DEVELOPMENT OF A MODIFIED ION EXCHANGE RESIN COLUMN FOR REMOVAL OF GADOLINIUM FROM THE MODERATOR SYSTEM OF PHWRS

摘要

Gadolinium is used as neutron poison in PHWRs. It is injected to the heavy water (D_2O) moderator at 15 ppm concentration during emergency shut down and at 2 ppm during the start up of the reactor. As gadolinium precipitates under neutral and alkaline conditions, pH_(app) range of 5 - 5.5 is recommended for maintaining gadolinium in the solution and to minimize corrosion of moderator system structural materials and radiolysis. Gadolinium has to be progressively removed as the reactor goes to power. It is removed by the purification system containing ion exchange resin columns. It has been observed that gadolinium precipitates in the ion exchange columns. Experimental investigations were carried out to understand the phenomenon of precipitation of gadolinium in the weak base ion exchange resin. It was found that the strong base groups present in the weak base resin was responsible for the precipitation of gadolinium. Hence, attempts were made to selectively cap the strong base groups present in the weak base anion exchange resin. It was found that the strong base groups of weak base resin can be selectively saturated by treating the resin with 0.2 M NaNO_3 and it prevented Gd precipitation. Based on studies carried out on gadolinium pick-up and radiation decomposition with weak base anion exchange resins, a simple mixed bed consisting of nitrate treated weak base resin of poly acrylic based macroporous resin and a strong acid cation exchange resin has been recommended for the removal of gadolinium for both 2 ppm and 15 ppm concentrations as it does not precipitate gadolinium and gives optimum chemistry parameters in the outlet.