to dim

If your turbine can't change how hard it tries to turn the generator then, as the load on the generator changes, the speed of the turbine and generator also change.

This is a problem because it means the voltage changes the whole time.  So the more lights you turn on the slower the generator and the more the lights dim.

A power grid helps keep all the generators running at the same speed

Now imagine that our generator was supplying electrical energy to a city to run all the lights, kettles, fridges, factory machinery and so on.  We need to keep the voltage as constant as possible otherwise things won’t work properly.

We could keep on adjusting how hard we turned the generator to keep the speed the same.  But in fact a better solution is to have lots of generators connected in parallel with each other.  This is called a ‘grid’.

So when an extra light (or toaster or kettle, etc) is turned on, each generator is only a bit more difficult to turn.

Demand for electrical energy changes throughout the day

Over the course of the day demand for electrical energy changes.  This means generators start and stop all the time.  It may take several days to start up a nuclear power station so they tend to be run at full power all the time.

This is called ‘base loading’.  So even though we'll learn how to change the output of nuclear power stations, this rarely happens.

A nuclear reactor can provide the heat to boil water into steam

So far we’ve learned that to supply electrical energy we need to use a generator.  The generator can be spun using a steam turbine.  A nuclear reactor can provide the heat energy to boil water into high-pressure steam.

So how does the nuclear reactor boil the water?

The water that is boiled into steam is not heated by the nuclear reactions directly.  The nuclear reactions heat different water or gas or even liquid metal first.  These ‘coolants’ carry heat energy from the reactor to a boiler.  The turbine water passes through the boiler in pipes.

The coolant transfers its heat energy through the boiler pipes to the turbine water, which is boiled into steam.

The steam going through the turbine doesn't go through the reactor

The coolant contains radioactive substances because it has passed through the reactor.

But the turbine water isn’t radioactive because it hasn’t been in contact with the reactor.

Fuel rods in the reactor act like heating elements in an electric kettle

So how do nuclear reactions heat the coolant?

The coolant is heated by the metal fuel rods.  Nuclear reactions inside the fuel rods cause them to get hot.  They act like the heating element in a kettle.  Heat energy is transferred from the fuel rods to the coolant.

Fuel assemblies contain fuel rods contain fuel pellets

The fuel rods are just hollow metal cylinders.  Inside are the fuel pellets. These are where the nuclear reactions actually take place.  A fuel pellet is like a little black shiny tube a bit narrower than the tip of your finger.

The fuel pellets are very small so the fuel rods are very thin.  The fuel rods are bundled into ‘fuel assemblies’.  The fuel assemblies are carefully engineered to make it easy to fuel and unload the reactor.

We’ll just call the whole thing a ‘fuel rod’ but it’s worth remembering that it’s the pellets that are the actual fuel.

Unused nuclear fuel pellets are only very weakly radioactive

The fuel pellets are mostly uranium-238 with about 4% uranium-235.  The uranium is bound into the pellet with other chemicals.

Both uranium isotopes are very weakly radioactive.  But their slight radioactivity isn’t really important.

Neutrons can cause uranium-235 to fission

What is important is that uranium-235 nuclei fission when they absorb a neutron.  The fission releases energy.

It's this energy that appears as heat, heats the fuel rods that heats the coolant that boils water into high-pressure steam that turns a turbine that turns a generator that transfers electrical energy so that your lights and fridge and PC all work.

Nuclear chain reactions

A uranium-235 nucleus won’t fission without absorbing a neutron.  We can get neutrons by making a neutron source.

When a uranium-235 fissions it tends to split into two larger bits plus typically two neutrons.  The exact composition is random and can happen in lots of different ways.

When one of these neutrons is absorbed by another uranium nucleus then it can fission in turn, releasing yet more neutrons.  This is called a chain reaction.  Whether a chain reaction keeps going depends on the shape and the amount of uranium.

Sub-critical chain reactions fizzle out

If there isn't enough uranium-235 or it's very spread out for some reason then most of the released neutrons escape and don't cause further fissions.  In this case the chain reaction stops, often fairly quickly.

This is called a sub-critical chain reaction.

Critical chain reactions keep going

With a critical chain reaction each fission causes on average exactly one further fission.  For this to happen you need enough uranium-235 arranged in the right shape.

Nuclear reactors are carefully controlled so that they stay just critical.

Super-critical chain reactions run away

If you have enough uranium arranged into the right shape then each neutron released causes more than one further fission.  If it causes much more than one then the chain reaction can quickly increase rapidly.

When a nuclear reactor is being powered up the reaction is arranged to be slightly super-critical until it gets to its operating level.  The uranium-235 is not concentrated enough in nuclear fuel to allow it to become very super-critical.

In a fission (atomic) bomb the chain reaction is designed to become highly super-critical very quickly.  For this you need to make sure you have enough very pure uranium-235 in the right shape.

Gun-triggered nuclear weapons

A gun-triggered nuclear weapon is a simple fission device (or atom bomb).

You take two pieces of highly enriched uranium which are too small to produce a super-critical chain reaction by themselvs.  Then you use conventional explosives to fire one into the other so that the size and shape is right for a highly super-critical chain reaction.

This was the design of the Little Boy weapon that was dropped on the Japanese city of Hiroshima by the American Air Force in the Second World War.

Gun-triggered weapons are the simplest to build and you can pretty much guarentee you'll get some sort of explosion.  However they're not very efficient and aren't used by any of the major nuclear powers.

Implosion-triggered nuclear weapons

An implosion-triggered nuclear weapon is a more sophisticated fission device (or atom bomb).

With an implosion-triggered weapon you have a hollow shell of highly enriched uranium-235 (or more often plutonium) surrounding a smaller sphere.  Conventional explosives force the shell into the inner sphere making a bigger sphere of the right size to cause a super-critical chain reaction.

An implosion-triggered device has a much bigger yield than a gun-triggered bomb but it’s very difficult to make the implosion happen in an exact sphere.

The H-bomb: use a fission bomb to start a fusion reaction

An implosion-triggered fission bomb can be used to set off a fusion reaction.  Fusion bombs are sometimes called hydrogen bombs or ‘H-bombs’.

The fusion fuel is a chemical with lots of hydrogen in it, for example lithium deuteride.  Remember that deuterium is an isotope of hydrogen.  A fusion bomb is many times more powerful than a fission bomb.

Controlling reactor chain reactions using control rods

So nuclear weapons use super-critical nuclear reactions.  But in a nuclear reactor we want the reaction to stay just critical.  This means each fission causes on average just one further fission.

The fuel rods hold the fuel pellets containing uranium-235.  The fuel rods get hot when fission happens in them.  Neutrons from one fuel rod cause fission in the pellets of neighbouring fuel rods.

Control rods are made of a material that absorb neutrons.  By lowering the control rods between the fuel rods you can absorb neutrons and reduce the chain reaction.  By lifting the fuel rods you can increase the chain reaction.

The Safety Control Rod Axe Man (SCRAM)

The first experimental nuclear reactor was built in 1942 at the University of Chicago by Enrico Fermi and Leo Szilard.

They wanted to be able to drop the control rods in very quickly if the nuclear reaction became super-critical.  The control rods were suspended by a rope above the reactor.  In an emergency the rope would be cut and the control rods would drop down between the fuel rods killing the chain reaction.

The story is that this was the job of the Safety Control Rod Axe Man.  So doing an emergency reactor shut-down is to this day called a SCRAM.

Fission is about absorbing not smashing

So now we’ve seen how fission of uranium-235 in the fuel rods causes them to get hot.  We’ve also seen how the control rods absorb neutrons to control the chain reactions between fuel rods.

Fission releases neutrons.  They move very quickly, typically 20 000 KILOMETRES per second. They are called ‘fast neutrons’.  However fission of uranium-235 is NOT about smashing the nucleus to pieces with a neutron.

A neutron has to actually be absorbed by the nucleus.  The nucleus becomes unstable and finally it splits.  The neutrons need to be slowed down a lot for this to happen - to around 2 kilometres per second.

This is roughly their speed because of the surrounding temperature.  So slow neutrons are called ‘thermal neutrons’.

The moderator slows neutrons down so they can cause fission

So how do you slow down fast neutrons enough for them to become thermal neutrons?  You make them collide with something.

What’s the ideal size for the colliding particle?

The ideal mass for the colliding particle is about the same as the mass of the neutron.  Protons have a very similar mass to neutrons.  A proton is just a hydrogen nucleus.  Water has lots of hydrogen atoms so water is good at slowing fast neutrons down so they can cause more fissions.

A substance that slows fast neutrons down to thermal speeds is called a ‘moderator’.  In a pressurised water reactor the water coolant transfers energy and also acts as a moderator.

Where are all the uranium mines?

Now let’s look at how we make a fuel rod for a nuclear reactor.

The first thing we need to do is dig up some uranium ore.  Uranium is fairly common in soil and rocks all over the world.  Remember that that's where the radon gas comes from that can be a health hazard in the home.

But uranium mines cause great scars on the landscape so only fairly empty parts of the world, for example Australia and Canada have large-scale uranium mining.

From uranium ore to yellow cake

Let’s see how we mine the uranium ore and purify it into uranium oxide.

First huge digging machines dig up tonnes of uranium-bearing rocks.

Then these uranium-bearing rocks are crushed up into dust.  Next acid is used dissolve the uranium compounds.  You can then drain off this uranium solution leaving the insoluble rock dust behind.  More chemicals are added to the solution which precipitates out little particles of uranium oxide.

All you need to do now is catch the uranium oxide particles using a filter then dry and compress them into a small brick of solid uranium oxide, which is called yellow cake.

Yellow cake still isn't that much use

Yellow cake is a mixture of uranium-238 and a very small amount of uranium-235.  It’s uranium-235 we need for our nuclear reactor.  Uranium-238 is no good for fissioning.

Unfortunately, for every 150 or so atoms of uranium only 1 will be uranium-235.  This is not enough for a critical chain reaction.

So somehow we have to increase the concentration of uranium-235.  This is called ‘enrichment’.  To make a fuel rod you need around 4% uranium-235.  To make a bomb you need around 90% uranium-235.

Conversion to UF6: to enrich uranium you need it to be a gas

To enrich the uranium you need it to be a gas.  But yellow cake is a powdery solid.

So we need to do some chemistry to produce a uranium compound that is a gas.  This is called ‘conversion’.  After the chemical processing you end up with crystals of uranium hexafluoride.  This is a uranium compound that would be a gas on a hot day.

The crystals can melt (they can also sublime) then turn into a gas.  If the gas is left to cool it will recrystalize.

Enrichment: increasing the concentration of uranium-235

So we’ve got a uranium compound that we can easily turn into a gas.  Now we want to increase the concentration of uranium-235.

A uranium-235 atom has a slightly lower mass than a uranium-238 atom.  At the same temperature both atoms (strictly molecules of uranium hexafluoride) have the same average kinetic energy, given by 1/2 mass x velocity squared.  If the mass is slightly lower then you need a slightly higher velocity to keep the energy the same.

So uranium-235 hexafluoride molecules will have a slightly higher velocity.  This means that uranium-235 hexafluoride molecules diffuse slightly faster than uranium-238 hexafluoride molecules.

By using dozens of diffusion stages the concentration of uranium-235 can be gradually increased.  The more enrichment the more stages you need.

Modern methods use high speed centrifuges.  They spin about a hundred times faster than a spinning washing machine.  The uranium-235 tends to collect at the centre of the centrifuge.  These centrifuges are highly sophisticated and difficult to make.

Centrifuges can be used to enrich uranium for nuclear reactors.  But they can also be used to continue the enrichment for nuclear weapons.

You need thousands of centrifuges to enrich uranium at a useful rate.  The process is very expensive and time-consuming.  But because centrifuges are quite small they are easy to hide.  The international community likes to keep a very careful eye on the enrichment of uranium.

Depleted uranium is a bi-product of enrichment

So now we have some uranium hexafluoride with extra uranium-235.  And also some with reduced uranium-235.  In other words even more uranium-238.  This is called ‘depleted uranium’.

We can extract the uranium metal from the uranium hexafluoride.  Both depleted and enriched uranium are only very weakly radioactive.  Uranium metal is extremely dense and can be made very hard.

Depleted uranium is quite cheap and has many uses where hard, dense materials are useful, for example yacht hulls, radiation shields, artillery rounds and armour for tanks.

It's not so much that there's anything special about the fact that it's depleted of the tiny amount of uranium-235 it started with.  Simply that it isn't very useful for anything else and you've already gone to the time and expense of digging it out of the ground and processing it.

Fuel fabrication - making fuel pellets

Let’s say we have some 4% enriched uranium hexafluoride.  It needs to be made into fuel pellets for our reactor.  This final step is called ‘fuel fabrication’.

Chemical processing turns the uranium hexafluoride into uranium oxide again (this time, though, it's enriched uranium oxide) in the form of a pellet.  The pellets are baked in an oven to harden them and remove any holes.

The pellets are then dropped into fuel rods and then fuel rods bundled together into fuel assemblies so they can fit into the reactor.

All steam turbines need lots of water for cooling

So we should now have an idea about how nuclear power plants work and how uranium is processed to fuel them.  We’ll finish this lesson by looking at some of the environmental issues surrounding nuclear power.

First let’s look at where to build a nuclear power plant.

There are two questions.  Where is it possible to build one?  And where do people find it acceptable to build one?

There are over 400 nuclear power plants worldwide in over 30 countries.  Most of them are in Europe and the US.  France gets 80% of its electricity from nuclear power.

Nuclear power plants need to be near water.  In fact all power plants that use steam turbines need water for cooling.

As well as the water that flows through the reactor to take heat energy to the boiler, there are two other completely separate water 'circuits'.  The turbine circuit, which is closed, and the cooling circuit, which generally uses new water all the time.

Steam that comes out of the back of a turbine is cooled by water in the cooling circuit and condenses back into water.  This makes the turbine more efficient.  Once the turbine is up and running the steam and water in the turbine circuit just cycle round and round.  No new water is added.

To condense the spent steam, cold water from the sea or a river is often used.  This water is purely for cooling.  It never goes near the reactor.

Some power plants return the slightly warmed water to the sea or river.

The warmed water can also be cooled using a cooling tower.  This means you’re not pumping warmed water into the sea or river.  Warm water can affect the plants and animals living there.

The white cloud is caused by the warm moist air which rises out of the tower.  When the warm moist air hits the cold air above the tower the water vapour condenses into tiny droplets of pure water, forming clouds.

Where to build a nuclear power plant?

So now we know how a nuclear power station works.  Where is the best place to build one (assuming you want to build one at all!)?  There are at least four concerns that need to be addressed:

  1. Technical (can you build it here?)
  2. Safety (can you minimise the risk to people?)
  3. Economic (will the cost of the electricity make it worthwhile?)
  4. Environmental (will people allow it to be built here?).

Nuclear power plants are built away from urban areas just in case there is a major accident.  Chernobyl has so far been the only one.  They basically don’t increase the radioactivity of their surroundings.  But they can’t be too far away from where people live otherwise too much energy is lost as heat in the power lines.

Balancing costs and benefits

The decision to build a nuclear power plant depends on a careful balancing of costs and benefits.

For example:

Fuel pellets last for a few months

New fuel pellets are hardly radioactive at all because uranium isotopes have very long half-lives.

In the reactor the uranium nuclei fission into smaller nuclei.  These have nowhere to go and stay in the fuel pellets.  Eventually, after a few months, the fission products ‘poison’ the uranium-235 chain reaction.

The whole fuel assembly is removed for the pellets to be reprocessed.  We’ll look at this in a minute.

There are two processes that cause radioactive nuclear waste:

Nuclear waste: fission products: beta emitters, short half-lives

The first is the fission of the uranium-235 nuclei into smaller nuclei.  This is what we want to happen because it releases energy.  These smaller nuclei (called fission products) are almost always radioactive.

The fission products tend to be beta emitters.  This is because they have ‘too many’ neutrons to be stable.  Neutrons change to protons.  They sit at the top of short decay chains.  Many of these smaller nuclei have half-lives of the order of hours or days.  Some have half-lives of decades and a tiny proportion of the order of thousands of years.

Nuclear waste: transuranic elements: alpha emitters, some with very long half-lives

The second is the production of heavy nuclei of the tranuranic elements.  This normally happens when a uranium-238 nucleus absorbs one or more neutrons.  One heavy nucleus is plutonium-239.  This is also fissionable and contributes up to 30% of the energy released as the fuel rods age.

The transuranic nuclei, like neptunium, protactinium and plutonium are alpha emitters.  This is because the nuclei are too big to be stable.  They sit at the top of long decay chains.  Their half-lives can be of the order of tens or hundreds of thousands of years.

New and used fuel pellets are still mostly unused uranium-238

The fuel pellets started off with roughly 96% uranium-238 enriched with 4% uranium-235.  It is the uranium-235 that fissions.

Almost all of the uranium-238 is left intact.  Some has absorbed a neutron and ended up as plutonium-239.  About 1% typically remains.

About three quarters of the 4% of uranium-235 will have fissioned.  This means 1% of the fuel pellet will be uranium-235 and 3% will be fission products.

High-level nuclear waste: very radioactive but short half-life

So how do we deal with the used fuel rods after they’ve been taken out of the reactor?

The fuel rods are stored underwater to keep them cool.  Their radiation causes the blue glow in the water.  The water also shields nearby workers from the radiation.

The fission products in the fuel rods are very radioactive.  This means the fuel rods still produce about 10% of the heat energy they did in the reactor.  Because the radioactivity is high enough to produce significant heat the used fuel rods are classified as ‘high-level’ waste.

High radioactivity means short half-life.  After one day the radioactivity of the fuel rods has dropped to 10% of its initial value.  After about a year it’s down to 1%.

Reprocessing the fuel pellets

After five years the radioactivity has dropped enough for the fuel rods to be broken open and the fuel pellets treated.

The fuel pellets are dissolved in acid and then chemical reactions are used to separate out the different isotopes.  Uranium makes up about 96% of the spent fuel.

This can be converted back into uranium hexafluoride, enriched and then re-used.  This is called ‘reprocessing’.

The plutonium-239 is also fissionable.  It can be mixed with the enriched uranium to produce mixed oxide fuel or ‘MOX’.  The plutonium-239 can also be used to make nuclear weapons, which is another objection to nuclear power.

So only around 3% of the spent fuel consists of isotopes that can’t easily be reprocessed.  These are some transuranic elements and fission products.  They have half-lives of around one year to hundreds of thousands of years.

Intermediate-level waste

This is now classified as intermediate level waste.  This is because nearby people still need to be shielded from the radiation but there is not enough heat generated to need cooling.

Some countries don’t reprocess their waste at present so the waste may consist of uranium and plutonium as well.

The health risks of plutonium-239

There is a great deal of public concern about the risk of plutonium-239 and other long-lived isotopes.  The risks are certainly there but some claims are not scientific.

For example, this statement taken from a national newspaper is misleading:  'A tiny speck of the fine powder [plutonium-239] can cause lung cancer in anyone who inhales it.'

Any single alpha or beta particle ‘can’ cause cancer but no amount will definitely cause cancer.  Cancer is completely random.  It is only scientific to talk about how much cancer risk increases with increased dose.

It’s important to remember the relationship between how radioactive an isotope is and how long it will be around for.  For example this statement taken from another national newspaper is contradictory: 'Plutonium-239 is highly radioactive and will be dangerous for hundreds of thousands of years.'

The longer the half-life the lower the radioactivity.  Atom for atom, long-lived isotopes have low radioactivity.  But if you have a lot in one place the radioactivity is still high.

The danger is one of contamination rather than irradiation.  In practice plutonium oxide dust does not stay airborne for long and is less soluble in water than sand.

Long-term burial of nuclear waste

But different elements behave in different ways.  So we want to reduce the risk of contaminating the environment if we can.  It’s easier to keep track of everything if all the nuclear waste stays concentrated in one place.

The first step is to mix the radioactive waste with special molten glass.  This is called ‘vitrification’.  When the glass cools it can’t be dissolved by water and is very hard and difficult to make into dust.

To protect the glass from chipping, it’s surrounded by a metal casing.  The canisters are then buried in carefully constructed caverns up to 500 m underground.  The caverns are filled in for stability.

Many scientists believe that this would pose a very low risk to the environment.

However no government has yet approved an actual plan to bury the encased glass, so all nuclear waste remains above ground.

Low-level nuclear waste: very low radioactivity

Low level nuclear waste like clothing and rags is disposed of in special landfill sites.

The main arguments against nuclear power

There are three main areas where many environmentalists argue that nuclear power poses dangers that make it unacceptable.

The first fear is that accidents at nuclear power plants are quite likely.  The second is that there could be a leak while transporting or storing nuclear waste.  The third is that nuclear waste buried underground could eventually contaminate the environment.

We have discussed at length how people evaluate risks like this in lesson 6.

Back to Summary of Radioactivity and Atomic Physics Explained