Do silt fences stop radioactive particles in water?

Silt fence at intake canal unit 2. Curtain-like material hangs from floats and reaches to the bottom of the harbour. TEPCO photo

TEPCO  claims that  six ‘silt curtains’ or ‘silt fences’ in the inner port of Fukushima harbour have prevented radioactive particles passing through them, even though sea-water does go through.   This is a very dubious claim, and if it is not true, a lot more radiation has leaked to the ocean than previously announced.

In early April, highly radioactive water leaked from the turbine building of reactor 2 into its intake canal,   in the inner port of the harbour.   After this, TEPCO installed  silt fences near the four reactors’ water intakes, and two more at each end of the inner port.   The north end of the inner port is open to the Pacific ocean.

On May 11  about 250 tons of radioactive water spilled from the plant near unit 3, into the inner port.   Two weeks later  TEPCO said  (Press release May 21, para 2) “it is considered that most of the Unit 3 contained water is staying in the port now because of the nonproliferation measure such as silt fence set near intake channel”.

Although Unit #3 was near to overflowing  just like #2 had been before it spilled to the sea,  TEPCO was not monitoring radiation inside the silt curtains (!!),  so we have no data from there. However, the regular morning samles outside the fence were recorded, and after it spat out contaminated water on May 11,  extra samples OUTSIDE the #3  silt fence were taken that that evening,  at three places up to  several hundred metres away  (all in the inner port,  see table here).  In those eleven hours, levels of radioactive cesium at those three places rose by about 400 or 500%.   This would indicate that the  silt fence off Unit 3 did not prevent radioactive particles migrating through it to the other end of the intake canal.  A longer time sequence shows a 15 fold rise at the north end of the inner port over the next few days…

So if the silt fence off Unit 3 did not prevent radioactive water leaking into the inner port, did the second line of defence,  the silt fence between the inner port and the outer port stop radiation leaking to the open ocean?   It seems not, because radiation in the outer port rose about 10-fold over the next few days, as measurements at the shallow-draft quay show:

The Japanese nuclear regulator and most of the press (and some researchers????) have accepted TEPCO’s assurance that silt fences stop radioactive water passing through them.  The government reported to the  IAEA that  20 TBq of radiation leaked from the power station at #3 (as TEPCO estimated), but did not say if this was contained in the port by the silt fences.   TEPCO regularly  briefs the press  that silt fences work:  for example “Highly radioactive water could flow out to sea when the silt fences are opened. The fences are designed to prevent contaminated water from flowing out beyond the levee” or again from September “Opening and closing of silt fences were performed due to the blocking work by steel plate to prevent contaminated water from diffusing at the north side of the intake of Units 1 to 4 (from 09:25 to 09:55, September 18)”.

Does it matter if silt curtains stop radiation?

If TEPCO is wrong and silt curtains have not contained radiation in the port, is this a problem?   Well, firstly, it would mean that the 1.2 TeraBecquerels that TEPCO assumes is in the port could actually have leaked to the ocean, and Japan has failed to inform the IAEA and neighbouring countries.

Secondly, it would mean that other leaks will not be kept in the port.    TEPCO expects such leaks  – from the reactor buildings to the sea through underground permeable rock.  And there seem to have been several other leaks already, which TEPCO has not acknowledged.

Thirdly, it would be yet another scandalous oversight by the regulator… to accept TEPCOs word that silt fences are stop radiation, without looking closely at the data or apparently testing this theory in the laboratory.

Lastly, it is important to know if silt fences actually work, because their deployment at Fukushima might lead to them becoming a standard countermeasure in any future accident.

I don’t know what size of particles these silt fences are designed to allow through, or what is the size of the cesium particles.   Some people seem to think that the fences are impregnated with zeolite, but this seems unlikely.   Perhaps the IAEA can do some testing?

How to test if silt fences work.

It would be simple to test in the laboratory if radioactive particles are small enough to pass through silt fences.   A real-world test in the Fukushima port could be made by comparing the radioactivity in soil at the bottom of the inner harbour on one side of the silt fence with a soil sample from just outside the silt fence in the inner harbour.  TEPCO seems to assume that most of the particles released by the May 11 leak from Unit 3 have settled to the bottom of the inner harbour, so soil inside the fence should be much more radioactive that just outside it. Of course, the best way to tell if radiation has been leaking from the harbour would have been regular sampling at the harbour entrance as the tide was going out … but for some reason, samples from that location have only been take a few times, often when the tide was coming in.

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