Výběr zpráv ze sítě NucNet - 10. týden 2011

Tepco Considers Controlled Venting At Fukushima-Daiichi

12 Mar, 0:51 (NucNet): Tokyo Electric Power Company (Tepco) has installed a mobile power generation unit at its Fukushima-Daiichi nuclear power plant in an effort to ensure the function of essential pumps for cooling reactors that automatically shut down during an earthquake yesterday.

The back-up power is needed because the plant’s cooling system failed when the earthquake caused a power outage and emergency diesel generators stopped working after less than an hour for an as yet unknown reason.

The utility said pressure in the containment of the oldest unit on the site, unit 1, which has been in commercial operation since 1970, has continued to increase.

At 00:00 local time, the pressure was 600 kilopascal (6 bar) and according to Japan’s Nuclear and Industrial Safety Agency (NISA) it could have increased to 840 kilopascal (8.4 bar) in the meantime. NISA said the design pressure for the unit’s containment is 400 kilopascal (4 bar).

At 04:00 local time Tepco said it had decided to implement measures to reduce the pressure in the reactor containment vessel “for those units that cannot confirm a certain level of water injection by the reactor core isolation cooling system”.

Tepco is considering a controlled containment venting in order to avoid an uncontrolled rupture and damage to the containment itself.

Tepco said there has already been a leak of a small amount of radioactive substances.

Earlier, authorities in Fukushima prefecture evacuated residents living within three kilometres of the plant.

Japanese officials also told the International Atomic Energy Agency (IAEA) that pressure is increasing inside unit 1’s containment and they have decided to vent the containment to lower the pressure. The controlled release will be filtered to retain most radioactive substances within the containment, the IAEA said.

Three reactors at the plant were operating at the time of the earthquake, and the water level in each of the reactor vessels remains above the fuel elements, according to Japanese authorities.

The Japan Atomic Industrial Forum (JAIF) said earlier today that unit 2 at the six-unit plant suffered a loss of feeding water for its cooling system, caused by the cut-off of power supply. JAIF said Tepco had earlier reported to NISA that two emergency diesel generators were out of order at Fukushima-Daiichi.

Eleven nuclear reactors in areas of Japan affected by today’s earthquake have all shut down automatically and so far there have been no reports of radioactive release, the Ministry of Economic Trade and Industry (METI) has said.

The 11 nuclear units are in three northern prefectures: Miyagi, Fukushima and Ibaraki. The reactors that automatically shut down because of the quake are:

- All three units at the Onagawa plant;

- Units 1, 2 and 3 at the Fukushima-Daiichi plant (units 4, 5 and 6 were undergoing a scheduled inspection and already shut down);

- All four units at the Fukushima-Daini nuclear plant;

- Unit 1 at the single-unit Tokai plant.

Earlier, NISA said no damage to nuclear plants has been reported “at this stage” following the earthquake

The US Geological Survey said the quake struck at 14:46 local time and was magnitude 8.9 on the Richter scale. The epicentre was about 70 kilometres east of Honshu and the depth was about 24 km.

The epicentre was about 80 km east of the Onagawa nuclear site and about 150 km north-east of both Fukushima sites.


Evacuation Zone Increased Around Fukushima-Daiichi, Explosion Reported

12 Mar, 15:24 (NucNet): Japan ordered the evacuation of a 10-kilometre zone around the Fukushima-Daiichi nuclear power plant because there is still not enough emergency power on the site to operate essential pumps for cooling reactors that automatically shut down during the earthquake yesterday.

The back-up power is needed because the plant’s cooling system failed when the earthquake caused a power outage and emergency diesel generators stopped working after less than an hour for an as yet unknown reason.

Fukushima-Daiichi-1, -2 and -3 shut automatically after the 8.9 magnitude earthquake struck. Tepco confirmed that emergency diesel generators stopped working and left the reactors with no power for cooling.

The other three units at the plant were already shut down for inspection when the earthquake hit.

Here are the main developments so far:

- An explosion has been reported at the Fukushima-Daiichi site and a building that authorities said may house one of its reactors appeared to have suffered damage.

- Japanese media is reporting that the evacuation zone around Fukushima-Daiichi has been extended to 20 kilometres. Earlier, evacuation had taken place of residents living within 10 kilometres of the plant.

- Tepco said one of its employees working at unit 1 was irradiated at over 100 millisieverts (mSv). He received medical treatment from a special physician. Statutory dose limits vary from country to country, but in Europe are 20 mSv a year and in the US 50 mSv a year.

- At 09:00 local time Japanese authorities confirmed they had started preparations for the venting of the containment of unit 1 reactor at the Fukushima-Daiichi plant through a controlled release of vapour.

- Preparations were also being made to vent the containment of units 2, 3 and 4 at the plant.

- Tepco said the increase in reactor containment vessel pressure was thought to be due to a leakage of reactor coolant. Tepco later said this did not appear to be the case.

- The operation was intended to lower pressure inside the reactor containment.

At 00:00 local time, the pressure in unit-1’s containment was 600 kilopascal (6 bar) and according to Japan’s Nuclear and Industrial Safety Agency (NISA) it could have increased to 840 kilopascal (8.4 bar) in the meantime. NISA said the design pressure for the unit’s containment is 400 kilopascal (4 bar).

Tepco said temperatures in the containment system of units 1 and 2 at Fukushima-Daini were rising.

Tepco said in a statement 0600 local time on 12 March that it has detected radiation levels higher than normal at the plant.

Radiation levels next to the machine building have increased from 0.07 millisieverts per hour (mSv/hr) to 6.7 mSv/h and around the buildings measurements have shown an increase from 0.7 mSv to 5.3 mSv. Normal background radiation levels would be 0.07 mSv/hr.

Japanese officials also told the International Atomic Energy Agency (IAEA) that pressure is increasing inside unit 1’s containment and they had decided to vent the containment to lower the pressure. The controlled release would be filtered to retain most radioactive substances within the containment, the IAEA said.

Eleven nuclear reactors in areas of Japan affected by the massive earthquake have all shut down automatically and so far there have been no reports of radioactive release, the Ministry of Economic Trade and Industry (METI) has said.

The 11 nuclear units are in three northern prefectures: Miyagi, Fukushima and Ibaraki. The reactors that automatically shut down because of the quake are:

- All three units at the Onagawa nuclear plant;

- Units 1, 2 and 3 at the Fukushima-Daiichi plant (units 4, 5 and 6 were undergoing a scheduled inspection and already shut down);

- All four units at the Fukushima-Daini nuclear plant;

- Unit 1 at the single-unit Tokai plant.

The US Geological Survey said the quake struck at 14:46 local time on 11 March 2011 and was magnitude 8.9 on the Richter scale. The epicentre was about 70 kilometres east of Honshu and the depth was about 24 km.
The epicentre was about 80 km east of the Onagawa nuclear site and about 150 km north-east of both Fukushima sites.


Tepco Confirms Venting Of Unit 1, ‘Reactor Not Affected’ By Explosion

12 Mar, 15:24 (NucNet): Tokyo Electric Power Company (Tepco) has confirmed that it has successfully vented the containment of unit 1 at the Fukushima-Daiichi nuclear power plant in northern Japan.

Meanwhile, chief cabinet secretary Yukio Edano has told a press conference that there was an explosion at Fukushima-Daiichi at 15:36 local time, but he said it has not affected the reactor’s primary system or its containment.

Mr Edano said there was a hydrogen explosion in the space between the concrete container and the reactor’s primary system. However, the explosion did not damage the containment function or the reactor system, he added.

In a statement, Tepco said the venting of unit 1 had been successful and it was preparing to vent units 2 and 3, which also shut down automatically when the 8.9-magnitude earthquake hit yesterday.

The utility also confirmed that the national government had ordered an evacuation for residents within a 10 km radius of the plant’s periphery.

Tepco has not yet made any comment on the explosion at the plant, although confirmation of successful venting would appear to contradict media reports that the containment has been breached.

Venting is designed to reduce pressure in the containment. It is not yet known why the pressure increased.

Earlier, Tepco said the pressure in unit-1’s containment was 600 kilopascal (6 bar) and according to Japan’s Nuclear and Industrial Safety Agency (NISA) it could have increased to 840 kilopascal (8.4 bar) in the meantime. NISA said the design pressure for the unit’s containment is 400 kilopascal (4 bar).


Radiation Leak At Fukushima-Daiichi Rated INES Level 4

12 Mar, 17:28 (NucNet): Radioactivity at the boundary of the Fukushima-Daiichi nuclear power plant in northern Japan has exceeded statutory limits and the incident has been rated as level 4 on the International Atomic Energy Agency’ International Nuclear and Radiological Event Scale (INES).*

The Japanese authorities did not give any radiation measurements in their INES report to the IAEA, but plant operator Tokyo Electric Power Co. (Tepco) said earlier that radiation levels next to unit 1’s machine building had increased from 0.07 millisieverts per hour (mSv/hr) to 6.7 mSv/h and around the buildings measurements have shown an increase from 0.7 mSv to 5.3 mSv. Normal background radiation levels would be 0.07 mSv/hr.

Tepco confirmed that it has successfully vented the containment of unit 1 at Fukushima-Daiichi and was preparing to vent units 2 and 3, which also shut down automatically when the 8.9-magnitude earthquake hit yesterday.

Venting is designed to reduce pressure in the containment. It is not yet known why the pressure increased.

Noriyuki Shikata, deputy cabinet secretary for public relations and director of global communications at the prime minister’s office, said Tepco’s efforts to depressurise unit 1’s containment was successful. “Additional measures are now being taken tonight using sea water and boric acid,” he said, commenting on methods being used to cool the plant’s reactors and containments.

He said an explosion earlier today at the plant was caused by accumulated hydrogen combined with oxygen in the space between containment and outer structure. He said there was no damage to the containment.

* The International Nuclear and Radiological Event Scale

The International Nuclear and Radiological Event Scale (INES) was
developed by the IAEA and the OECD in 1990 to better communicate and standardise the reporting of nuclear incidents or accidents to the public.

INES explains the significance of events from a range of activities, including industrial and medical use of radiation sources, operations at nuclear facilities and transport of radioactive material.

Events are classified on the scale at seven levels: Levels 1–3 are called “incidents” and Levels 4–7 “accidents”. The scale is designed so that the severity of an event is about 10 times greater for each increase in level on the scale. Events without safety significance are called “deviations” and are classified Below Scale / Level 0.

Chernobyl rated as 7 (Major Accident) on the scale and Three Mile Island rated 5 (Accident with Wider Consequences).

7 Major Accident
6 Serious Accident
5 Accident With Wider Consequences
4 Accident with Local Consequences
3 Serious Incident
2 Incident
1 Anomaly
0 Below Scale/No Safety Significance


Japan Says Containments Are Intact At All Fukushima-Daiichi Units

13 Mar, 0:35 (NucNet): Japanese authorities have confirmed that the explosion at unit 1 of the Fukushima-Daiichi plant occurred outside the primary containment vessel (PCV), not inside. The plant operator, Tokyo Electric Power Company (Tepco), has confirmed that the integrity of the primary containment vessel remains intact.

Tepco has also confirmed to the International Atomic Energy Agency (IAEA) that containment remains intact at Fukushima-Daiichi units 1, 2 and 3. Units 3, 4 and 5 were already shut down for scheduled maintenance when the earthquake struck.

As a countermeasure to limit damage to the unit 1 reactor core, Tepco proposed that sea water mixed with boron be injected into the primary containment vessel. This measure was approved by Japan’s Nuclear and Industrial Safety Agency (NISA) and the injection procedure began at 20:20 Japan time.

The IAEA said in a statement that Japan has reported that four workers at Fukushima-Daiichi were injured by the explosion. Tepco said in a statement at 02:00 Japan time on 13 March 2011 that two workers at the site were unaccounted for.

NISA has confirmed the presence of caesium-137 and iodine-131 in the vicinity of Fukushima-Daiichi unit 1. NISA reported an initial increase in levels of radioactivity around the plant earlier today, but these levels have lessened in recent hours.

Evacuations around two affected nuclear plants have begun. In the 20-kilometre radius around Fukushima-Daiichi an estimated 170,000 people have been evacuated. In the 10-kilometre radius around the Fukushima-Daini plant an estimated 30,000 people have been evacuated. Full evacuation measures have not been completed, the IAEA said.

Radioactivity at the boundary of Fukushima-Daiichi has exceeded statutory limits and the incident has been rated as level 4 on the IAEA’s International Nuclear and Radiological Event Scale (INES).

Japan has also confirmed the safety of all its nuclear research reactors.


Cooling System Fails At Fukushima-Daiichi-3, Venting ‘A Success’

13 Mar, 13:54 (NucNet): Tokyo Electric Power Company (Tepco) says it successfully vented the inner or primary containment vessel (PCV) at Fukushima-Daiichi unit 3 this morning after the high pressure reactor core coolant injection system stopped working.

Tepco said the venting was completed at 08:41 local Japan time. Venting is a controlled release of gases including radioactive substances via a filter system to the outside air in order to stop pressure building up.

Tepco said it then began injecting water containing boric acid, which absorbs neutrons, into the reactor system using a fire pump.

A statement said workers at the plant in northern Japan had attempted to restart the unit’s cooling system, but had failed.

Unit 3 at the plant, a boiling water reactor of 760 megawatts, uses mixed-oxide (MOX) fuel and according to the Japan Safety Agency coolant levels were 2 to 3 meters below the top of the MOX fuel rods.

Tepco said it has been seeing a rise in pressure at unit 3 and there is a risk of damage to the facility, but it is working to prevent this.

Chief cabinet secretary Yukio Edano said the core of the unit-3 reactor may have been deformed due to overheating, but denied it had led to a “meltdown”, a critical situation where fuel rods have melted.

Mr Edano said at a news conference that a hydrogen explosion might occur at the unit 3 reactor building because hydrogen may have accumulated during a period when cooling of the reactor was insufficient.

He said there might be a similar explosion to the one which occurred at unit 1 yesterday, but said even if an explosion occurs, the impact should not affect the pressure vessel and containment vessel.

Unit 1 at the six-unit plant has been shut down and is being inspected following yesterday's explosion. Japanese authorities confirmed last night that the explosion occurred outside the PCV, not inside. Tepco said the integrity of the PCV remains intact.

Tepco said it has been injecting seawater mixed with boric acid into the reactor in an effort to cool the nuclear fuel. This measure was approved by Japan’s Nuclear and Industrial Safety Agency (NISA) and the injection procedure began at 20:20 Japan time.

Unit 2 is shut down but the backup cooling system is not working, Tepco said. Workers are trying to install equipment in order to enable cooling with seawater.

Tepco also confirmed last night that containment remains intact at Fukushima-Daiichi units 2 and 3. Units 4, 5 and 6 were already shut down for scheduled maintenance when the earthquake struck.

Unit 2 is shut down and the reactor core isolation cooling system has been injecting water to the reactor. Tepco said the reactor water level is lower than the normal level, but the water level was “steady”.

Radioactivity at the boundary of Fukushima-Daiichi has exceeded statutory limits and the incident has been rated as level 4 on the IAEA’s International Nuclear and Radiological Event Scale (INES).

Japan has also confirmed the safety of all its nuclear research reactors.

Zdroj: NucNet

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