Bungled replacement work and a chance opening in a separator gate very likely saved the No. 4 reactor at the Fukushima No. 1 nuclear power plant from descending into a nightmarish situation, it has been learned.
The governments of Japan and the United States were worried that overheating and the decay of spent fuel rods at the No. 4 reactor could trigger catastrophic developments following the disastrous events of last March 11.
It appears now that the coincidental occurrence of the delayed replacement work and the opening in the separator helped avert the potential scenario of a full-blown meltdown.
The core shroud, a large structure in the reactor core, was undergoing replacement at the No. 4 reactor at the time of the tsunami. The reactor was undergoing its most extensive upgrade since it entered commercial operation in 1978.
The reactor well, the space directly above the reactor, and the dryer and separator (DS) pit, where equipment tainted with radioactivity is stored temporarily in water, were filled with 1,440 cubic meters of water during the work. Under normal circumstances, those spaces are dry.
To minimize exposure to radiation, the core shroud was to be kept underwater while it was cut into pieces and transferred to the DS pit. A separator gate was to be installed between the reactor well and the DS pit, and the water was to be removed from the reactor well to proceed to the next phase of the job. All those tasks were to be completed by March 7, according to the initial plan.
When workers were about to insert a shroud-cutting tool into the reactor core, however, they discovered that auxiliary equipment to guide that tool into the reactor core was the wrong size. Retooling that equipment caused a delay in the process and, as a result, the reactor well was still filled with water on March 11, the day the Great East Japan Earthquake struck.
The large tsunami spawned by the magnitude-9.0 temblor led to a loss of power and the capacity to cool down the spent fuel storage pool in the No. 4 reactor building. The water in the storage pool began evaporating due to decay heat from the fuel.
A decrease in the water level could have caused exposure and overheating of the nuclear fuel and a massive discharge of radiation and radioactive substances. That would not only have made the entire Fukushima No. 1 nuclear plant inaccessible, but also could have led to the abandonment of the Fukushima No. 2 plant and other nuclear power plants located nearby.
Worst-case scenarios envisaged by the governments in Tokyo and Washington involved the evacuation of residents from the Tokyo metropolitan area.
In reality, however, a displaced separator gate between the spent fuel storage pool and the adjoining reactor well apparently created an opening, allowing about 1,000 tons of water to flow from the reactor well into the storage pool, it was learned later.
The injection of outside water into the storage pool began on March 20. As a result, the fuel in the pool was kept at relatively safe levels during the crisis.
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