GADOLINIUM DEPLETION EVENT IN A CANDU? MODERATOR - CAUSES AND RECOVERY

摘要

Gadolinium nitrate is added to the moderator of CANDU units to maintain the reactor in a guaranteed shutdown state (GSS). In April 2008, after being in stable GSS for over 30 hours, one of Ontario Power Generation's Pickering-B units showed a gradual depletion of the dissolved gadolinium, despite purification being isolated. Further additions of gadolinium stabilized the moderator gadolinium concentration, however, since the root cause of the depletion was not immediately identified, the unit was placed in the drained shutdown state, per established procedures. The cumulative gadolinium depletion amounted to about 3200 grams, the equivalent of about 12ppm.Analysis showed the presence of oxalate in the moderator water. It is well-known that gadolinium forms a very insoluble oxalate (log K_(sp) = -29.1). Although sub-micron filtration of water samples did not show the presence of gadolinium particulate, the measured levels of oxalate, 1.2 to 2 ppm, were sufficient to react with 1.4 to 2.4 ppm of gadolinium.The source of oxalate was traced to radiolysis of dissolved CO_2 species. This unit had been experiencing chronic low-level ingress of CO_2 from the Annulus Gas System. Free oxalate ion is normally susceptible to radiolytic breakdown back to CO_2, but Gd~(3+) provides a stable sink for radiogenic oxalate, 2 Gd~(3+) + 3 C_2O_4~(2-)→Gd_2(C_2O_4)_3. Subsequent testing confirmed that gadolinium oxalate is quite stable with respect to gamma irradiation.Inspections showed well-crystallized gadolinium oxalate deposited on moderator system surfaces. Estimates indicated that about 1200 grams of gadolinium could have deposited on in-core surfaces, including the outside of the calandria tubes. That amount of negative reactivity was a concern, since it would prevent re-start of the unit.OPG, with support from AECL-Chalk River and Kinectrics, embarked on a two-pronged chemistry recovery program aimed at 1) developing solvents for dissolution of the gadolinium oxalate deposits and 2) assessing the potential for radiolytic (or other) degradation of in-core gadolinium oxalate. As expected, the oxalate was found to be readily dissolved by chelant solutions, especially the ammonium salts of EDTA. However, EDTA was less suitable for in-core cleaning, since the shutdown gamma fields (2 kGy/hour) rapidly degraded the chelant and resulted in radiogenic H_2. Inorganic solvents, including acidic solutions of cerie ammonium nitrate, persulphate salts and permanganate were also evaluated. Testing indicated that the inorganic solvents were prone to some radiolysis effects and exhibited relatively high corrosion rates for some moderator system materials.Subsequent remote inspection of in-core surfaces and characterization of the deposits on a calandria tube removed from this unit showed that the gadolinium oxalate had converted to the readily-soluble gadolinium nitrate, by mechanisms that are currently under investigation. As a result, the decision was made to refill the moderator, during which virtually all of the in-core gadolinium was found to re-dissolve. The unit was successfully returned to service, with the normal approach-to-critical confirming the absence of residual in-core gadolinium deposits. Investigations continue, under the auspices of the CANDU Owners Group, into the specific conditions favouring the formation of oxalate in the CANDU moderator and the factors responsible for the in-core conversion of gadolinium oxalate to the nitrate during the outage. Chemistry control specifications have been developed to limit the allowable levels of CO_2 species in the moderator water and cover gas.