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More on the subject of clay and cesium

Dear Reader,

It has come to my attention that only a small fraction of the debris created by the tidal wave in Japan has been disposed of. A report from Japan indicates that only 6 % of the waste has been disposed of, part of the problem it that due to concerns that the debris is radioactive the disposal actions (land filling and incineration) has not been done.

I hold the view that the national government in Japan needs to devise a safe, environmentally acceptable and cheap plan for dealing with the waste. Then it should impose the solution using national law, my worry is that if consent needs to be obtained from each municipality then it is likely that nothing will ever happen. If the Japanese do nothing then after 300 years the waste will be almost totally non radioactive (regardless of the radioactivity in it today) but I do not think that they can wait 300 years for the problem to go away on its own.

I think that the debris should be sorted according to its radioactivity, the debris below 400 Bq per kilo should be treated as non radioactive waste. I hold the view that if the waste is slightly over 400 Bq per kilo but bulk of the additional radioactivity is due to short lived radioisotopes then the waste should be either be allowed to stand for some time before being burnt in a normal incineration plant or it should be land filled.

I think that the contaminated top soil from schools, residential areas and commercial areas (shopping areas) is an important thing to deal with. While in the ideal world it would be best if this decontamination waste was either made to vanish into thin air by a magic genie but in the world we are stuck with someone needs to do something with the soil. While a good quality hazardous waste landfill would be the best place to put contaminated soil, I think that for low level beta active decontamination waste it is better to put it in an ordinary landfill rather than leaving this waste at the site it was dug up from.

If the waste is a lot more than 400 Bq per kilo then it needs to be managed as radioactive waste. One option is to land fill it. The land fill must be equipped with a barrier layer to prevent the radioactivity leaking out of the landfill. If it is cesium then one of the best barrier layers is clay. The cesium will bind to clay, I have found in the literature a crystal structure of a cesium exchanged clay. (D. Gournis, A. Lappas, M. A. Karakassides, D. Többens and A. Moukarika, Phys. Chem. Minerals, 2008, 35, 49-58).

The clay is a layered solid, it has an set of anionic layers of aluminium, oxygen and silicon. These layers are like slices of bread, and the cationic cesium ions go between the slices of “bread” to form layers. I think that the strong binding of clay to cesium will slow the movement of cesium in the barrier layer so that the cesium will decay by the time it escapes from the landfill.

Here is a picture of the clay with the cesium in it. Aluminium is green, oxygen is orange, silicon is purple and the cesium is blue.


The clay with cesium cations in it.


If the waste is very radioactive then it may be best to put it into drums, add some cement to make the waste into a hard block (less dust and no chance of a liquid spill). The steel drum will then improve the safety of the transport of the waste to the landfill and it will give some decades of additional containment. If the waste is only cesium-137 then assuming that we choose the right drum then it could last for several half lives. A carbon steel is normally passive if it is in contact with cement so we will not get much corrosion on the inside of the drums. If the outside of the drums are painted or better still galvenised then the drums will be very long lasting.

Now whatever the extreme greens say what you need to do is to isolate the waste from humans for a finite time, they hard part is making a choice of how low the threat has to be at the time when you lose the ability to contain the waste. Once the experts work out how much much activity the dump can release per day without breaching the limit then we can work out a design for the land fill.

For example if our release limit is 1 kBq per day and we know that the dump will release 1 % of its contents per year after the barrier has failed. Then if the dump starts with 1 GBq of activity then we can work out how long the dump must contain the waste.

1 kBq per day works out as 365.25 kBq per year.

So if the dump must not start to leak until the total activity is 36525 KBq (or 36.525 MBq).

So the waste must decay by a factor of 27.379 before it starts to leak.

As A = Ao exp -λt

We can work of how long t has to be, if the half life is 1 year, then λ will be 0.693 year-1

exp -λt must be equal to 0.03652 at the time when leaking can start.

So we start with

exp -λt = 0.03652

-λt = ln (0.03652)

-t = ln (0.03652) / λ

t = ln (0.03652) / -λ = -3.3098 / (-1 x 0.693 year -1)

t = -3.3098 / – 0.693 year -1 = 4.77 years

So if we can build a waste store which will not leak for five years then we can be sure that not release too much radioactivity per day. I choose a half life of one year to make the maths easy but the same ideas can be used with real waste dumps.


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