Concern rises over Fukushima I reactors

In an unprecedented move in the wake of Friday’s powerful earthquake, the Japanese government has declared an “atomic power emergency” and evacuated some 3000 residents living close to Fukushima I Nuclear Power Plant . Located in northeastern Japan, the plant is one of the world’s largest. As a safety precaution, plant operators are expected to vent radioactive steam in the next few hours.Japan has 54 nuclear power plants, which provide about 30% of that nation’s electricity. All the structures are designed to withstand earthquakes to a certain degree, but yesterday’s 8.9-magnitude quake off the coast has caused significant damage to Japan’s infrastructure through a combination of ground shakes and a tsunami. As a precaution, the 10 nuclear plants close to the epicenter were automatically shut down.However, at the Fukushima I plant, which has eight reactors, the backup safety systems for the number 1 and number 2 reactors, which provide water and power to keep the fuel rods cool, failed.The design, based on a General Electric water-boiling reactor, requires the electrical pumping of water to cool the fuel rods. Even when a chain reaction has been stopped, radioactive waste builds up in the rods while they burn, making them radioactively hot.Usually reactors have a number of backup power systems. They can obtain power from the national grid, diesel generators, or backup batteries. In this particular case, power from the grid and diesel generators failed, which allowed the water level in the reactor to go down.After a short while the battery system kicked in and started supplying fresh water to the cooling system, which has now stabilized the water level a few meters above the fuel rods. Although the batteries generally last only about 24 hours, they are buying some valuable time to bring other supplies and personnel to the plant. US Air Force planes based in Japan have already delivered emergency coolant to the plant, says the US State Department.Typically, nuclear power plants are designed to run for 2472 hours on passive systems, such as steam-based pumps, before the reactor goes critical and the fuel rods melt.Despite the use of the passive systems and the battery backup, the cooling water would still be boiling inside the reactor chamber, reducing the water level further, and increasing pressure on the containment vessel. In this instance, the authorities have a difficult decision to make: Release the pressure by venting some of the hot water vapor, or hope the power comes back on before the vessel reaches its safety limits.According to Tatsujiro Suzuki, vice chairman of the Japan Atomic Energy Commission, that decision has now been made. The valve to release steam from unit 1’s containment vessel will shortly be opened, which may release some radioactive material."It seems that the situation at Unit 2 has been stabilized,” says Suzuki. He cautions, however, that operators may yet have to release steam from that vessel too.Paul Guinnessy