Spent fuel ponds become a concern at Fukushima nuclear power plant

March 18, 2011 — Workers at Fukushima Daiichi nuclear power plant in Japan have been battling to prevent fuel in the reactors from overheating since the magnitude 9.0 quake and subsequent tsunami.

The current situation at Units 1, 2 and 3, whose cores have suffered damage, appears to be relatively stable. Seawater is being injected into all three units using fire-extinguishing hoses. Containment pressures are fluctuating.

Unit 4 remains a major safety concern. No information is available on the level of water in the spent fuel pool. No water temperature indication from the Unit 4 spent fuel pool has been received since March 14, when the temperature was 84 °C. No roof is in place.

On March 17, military helicopters carried out four water drops over Unit 3 to try to prevent fuel rods melting, but it is unclear how great an impact this had.

The Tokyo Electric Power Co., which runs the plant, has been attempting to connect one of its damaged units to the main grid via a 1-km (0.6-mile) electricity cable.

The International Atomic Energy Agency said workers began work on the cable March 17, and that engineers plan to reconnect power to the reactor once workers have finished spraying seawater over reactor 3.

Japan has imposed a 20-km (12-mile) exclusion zone around Fukushima and has urged people living up to 30km away to stay indoors. Some countries have advised their nationals in Japan to stay up to 50 km away.

Ranking the disaster

Japanese authorities have assessed that the core damage at the Fukushima Daiichi 2 and 3 reactor Units caused by loss of all cooling function has been rated as 5 on the International Nuclear and Radiological Event scale.

A rating of 5 is an "accident with wider consequences." On the INES scale, the Chernobyl disaster ranks a 7, and the Three Mile Island accident ranks a 5.

Japanese authorities have assessed that the loss of cooling and water supplying functions in the spent fuel pool of the Unit 4 reactor has been rated as 3.

Japanese authorities have assessed that the loss of cooling functions in the reactor Units 1, 2 and 4 of the Fukushima Daini nuclear power plant has also been rated as 3. All reactor Units at Fukushima Daini nuclear power plant are now in a cold shut down condition.

Spent fuel ponds

Japanese authorities have informed the IAEA that, prior to the earthquake of 12 March, the entire fuel core of reactor Unit 4 of the Fukushima Daiichi nuclear power plant had been unloaded from the reactor and placed in the spent fuel pond located in the reactor's building.

Spent fuel removed from a nuclear reactor is highly radioactive and generates intense heat. Nuclear plant operators typically store this material in pools of water that cool the fuel and shield the radioactivity.

Water in a spent fuel pool is continuously cooled to remove heat produced by spent fuel assemblies. According to IAEA experts, a typical spent fuel pool temperature is kept below 25 °C under normal operating conditions. The temperature of a spent fuel pool is maintained by constant cooling, which requires a constant power source.

Given the intense heat and radiation that spent fuel assemblies can generate, spent fuel pools must be constantly checked for water level and temperature.

If fuel is no longer covered by water or temperatures reach a boiling point, fuel can become exposed and create a risk of radioactive release. The concern about the spent fuel pools at Fukushima Daiichi is that sources of power to cool the pools have been compromised.

Concern about spent fuel storage conditions has led Japanese officials to drop and spray water from helicopters and trucks onto Unit 3 at Fukushima Daiichi nuclear power plant.

According to TEPCO, water discharge by Self Defense Force helicopters was conducted from 9:48 a.m. in the morning March 17. More water discharge followed at 7 p.m., and used riot police water cannon trucks and Self Defense Force fire engines.

Japan's Nuclear and Industrial Safety Agency has reported increasing temperatures in the spent fuel ponds at Units 5 and 6 since 14 March. An emergency diesel generator at Unit 6 is now powering water injection into the ponds at those Units, according to NISA.

The IAEA reports that the temperatures of the spent fuel ponds in Units 4, 5 and 6 are lingering around 62 to 65 °C.

Power line

Japanese engineers have begun to lay an external grid-linked power line cable to Unit 2 in an effort to restore power.

Restoring power should enable engineers to restart the pumps that send coolant over the reactor.

Earlier reports that the line had already been completed were premature. The line is still under construction as of March 18.

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