Working conditions improve at Fukushima unit

22 June 2011

Workers at the damaged Fukushima Daiichi nuclear power plant have entered the reactor building of unit 2 to assess the working environment. Ventilation of the building has led to a drop in humidity. Meanwhile, the system to decontaminate water on the site is in operation. 


Plant owner Tokyo Electric Power Company (Tepco) announced that the airlock to the reactor building was slightly opened at 8.51 pm on 19 June. It was opened fully at 5.00 am on 20 June. The truck bay door to the reactor building was also subsequently opened, allowing the building to be ventilated.


These actions have helped to reduce the level of humidity within the reactor building, improving the working environment. The humidity had earlier been measured at 99.9%, originally thought to be due to the warm cooling pond in the top of the sealed building. An air filtration system will also be installed in the building to reduce airborne radiation levels.


An alternative circulation system for the used fuel pool at unit 2 began operation on 31 May and brought temperatures down to 31°C by 10 June - much closer to normal temperatures of around 25ºC. This had been expected to reduce humidity within the building but Tepco now thinks the primary source of humidity is the damaged torus suppression chamber in the basement of the building.


On 21 June, ten workers (seven Tepco employees and three contractors) entered the reactor building of unit 2. Whilst there, they took radiation, temperature and humidity measurements on the ground and first floors of the building. A maximum radiation level of some 430 millisieverts per hour was detected with the temperature within the building 20-27°C and the level of humidity down to 50-65%.


Tepco said that it plans to install equipment - including water level gauges, a water circulating injection system, nitrogen injection pipes - in unit 2's reactor building in the near future.


Water decontamination 


A system to decontaminate highly-radioactive water at the site went into full operation on 17 June. However, after just five hours the system was halted as higher-than-expected radiation levels were detected around a caesium absorption device, which is also designed to remove oil and technetium.


Tepco said that greater-than-expected amounts of radioactive substances flowing into the system may be the cause of the problem. The company is considering adding more equipment to remove oil or lower the amount of water flowing into the system.


Earlier, a test run of the system on 16 June was stopped when the caesium absorption device was found to be leaking water. Tepco found that a mistakenly closed valve in the pipes had clogged the water, which then damaged a pressure-easing safety valve in one of the caesium absorption units and caused the leak.


Tepco said that it replaced the caesium absorption device on 19 June and restarted the test run of the system. However, a pump stopped working on 21 June and the company again had to shut down the system. After adjusting some valves, Tepco was able to restart the system, which now appears to be operating properly. As of 7.20 am on 21 June, some 750 tonnes of water had been treated.


Worker doses 


Tepco said that it aims to examine the radiation exposure of some 3700 workers who have worked at the Fukushima Daiichi plant since 11 March. So far, medical checks have been made on 3514 workers.


These examinations showed that 124 of them had received radiation doses above 100 mSv. Of these, 107 workers had received doses between 100 and 200 mSv, while eight workers had received doses of 200 to 250 mSv. Nine workers have now been found to have received radiation doses over the government-set legal limit of 250 mSv.


Tepco said that it plans to complete medical examinations of all the workers and produced a detailed evaluation of the results by the end of June.


Fact-finder's verdict 
Tsunami preparation at Fukushima Daiichi was criticised in the report of the International Atomic Energy Agency fact-finding mission, released this week.
Without disclosing the full story of tsunami consideration in Japan, mission head Mike Weightman concluded that "there were insufficient defence-in-depth provisions" for tsunami hazard.
The risk had been underestimated at the stage of site evaluation and plant design in the 1960s, and this remained the case despite a revision around 2002 that increased sea defences to 5.7 metres. 
Another evaluation was conducted after 2002 and further protective measures taken but these "were not sufficient to cope with the high tsunami run-up values and all associated hazardous phenomena" such as heavy impacts from floating debris. "Moreover, those additional protective measures were not reviewed and approved by the regulatory authority." The waves that actually struck the plant in 2011 measured over 15 metres.
The nature of flooding means that systems and components can fail suddenly rather than gradually and this left the plant unable to cope with tsunami above its defences, "leading to cliff-edge effects" from which recovery was impossible.



Researched and written

by World Nuclear News