Pick up of cesium and cobalt activity by oxide sludge in steam generator

A pinhole developed near the tube sheet in the steam generator (SG) tube of a pressurised heavy water reactor (PHWR) caused leak of primary coolant containing radioactive contaminants (¹³⁷Cs, ¹³⁴Cs and ⁶⁰Co) to the shell side. The sludge collected from the tube sheet region was found to have adsorbed these radionuclides at the high temperature (230-240 °C) that prevailed in the SG. An attempt has been made to evaluate the quantity of activity retained in the various oxide phases that constitute the sludge and their mode of pick-up. The sludge was characterized by XRD and XPS, which showed the presence of various oxides of iron, copper and nickel along with the silicates of calcium, magnesium and aluminum. Gamma-spectrometry of the sludge confirmed the presence of ¹³⁷Cs¹³⁴Cs and ⁶⁰Co to an extent of 7.6, 1.3 and 0.9 mCi/g of sludge, respectively. Selective dissolution in various EDTA based formulations and equilibration with nitric acid and magnesium chloride solutions helped to understand the quantity of activity adsorbed by various constituents of the sludge. It was concluded that a major portion of cesium was picked up by a reversible ion exchange process on various oxide constituents and about 10% by an irreversible specific adsorption process on insoluble silicates. Also, it was proved that ⁶⁰Co was specifically adsorbed over the oxides of iron and nickel.