Tipping Point at Fukushima?

Faced with potentially lethal radiation doses, workers battle to avert a full-scale disaster at Japanese nuclear complex.

The BBC and the Washington Post are reporting [Tuesday evening, Pacific Time] that elevated radiation levels at the crippled Fukushima Daiichi nuclear plant have forced at least a temporary suspension of efforts to stabilize the facility.

The crippled Japanese nuclear complex.

At least 750 workers had been evacuated from Fukushima on Tuesday morning after an explosion breached the inner containment building of the Unit 2 reactor at the plant. Since then, a heroic skeleton crew of approximately fifty workers has been struggling to keep cooling water flowing to the facility’s reactor vessels and fuel storage pools.

There are six reactors at Fukushima Daiichi, three of which (Units 1, 2, and 3) were operating at the time the 8.9 magnitude earthquake struck Friday. Since then there have been explosions at all three plants and reports of fires at another reactor (Unit 4) where spent fuel is stored. Although conditions at the complex seem to change by the hour, much of the concern is on Unit 2 where TEPCO, the plant’s owner, has reported that fuel in the core of the Unit 2 was completely uncovered for a matter of hours.

In a lengthy and sobering conference call with reporters Tuesday, experts with the Union of Concerned Scientists delved deeply  into the complexity of the difficulties that embattled workers are facing as they try to contain the immense radioactive hazards at the plant in an increasingly hostile work environment.

For example, this is how UCS Nuclear Safety Program director Dave Lochbaum, a nuclear engineer, answered a reporter’s question about what causes spent fuel (stored in deep pools of water) to catch fire:

“The spent fuel pool is approximately 45-foot deep. The spent fuel is stored in the lower 15 feet of that 45-foot deep pool. The spent fuel is stored in metal racks that are approximately six inches off the bottom of the floor. They have little legs on them that hold the bottom of the rack, about six inches off the bottom. That allows water to circulate up through the bottom of the racks, past the fuel assemblies to cool them, and then that water is removed, cooled, and returned to the pool, when things are working right.

If the water boils away or drains away such that it drops down to about the lower — the lower regions of the pool, what happens is you don’t get enough water flow. You don’t have water flow removing heat anymore, and the steam that’s being boiled away from the surface of the pool isn’t enough to cool the top portions of the exposed fuel bundles. So, they heat up, heat, and as they heat up, at some point, there is actually — they reach the ignition temperature or the point at which the metal cladding catches on fire. When that occurs, the contents of the fuel rods are released into the airspace that’s going around there.

The spent fuel pools, as I mentioned earlier, are located in secondary containment buildings that aren’t as robust and aren’t as effective at containing that radioactivity, so a lot of it does get to the environment.”

Lochbaum also described in chilling detail what plant workers would face trying to manually refill the fuel storage pools at the plant.

“There were some studies done in the United States of what would happen if the water level in the spent fuel pools were to be drained away for whatever reason, and I recall one study for the plants in Connecticut that said if the water level dropped down to where the top of the fuel was — that’s even higher than it would be for a fire (inaudible) started — the dose rates on the railing of the spent fuel pool, if you were looking down into the pool, would be high enough that you would receive a lethal dose in something like 16 seconds.

“So, as Ed [physicist Dr. Ed Lyman) suggests, when the plant's in that configuration, the high dose rates preclude a lot of worker actions or turn them into suicide missions.”

Wednesday Japanese Cabinet Secretary Yukio Edano told reporters that the skeleton crew was being withdrawn because of elevated radiation levels."

“Workers cannot carry out even minimal work at the plant now,” Edano is reported as saying. “Because of the radiation risk, we are on standby.”

The Fukushima Daiichi plant, with its six nuclear power reactors, is located on the east coast of Japan, 140 miles northeast of Tokyo.

Characteristic of the confusion that has accompanied the harrowing events at the nuclear facility since Friday’s devastating earthquake and tsunami, the New York Times followed the BBC and Washington Post reports with an on-line post reporting that an English translation of Edano’s remarks was in error and that “a small group of workers” remained at the plant.

It remains unclear, however, what the workers would be able to do given the escalating radiation hazards at the complex.

Earlier, the Times had reported that Japanese authorities said there now appear to be ruptures in two of the reactors’ primary containment structures.

“Taking shelter when possible in the reactor’s control room, which is heavily shielded from radiation, they [the plant’s workers] struggled through the morning and afternoon to keep hundreds of gallons of seawater a minute flowing through temporary fire pumps into the three stricken reactors, Nos. 1, 2 and 3, where overheated fuel rods continued to boil away the water at a brisk pace,” the Times reported.

At least as ominously are the continued reports of fires in Unit 4. Unit 4 was shutdown at the time of the earthquake/tsunami but there is a large quantity of extremely hot and highly radioactive fuel stored outside the reactor’s primary containment structure. A fire inside the building was reported Tuesday, and then another reported on Wednesday.

Reuters news service is compiling a running timeline on the unfolding nuclear disaster in Japan. You can link to it here.

The Columbia Generating Station at Hanford.

There are basic similarities in the design and vulnerability of the Fukushima Daiichi reactors and the Columbia Generating Station (CGS) at Hanford, the Northwest’s only commercial nuclear reactor. The  Seattle Times’s took a first look at this issue in Tuesday’s edition . The story didn’t get deeply into the containment technology issue that has been a key focus following the nuclear accident at Three Mile Island in 1979 and the Chernobyl disaster in 1986. But the issue was addressed in an article by MSNBC investigative reporter Bill Dedman.

An information sheet distributed earlier this week by the public interest watchdog group Hanford Challenge raised concern about the location of the CGS spent fuel storage pool and the lack of a containment structure for the pool.

–Tim Connor