Decay heat and emergency cooling

Shutting a reactor down is easy: the control rods drop in seconds and the chain reaction stops. What cannot be shut down are the fission products accumulated in the fuel, which keep decaying and releasing energy. In a reactor that has run at power for a long time, that decay heat is about 6.5% of thermal power at the instant of shutdown: some 200 MW in a 3,000 MWt plant. It falls fast at first, to 1.5% after an hour and 0.4% after a day, and then slowly: 0.2% after a week and still a few hundred kilowatts after a year. The curve was described by Way and Wigner's formula in 1948.

With that heat and no cooling, the water in the vessel boils, the level drops, the fuel is uncovered and the zirconium cladding reacts with steam above 1,200 °C, producing hydrogen; at about 2,800 °C the uranium oxide melts. That is why every plant has emergency cooling systems with pumps, diesel generators and batteries, and why the central design question is: what happens if all electricity is lost?

Fukushima Daiichi answered that question on March 11, 2011. The magnitude 9.0 earthquake shut the reactors down as designed and cut the grid; the diesels started. Forty minutes later a tsunami of about 15 meters, against a design basis of 5.7, flooded the turbine halls and disabled 12 of the 13 generators and the seawater heat exchangers. The batteries were flooded or ran out. One hour after shutdown the cores were still generating 1.5% of their power: about 22 MW in unit 1 and 33 MW in units 2 and 3. In unit 1 the water reached the top of the fuel three hours in; the zirconium produced the hydrogen that blew up the buildings and all three cores melted. At units 5 and 6, a single surviving air-cooled diesel was enough to keep them cool.

The lesson translated into passive systems: removing decay heat by gravity, convection and evaporation, with no pumps and no electricity. Isolation condensers condense the vessel's steam in a pool and return the water by its own weight; elevated tanks flood the core; steel containments are cooled by water falling from a tank on the roof. The AP1000 promises 72 hours without operator action, the ESBWR likewise, and NuScale an indefinite time with the module submerged in a pool. Older reactors added mobile generators, portable pumps and flood protection, the so-called FLEX equipment in the United States.