Fukushima: Robot Detects 270 Millisieverts Per Hour In No. 1 Reactor Building

High levels of radiation discovered at the Fukushima No. 1 nuclear power plant could disrupt Tokyo Electric Power Co.’s timeline for a cold shutdown of the crippled facility, TEPCO officials acknowledged.

On April 18 unexpectedly high levels of radiation were detected in water in the storage pool containing spent fuel rods in the No. 2 reactor, the officials said.

TEPCO officials believe the radiation may have been triggered by damage to the spent fuel rods. One possibility being looked at is the damage was caused by debris falling into the pool when the Great East Japan Earthquake struck on March 11.

An analysis of water samples taken from the storage pool on April 16 found cesium-134 at 160,000 becquerels per cubic centimeter, cesium-137 at 150,000 becquerels and iodine-131 at 4,100 becquerels.

Ordinarily, the level of radioactivity in the pools is much lower.

Another problem area is the building housing the No. 1 reactor. TEPCO officials used a U.S.-made robot on April 16 to measure radiation levels and detected radiation of 270 millisieverts per hour in the No. 1 reactor building.

That level of radiation means a worker could spend less than an hour in the area before exceeding the allowable dosage.

The exposure would be so high workers could not re-enter the area for several years, officials said. If radiation levels remain at high levels, TEPCO’s experienced workforce would all quickly reach maximum radiation exposure levels, severely slowing the effort to stabilize the plant.

Radiation measurements were also taken at another entrance to the No. 1 reactor building and found levels of 49 millisieverts per hour. Radiation at an entrance to the No. 3 reactor building was also measured at 57 millisieverts per hour.

Those are still high levels and workers who remain in that environment for five hours will reach the maximum amount of radiation exposure allowed.

Huge volumes of water contaminated with radiation are also expected to slow work to bring the Fukushima reactors under control.

Officials of the Nuclear and Industrial Safety Agency (NISA) said April 18 that a pool of water about five meters deep had been found in the basement of the building housing the No. 4 reactor.

Radiation levels as high as 100 millisieverts per hour were detected on the water’s surface.

About 54,000 tons of radiation-contaminated water also sits in the basements of the turbine buildings for the No. 1 to No. 3 reactors.

The radiation level in the basement of the turbine building for the No. 2 reactor is especially high.

Finding storage space for the contaminated water is a pressing issue, as is what to do with the rubble on the plant grounds that is also contaminated with radiation.

NISA official Hidehiko Nishiyama said, “The situation is very serious. It is desirable to lower the level of radiation workers are exposed to by using anything that will shield the radiation as well as by decontaminating the workers. We will have to think of ways to carry that out from now.”

TEPCO officials have plans to fill the core containment vessels at the No. 1 and No. 3 reactors with water to submerge the pressure containers that hold the fuel rods.

To achieve TEPCO’s road map objective of a cold shutdown of the reactors after six to nine months, officials are seeking to restore the cooling system rather than depend on pumping in water to the reactor cores.

However, the main equipment and piping used in the continuous cooling system are all located within the reactor buildings. Workers will have to work in those buildings to inspect and repair the equipment and piping.

As a contingency plan in the event the cooling system cannot be restored, preparations are being made to install heat exchangers that use cool air rather than water. That installation work will also require workers to enter the reactor buildings.

At the No. 2 reactor, holes have opened in the suppression pool connected to the containment vessel so repairs will be needed before the No. 2 reactor can be submerged. However, there is the possibility that radiation levels of several dozens of sieverts are present near the suppression pool. Such levels would lead to immediate health problems for workers.

Plans are being considered to use robots for inspection and simple tasks to reduce radiation exposure among workers.

Another factor that has slowed work at the Fukushima plant is the frequent aftershocks. Some have led to tsunami watches that have meant workers have had to be evacuated.

An aftershock measuring an intensity of lower 6 on the Japanese scale of 7 hit Fukushima Prefecture on April 11. That caused a power outage that stopped the pumping in of water for about 50 minutes.

Another tsunami triggered by an aftershock could also flood the plant site, damage equipment and lead to the leaking of highly contaminated water.

Meanwhile, NISA officials on April 18 for the first time publicly admitted that some of the fuel rods in the No. 1 to No. 3 reactor cores had melted in the wake of the March 11 quake and tsunami.

NISA officials gave a report to the government’s Nuclear Safety Commission of Japan.

NISA officials had alluded to the possibility of a melting of the fuel rods, especially after hydrogen explosions rocked the No. 1 to No. 3 reactors. However, no NISA official had actually stated that some of the fuel rods had melted until April 18.

NISA officials said that based on an analysis of the radioactive material collected and their radiation concentration, there has likely occurred a melting of the fuel pellets contained within the fuel rods.

2011/04/20

Source: Asahi

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