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THE NUCLEAR FISSION REACTOR

The possibility of producing large amounts of energy from nuclear fission lead to the construction of the worlds first nuclear reactor in a squash court in Chicago in 1942. Since then many commercial nuclear power stations have been built around the world. The neutrons produced in a chain reaction are moving too fast to cause further fission in U235 nuclei and they have to be slowed down. This is done by graphite or heavy water and these materials are called moderators. As the neutrons collide with atoms of the moderator they slow down from 106 ms-1 to 104 ms-1 and at this speed they are known as thermal neutrons. Rods of boron steel are used as control rods to control the rate of the reaction since they gobble up neutrons without fissioning. Lowering them into the reactor core will slow down the reaction. They are held on electromagnetic clamps so that if there is a dangerous increase in core temperature they can be dropped into the reactor and so shut down the chain reaction.

A nuclear reactor, in fact any device using a nuclear chain reaction needs a minimum amount of fuel called the critical mass (about 15 kg for pure U 235). This is the mass of pure uranium 235 needed to sustain a chain reaction. Anything less than this and the loss of neutrons from the surface will be too great and the chain reaction will stop. Masses below this are called subcritical while those greater than this are known as supercritical. The exact value of the mass needed depends on the shape and purity of the sample as well as the material.

Problem
1. Why is graphite or heavy water used as a moderator? HINT: What makes the size of these atoms suitable for slowing down neutrons.

DEVELOPMENT OF NUCLEAR POWER REACTORS IN BRITAIN

(a) the thermal reactor (Magnox)
In Britain there are two types of reactor in service: (b) the advanced gas cooled reactor (AGR)
(c) pressurised water reactor (PWR)

Thermal (Magnox) Reactors

A typical reactor of this type was built at Hinkley Point in Somerset in 1965, it is now called Hinkley A.

There are two reactors, each reactor core contains 355x103 kg of uranium fuel (99.3% U238, 0.7% U235) in 36 000 fuel rods, each one a metre long, packed in magnesium alloy (magnox) cans, placed eight high in 4500 channels in the 1891 tonnes of graphite moderator. The magnesium alloy is used because it has a low neutron absorption. The core (14m high and 8 m diameter) is contained in a 20m diameter, 7.62 cm thick steel pressure vessel and heat is drawn off by carbon dioxide gas at 1.28x106 Pa(185 p.s.i) which is blown through the core. The temperature of this gas rises from 180oC to 360o as it passes through the core. This is used to turn water into steam in heat exchangers and the steam drives turbines giving an output of 215 MW of electrical power to the national grid.
(many of these older reactors are being decommissioned).

Advanced Gas Cooled Reactors

(Hinkley B Station)

This has a pressure vessel 18.9m diameter, 19m high, made of 5m thick pre-stressed concrete lined with 16mm steel containing a core that gives 1500 MW of power output.
The fuel is 114.2 x 103 kg of slightly enriched uranium oxide in fuel elements placed eight high in 308 channels in the 1384 metric tons of graphite moderator. The coolant is still carbon dioxide but this time it is blown through the core at a greater pressure (3.97x106 Pa (576 p.s.i)) and with a flow rate of 3790 kgs-1.
The inlet temperature of the gas is 284oC and this rises to an outlet temperature of 630oC when it leaves the core.


Pressurised Water Reactors

(Sizewell B - Suffolk)
These reactors were developed mainly in the USA and the Soviet Union and have a much smaller core than either the Magnox or AGR types. The other main difference is that the reactor uses light water as both a moderator and coolant.

Fast reactors

These reactors use fast neutrons, natural uranium enriched with 20% plutonium as the fuel and liquid sodium as the coolant. The liquid sodium can be circulated with electromagnetic pumps.

Summary of properties of a variety of nuclear fission reactors


Type Fuel (%U235) Moderator Coolant Temperature (oC Efficiency (%) Pressure (psi)
Magnox 0.7 graphite carbon dioxide 400 31 300
AGR 2.3 graphite carbon dioxide 650 42 600
PWR 3.2 water water 324 32 2250
Fast 20 Pu none sodium 620 44 5
BWR 2.6 water water 386 32 1050
RBMK 2 water water 284 31 1000
SGHWR 2.24 heavy water water 272 32 900

Commercial problems associated with the exploitation of nuclear fission

There are various problems associated with the commercial use of nuclear reactors.
(a) Disposal of nuclear waste
transport facilities environmental aspects
industry and commerce hard rock site
centres of population

(b) Decommissioning a nuclear reactor - what do we do with it after it has ended its useful life?

Nuclear waste

In Britain low level waste (LLW) such as gloves, cast off clothing, over shoes etc. is packed into 200 litre steel containers about the size of a kitchen fridge - squashed to 0.2 of their original volume and then packed into 3 cubic metre boxes

The intermediate level waste (ILW) such as fuel containers is packed in 500 litre steel drums.
In future ILW from decommissioning will be stored in 12 cubic metre steel boxes - the spaces between items packed with concrete
ILW is now stored, about 50 000 cubic metres currently

Typical amount of waste in store in the UK by the year 2030
LLW 1.5x106 cubic metres
ILW 0.25 x 106 cubic metres

Table of the contents of ILW and HLW to be disposed of by NIREX by 2030.

Isotope Low level waste (Bq) High level waste (Bq)
Hydrogen 3 0.3x1014 6.8x1015
Carbon 14 4.8x1013 6.6x1015
Calcium 41 5.8x1011 1.3x1012
Cobalt 60 3.7x1013 2.9x1018
Nickel 59 6.0x1010 2.9x1018
Caesium 137 2.9x1013 2.3x1018
Thorium 232 3.7x109 3.0x1012
Uranium 238 2.5x1013 2.0x1014
Plutonium 239 2.9x1012 1.7x1016
Americium 241 2.8x1012 5.0x1016

The most radioactive is the high level waste (HLW), and the government has decided to store it for 50 years before it is disposed of. HLW is the responsibility of UKAEA and BNFL and is mainly irradiated fuel taken from reactors. Fuel has been reprocessed to take out uranium and plutonium but it still leaves a very toxic waste behind. This is being converted into glass blocks for storage. Typically 1000 times more radioactive than ILW, about 100 cubic metres of HLW is produced every year.

Nuclear power and the Greenhouse effect - it has been calculated that if nuclear power were expanded CO2 emissions could be reduced by up to 30%, thus lowering global warming by 15%.


In 2011 the future of nuclear power is being re-examined by the UK government and a new rector is propsed for the Hinkley Point site.
 
 
 
© Keith Gibbs 2011