1Full power on a stormy evening
The plant is making 3,411 megawatts of heat and sending about 1,150 megawatts of electricity to the grid. Its own pumps, about 50 megawatts of them, also run on power taken from the grid.
2The grid is lost
Lightning and wind trip the transmission lines, and the switchyard breakers open. The generator suddenly has no load, so the turbine stop valves slam shut within a fraction of a second to stop it overspeeding. The plant’s own electrical supply, fed from the same grid, goes dead: the coolant pumps, the main feed pumps and the condenser’s cooling-water pumps all lose power.
3Reactor trip: the rods drop
The protection system sees the turbine trip and the coolant pumps slowing, and cuts power to the drive mechanisms. Their grippers open and all 53 rod clusters fall into the core in about two seconds. The chain reaction dies away, but the fission products in the fuel keep decaying and heating it: about 7 % of full power just after the trip, still over 200 megawatts.
4The emergency diesels start
On the loss of voltage both emergency diesel generators start by themselves and reach full speed within about ten seconds. A sequencer then switches the safety loads onto them in steps: the safety cooling-water pumps, the auxiliary feed pumps, the pressuriser’s backup heaters and the control room. The batteries kept the instruments and controls alive in the gap.
5The coolant pumps coast down
The coolant pumps are not on the diesels: they need far more power than the diesels have. Their flywheels keep them spinning for a minute or two, so the flow fades gradually while the decay heat is already falling. As the coolant cools after the trip it shrinks, the pressuriser level drops and the pressure dips to about 136 bar before the backup heaters bring it back.
6Steam to the sky, water from the tank
With no cooling water the condenser is lost, so the steam from the steam generators is let out to the air through the atmospheric relief valves: the white plume over the steam line. It is clean steam from the secondary side. The auxiliary feed pumps, two on diesel power and one driven by a small steam turbine, refill the steam generators from the condensate storage tank.
7Natural circulation takes over
With the pumps stopped, the coolant keeps moving by itself. Water heated in the core becomes lighter and rises up the hot leg; it cools in the steam generator tubes, which stand higher than the core, becomes denser and sinks back down to the vessel. That density difference drives a flow of about 4 to 5 % of normal, enough for the decay heat. The temperature rise across the core grows to 15 to 20 degrees, but the water stays well below boiling.
8An hour on: still 45 megawatts of decay heat
An hour after the trip the decay heat is about 1.3 % of full power, enough to boil about 17 kilograms of feedwater a second. The plant can sit like this for days on its diesels and the storage tank, or the operators can cool it down slowly by letting out more steam, then hand over to the residual heat removal system below about 175 °C.
9How plants are built to cope with losing the grid
- Shut down without power: the control rods hang on electromagnets, so losing power drops them. Boron can also be injected to keep the core shut down as it cools.
- Decay heat cannot be switched off: about 7 % at the trip, 1.3 % after an hour, 0.5 % after a day. Every design is judged on how it removes it.
- Redundant emergency power: two or more diesels in separate buildings, each able to carry the safety loads alone, started and loaded in monthly tests; batteries for the hours in between.
- Cooling without any AC power: the steam-driven auxiliary feed pump, natural circulation and relief valves need only battery power for their controls.
- Fukushima Daiichi (2011) lost its grid and then, in the tsunami, its diesels and most batteries. Since then plants keep extra portable pumps and generators, some of them off site, and many have added filtered containment vents.
- Newer designs such as the AP1000 rely on gravity, natural circulation and stored gas pressure, and need no AC power for 72 hours.