P2 NUCLEAR FISSION

NUCLEAR FISSION P2HRadioactive isotopes emit small particles from their nuclei which only change the mass and atomic numbers slightly.

22286Rn

21884Po

42α+

Alpha decay:Mass - 4, Atomic - 2

Neutrons - 2, Protons – 2

87

136

137

134

135

858483 86

N

P

α

ALPHA2+

Beta minus (electron) Beta plus (positron)

41

52

53

50

51

393837 40

N

P

β־

-

Beta - decay:Mass no change, Atomic +1

Neutrons - 1, Protons +1

90

38Sr

90

39Y+ 0

-1β

15

12

13

10

11

131211 14

N

P

β+

24

13Al

24

12Mg + 0

+1β

+

Beta + decay:Mass no change, Atomic -1

Neutrons +1, Protons -1

Rather than emitting small particles, nuclear FISSION involves a nucleus SPLITTING into two LARGE FRAGMENTS.

Although some isotopes do fission by themselves, most need to be “induced” to split by absorbing another particle, usually a neutron.

The isotopes most used in nuclear reactors are Uranium-235 and Plutonium-239

FISSION OF 235U

235U

Fast neutron

Fast neutrons pass through the uranium nucleus, without causing it to split

Slow neutron

Slow neutrons are absorbed by the 235U nucleus, becoming an unstable 236U nucleus which then splits into two smaller nuclei and three FAST neutrons

14456 Ba +

9036 Kr

10 n3+

23692 U

23592 U

10 n+

The nucleus can fission in more than one way

14857 La +

8535 Br

10 n3+

23692 U

23592 U

10 n+

Reactants Products

The PROUCTS have a mass about 0.088% LESS than the REACTANTS. This ‘missing’ mass or MASS DEFECT is converted into the KINETIC ENERGY of the reactant particles. On a large scale this appears as HEAT.

Some mass

‘missing’

“Missing” mass

converted to ENERGY

The amount of energy E produced from the mass defect m

is given by Einstein’s famous equation E = mc2

Where c = speed of light (3 x 108 or 300 million m/s).

If 1kg of U-235 atoms ALL split then there would be about 0.88g of mass ‘missing’ which would be converted into 8.19 x 1013 J (82 trillion J), enough to run a 2 bar electric fire for about 1300 years!

Chain ReactionIf the FAST neutrons produced in the reaction could be slowed down then they could be used to induce further fission in other U-235 atoms.

Graphite (CARBON) MODERATOR Collision with C atoms slows down the fast neutrons

KE OF NUCLEI RISES

RAPIDLY SO RAPID

TEMPERATURE RISE

CRITICAL MASSTo keep a chain reaction going, there must be enough uranium to prevent too many neutrons escaping from the sides. The mass of uranium needed to keep the reaction going is called the CRITICAL MASS.

Too many neutrons escape without colliding with U atoms, reaction slows down Neutrons have much higher

chance of colliding with U atoms so reaction keeps going – uranium stays HOT

CONTROLLED & UNCONTROLLED FISSION

UNCONTROLLED CHAIN REACTION

If the mass of uranium is too large, the rate of the fission reaction increases very rapidly, the temperature rises by thousands of degrees in a fraction of a second and a NUCLEAR EXPLOSION results.

CONTROLLED CHAIN REACTION

If the mass of uranium is a little lower and a material is used to ABSORB EXCESS NEUTRONS, then the rate of fission can be CONTROLLED.

This means the uranium can be kept hot (few hundred degrees) – for years!

“everlasting coal”

What is needed to make use of the energy from nuclear fission?

Uranium-235. Natural uranium from mines is mainly U-238 and so must be ENRICHED to raise the % of U-235

1. FUEL RODS – enriched U-235

2.CONTROL RODS (BORON) – ABSORB NEUTRONS.Lowered – fuel rods cool downRaised – fuel rods heat up

3.MODERATOR (CARBON) to slow down fast neutrons so they can keep the reaction going

4.Some means of TRANSFERRING the HEAT generated into ELECTRICTY

REACTOR CORE

Schematic Nuclear Reactor (MAGNOX type)

THICK CONCRETE BIOSHIELDING NEEDED

The PRIMARY COOLANT (which carries heat from the core) can be GAS or LIQUID. It becomes RADIOACTIVE

WATER is usually used as the SECONDARY COOLANT. The steam generated turns turbines. It is not radioactive.

This UNFILLED CORE is made from GRAPHITE

FUEL RODS (U) CONTROL RODS (B)

An operating reactor with a water moderator

Nuclear Energy Issues 1: Solution to Energy Shortage

Enriched uranium releases millions of times the energy released by the same mass of fossil fuel.

Energy is released by the same fuel over a long time period.

If the reactor is properly contained and shielded there are no emissions into the environment.

Nuclear Energy can make a major contribution to supplying our electricity without the emission of GREENHOUSE GASES

Nuclear Energy Issues 2: Cost

Due to the need for:

•containment of radiation

• safety systems with multiple ‘back ups’

•security against terrorism

nuclear power stations are very expensive to build, maintain and ‘decommission’

Nuclear Energy Issues 3: Waste

Uranium-238 & 235 in fuel are low level alpha emitters

The fission products AND their decay products contain many intense short and long half life α,β,γ emitters

Spent fuel rods comprise HIGH LEVEL WASTE

When a reactor is ‘decomissioned’, the coolant, pipework, reactor vessel materials comprise MEDIUM and LOW LEVEL WASTE

Nuclear Energy Issues 3: Waste (ct’d)

Spent fuel rods are stored under water for about 5 years for the most intense radiation to decay, before being converted into ‘glass’ and encased in concrete and steel containers

Low level waste is packed into steel drums and stored on the power station site

Nuclear Energy Issues 3: Waste (ct’d)

Nuclear waste remains radioactive for thousands of years. There is no treatment, chemical or physical, that can reduce the radioactivity or speed up the decay process.

It is planned to store the waste UNDERGROUND long term. To date, very few storage sites have been built, due to expense and local opposition.

Nuclear Energy Issues 4: Accidents

Although the POSSIBILITY of an accident is very SMALL, the potential DAMAGE an accident could cause is very LARGE.

There is NO possibility of a nuclear explosion (due to the smaller % of U-235 in a reactor compared to a bomb). The main danger is LEAKAGE OF RADIOACTIVE ISOTOPES INTO THE ENVIRONMENT.

Leakage of radioactive steam from a Japanese reactor following an earthquake.

Nuclear Energy Issues 4: Accidents

The most serious nuclear accident was in 1986 at Chernobyl in Ukraine.

“The disaster occurred at reactor number four during an unauthorized systems test. A extreme power output surge took place, which led to a reactor vessel rupture and a series of explosions.

This exposed the graphite moderator components to the air and they ignited. The resulting fire sent a plume of radioactive fallout into the atmosphere over large parts of the western Soviet Union, and much of Europe.

As of December 2000, 350,400 people had been evacuated and resettled from the most severely contaminated areas of Belarus, Russia, and Ukraine” Wikipedia

Nuclear Energy Issues 4: Accidents

Distribution of Caesium-137 fallout over Europe a few weeks after the Chernobyl incident The city of Pripyat was evacuated

and is still abandoned

Nuclear Energy Issues 5: Proliferation

Both Uranium-235 and Plutonium-239 can be used to make nuclear weapons. At present, only a small number of governments have nuclear weapons, but the number may be growing. The U-235

bomb dropped on Hiroshima in 1945