#Fukushima Reactor 4 Skimmer Surge Tank Latest: Earthquake Caused the Water to Go from SFP to Reactor Well Instead, Says TEPCO

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.

(For the last announcement from TEPCO, see my post here.)

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.)

１月１日午後５時30 分頃、４号機使用済燃料プールのスキマサージタンク＊１の水位が午後２時から午後５時までの３時間で約240mm 低下していることを確認（これまでの運転実績では３時間で約50mm 程度の低下）。その後、現場確認を行った結果、４号機原子炉建屋外廻りおよび同号機使用済燃料プール代替冷却システムの一次系配管接続部や設置エリア等に、漏えいは確認されなかった。

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.

なお、１月１日午後５時現在の４号機使用済燃料プール水の温度は23℃（１月２日午前５時現在22℃）であり、現在も使用済燃料プール代替冷却システムは運転しているため、同プールの冷却に問題はない。また、使用済燃料プールの水位も維持されており問題はないものの、スキマサージタンクの水位低下は継続しているため、１月１日午後10 時27 分から同日午後11 時13 分にかけてスキマサージタンクの水張りを実施。現在のスキマサージタンク水位低下は１時間あたり約90mm で継続中であり、スキマサージタンクの水位確認を３時間に１回から１時間に１回に強化する等の監視強化を継続。なお、現時点では建屋外への漏えいは確認されておらず、建屋内の滞留水の水位にも顕著な変化は確認されていない。

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.

その後の調査により、スキマサージタンクの水位低下に相当する減少量と原子炉ウェル＊２の水位上昇に相当する増加量がほぼ同等であること、および原子炉ウェル水位が使用済燃料プール水位より低いことを確認。これらのことから、１月１日午後２時30 分頃に発生した地震の影響で原子炉ウェルと使用済燃料プール間のゲートの隙間の状態が変化し、使用済燃料プールから原子炉ウェル側への水の流入量が増加したことにより、使用済燃料プールからスキマサージタンクへのオーバーフロー量が低下し、スキマサージタンクの水位低下が通常よりも多くなったことが原因であると推定。

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.

原子炉ウェルと使用済燃料プールの水位差を低減させるため、１月２日午前11 時50 分から午前11 時59 分にかけて原子炉ウェルへの水張りを実施したところ、午後４時現在、スキマサージタンクの水位低下は確認されていない。今後も引き続きスキマサージタンク水位の監視を実施予定。

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.

＊１ 使用済燃料プールからオーバーフローした水を受けるため設置されているタンク。使用済燃料プールの水は、通常、燃料集合体の冷却および水の不純物を取り除くため、スキマサージタンクへオーバーフローさせ、熱交換器およびフィルタを通した後、再び使用済燃料プールへ戻している。

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.

＊２ 原子炉ウェルは、原子炉圧力容器および原子炉格納容器の蓋を収納している空間で、定期検査中はこの空間を満水状態にし、燃料交換などを行う。

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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