The New York Times has a useful summary, and we’re getting a more detailed picture from the Union of Concerned Scientists, which includes scientists and nuclear engineers with direct familiarity with the 1970s era General Electric boiling water reactor designs used at the station. The UCS are more prone to explain the risk analysis than government officials understandably concerned about public panic and safety.
Here’s what appears to be happening.
1. As soon as the earthquake began, all the Japanese nukes began their automatic shutdown sequences. That’s what they’re supposed to do. Control rods that help separate the radioactive fuel rods/elements and reduce the heat buildup were immediately inserted. As far as we know, this worked at every reactor.
2. When this happens, the cooling water system, run by electric pumps, is supposed to continue circulating water through the reactor vessel containing the fuel rods, to continue to remove their heat. However this system runs on electricity from the grid or from backup generators. The grid was damaged, so the plant was isolated.
3. With grid electricity unavailable, all plants automatically activate back-up, on-site generators, probably fueled by diesel. The diesel generators provide enough electricity to keep the control rooms functioning and to operate the electric pumps that continue to circulate water over the reactor core to continue its cooling. This normal back-up system worked at almost all plants until . . .
4. The tsunami hit the Fukushima station and apparently damaged the back-up diesel generators. At this point, Units 1 and 2 were without power from either the grid or back-up generators. It was flying blind.
5. When this happens, there is a fail-safe mechanism attached to the reactor that runs on steam that can still force cooling water across the core and continue the cooling process . . . for a while. This system is controlled by batteries with a limited life. The UCS think this is about 8 hours. That time has probably lapsed.
6. The Japanese operators are trying to bring in replacement batteries to keep the emergency fail-safe cooling system functioning. We don’t know the status of that effort at this time.
Bottom line: There may be not one but two units at risk. They’re both on last-ditch, fail-safe systems that rely on limited-life batteries to keep cooling water flowing and covering the core. The operators are in a race against time to replace them or to get electric power either from repaired or replaced back-up generators or restored access to the grid. We don’t know the status of any of these efforts.
Without continuously circulating cooling water, the still very hot reactor core will slowly (over hours) boil away the remaining cooling water, and that could eventually leave the reactor core and its radioactive fuel rods uncovered. We don’t know how far along we are in that sequence. What happens after that can lead to an uncontrolled meltdown and releases of radiation.
“Controlled” radiation releases, through filters (we don’t know their effectiveness), have already been used to relieve pressure inside the reactor. [There's a report they've lost any other ability to control pressure.] suspect most has been contained inside a massive containment structure, which is designed to withstand everything except the things they didn’t plan for, like the loss of everything. We’re there.
There have already been pressure buildups inside the reactor (or containment?) that exceed its design capacity. We don’t know what it’s real limits are, and we don’t know what damage the earthquake caused to its integrity.
Evacuations are underway, in increasingly larger areas. Stay tuned.
UCS: Nuclear crisis at Fukushima
NYT: Japan expands evacuation around nuclear plant
Nuclear plant under state of emergency
Design features of a boiling water reactor
Risks of nuclear catastrophe escalates . . .
Radiation 1000 times higher than normal
Update: keep in mind there are two multi-unit nuclear power stations in play. In all, five units are at risk at these two stations.
Fukushima I (Daiichi) Nuclear Power Station = six units built in the 1970s
Fukushima II (Daini) Nuclear Power Station = four units built in the 1980s
At Daiichi, Unit 1 had the explosion on Saturday; Unit 2 is also at risk.
At Daini, Units 1, 2, and 3 are also at risk.
Disclosure. I once consulted for Tokyo Electric Power Company, the plant owners, on an unrelated matter, explaining US grid coordination. I’m not a nuclear expert.