Nuclear reactor: parts and working
A reactor keeps a controlled chain reaction going, and its heat makes steam to spin a turbine. Moderator slows neutrons; control rods absorb them; coolant carries heat away.
Parts of a nuclear reactor and their jobs
| Part | Made of | Job |
|---|---|---|
| Fuel rods | Uranium (²³⁵U, natural or enriched) or plutonium-239 | Undergo fission and release energy |
| Moderator | Heavy water, graphite or ordinary water | Slows fast fission neutrons so they can split ²³⁵U easily |
| Control rods | Cadmium or boron | Absorb extra neutrons; pushed in to slow the reaction, pulled out to speed it up |
| Coolant | Water or heavy water (liquid sodium in fast reactors) | Carries heat from the core to the heat exchanger |
| Reflector | A layer such as graphite round the core | Sends escaping neutrons back into the core |
| Pressure vessel | Thick stainless steel | Holds the core and coolant under pressure |
| Shield | Thick concrete walls | Protects workers from radiation |
| Reactor building | Air-tight building kept slightly below atmospheric pressure | Stops any radioactive air from leaking out |
How a nuclear power station works
- 1In the core, a controlled chain reaction splits ²³⁵U nuclei and releases heat.
- 2The moderator slows the neutrons so the chain reaction continues; the control rods keep it steady.
- 3A coolant is pumped through the core and becomes very hot.
- 4In a heat exchanger, the hot coolant boils a second supply of water into high-pressure steam.
- 5The steam spins a turbine joined to a generator, which produces electricity.
- 6★The steam is cooled in a condenser and pumped back. It works like a coal power station, but the heat comes from fission.
★ key step
Thermal and fast reactors
Worked example: fuel used by a reactor
A reactor produces 300 MW of heat from the fission of ²³⁵U, each fission releasing 200 MeV. Find (a) the number of fissions per second, (b) the mass of ²³⁵U used in one day. (1 MeV = 1.6 × 10⁻¹³ J, Avogadro number = 6.02 × 10²³ per mol)
- Energy per fission = 200 × 1.6 × 10⁻¹³ J = 3.2 × 10⁻¹¹ J.
- (a) Fissions per second = 300 × 10⁶ J/s ÷ 3.2 × 10⁻¹¹ J = 9.4 × 10¹⁸.
- (b) Fissions in one day = 9.4 × 10¹⁸ × 86400 = 8.1 × 10²³.
- Moles of ²³⁵U = 8.1 × 10²³ ÷ 6.02 × 10²³ = 1.345 mol.
- Mass = 1.345 mol × 235 g/mol ≈ 316 g.
(a) about 9.4 × 10¹⁸ fissions per second; (b) about 316 g, roughly a third of a kilogram, each day
How to draw a nuclear power reactor
- Draw a large box labelled reactor core inside a thicker outline labelled concrete shield.
- Inside the core, draw vertical rods labelled fuel rods (uranium) in a shaded region labelled moderator, and rods coming down from the top labelled control rods.
- Draw a pipe carrying hot coolant from the core to a box labelled heat exchanger, and a pipe with a pump bringing cool coolant back.
- From the heat exchanger, draw steam going to a turbine joined to a generator.
- Draw the steam passing on to a condenser, and a pump returning the water to the heat exchanger.
India follows Dr Homi Bhabha's three-stage plan. Heavy water reactors burn natural uranium and make plutonium, and fast breeder reactors then use the plutonium. Later reactors will use India's large reserves of thorium.
5 marks: “Describe the main parts of a nuclear reactor with a labelled diagram and state the function of each.” Fuel rods (²³⁵U) undergo fission; moderator (heavy water or graphite) slows neutrons; control rods (cadmium or boron) absorb neutrons; coolant carries heat to a heat exchanger that makes steam for a turbine; reflector, steel pressure vessel, concrete shield and air-tight building for safety. Diagram: core, coolant loop, heat exchanger, turbine and generator.
2 marks: “Why does a nuclear reactor need (a) a moderator and (b) control rods?” (a) The moderator slows fast neutrons so that they cause fission of ²³⁵U. (b) Control rods absorb extra neutrons to keep the chain reaction steady, or to stop it.
1 marks: “Which type of reactor is mainly used in India to generate electricity?” The pressurised heavy water reactor (PHWR).
Fuel splits, Moderator slows, Rods control, Coolant carries.