A2 – nuclear power Garfield Graphic with kind permission from PAWS Inc – All rights reserved.

Terminology to be aware of! Induced fission – fission we make happen by firing a neutron at a nucleus. It is the splitting of nucleus into two smaller nuclei (called fission fragments NOT daughter nuclei) brought about by bombardment of the nucleus by a neutron. This makes the nucleus split into two fragments and release more neutrons which can go on to produce more fissions. Spontaneous fission occurs without our interference.

Terminology to be aware of! In enriched uranium the proportion of U-235 is greater than in natural uranium.

Terminology to be aware of! Thermal neutrons have low energies or speeds (e.g. 0.03 eV) Neutrons from fission are fast (high energy) neutrons (e.g. 2 MeV)

Terminology to be aware of! A self sustaining chain reaction occurs when a fission reaction gives out neutrons that go on to cause further fissions For this to happen there must be a critical mass of fuel.

Control Rods Control of the reaction involves limiting number of neutrons that go on to produce fission. The excess neutrons can be absorbed by control rods (e.g. boron, cadmium). Control rods are inserted into reactor to slow reaction rate (and vice-versa)

Control Rods Control rods can be used to keep the neutron flux [or power or reaction rate] constant by raising or lowering control rods so that one neutron per fission goes on to produce another fission.

Control Rods As each fission produces two or three neutrons on average and some neutrons escape or are absorbed by U-238, or fission fragments without fission occurring the level has to be found by observing output. This is done using electronic monitoring equipment together with a computer program. They can be rapidly dropped deep into the reaction vessel in emergency to stop the reaction completely.

Moderation Neutrons from fission are fast (high energy) neutrons (e.g. 2 MeV) As fission is most probable with bombardment of low energy neutrons moderation involves slowing down neutrons. This is done by collision with moderator atoms (e.g. graphite or water) A large number of collisions required (roughly 50). The collisions are elastic – all the kinetic energy is transferred to atoms The moderator must not absorb many neutrons.

Why does the moderator get hot? Fission fragments repel (same charge) when formed and collide with other atoms in fuel rod so moderator atoms gain Ek due to collisions (and vibrate more) high energy fission neutrons enter moderator [or collide with moderator atoms] temperature depends on the average Ek of (vibrating) atoms – therefore it gets hotter!

Why is the fuel in separate pellets rather than a big lump? Neutrons need to pass through a moderator to slow them (in order to cause further fissions or prevent U-238 absorbing them) – moderator can be between fuel rods (line of pellets) Neutrons that leave the fuel rod (and pass through the moderator) are unlikely to re-enter the same fuel rod – but can enter another one (or need to be absorbed by the concrete shielding. It makes it easier to replace the fuel in stages and handle it.

Fuel Rods As time progresses fuel rods become less effective for power production because: - amount of (fissionable) uranium (235) in fuel decreases - fission fragments absorb neutrons – but no fission then occurs from them

Fuel Rods Fuel rods become more dangerous to handlers as time progresses because: - fission fragments are radioactive or unstable (emit a, b and g) and - some fission fragments have short half-lives resulting in higher activities than the parent had.

Spent Fuel Rods They are removed from the reaction vessel by remote control and placed in cooling ponds for several months to allow short half life (most active) isotopes to decay. The remaining long half life ones then have to be dealt with.

Spent Fuel Rods Transport precautions have to be taken e.g. impact resistant flasks – slow moving lorries – hazard marking etc. The unspent fuel is then extracted by chemical separation of uranium from the active wastes. High level waste stored (as liquid) and then undergoes vitrification

Spent Fuel Rods High level waste stored (as liquid) and then undergoes vitrification (converting the waste into a large glass block)

Spent Fuel Rods A vitrification plant at Sellafield enables reprocessed high level waste to be solidified in glass blocks, sealed in stainless steel cans and placed in a carefully controlled storage facility. (Must take storage precautions, e.g. use shielded tanks or monitor regularly for leaks) They can be put in ponds to cool and then finally be buried deep underground geologically stable site.