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Table 4. Corrosion Testing of reactor materials in decontamination chemicals Austenitic Stainless Nickel Low Carbon Non Steel Base Alloy Steels Steels Metallic Welds* Process CAN-DECON CAN-DECON (Fe) 18,19,20 18,19,20 18,20,21 18,19,21 28 19,20 or EDTA and 21,14,22,25, 21,22,28 22,28 22,28 28 25 Improvements 26,28 AP/CAN-DECON 17,18,19 19 28 19 21,14,26 20,27 25 24 19 CITROX and 19,20,21, 16,20, AP/CITROX 14,23,24, 23,24,27 25,27 15,19,25, 26 LOMI 12,15,18,19, 1,11,18,19, 18,19 18,19,21,26 24 10,21,24,25, 20,21,24,26 20,21,26 26 AP/LOMI 12,15,17 12,23 15 15,19 NP/LOMI 12,15,18,16, 12,15 18,8 12 21 Numbers shown denote references. *Mostly type 304, 316 stainless and Inconel 182
Chemical decontamination technology 55 Corrosion issues require data dealing on all materials to be wetted by the decontamination chemicals. In particular, avoidance of IGSCC must be established with a high degree of reliability. 4.13 Discussion Substantial progress has been made in recent years on documenting corrosion effects of decontamination solvents on B~ plant materials and on understanding the observed effects. LOHI continues to appear innocuous and is fully qualified for BWR piping system applications. Some adverse effects (IGA, pitting, rapid general attack) have been observed with dilute organic acid reagents. These effects can be minimized by tight control of reagent chemistry, including ferric ion content, but creviced configurations remain of concern, because chemistry control is difficult in crevices. Laboratory studies suggest that the presence of oxalic acid is necessary for IGA to occur. Additional tests on low alloy steels are in progress. Recent data indicate minor cracking of low alloy steel exposed to the nitric permauaganate (NP) preoxidatton process. There was no cracking on identical specimens exposed to the alkaline permanganate (AP) or LOMI processes. Here again, utilities have the choice of using a preoxidizing step before LOMI to get the best possible decontamination factor, or using IX)HI alone, which is quicker and cheaper with no corrosion concerns, but which will not be so effective on high chromium oxides. A major step forward has been the generic approval given to the LOMI process by GE Nuclear Energy for full system decontamination. This resulted from an in-depth review of corrosion data; the stated provisos, including a plant specific review, should not delay the implementation of the technology. GE did not endorse the NP or AP preoxidatlon steps, but these should not be required for BWR full system decontamination in view of the improved efficiency that results from the use of the reactor coolant pumps. In summary, process restrictions to minimize corrosion have been defined and there is ample data available now for utilities to select a process for their particular application. 5. RECONTAHINATION 5.1 Introduction Recontaminatlon occurs when the freshly-decontamlnated surface picks up activity, as the surface reestablishes its protective corrosion film. Inltlally, corrosion is rapid, but it slows down as adherent passive oxides cover exposed surfaces. When part-system decontamination has taken place, the reactor coolant will contain high concentrations of radioactive species, which were not present during the original plant commissioning time period when corrosion films were inltially developed on these surfaces. Consequently the new, rapldly-developlng corrosion film will incorporate significantly higher concentrations of radioisotopes and cause a rapldly-lncreaslng radiation fleld in the vicinity. Early part-clrcuit decontaminations of B~/Rs that used concentrated reagents resulted in extremely rapid recontamlnatlou rates. In one case, a reactor pilot loop reached radiation levels higher than before the decontamination in 500 hours. Fortunately, the recontamlnatlon problem, although still slguiflcant, is less severe when dilute chemical processes are used. The most likely explanation of the high recontamlnatlon rates observed after use of concentrated reagents is that these reagents were actually corrosive, removing the entire protective film and roughening the underlying surface. A similar phenomenon was observed on steam generator channel heads decontaminated using a dry grit-blastlng process, which exhibit increased buildup of radioactive materials after return to power(38). In several cases, radiation fields in the decontaminated channel heads became slgnlflcantly higher within a few years than equivalent channel heads that had not been decontaminated. Recontamlnatlon rates in channel heads following dilute chemical decontamination are generally not so great. 5.2 Recontamlnation rates As a general rule, recontaminatlon rates following dilute chemical decontamination ere similar to or slightly greater than Inltlal radiation buildup rates on new plants, but less than buildup rates ou new components such as replacement reclrculatlon piping in BNRs or replacement steam generator channel heads in PNRs.
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