See also: https://www.fortum.com/products-and-services/power-plant-ser...
1) DF doesn't add up like that at low concentrations. Then it comes down to selectivity of the material. Sodium and cesium have similar chemical characteristics, but CsTreat is very efficient in selecting Cs over Na.
2) Material you are using as filter becomes radioactive waste. You want to keep the volume at minimum.
3) With CsTreat you already reach radioactive Cs levels below measurable limit. There is no reason to try to remove more Cs from that water :)
> 2) Material you are using as filter becomes radioactive waste. You want to keep the volume at minimum.
This is very reasonable, but storing sand for 50 years somewhere is technically feasible and maybe even cheaper than storing water. You can put the sand in concrete and obtain a solid block. Then put the package in some kind of coating and bury it in a safe vault. Water instead has the bad habit to leak, run away, and enter in the life chain when bad weather, monsoon or accidents happen. A huge concrete block is also less vulnerable to terrorism than a water tank. Is not so easy to steal a jar of it, for example. Even if you could make a hole in the coating, the product will not just flow out.
As far as I know, this is because about 10 PBq of Cs-137 was released during the accident and the released material is transferred by ocean currents. CsTreat is being used for purification of the water, which is pumped into and out the containment to keep core remnants cool. Nowadays the releases are very limited if any.
>This is very reasonable, but storing sand for 50 years somewhere is technically feasible...
You're right. Also CsTreat is solid, rock/sand-like material. It can be stored as mixed into concrete and disposed as low/intermediate-level radioactive waste.
Now you probably want to ask: Why there is huge amounts of radioactive water stored at the plant site? Because of tritium, which is chemically equal to hydrogen and is very expensive to separate from water. Under normal operating licenses of NPPs, you could release such water into environment, but at Fukushima standards are nowadays peculiar.
Yes, I though also about it, but I will add something to the idea of a sand filter. This is not our average dead sand, is marine sand, cleaned and oxigenated constantly by the tides. If we discard an effect of grain size, and sand composition, the really interesting hot-stuff is IMHO probably in what remains, the bacterial biofilm coating the sand.
Would it be feasible to stack filters? (E.g., 0.01 * 0.01 * 0.01...)
It doesn't involve the China Syndrome speculation though, what it does find is much much more interesting.
The paper reports that beach sand is capable of 99% adsorption of Cs137. And then goes on to discuss how it gets de-adsorbed (up to 11%) when Cs137 free sea water is mixed with it. What this says to me is that you can use really cheap beach sand to filter the discharge water from a nuclear reactor and collect 99% of the Cs137 in the water which will stick to the sand. The sand in your filter can be replaced and the cesium rich sand can be sequestered for the Cs137 to decay over time.
This is an excellent finding that suggests reactors have a reservoir of sand for pumping cooling water through especially during reactor compromise. Knowing this a future even would release significantly less Cs into the ocean, collecting it instead in the sand filter.
[1] The experiments showed that beach sands have an adequate ion exchange capacity for this level of 137Cs, with an average adsorption fraction of 99% regardless of the salinity, sediment grain size, or mineralogy (Table S3). ---https://www.pnas.org/content/early/2017/09/26/1708659114