The temperature of the reactor core of unit 3 of the damaged Fukushima Daiichi plant is expected to drop significantly with the commissioning of a more efficient cooling system, according to Tokyo Electric Power Company (Tepco).
The company said that it plans to put into operation a system for spraying cooling water directly into the core of the reactor. This core spray system - which is positioned above the fuel - would be used in parallel with the feed water system currently in use, which injects water into the bottom of the reactor pressure vessel (RPV). Some six cubic metres per hour of water is currently being injected into the RPV using this system.
|The core spray system will work in tandem with the feed water system (Image: Tepco)
Tepco said that operating the core spray system would offer "efficient cooling potential." It said that it had confirmed that the injection valve of the existing system could be operated electrically. The company also said that a gauge would be installed to allow the flow of water through the jet pump to be measured.
The core spray system is set to be commissioned on 25 August.
Tepco said that, while the feed water system continues to inject 6m3/h of water into the RPV, the flow of water through the core spray system will be increased incrementally to 3m3/h over the first three days of operation. This total level of water injection - 9m3/h - would then be maintained over two days and the temperature of the RPV monitored. Then, over the next four days, the flow of water through the feed water system would be gradually reduced to 3m3/h, again with the RPV temperature being monitored. After that, both systems would be injecting 3m3/h of water into the reactor, for a total flow of 6m3/h of cooling water.
The temperature at the bottom of the reactor vessel of unit 3 is currently around 108°C. However, Tepco said that it expects the use of the core spray system to reduce the temperature of the fuel and core internals sufficiently so that they are being cooled directly by water rather than steam, making the cooling system more efficient and resulting in a significant reduction in temperature inside the RPV.
Tepco suspects that most of the fuel in unit 1 at the plant melted and fell to the bottom of the reactor vessel, where it has solidified and been kept adequately cooled. Fuel melt has also occurred to a lesser extent in units 2 and 3, with most of the fuel in those units thought to still be on the support plate. The temperature at the bottom of the reactor vessel of unit 1 is now around 85°C, while that of unit 2 is still over 114°C.
Cold shutdown at all three reactors - when the heat from radioactive decay has been removed effectively enough for coolant water to be below 100ºC - is the goal of the second phase of Tepco's roadmap for stabilisation.
Researched and written
by World Nuclear News