I have been speaking to a UK company producing ion-exchange resins, to see how the filtration process would be assembled.
Once a duct has been fixed to the window(s) of Reactor 2 (the one with intact secondary containment) the gases will first need to be cleansed of H2 because of the explosion risk.
An oxidant solution such as chlorine should meet this need.
Second, a simple particle filter would be needed. The gases should then be passed through water to dissolve the radionuclides. this would also cool them as they need to be close to 40*C for ion exchange filters to work.
Next, chemical preciptiation could bring most radionuclides out of solution.
Finally, ion-exchange resins would be used for entrapment of anything left. The exact form of resin would depend on the mix of ions left in solution.
All filters and precipitates would have to be stored as radioactive waste.
Once experience has been gained with Reactor 2 gases, 1,3 and 4 will need to be clad in fabric and have ducts inserted in order to do the same, but at larger scale, with them.
The ducts can lead to manifolds, so that more filter processes can be added on as necessary.
It is therefore pretty simple technology. At the end of the treatment, the cleaned water can be cooled further, and passed back into the cooling circuit.
The technology is simple. Persuading the decision makers to do it is the difficult aspect.