17 Décembre 2015
December 17, 2015
By MASANOBU HIGASHIYAMA/ Staff Writer
A set of critical valves designed to release pressure inside the No. 2 reactor at the Fukushima No. 1 nuclear power plant likely failed due to surging temperatures and extreme stresses that built up in the early stage of the 2011 disaster, the plant’s operator said Dec. 17.
In its report on this specific aspect of the catastrophe, Tokyo Electric Power Co. concluded that the failure of the valves to reduce pressure likely prevented water from being injected into the reactor.
The accident was triggered by the magnitude-9.0 Great East Japan Earthquake that struck March 11, 2011, and spawned towering tsunami that inundated the site.
Workers at the plant attempted to open eight “pressure-releasing safety valves” early on the morning of March 15, but failed to release pressure inside the No. 2 reactor's containment vessel, TEPCO said. The pressure level was reduced around 1 a.m. only after one of the valves finally opened.
TEPCO, until now, has tried to confirm whether the valves, a key safety feature, functioned properly.
After a thorough analysis, the utility concluded that the valves did not function as pressure inside the No. 2 reactor was too high.
The valves are designed to open under the pressure of nitrogen gas piped in from tanks and other sources.
TEPCO concluded that the valves initially functioned properly after the earthquake and tsunami hit.
But after the meltdown of nuclear fuel inside the reactor, heat and pressure levels rose significantly in the late hours of March 14, preventing the valves from opening.
The rising heat inside the reactor likely caused sealing materials and other bits of equipment to disintegrate, apparently leading to nitrogen gas leakage.
Dec. 17, 2015 - Updated 00:24 UTC+1
The operator of the Fukushima Daiichi power plant says excessive heat from nuclear fuel at one of its damaged reactors may have caused some key parts to melt.
Officials at Tokyo Electric Power Company believe the loss of those components made it difficult to stop the Number 2 reactor from melting down and spewing out large amounts of radioactive substances following the March 2011 accident.
Four days after the onset of the accident, the emergency cooling system at the reactor stopped working.
In an attempt to cool fuel inside the reactor, the workers had no choice but to pour in water from a fire engine.
But they were unable to inject water as planned because of the high pressure that had built up inside the reactor.
The officials also could not use safety relief valves to release pressure. Those valves were designed to open when hit by high-pressure gas from a tank.
The TEPCO analysis determined that key parts of the gas feeding system may have melted, leading to a gas leak. The analysis says the temperature inside the reactor exceeded 200 degrees Celsius, far beyond the limit of the parts' durability.
Those parts are present in all other reactors in the model line used at the Fukushima Daiichi plant. The utility plans to replace the parts at its other nuclear power plant in central Japan with heat-resistant ones.