Possible Causes Of Fukushima Frozen Wall Failures

TEPCO reported last week that roughly 10% of the frozen wall is not working. Currently only certain sections are being actively frozen with other sections to possibly be frozen as they determine the cause and effect of the initial work.

One of the sections being frozen with some problems is near unit 1 on the sea side. TEPCO claimed that accumulated groundwater and gravel in the area may be contributing to the problem. TEPCO also indicated the wall is “riddled with holes”. These problems appear to be delaying the start for freezing of other sections of the wall. One action being considered is to put cement or other solidification agents in the problem areas. We  looked at these problem areas and what factors may be playing a role.

These thermal diagrams by date show what sections of the sea front area of the frozen wall near unit 1 are struggling to freeze.

Ice_Wall_U1_U2_0419to0517

This section has had some progress but a section on the left side at OP 0 (this is an elevation level equal to sea level) shows red as it is struggling to freeze over just short of a month. One problem with this thermal reporting is that the top of the scale is anything over 10c. How much over 10c is then unknown for these specific spots. The temperature graphs are not produced in a way to correspond to these small holes.
The May 19 handout in English can be found here.

A newer handout has since been released dated May 24 and it appears to show some small progress in this area compared to the May 17 diagram.

May24_section1

A few red sections shifted to orange and many of the yellow sections have shifted to blue. This newer handout also includes rates for pumping up groundwater out of the vertical shaft b of the unit 1 seawater pipe trench. This is the trench indicated in white in these thermal handouts. This trench runs through the area of concern in the frozen wall. Since the land side section of this area has not been frozen yet, it may be allowing water uphill to flow through the area.

We took the vertical thermal diagrams and matched them up on a larger map of this area to show where these hot spots are related to everything else.

frozenwall_warmpoints_u1_u2 Red boxes are the warm spots at OP 0 along the frozen wall shown as blue lines. Most of these spots are north of unit 1’s turbine building.

TEPCO explained some of the groundwater flow paths in this area back in 2013. This was related to a plume of contamination that is migrating towards the sea among the groundwater. TEPCO noted how the contamination at the level of OP 0 would migrate over the first three years of the disaster. We took two of these contamination plume maps and did an overlay on the frozen wall map.

turbineleak_plumes

This shows the color key concentrations of contamination three years after the initial disaster. Both the concentrations and flow paths seem to migrate through this area where the wall fails to freeze. Red indicates the areas of higher contamination.

frozenwall_plume These sweeping lines on the graphic above show water flows and potential contamination migration. Blue indicates the earlier locations and the yellow to red signifying the 3rd year locations. These water flow paths show a concentration where the wall is struggling to freeze. As mentioned above, this is also the general area where they are now pumping out increasing amounts of groundwater out of the unit 1 seawater pipe trench vertical shaft. On this graphic above, a small green box is shown on the side of the unit 1 turbine building. This is the rough location of an opening in the lower levels of the building where highly contaminated water leaked out. TEPCO later reported they cemented in this section where some pipes exited the building basement.

Another concern we looked at was the potential for a heat source to contribute to this problem. Japan has many areas with active hot springs that can bubble up, usually into rivers. While there is no data confirming that there is a hot spring in the area or that one could be contributing to the temperature of the groundwater that flows through the plant, it is a distant concern. There have been some water temperature problems discovered at the plant. There was an early effort to freeze a trench that exited the unit 2 turbine building. This effort failed and the trench was eventually concreted in. Water exiting the turbine building into the trench was warmer than water standing in the trench. The exact mechanism of the water being heated was not explored by TEPCO. Thermal retention of the building or influence from the corium could contribute to slightly higher temperatures that caused this problem.

Along with this problematic section north of Unit 1, a section south of Unit 4 had a few problems with fully freezing but on a smaller scale than the one near Unit 1. This could be again, a problem of concentrated groundwater flows known to be in that area.

TEPCO’s plan to dump concrete in these areas that are struggling to freeze may just result in re-routing the cause of the problem to another section of the frozen wall. Attempting to freeze the land side section of this region of the frozen wall system could help the sea side freeze by reducing incoming water flows. It could also show if there is enough groundwater flow on the land side to make that wall section also problematic.

 

 

 

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