Nuclear Power and Safety

Nuclear Power and Safety

Info: Board Coverage AQA Paper 3 (Section B) | Edexcel CP6 | OCR (A) Paper 2 | CIE P4

the induced fission of uranium-235 releases about 200 MeV per event. A reactor is an engineering arrangement for releasing that energy at a controlled, steady rate and turning it into electricity. Every component in the core exists to solve one of three problems: keep the chain reaction going, slow the neutrons down, or absorb the excess.

1. The Fission Chain Reaction

A thermal neutron absorbed by ${}^{235}_{92}\mathrm{U}$ produces the unstable ${}^{236}_{92}\mathrm{U}$, which splits into two unequal fragments plus two or three fresh neutrons:

Each released neutron can induce further fissions. If, on average, exactly one neutron from each fission goes on to cause another fission, the reaction is critical: steady power. Fewer than one: the reaction dies out (subcritical). More than one: it grows exponentially (supercritical) — fine inside a bomb, catastrophic in a power station.

2. The Moderator

Fission neutrons emerge at about 2 MeV (“fast” neutrons). Uranium-235 fissions much more readily when hit by slow, thermal neutrons (around 1-2 eV), so the neutrons must be slowed between fissions. The moderator does this by elastic collisions.

• In thermal reactors (e.g. the AGR and PWR) the moderator is water or graphite.
• The best moderator has a low mass number: a neutron loses the largest fraction of its kinetic energy in a head-on collision with a particle of comparable mass. This is why ${}^{1}_{1}\mathrm{H}$ (in water) and ${}^{12}_{6}\mathrm{C}$ (graphite) are used, and why deuterium (${}^{2}_{1}\mathrm{H}$) is slightly worse but still used in heavy-water reactors.
• A moderator must not absorb neutrons appreciably.

In about 25 collisions with water a 2 MeV neutron reaches thermal energies. A fast-breeder reactor, by contrast, has no moderator: it relies on fast neutrons converting ${}^{238}_{92}\mathrm{U}$ into plutonium-239 faster than the plutonium is consumed.

3. Control Rods and Criticality

Control rods (boron or cadmium) are raised and lowered into the core to absorb neutrons. Their job is to make the reactor critical rather than supercritical: rod depth is adjusted so the effective neutron multiplication factor $k$ sits at $k = 1$ at steady power.

Two facts examiners reward:

• Control rods are moveable and rapidly adjustable; the moderator is fixed; the coolant circulates. Confusing these three roles is the single most common marking-point loss.
• Dropping the rods fully (SCRAM) shuts the fission chain reaction down in seconds. The reactor still produces decay heat afterwards, which is why cooling must continue.

The fuel is enriched to roughly 3-4 % ${}^{235}\mathrm{U}$ — far below weapons-grade — precisely so that it cannot explode like a bomb; a bomb requires near-pure ${}^{235}\mathrm{U}$ assembled supercritical almost instantly.

4. The Coolant

The coolant carries the fission energy (mostly the kinetic energy of the fragments) from the core to the boilers. Requirements: high specific heat capacity, chemically stable at high temperature, and low neutron absorption.

• PWR: ordinary water acts as both moderator and coolant in one loop, kept at about 150 atmospheres so it stays liquid at 320 °C.
• Gas-cooled reactors: carbon dioxide gas through the core.
• Liquid sodium coolant in fast reactors.

A steam generator transfers energy from the primary (radioactive) loop to a secondary loop; the steam drives turbines, and the reactor is essentially a way of boiling water.

5. Safety Systems and Shielding

• Shielding: several metres of concrete surround the core, absorbing neutrons and gamma radiation. The fuel stays inside the shielding while in use; the coolant loop may be inside or outside depending on design.
• Emergency cooling: independent backup systems inject water if the primary coolant loop fails. Loss-of-coolant accidents (LOCA) are the design-basis concern, because decay heat continues after shutdown.
• Reactor containment: the building is designed to contain steam and any released radioactivity.

6. Radioactive Waste

Spent fuel rods contain fission products with a huge range of half-lives, plus unreprocessed ${}^{235}\mathrm{U}$ and bred ${}^{239}\mathrm{Pu}$.

• High-level waste (spent fuel, fission products): thermally hot and intensely radioactive. Stored in cooling ponds for months to years while short-lived isotopes decay, then vitrified (locked in glass) for long-term storage deep underground.
• Intermediate-level waste (reactor components, chemical sludges): sealed in concrete or resin in steel drums.
• Low-level waste (gloves, clothing, filters): compacted and buried in shallow lined repositories.

The activity of stored waste falls with the half-lives of its isotopes; show you can use $A = A_0 e^{-\lambda t}$ with $\lambda = \frac{\ln 2}{t_{1/2}}$ to justify how long a category of waste must be stored before handling becomes safe.

7. Common Pitfalls

• The moderator slows neutrons; control rods absorb them. Swapping these two roles loses both marks.
• “The water prevents the reactor melting” is not the moderator’s job — the coolant removes thermal energy; the moderator slows neutrons.
• Neutrons are slowed by elastic collisions with nuclei, not by “friction” with the water.
• Enrichment makes the fuel economical to use, not safe to explode: a civil reactor cannot detonate like a fission weapon.
• Activity after shutdown: the chain reaction stops within seconds, but decay heat continues for years.

8. Summary

Cross-References

• Nuclear Physics — fission energetics, mass defect and the decay law
• Particle Physics — quarks and the standard model
• Thermal Energy Transfer — coolant heat transfer mechanisms