Fukushima Update #73 – January 2, 2012
TEPCO admits the January 1, 2012 earthquake caused it, and no it wasn't a leak but the water from the Spent Fuel Pool went the other way to the Reactor Well, instead of going to the Skimmer Surge Tank.
I started reading the Mainichi Shinbun article, but it was very short on details, and knowing TEPCO couldn't have held a press conference on January 2 I went to look for TEPCO's own words somewhere. It was in the handout for the press on January 2, 2012.
The handout is in Japanese only for now, so the following is my translation. It was clearly written over time as new information came in. (I will put up the English reference handout when TEPCO issues one.)
At 5:30PM on January 1, it was observed that the water level of the Skimmer Surge Tank of the Spent Fuel Pool of Reactor 4 dropped by 240 millimeters during the 3 hours from 2PM to 5PM (while the normal decline is 50 millimeters in 3 hours). Later we surveyed the facilities, but found no leak outside the reactor building, at the pipe joints of the SFP cooling system or at the location where the cooling system was installed.
As of 5PM on January 1, the temperature of the water in the Reactor 4 SFP was 23 degrees Celsius (as of 5AM on January 2 it was 22 degrees Celsius). The SFP cooling system is in operation and there is no problem in cooling the pool. While the water level in the SFP remained the same, the water level in the Skimmer Surge Tank continued to drop. From 10:27PM to 11:13PM on January 1, we filled the water in the Skimmer Surge Tank. Currently the water level is declining at about 90 millimeters per hour. We are monitoring the water level every hour instead of every 3 hours. As of now there is no leak outside the building, and there is no noticeable change in the water level of the contaminated water inside the [reactor] building.
Later inspection revealed that the amount of water decreased in the Skimmer Surge Tank was about the same as the amount of water increased in the Reactor Well, and that the water level in the Reactor Well was lower than that in the SFP. Therefore, our hypothesis is that: the earthquake on January 1 at 2:30PM tweaked the space in the gate between the Reactor Well and the SFP; water flowed from the SFP to the Reactor Well, and the overflow water to the Skimmer Surge Tank decreased, causing the water level of the Skimmer Surge Tank to decrease more than normal.
[Nelle's comment: So the earthquake broke something and allowed cooling water to flow into the reactor well instead of the tank set up to catch it for decontamination and recirculation. Hmmm…my first question is: Is this the only damage to any of the reactors?]
In order to decrease the difference in the water levels between the Reactor Well and the Spent Fuel Pool, water was poured into the Reactor Well from 11:50AM to 11:59AM on January 2. As of 4PM, there is no decrease in the water level in the Skimmer Surge Tank observed. We will continue to monitor the water level in the Skimmer Surge Tank.
Skimmer surge tank is a tank set up to collect the overflow from the Spent Fuel Pool. In order to cool the spent fuel bundles and to remove impurities from the water, the water in the Spent Fuel Pool is overflowed into the skimmer surge tank, and goes through the heat exchanger and the filter before it goes back into the Spent Fuel Pool.
The Reactor Well is a space to store the Reactor Pressure Vessel and the Containment Vessel Lid. During the regular maintenance [which Reactor 4 was undergoing at the time of the accident], this space is filled with water to conduct work such as exchanging the nuclear fuel.
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Here's a comment from Physics Forum:
Re: Japan Earthquake: nuclear plants
________________________________________
Originally Posted by Yamanote
Let's get back to presence, they are now dealing with a leak in the skimmer surge tank and/or the spent fuel pool of unit 4:
http://www.tepco.co.jp/en/press/corp-com/release/12010201-e.html
I think this could become a quite dangerous incident.
Indeed, here is relevant part:
"At approx. 5:30 pm on January 1, 2012, we observed approx. 240 mm
decrease in the water level of the skimmer surge tank*1 of Unit 4 spent
fuel pool in the three hours between 2:00 pm to 5:00 pm (According to the
operation record so far, there had been an approx. 50 mm decrease.). As a
result of the site investigation later, we did not observe any leakage
around the Unit 4 reactor building, connecting points of primary system
pipes of Unit 4 spent fuel pool alternative cooling system, or its
installation space. The water temperature of Unit 4 spent fuel pool as of
5:00 pm on January 1 is 23°C (22°C as of 5:00 am on January 2). The spent
fuel pool alternative cooling system is still in operation and there is
no problem in cooling the reactor. Though the water level of the spent
fuel pool is kept stable without any problem, the water level in the
skimmer surge tank continues decreasing. Therefore, from 10:27 pm to
11:13 pm on January 1, we filled water in the skimmer surge tank. At
present, the water level in the skimmer surge tank keeps decreasing
approx. 90mm/hour. We will strengthen surveillance on observation of the
water level in the skimmer surge tank, increasing the frequency to once
in an hour from once in three hours. At the same time, we will continue
examination of the causes of the decrease in the water level, such as
site investigations based on such possibilities as the water may flow
into the reactor well*2 from the spent fuel pool.
At present, no leakage is observed outside the building. No significant
change is observed in the water level of the accumulated water in the
building.
*1 The tank installed in order to receive the water overflows from the
spent fuel pool. The water in the spent fuel pool is overflowed into
the skimmer surge tank in order to cool the fuel assembly and remove
impurities in the water. The water returns to the spent fuel pool after
passing through the heat exchanger and the filter.
*2 The reactor well is the space which contains the reactor pressure
vessel and the lid of the reactor containment vessel. During the
periodical inspections, the space filled with water and the fuel is
changed.
-At this moment, we don't think there is any reactor coolant leakage
inside the primary containment vessel."
Here is my own commentary. I found some drawings in an IEAE Report that will help to understand what is going on.
http://www.iaea.org/newscenter/focus/fukushima/japan-report2/chapter-2-2.pdf
Page 48, Figure II-2-50 Installation of Support Structure at Bottom of Spent Fuel Pool of Reactor Building of Fukushima Dai-ichi Unit 4
The SFP is on the upper left with the red concrete-encased steel pillar below it. The reactor well and pressure vessel are in the middle shown full of water as explained in the TEPCO press release above.
Pages 44 and 45 of this document contain detailed drawings of Unit 3 and it's alternate cooling system which would be similar to Unit 4 arrangement.
They show the Gate, the SFP, the Reactor Well and the Skimmer Surge tank as discussed in the press release.
The schematic on page 44 shows the skimmer surge tank adjacent to the SFP
The GATE where TEPCO says the earthquake "tweaked the space in the gate between the Reactor Well and the SFP; water flowed from the SFP to the Reactor Well…"
As you can see from the diagram, a decrease in water pressure in the SPF allows water to flow from WELL into the SFP. This shows what happened on March 12th or shortly thereafter.
So for water to flow the other way, the reactor well level would have to be well below the SFP level and the gate would would have to be damaged or shifted by an earthquake (as it was).
Since they are keeping the SPF full and letting it overflow into the Skimmer Surge Tank, we must assume the reactor well was empty,for it to be possible the water flowed from the SPF to the Reactor Well.
Since they have now refilled the well:
"In order to decrease the difference in the water levels between the Reactor Well and the Spent Fuel Pool, water was poured into the Reactor Well from 11:50AM to 11:59AM on January 2. As of 4PM, there is no decrease in the water level in the Skimmer Surge Tank observed. We will continue to monitor the water level in the Skimmer Surge Tank."
Questions: Was the gate actually permanently damaged or just shaken open and is now closing properly?
The water flow dynamics are questionable. Hopefully someone will explain this better.