Units 1 to 3 at the Fukushima Daiichi plant are all on track to be declared in cold shutdown by the end of the year, in line with the schedule set in the restoration roadmap, according to plant owner Tokyo Electric Power Company (Tepco).
|The reactor building cover is now in place at unit 1 (Image: Tepco)
Cold shutdown, Tepco said, would be declared once the temperature at the bottom of the pressure vessel of each reactor is being effectively maintained at below 100ºC and the release of radioactive materials from the units is "under control and public radiation exposure by additional release is being significantly held down." These conditions - the goal of the second phase of Tepco's roadmap for stabilisation - have now almost been met.
According to Tepco, the temperature at the bottom of the reactor pressure vessels (RPVs) of units 1 and 3 have stabilized below100ºC: as of 17 October, the temperatures were 73.7ºC and 72.8ºC, respectively. The temperature of the RPV of unit 2 remains slightly higher, at 82.3ºC. However, Tepco said, "By changing the water injection volume on a trial basis, it has been verified that unit 2’s RPV bottom temperature can stabilize below 100ºC." The company said that it is currently injecting water into the units at a volume of some 3.7 cubic metres per hour (m3/h) at unit 1, about 10.4 m3/h at unit 2 and around 10.2 m3/h at unit 3.
The amount of electricity generated in Japan during the first half of fiscal 2011 was 8.1% down from a year earlier, according to data from the Federation of Electric Power Companies.
Some 456.06 terawatt-hours (TWh) of electricity were generated by Japan's ten electricity utilities between April and September 2011. Nuclear generation fell 46.2% to 73.98 TWh as a result of extended reactor shutdowns following the Fukushima Daiichi accident. In turn, the capacity factor of Japan's power reactors fell 67% to just 34.9% during the six month period.
The reactor shutdowns led to a 12.6% increase in output from thermal (gas and oil) power plants, to 267.52 TWh.
Tepco has now installed a system that enables various parameters of the damaged reactors to be centrally monitored from the main anti-earthquake building on the Fukushima Daiichi plant site. The system can monitor such parameters as the water injection volume, injection pressure, buffer tank water level and the operational status of the accumulated water treatment system.
With regards to the release of radioactive materials from the damaged reactors, Tepco said that these are now estimated to be eight million times lower than at the height of the accident. By measuring the airborne radioactivity levels in the upper parts of the reactor buildings as well as in the sea, the company estimates that the current maximum total release rate is some 0.1 billion Bq/h, compared with the 800 trillion Bq/h estimated on 15 March.
Tepco announced on 14 October that it had completed building a cover over the damaged reactor building of unit 1 in order to reduce the dispersion of radioactive materials from it. The company aims to complete the installation and testing of water injection pipes for the unit’s spent fuel pool, near the top of the reactor building, by the end of October. Work to clear debris from the top of the damaged reactor buildings of units 3 and 4 has already begun in preparation of the construction of similar covers for those.
Tepco has been working towards a target of reducing radiation exposure at the site boundary to no higher than 1 millisievert per year (mSv/y), excluding the effect of the radioactive materials already released up until now. Readings from monitoring points around the plant indicate that the radiation exposure at the site boundaries is 0.2 mSv/y.
A total of some 128,140 tonnes of contaminated water from the tsunami itself, the injection of water to cool the reactors and from heavy rains has now been treated. Tepco said that the accumulated water level is being maintained at the present target level, which it says is sufficient to withstand further heavy rains as well as long-term processing facility outages.
Tepco said that it would ensure that cold shutdown conditions are met by "carefully assessing the reactor pressure vessel bottom temperatures, current release rate of radioactive materials from primary containment vessels, together with the radiation exposure due to this release and the securement of the mid-term safety of the circulating cooling system."
Researched and written
by World Nuclear News