(New York Times, 1:45PM, March 16, 2011)
(Photo: Fukushima, 6:35AM PDST, 3/16 [Digital Globe])
One issue for the Japanese government, public, and nuclear experts and engineers around the world, is that the Tokyo Electric Power Company (TEPCO) has apparently been less than forthcoming with accurate information about the emergency at its Fukushima Daiichi nuclear power plant. What we do know isn't good; reporting from Japan sketches an image of controlled panic, real bravery, but ultimately a deteriorating situation. The only real question seems to be how bad it will actually become.
The photo above (via Digital Globe), shows all four reactors at the Fukushima plant site as of 6:35AM PDST today; I've added labels and enhancements to see the reactor structures more clearly.
I'm not an expert on systems engineering. I have read some things; I've written some things, years ago, about nuclear power that required me to understand the basics of generating that kind of power and the differences in reactors to do so. Some things I've read or seen stand out as singular, and at the risk of posting something which reads like a high school science fair exhibit, this is what we know:
What We Know: The Reactors
The TEPCO Fukushima Daiichi power plant consists of four nuclear reactors, designed and built by General Electric. Each is a "hot water" reactor, producing power by pumping fresh water around bundles of fuel rods (filled with pellets of Uranium-238) suspended inside a steel containment vessel. The water is heated, produces steam, and drives a turbine to generate electricity.
(Interactive Graphic At New York Times Online)
The temperature of the water is regulated by control rods around each fuel bundle, made of metal which absorb neutrons created by radioactive Uranium-238. The rods are pushed in to reduce the chain reaction and lower water temperature, and pulled out to raise it and generate more heat to produce steam. But the control rods alone won't prevent the fuel assemblies from overheating -- water as a coolant is critical.
Each steel containment vessel in each reactor building is surrounded by a thick, reinforced concrete structure (see the graphic above) and, like 19th century construction of powder magazines and armories, the roofs are comparatively flimsy; the force of an explosion inside the reactor building would be directed up, not contained inside, where it would cause more damage.
Another feature of each reactor building is a large, reinforced concrete tub, built into the main structure just under the roof. This is filled with circulating water, much like the containment vessels, and are used to store "spent fuel" -- fuel rods full of Uranium-238 that have reached the end of their useful life and will eventually be removed (in a complicated series of steps) for storage as nuclear waste.
Without consistent water flow and continual heat exchange, temperatures inside the reactor's containment vessels (or in the spent fuel pools) rise; the water is boiled away, fuel rods overheat and begin to melt. The nuclear fuel will literally begin to burn through concrete and steel. A cascade of system failures and "incidents" begins. If they can't be controlled... well, you should go for pizza and not come back for, oh, 25,000 years or so.
(Graphic: Aohi Shinbun, Illustrating The Problem)
What We Know: Incidents At The Reactors
When the earthquake and tsunami struck (U.S. time) on Thursday, March 10, the four reactors were apparently shut down -- "Scrammed", a procedure where control rods are lowered rapidly into the reactor core and water levels raised; it's an emergency measure. They depended on those cooling systems to maintain temperatures in the reactor cores.
However, the cooling system pumps were fueled by several above-ground diesel gasoline tanks, and while they apparently survived the earthquake intact, one or more of them were damaged by the tsunami when it swept through the plant area. The reactors in one or more building immediately began to overheat.
Reactor No. 1: On Saturday, March 12th, less than 24 hours after the earthquake and tsunami, an explosion blew the wall panels and roof off the number one reactor building. This may have been the result of hydrogen gas, generated when the level of coolant water circulating around bundles of fuel rods dropped far enough that the rods were exposed.
The hydrogen may have built up inside the steel containment vessel until the pressure blew a seal, tripped a valve, or found a way out. Once it did in enough quantity, it spontaneously exploded. A live feed video camera, set up several kilometers away by Japan's national news agency, NHK, captured the blast.
- (Status Of No. 1 Reactor Now: Fuel rods may have partially melted. The containment vessel may have been cracked, and is being flooded with seawater drawn from the ocean directly adjacent to the plant.)
- (Status Of No. 3 Reactor Now: Its core also being flooded with seawater, the number three containment vessel appears to be cracked as well and leaking radioactive steam; it can be seen clearly in the photo at the top.)
- (Status Of No. 2 Reactor Now: Its core is also being flooded with seawater. The status of the containment vessel is unknown, but a small plume of radioactive steam can be seen in the photo.)
The same fire, or a new one, started in the building again today; again, public statements from TEPCO did not identify the cause.
- (Status Of No. 4 Reactor Now: Its core is also being flooded with seawater. The status of the containment vessel is unknown. TEPCO has reported 'concerns' about the status of spent fuel rods in the pool on the top of the reactor building, which may be massive understatement.)
At least since Saturday, all four reactors have cooled by seawater pumped into the containment vessels, and the spent fuel pools on the top of the structures (and in case anyone was wondering, corrosive seawater will irreparably damage the physical plant). This is the only thing preventing a more massive release of radiation, and real catastrophe, at the moment.
Cooling the reactor cores and spent fuel in this way releases radioactive steam into the atmosphere. Even so, the type of radiation released is relatively harmless (very short half-lives) and the amounts produced have been very low. Compared to how much worse it could get, this is "Relatively Good".
Worse would be the fuel rods -- in the reactors, or spent fuel -- getting hot enough to melt down further; they will burn through the containment structure. This would release different kinds of particles, carried in the smoke from fires -- much more radioactive, and dangerous. This is "Bad".
And as radiation levels rise or fires gets worse, conditions for human beings to survive (long enough to do anything to prevent a full-on meltdown of all four reactor cores and the spent fuel) deteriorates. This would be Bad, on the order of "Central Japan Should Go For Pizza For 25,000 Years" kind of bad.
The New York Times online posted the graphic (below) of the radiation levels, reported since the earthquake and tsunami on Thursday, from just outside the Fukushima plant.
(Graphic: New York Times Online, March 16, 2011 - Click To Enlarge)
I've added a trend line in red; you can see where it's going. In the beginning, even with the explosions on Saturday and Monday, levels of released radiation are very low. A spike appears on Monday night -- and then after the explosion of Reactor 2 and the fire at Reactor 4, an escalating series of spikes of higher radiation that are not linked to any reported fire or other event. And, you can see the intervals between rises in the level of radiation appear as almost regular pulses. If true, I couldn't speculate what the cycles are tied to.
What this does say is that something is happening, with frequency, to release larger amounts of more highly radioactive material. Whatever is happening has not been fully, publicly reported by TEPCO or the Japanese government -- and, it may not be fully known or understood by TEPCO's engineerse, or even by any UN-IAEA or NRC experts brought in from outside. At this point trained personnel (by now exposed to large amounts of radiation) can't get close enough to the areas where the real problems are occuring.
UPDATE: From the New York Times, as of 1:45PM PDST:
Gregory Jaczko, the chairman of the [Nuclear Regulatory C]ommission, said in Congressional testimony that the commission believed that all the water in the spent fuel pool at the No. 4 reactor ... had boiled dry, leaving fuel rods stored there exposed and bleeding radiation...
If his analysis is accurate and Japanese workers have been unable to keep the spent fuel ... properly cooled — it needs to remain covered with water at all times — radiation levels could make it difficult not only to fix the problem at reactor No. 4, but to keep workers at the Daiichi complex from servicing any of the other problem reactors at the plant.
Mr. Jaczko said radiation levels may make it impossible to continue ... using fire hoses to dump water on overheated fuel and then letting the radioactive steam vent into the atmosphere.
Those emergency measures, implemented by a small squad of workers and firemen, are the main steps Japan is taking at Daiichi to forestall a full blown fuel meltdown that would lead to much higher releases of radioactive material.
The Emperor of Japan, Emperor Akihito, went on television for the first time to address his country, and say he was "very concerned" about the situation at Fukushima Daiichi. It's difficult to overstate how significant, in Japan's culture, this event is.