Abstract
Gadolinium is used as neutron poison in Pressurized Heavy Water Reactors (PHWR) at 15 ppm concentration during emergency shut down and at 2 ppm during start up of the reactor. The pHa range of 5–5.5 is recommended based on the acidity requirement 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 has been observed that gadolinium precipitates in the purification system consisting of ion exchange columns. Experimental investigations were carried out to understand the phenomenon of precipitation of gadolinium in the weak base ion exchange resin. Attempts were made to selectively cap the strong base groups in weak base resin with nitrate to prevent precipitation of gadolinium. Based on studies carried out on gadolinium pick-up and radiation decomposition of weak base anion exchange resins, a simple mixed resin column consisting of nitrate treated poly acrylic based macroporous weak base resin and a strong acid cation exchange resin has been recommended for the removal of gadolinium. It does not precipitate gadolinium and gives optimum chemistry parameters in the outlet.
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