OKUMA, FUKUSHIMA PREF. – “Now 1,000 microsieverts (per hour)!”
The shout by a Tokyo Electric Power Co. worker, in full face mask and seamless white protective suit, sent tension high among the 18 journalists aboard a slow-moving bus inside the compound of the crippled Fukushima No. 1 nuclear power plant.
As the bus moved near the badly damaged reactor 2 building, a radiation meter spiked to a level about 18,000 times higher than the everyday reading in Tokyo.
On Saturday, for the first time since the quake-tsunami disaster of March 11, reporters were allowed into the plant for a guided tour.
On that fateful day in March, 13-meter tsunami knocked out the plant’s power, triggering meltdowns in three reactors and leading to massive radiation emissions.
In just one hour, had the bus not moved, the 1,000 microsieverts an hour — the highest encountered on the media tour — would have exposed everybody to a dose in excess of the annual legal limit for ordinary citizens.
Still, the route was chosen carefully to keep radiation exposure as low as possible for the journalists, who were not allowed inside any structures except one quake-resistant headquarters building.
In August, Tepco found that the bottom of an exhaust pipe between reactors 1 and 2 was still emitting radiation greater than 10 sieverts (or 10 million microsieverts) an hour, the highest level detected so far. One-time exposure to 10 sieverts is lethal.
But Masao Yoshida, the plant’s general manager, who has led the effort by thousands of workers to contain the crisis since March, stressed to the reporters that the plant has been safely stabilized and the likelihood of a large-scale leak of radioactive materials is low.
At the same time he admitted the plant is still a very dangerous place for the workers.
The facility “has been stabilized sufficiently enough to ease the worries of local people (outside the no-entry zone). But some conditions are still very tough for working,” Yoshida said at the plant’s control center.
Indeed, although the temperature at the bottom of the reactor pressure vessels has been kept below 100 degrees for weeks, preventing coolant water from boiling off and spewing large amounts of radioactive materials into the atmosphere, reporters still saw many buildings left half-destroyed, their windows shattered.
In spite of mobilizing about 3,000 workers a day, Tepco has only been able to focus on fixing and maintaining the critical ad hoc water-circulation system to cool the melted fuel in the cores of three reactors.
Although nuclear fission ceased when the reactors were automatically shut down shortly after the monster earthquake shook the plant, melted fuel will continue to emit decay heat for years to come. Even this relatively weak product of radioactive decay is powerful enough to melt the fuel fragments again if the injection of coolant water is disrupted long enough.
A worst-case scenario was released by Tepco on Oct. 1, along with a plan for dealing with it.
According to a computer simulation run by the utility, if the cooling system is disrupted for any reason, the temperature of the fuel fragments would reach 2,200 degrees in 38 hours.
At that point, the fuel fragments would melt again, releasing large amounts of radioactive materials into the environment, according to the scenario.
Tepco, however, has already set up multiple backup pumps and water lines as well as placing firetrucks on standby to inject water into the reactors as a last resort.
Even if multiple accidents occur, Tepco claims it would be able to resume injecting water within three hours, saying the probability of another meltdown is extremely small.
Some backup systems have been set up nearby, on land 35 meters above sea level to survive monster tsunami.
Kenji Sumita, a professor emeritus of nuclear engineering and a former deputy chief of the Nuclear Safety Commission, basically agrees with Tepco’s views on the safety of the reactor and cooling system.
“I don’t think any large leakage of radioactive materials is likely any more unless (Tepco) makes a very big blunder” in dealing with an emergency, he said.
Still, concerns that a big temblor could wreak havoc can’t be dismissed as Tepco spends at least the next 10 years cooling the reactors, waiting for the decay heat to fall off far enough for the melted fuel to be removed.
However, Tadashi Narabayashi, a professor of reactor engineering at Hokkaido University, also believes a serious accident at the Fukushima plant is unlikely.
But he recommends that Tepco continue to improve the reliability of the backup systems so they can survive any quake or tsunami.
“In the medium to long term, Tepco should replace the current equipment with more robust gear,” he said.
Saturday’s media tour started out from the J. Village complex, which is situated 20 km south of Fukushima No. 1. The former soccer training center now serves as base camp the for thousands of workers shuttling to and from the troubled power station.
During the 40-minute ride to the plant, reporters saw three completely abandoned towns: Naraha, Tomioka and Okuma.
Every restaurant, shop, home and office was deserted. The government has designated the area within 20 km of the plant as a no-go zone, forcing most of the 33,000 residents of the three towns to evacuate.
The radiation reading inside the bus when it was parked at J. Village was 1.5 microsieverts per hour, higher than the outside reading of 0.5 microsieverts. That meant the interior of the bus was more contaminated than the outside environment at J. Village.
“That’s probably because the bus is regularly used” to shuttle plant workers back and forth between J. Village and the plant, a Tepco official explained.
As the bus moved along National Road No. 6 toward the plant, the reading inside the vehicle kept rising. During the ride to the complex, the reading peaked at 20 microsieverts per hour.
This occurred about 1.5 km from the plant’s main gate. Several abandoned houses were on the left side of the road, and a convenience store stood on the right.
“The dose decreases toward the main gate (along the route). In this area, hot spots are found here and there,” said a Tepco official who accompanied the journalists.
“The dose tends to be higher in places near woods.”
The journalists on the tour were all required to wear single-use protective wear, just as the plant workers do.
A tight-fitting face mask is required to keep all radioactive substances from being inhaled. Such particles are particularly dangerous because they stay inside the body for a long period of time and directly damage organs.
By the end of September, 162 workers had been exposed to cumulative radiation of 100 millisieverts or more, with six exposed to at least 250 millisieverts. The highest dose was 678.1 millisieverts.
According to the International Commission on Radiological Protection, an advisory body of scientists, a cumulative dose of 100 millisieverts increases the risk of dying of cancer by 0.5 percent. A single high dose further increases the risk, yet some workers were not carrying a dosimeter in the early stages of the crisis.
“Some workers have been here since March, undergoing horrible conditions,” Yoshida said Saturday at the plant’s headquarters. “So they have felt a mental burden.” He added that bringing them some relief is a priority.
At the plant, two workers were drowned by the tsunami and three others have died since March, but Tepco says none of the deaths were related to their work or radiation, including a man who suddenly died of acute leukemia not long after passing a physical test. A total of 63 have been injured on duty.
The legal limit for nuclear plant workers is 100 millisieverts, which was temporarily raised to 250 millisieverts after the meltdowns. The limit was returned to 100 millisieverts on Nov. 1.