6.4- Nuclear Fission & Fusion

nuclear reactions (fission, fusion, radioactive decay) can be a source of…

energy (used for electricity, transport, etc)

advantages of nuclear power

• produces no polluting gases e.g. carbon dioxide

• require far less fuel than fossil fuels (produce more energy)

• highly reliable (unlike some renewable resources)

disadvantages of nuclear power

• produces radioactive waste which is very dangerous and expensive to deal with

• risk: in the event of an accident e.g. a nuclear meltdown such as Chernobyl, can have catastrophic consequences on the environment and to the people in the surrounding area

• public perception is generally negative, people are very wary of nuclear power

• expensive to set up + shut down

nuclear fission

the splitting of a large unstable nucleus into two smaller nuclei

a neutron collides with an unstable nucleus, it splits, and emits 2 or 3 neutrons + gamma rays

the process behind nuclear power stations (usually uses Uranium-235 and Plutonium-239)

all products of nuclear fission are… so have to be…

RADIOACTIVE so have to be stored underground until they aren’t any more

fission of Uranium-235

used as a fuel in nuclear reactors

slow neutrons absorbed by nucleus (fast-moving e- just pass through), becoming an unstable U-236 nucleus

this splits into 2 smaller nuclei and emits 2 or 3 neutrons, releasing energy

products move away at high speed (high KE)

nuclear chain reaction

• only 1 extra neutron is needed to induce U-235 fission

• each fission produces 2 or 3 neutrons which move away at high speed

• each neutron produced can start another fission reaction, again creating further excess neutrons, which go on to induce further fission reactions

control rods (purpose in controlling nuclear chain reactions in a reactor)

Purpose: absorb neutrons

made of a material that absorbs neutrons without itself becoming unstable

number of neutrons controlled by varying depth of control rods in the fuel (lowering = decreased rate of fission, more neutrons absorbed)

adjusted automatically so only 1 fission neutron goes on to cause another fission

control rods lowered completely to shut down reactor

moderator (purpose in controlling nuclear chain reactions in a reactor)

purpose: slow down neutrons

material that surrounds fuel rods + control rods in core

fast moving neutrons produced by fission reactions slow down by colliding with the molecules of the moderator

slowed neutrons are in thermal equilibrium with moderator, ensuring they can react efficiently with U-235 fuel

how is thermal energy from chain reactions used to generate electricity in nuclear power stations?

1. KE from nuclear fission produces a large quantity of heat which is carried away from reactor by coolant

2. coolant used to heat a water source + turn it into steam (separate source used to prevent contamination)

3. steam turns turbines which turn generators, producing electricity

nuclear fusion

two light nuclei join to form a heavier nucleus

requires extremely high temps to maintain, mostly just occurs in stars, which produce energy from it

produces a huge amount of energy which comes from a small loss of mass, as mass is conserved as energy (according to e=mc2)

conditions for nuclear fusion

very high temp of fuel (millions of degrees C)

very high KE of nuclei to overcome repulsion

very high density/pressure to increase possibility of suitable collisions

why does nuclear fusion not occur at lower temps?

fusion requires the fusion of 2 nuclei, which contain protons (no e-) so are positively charged

protons repel each other as they are both +vely charged

very high KE (therefore extremely high temperature) required to overcome repulsion

why is fusion not used as a power source on Earth?

• it is extremely difficult to achieve & maintain high temps + pressures needed for fusion

• where these conditions have been achieved, they are hard to maintain, so only a small rate of fusion is produced

• this is not useful for current energy needs

Fusion vs Fission

Fission is process of breaking apart

Fusion is process of joining together

fission produces energy in nuclear reactors

fusion occurs in the cores of stars

fission uses large nuclei e.g. Uranium, produces smaller

fusion uses small nuclei e.g. Hydrogen, produces larger

fission provides less energy per kg of fuel than fusion

fission products are radioactive + very dangerous

fusion products are not radioactive + safer

fission requires high temps + neutron to induce

fusion requires extremely high temps