How The Fukushima Daiichi Sea Wall Was Damaged
TEPCO announced last week that the steel and concrete sea wall they installed and recently closed the last section of had been damaged. The wall was bowing outward due to increased groundwater pressure. A recent presentation to METI shows why.
Steel pilings to close the opening near unit 4 happened around September 22. The final concrete sealing work took place around October 26th.
TEPCO monitored water levels in the amount of ground water they removed from wells near the reactor buildings during this closure process. They also moved from daily manual water removal to an automated 24 hour water removal system over this time. What may have caused part of the miscalculation is the amount of rainfall received during the time they began the well pumping through the closure of the wall. In the graph below for unit 1, rainfall can be seen as the lower blue graph. Rainfall began to taper off before they began the wall closure and remained low during that entire procedure. This may have given a false reading of the effectiveness of the well pumping program.
October 2015 rainfall in the area was unusually low, with only 9 mm of rain when this month usually has roughly 48-400+ mm of rain in October. The only other single digit reading for this station in Fukushima City was back in 1898.
Sea front drains were added to the water removal process on November 5th. Even with pumping out groundwater near the reactors and near the sea wall was not able to mitigate the water build up near the sea wall.
These groundwater level maps show the progression of increasing groundwater levels as work took place in October to November.
The levels near the sea wall can be seen slowly rising over time while the land side section lowered in groundwater levels.
The hydraulic “head” pressure in the groundwater also increased near the sea wall quickly as seen in this illustration with the last date being November 9th.
Also mentioned in this report is the depth for the basemat of each reactor building. Unit 1 appears to be quite shallow with each newer unit built slightly deeper down into the soil levels. While this doesn’t impact the sea wall itself this is of note related to water issues at each reactor.
TEPCO had previously began operation of the land side section of the frozen wall. This appeared to be successful but was shut off in August. TEPCO announced in October that they would commence operation of the frozen wall by the end of 2015. No explanation for why they didn’t allow the land side section to stay in operation after what they now call a test run.
A combination of the low rainfall in October and allowing the frozen wall to go out of operation may have contributed to this increase in groundwater height and pressure at the sea front. While the sea front area has seen rises in groundwater and pressure, the land side sections have seen a slight decrease. Right now it is not clear if this will equalize over time or continue to rise at the sea front. Without some method to block or release sufficient amounts of water it could continue to be a problem or get worse. So far TEPCO has not given a clear explanation how they will confront this problem before the frozen wall is in full operation.
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