(or is supposed to work, considering numerous leaks...)
Spent Fuel Pools of Fukushima I Nuclear Power Plant, particularly that of Reactor 4, continue to be in the news inside and outside Japan.
Dr. Hiroaki Koide of Kyoto University spoke in New York on May 3 and held a press conference on the status of Fukushima I Nuclear Power Plant and radiation contamination in Japan as he sees it.
He said about the Reactor 4 Spent Fuel Pool (I'm reading off from the Japanese transcript),
So-called reinforcement work was done [on Reactor 4 SFP]. (My translation, not the translation at the press conference)
Then, one of my blog readers sent me a link to the post that has an audio clip of a radio program, in which nuclear expert Arnie Gundersen seems to be saying that TEPCO has been pumping water into the Reactor 4 Spent Fuel Pool from the top, the water drains to the basement, and TEPCO is pumping the water from the basement back into the SFP.
That's news to me, but then I haven't carefully checked how the cooling of the Spent Fuel Pools at Fukushima I Nuclear Power Plant has been done after TEPCO installed the temporary heat exchangers. So I decided to look for info, and here's what I've found.
Asahi Shinbun had an article on the temporary cooling systems for the SFPs back in June last year, and here's their simplified diagram of the system (missing the Skimmer Surge Tank):
Here's what TEPCO released on July 9, 2011 after they tested the pipes after installing the heat exchanger and the air cooling system (secondary cooling) for Reactor 4 SFP:
Here's TEPCO's diagram of the same system, released on April 16 this year after the Reactor 4 SFP cooling system stopped due to a leak (the information was released only in Japanese; I labeled "Heat Exchanger"):
As far as I can tell, it is basically a closed system though leaks have happened. The excess water from the Spent Fuel Pool goes to the Skimmer Surge Tank, which then is then goes through the heat exchanger to remove heat (0.9MW in the Reactor 4 SFP, as per TEPCO's press release above). The water is then pumped back into the SFP.
Back in February when the water level of the Reactor 4's Skimmer Surge Tank was dropping more than usual, TEPCO injected the water from the bottom of the Reactor Pressure Vessel (which is filled with water). That water went from the RPV to the Reactor Well to the SFP to the Skimmer Surge Tank.
Contaminated water in the turbine building basements in Reactors 1, 2, 3 and 4 gets treated through cesium absorption towers (Kurion or SARRY) and desalination systems, then pumped back into the Pressure Vessels but not to any of the Spent Fuel Pools.
But all my information is from publicly available sources (newspapers, TEPCO, etc.). Mr. Gundersen may have his own private sources of information that points to the collapsing Reactor 4 SFP or the SFP water boiling. (As far as I can gather, the temperature of the SFPs at Fukushima is below 30 degrees Celsius.)