THE NUCLEAR COMMUNICATIONS NETWORK News in Brief / No. 79 / 24 March 2011
Unit 1 Reactor Pressure Vessel Temperature Restored To Safe Level
24 Mar (NucNet): The temperature in the reactor pressure vessel (RPV) at unit 1 of the Fukushima-Daiichi nuclear power plant in Japan was restored to a safe level of less than 250 degrees Celsius today after the core temperature had reached almost 400 degrees Celsius, exceeding the design value of about 300 degrees Celsius, Tokyo Electric Power Company (Tepco) and the Japan Atomic Industrial Forum (JAIF) said.
The safe temperature was restored by increasing the volume of seawater injection yesterday. In order to keep the pressure in the RPV low, the seawater injection rate was then brought back to the previous level. However, a helicopter crew checking temperatures from above the plant reported seeing water vapour from unit 1 for the first time, which could indicate boiling water in the spent fuel pool or a release from the reactor’s primary loop.
JAIF also said work at units 1, 2, 3 and 4 was suspended after black smoke rose from unit 3 yesterday afternoon. Work resumed at around 05:30 Japan time (21:30 on 23 March central European time) with Tepco continuing water injection into the spent fuel pools of units 3 and 4.
The International Atomic Energy Agency (IAEA) said black smoke was seen rising from the unit 3 reactor building, spurring the temporary evacuation of workers from units 3 and 4. The emission of smoke has now “decreased significantly”, the agency said.
Spraying of the spent fuel pools at units 2, 3 and 4 continued most of the day yesterday with a number of planned interruptions and one unplanned interruption due to the smoke event at unit 3.
In unit 2, the pressure reading in the RPV is still not stable, but the core temperature is in the safe range below 250 degrees Celsius.
Tepco is continuing its efforts to restore external AC power to all six units at the plant, with external power available at units 2, 4, 5 and 6, JAIF said.
At approximately 11:30 Japan time today lighting was restored to unit 1’s main control room. Lighting is also available in the main control room at units 3, where backup batteries for essential instrumentation and control systems are operational.
JAIF said “integrity checks” of electrical equipment is being carried out in each unit and this must be completed before energising them.
Without electrical power, cooling systems at Fukushima-Daiichi’s six reactors cannot operate. Many of the problems facing the nuclear power plant stem from the loss of electrical power at the site following the massive earthquake and tsunami on 11 March. The earthquake cut off external power to the plant and the tsunami disabled backup diesel generators.
The IAEA said restoring external power to the plant does not mean the reactors will immediately resume normal safety function. The earthquake and tsunami may have inflicted considerable damage in addition to knocking out electricity supplies, the agency said. “Because the extent of this damage is unknown, it is not possible to accurately estimate a work schedule.”
Dan Yurman
Email: dan.yurman@usa.net
Mobile: 208-521-5726
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