NUCLEAR POWER STATION — FULL PLANT

Pressurised water reactor · primary loop → heat exchanger → turbine → generator → grid

NCEA L2 PHYSICS · AS91172

100%

Plant schematic — energy transformations

primary loop (radioactive, sealed) steam (secondary loop, clean) feedwater / condensate cooling water (tertiary) electricity

Plant readouts

REACTOR THERMAL POWER0 MW

CORE TEMPERATURE290 °C

PRIMARY PRESSURE155 bar

STEAM TEMPERATURE250 °C

TURBINE SPEED0 rpm

GRID SYNCOFFLINE

ELECTRICAL OUTPUT0 MW

WASTE HEAT REJECTED0 MW

EFFICIENCY0 %

Control room

CONTROL ROD INSERTION25 %

PRIMARY COOLANT PUMP100 %

GRID LOAD DEMAND100 %

STARTING UP — rods partly withdrawn, chain reaction building. Watch the core heat the primary water.

Energy accounting — where does the nuclear energy go?

ELECTRICITY

WASTE HEAT → COOLING TOWER

Nuclear energy released: 0 MW Useful electrical output: 0 MW · rejected: 0 MW

Try this in class

Withdraw the rods (slide toward 0%) — more neutrons survive, fission rate climbs, core gets hotter, more electricity. Push them in past ~75% and the chain reaction stops.
Hit SCRAM at full power — fission stops within seconds, but watch the thermal power not fall to zero: that leftover is decay heat from radioactive fission products, and it is why a reactor must be cooled even after shutdown.
Cut the primary pump to 0% at full power — the core temperature spikes, but notice the reactor power falls on its own. That is the negative temperature coefficient: a hotter core is a less reactive core.
Drop grid load to 20% — less steam is drawn off, so less energy leaves the plant, and the reactor settles at lower power to match demand.
Watch the energy bar — only about a third of the nuclear energy ever becomes electricity. Ask students where the rest goes, and why it cannot simply be engineered away.

Achieved — describe the energy chain

In the reactor core, uranium-235 nuclei undergo fission and release energy. This heats water in the primary loop, which is kept under very high pressure (about 155 bar) so it stays liquid even at over 300 °C.

The hot primary water flows to a heat exchanger (steam generator). It passes its heat to separate water in the secondary loop, turning it into steam. The two loops never mix, so the radioactive water stays sealed inside the reactor building.

The steam pushes the blades of a turbine, making it spin. The turbine turns a generator, which produces electricity that goes out to the grid. Used steam is cooled back into water in the condenser and pumped around again.

The energy chain is: nuclear → thermal → kinetic → electrical.

Merit — explain the design

The heat exchanger exists for safety, not efficiency. Water in the primary loop becomes radioactive as it passes through the core, so it is kept in a sealed circuit inside containment. Only heat crosses into the secondary loop — the steam reaching the turbine hall is clean.

Control rods are made of neutron-absorbing material (boron or cadmium). Pushing them in absorbs more neutrons, so fewer are left to cause further fissions and the chain reaction slows. Pulling them out lets the reaction build. They control the rate of fission, and therefore the thermal power.

The condenser and cooling tower are not wasteful add-ons — a heat engine can only produce work if heat flows from a hot place to a cold place. The condenser provides that cold end, and rejecting heat there is what allows the turbine to work at all.

Excellence — discuss efficiency and safety

Only about one third of the energy released by fission reaches the grid as electricity. This is not poor engineering — it is a fundamental limit set by thermodynamics. The maximum possible efficiency of any heat engine depends on the temperatures it works between: η_max = 1 − T_cold/T_hot (in kelvin). With steam near 290 °C and a condenser near 30 °C, that ceiling is about 46%, and real friction and turbine losses bring it down near 33%. Raising the steam temperature would raise the efficiency, but the materials and pressures required set a practical limit.

The reactor is designed to be inherently stable. As the core heats, the water expands and moderates neutrons less effectively, so reactivity falls automatically — a negative temperature coefficient. You can see this in the simulator: cut the coolant pump and the core temperature rises, but the reactor power drops by itself rather than running away.

Shutdown is not the end of the problem. Fission products in the fuel keep decaying and releasing about 6–7% of full power immediately after a SCRAM, falling slowly over hours. Cooling must continue regardless, which is precisely what failed at Fukushima in 2011 when the reactors had shut down correctly but lost the power needed to keep pumping coolant.