Two workers at the damaged Fukushima Daiichi plant have reportedly been exposed to radiation amounts exceeding the safety limit set by the government.
According to tests on two control room operators conducted by the National Institute of Radiology Sciences, a worker in his 30s internally absorbed an estimated 218 to 580 millisieverts of radiation, while another worker in his 40s received between 200 and 570 millisieverts. Most of the radiation, the institute said, was in the men’s thyroid glands. The two men are reported not to be showing immediate health problems.
An earlier test suggested the younger man received about 74 millisieverts of external radiation and the other about 89 millisieverts.
Shortly after the accident began, the Japanese government raised the radiation limit for workers at the site from 100 millisieverts to 250 millisieverts.
Plant operator Tokyo Electric Power Co (Tepco) said that safety measures, such as wearing protective clothing and masks, just after the accident may have been inadequate. It wants to conduct detailed tests on some 150 workers involved in similar operations to the two men.
Workers in protective suits are currently working in hot and humid conditions within the reactor buildings. Two workers have been treated for dehydration, while a further nine have suffered from heatstroke. Tepco is advising workers to wear vests containing cooling gels underneath their protective suits. As the summer weather arrives, conditions for the workers are likely to get tougher.
There are now some 105,000 tonnes of water across the six reactor units at the Fukushima Daiichi site: in reactor building basements, turbine building basements and trenches. The 14,000 tonnes of this at units 5 and 6 is uncontaminated, but much of the remainder presents a serious radiological hazard. The continued pumping of water to cool the reactors and recent heavy rain has exacerbated the problem of flooding.
Tepco is set to start operations on 15 June to filter the highly radioactive water. It has now begun testing the filtering system. However, the company warned that, in the worst case scenario, highly radioactive water that has built up in a trench at unit 2 could overflow by 20 June, or even sooner if there is more heavy rain.
The utility plans to treat 1200 tonnes of water daily at a storage facility and transfer the water to temporary tanks. The treated water will then be used to cool the damaged reactors. Tepco has already set up tanks for 13,000 tonnes of filtered water and will increase storage capacity by 20,000 tonnes per month.
A total of 370 tanks with a combined capacity of over 40,000 tonnes will be installed at Fukushima Daiichi. 170 of the tanks will have a capacity to store 120 tonnes of water, while the remaining 200 tanks can store 100 tonnes. The first two such tanks arrived were delivered to the site on 4 June.
Steam and radiation
Steam has been found to be escaping from the basement of unit 1 of the damaged Fukushima Daiichi plant into the first floor of its reactor building. Radiation levels in the vicinity are very high.
Tepco reported that a remote-controlled robot surveying the first floor of the reactor building of unit 1 discovered the steam leak on 3 June. The steam - coming through a hole in the floor through which a pipe passes - is thought to be from water at a temperature of some 50°C that has built up in the basement of the reactor building.
Radiation levels in the area near the leaking steam were found to be between 3000 and 4000 millisieverts - the highest level detected in the air at the plant so far. However, Tepco said that it does not expect the high radiation level to affect ongoing work in the unit as it is confined to a limited section of the reactor building, where workers do not yet need to enter.
The discovery of the steam and high radiation level further confirms that the reactor pressure vessel and containment vessel have been damaged by molten fuel in the unit.
Tepco said that the pressure inside the reactor vessel of unit 1 is now only slightly higher than the outside atmospheric pressure. Measurements from a new pressure gauge installed on 3 June showed that the internal pressure was just 1.26 atmospheres. The utility said that this proves that air inside the reactor is escaping. It also plans to install new gauges at units 2 and 3, where large-scale fuel melts are also suspected, to check their pressures.
Researched and written
by World Nuclear News