The Thermal Nuclear Reactor

Nuclear Physics Lesson 15

In the exam you are expected to be able to describe:  Induced fission by thermal neutrons; Possibility of a chain reaction; Critical mass; Need for a moderator in thermal reactors; Control of the reaction rate; Factors influencing choice of material for moderator,

control rods and coolant; Examples of materials; Safety aspects concerning fuel used, shielding, emergency

shut-down, production, handling and disposal of active wastes;

Production of man-made nuclides and examples of their practical applications, e.g. in medical diagnosis

Induced Fission We saw in the last topic that fission rarely

occurs spontaneously.

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. 

Question

 What kind of nucleus undergoes fission?  

A large nucleus (P)  

Of which the  diameter is about the size of the range of the strong force. (P)     

Question

Why do the neutrons need to be slow?

If they are too fast, they will go through the nucleus (P)   And get beyond the attractive region of the strong nuclear force. (P)

Chain Reaction

We also saw in the last topic that the fission of Uranium nuclei results in a chain reaction. 

Fine if what you want is a violent explosion.

But what if you want a controlled reaction, how is that achieved?

Critical Mass

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

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 in side fuel rods.  These are canisters of stainless steel which have fins to transfer the heat.

Fuel rods contain about 2-3% U-235.

Nuclear Power StationNuclear Power Station

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:

Controlling the Reaction 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.

Important Components II

For an AGR (Advanced Gas Reactor) 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.

Pressurised Water Reactor (PWR)

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.

Table…

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.

The disadvantages are: 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.

Summary

Nuclear Power Uses a controlled chain reaction. Reactor needs a moderator, control

rods and a coolant. Gives off no polluting exhaust gases; Produces very dangerous waste

materials.

Physics WorkshopPhysics Workshop

Every Wednesday 3.40pm-4.40pm in O8Every Wednesday 3.40pm-4.40pm in O8

Is this time good for most people?Is this time good for most people?

Learning Objectives

Describe the range of safety features Describe the range of safety features associated with a nuclear reactor.associated with a nuclear reactor.

Derive the equation relating half life Derive the equation relating half life and the decay constant.and the decay constant.

Practice calculations on radioactive Practice calculations on radioactive decay. decay.

High Energy Electron Diffraction

Any 6 from:- A beam of high energy electrons is directed at a thin sheet of an

element and accelerated through a potential difference of about 108 volts

(MeV/high energy) A detector measures the number of electrons diffracted at a

number of different angles. Scattering effects occur due the charge of the nuclei and electron

and this causes the count rate of the beam of electrons to decrease as angle increases.

The electrons are also diffracted by the nuclei in the sheet which causes minima and maxima to observed in the final pattern

as long the de Broglie wavelength of the electrons is of the same order as the size of the nucleus, which is about 10-15 m.

The diameter of the nucleus can be calculated using the angle to the first minimum, θmin

and the wavelength of the incoming beam λ using the diffraction equation R sin θmin = 0.61λ.

Safety FeaturesSafety Features The reactor is a thick steel vessel designed to The reactor is a thick steel vessel designed to

withstand the high pressure and temperature in withstand the high pressure and temperature in the core.the core.

The core is in a building with very thick concrete The core is in a building with very thick concrete walls which absorb the neutrons and gamma walls which absorb the neutrons and gamma radiationradiation

Emergency shut down system – designed to Emergency shut down system – designed to insert the control rods fully into the core.insert the control rods fully into the core.

Sealed fuel rods are inserted and removed using Sealed fuel rods are inserted and removed using remote handling devices.remote handling devices.

Spent rods are more radioactive than before use.Spent rods are more radioactive than before use.

How to Remember?How to Remember?

C - Concrete building.C - Concrete building. R - Remote Handling.R - Remote Handling. E - Emergency shut down system.E - Emergency shut down system. S - Steel vessel for reactor core.S - Steel vessel for reactor core. S - Spent fuel rods much more S - Spent fuel rods much more

radioactive.radioactive.

Spells Spells CRESS, CRESS, helps us remember?helps us remember?

Dangers of Nuclear Power

Chernobyl (1986) Wanted to see if

coolant pumps would keep operating if there was a loss of power.

When they pushed control rods into reactor, caused loss of power and reversed direction of the rods!

AftermathAftermath

Overheating caused decomposition of water Overheating caused decomposition of water into hydrogen and oxygen which gases into hydrogen and oxygen which gases collected at the top of the vessel.collected at the top of the vessel.

Ignited and blew the lid off the reactor and Ignited and blew the lid off the reactor and turned the vessel on its side.turned the vessel on its side.

Nine tonnes of caesium-137 floated across Nine tonnes of caesium-137 floated across Europe along with many other tons of Europe along with many other tons of radioactive material. radioactive material.

Caesium-137 is water-soluble and extremely Caesium-137 is water-soluble and extremely toxic in minute amounts (half life 30 years). toxic in minute amounts (half life 30 years).

Okay let’s try itOkay let’s try it

Describe the range of safety features Describe the range of safety features associated with a nuclear reactor (5 associated with a nuclear reactor (5 marks).marks).