Patent Number: 051475979
Section: description

A DETAILED DESCRIPTION OF THE INVENTION The present invention is concerned with coating the internal surfaces of steel members, generally pipes, which are used in the water system of light water nuclear reactors, with a chromium film of at least 500 .ANG. to reduce the amount of Cobalt 59 active corrosion products retained by the piping during normal use. The presently understood source of this cobalt isotope is the corrosion of the underlying steel pipe. Once cobalt from the underlying pipe enters the water coolant system through normal corrosion, it is subjected to conditions which lead to the formation of a radioactive isotope, Cobalt 59. In order to provide a film on the steel (or stainless) surfaces, the steel should first be prepared to receive the chromium film. The preparation steps typically include degreasing step followed by a brief residence time in the chromium plating bath for approximately 60 seconds prior to the application of current through the plating bath. Another preparation step involves anodic dissolution in the plating bath for approximately 90 seconds with a current density of approximately 40 amps per dm.sup.2. The plating step takes place in a common and well-known plating bath which uses chromic acid (CrO.sub.3) and sulfate (SO.sub.4), as sulfuric acid. The plating temperature is 50.degree. C. (122.degree. F.) and the current density applied to the plating bath is 40 A/dm.sup.2. Under these operating conditions, the current efficiency is claimed to be approximately 15%, which gives a plating speed of approximately 35 .mu.m per hour, that is, 97 .ANG. per second. Depending on local parameters, a deviation of up to 30% should be considered as normal. The thickness of an electrolytically plated layer is proportional to the time when the current density remains constant. However, some delay can be observed at the beginning of the deposition process due to phenomena in the cathodic layer. This delay has no importance in normal deposition where at least a few microns must be plated, but cannot be ignored in the present case. In order to evaluate this effect, it was decided to perform several tests with plating times ranging from 10 to 60 seconds and to measure the resulting thickness. These measurements were performed using the Auger Electron Spectroscopy technique. The sputtering time required for as thickness of 1,000 .ANG. of chromium was determined on a calibrated sample at the beginning and at the end of each series of measurements. It varied between 15 and 25 minutes from series to series but did not show differences larger than 5% from the beginning and the end of the measurement of samples from the same series. The results are presented in Table 1. TABLE 1 ______________________________________ Plating Time and Film Thickness Plating Time (in s) Thickness (in angstroms) ______________________________________ 10 900 20 1,900 30 2,850 60 not measured ______________________________________ Table 1 shows that in the present case, the initial delay is negligible and the deposition rate is 95 .ANG./second as shown in FIG. 1. The measurements described in Table 1 were taken at the center of the respective samples. At the periphery, a darker area was observed on the samples plated for 10 and 20 seconds. Beginning in approximately 5 mm from the edge, the thickness of the chromium layer on the first sample was 3200 .ANG., whereas there was 900 .ANG. thickness at the center. This is a consequence of the edge effect which is especially pronounced for chromium plating. To decrease the effect of this edge phenomena, several tests were performed using different types of metallic frames surrounding the sample. The best result was obtained with a cylindrical frame, 5 mm in diameter at a distance of 5 mm from the edges. In this case, the darkened area was much less pronounced coming closer to the edges than extended over a 5 mm maximum of smaller chromium thickness was expected on the test piece. As the total current remained the same as for a sample without the frame, a smaller chromium thickness was expected on the test piece. On a 316 steel specimen, the following thicknesses were measured using the Auger Electron Spectroscopy technique for a plating time of 10 seconds on samples at different places. See Table 2. TABLE 2 ______________________________________ Variation in Chromium Plate Thickness SAMPLE Thickness No. Side Position (in angstrom) ______________________________________ 1 Front 1/4 of the length 600 half of the width 2 Front 3/4 of the length 570 half of the width 3 Front corner, at 7 mm 1,300 from the edges 4 Rear center 620 ______________________________________ FIG. 2 shows the location of the sample site for each of the measurements reported in Table 2. The constancy of the thickness seems acceptable for the present application. If necessary, the periphery could be removed after exposure of the samples to the primary water. The chromium plating method of the present invention may be combined with the passivation techniques described in U.S. Pat. No. 4,636,266. Following coating of the surface with chromium, the chromium surface to be passivated is exposed to a gaseous oxygen source. The oxygen source may be oxygen itself, mixtures of oxygen with other gases, e.g., steam air, inert diluents and mixtures of these. It is essential that the oxygen source be in the gas phase and that the exposing step be carried out in the gas phase. The exposing must take place at a temperature which falls in the range from about 150.degree. C. to 450.degree. C. Preferably, the exposing step will take place at a temperature substantially equal to that of the water temperature within the light water reactor, namely temperatures between 250.degree. C. and 320.degree. C. The time of exposure is not critical but should be at least five hours. Generally, an exposure time from about 50 hours to about 3000 hours is preferred, although longer times can be used. The present invention can be used to provide a chromium coating on carbon steels and stainless steels, e.g., 304 stainless, 316 stainless and 347 stainless. In order to better illustrate the effectiveness of the chromium coating, the following examples are presented. WORKING EXAMPLES In June 1989, new and previous fuel cycle test coupons were installed in the steam generator at Doel-2 for activity buildup measurements. The test coupons were placed on the manway seal plates. Table 3 gives the coupon description and position in the steam generator channel head (i.e., hot leg and cold leg). The plant resumed operation in July, 1989, but an unscheduled shutdown was encountered in November, 1989. Since this was to be an extended shutdown for turbine work, the utility decided to perform a "COMBAT" type decontamination of the primary system. The decontamination consists of recirculating primary coolant with 2500 ppm boron concentration for seven days at a temperature of 120.degree.-140.degree. C. The utility agreed to drain the steam generator so that the coupons could be analyzed both before and after the decontamination. The specimens had been exposed for approximately 2500 hours upon plant shutdown. The pre-contamination measurements were made in January, 1990 with the date decay corrected to time of plant shutdown. After the first measurement, all but one of the specimens were re-installed to undergo the "COMBAT" treatment. Table 4 and 5 give the results of the gamma spectrographic analyses for the pre and post-"COMBAT" treatment, respectively. These tables do not show that the palladium coupon also had levels of Sb-124 and AG-110m of the same order of magnitude as the Cobalt 60 values. TABLE 3 ______________________________________ COUPON SPECIMEN LOADING AT DOEL-2, JUNE, 1989 ID* POSITION STATUS ______________________________________ 309L AR Hot Leg Second cycle exposure 309L EP/PV Hot Leg Second cycle exposure 309L Cr/PV Hot Leg New, first cycle exposure CF8M AR Hot Leg Second cycle exposure CF8M EP/PV Hot Leg Second cycle exposure CF8M Cr/PV Hot Leg New, first cycle exposure 316L Cr/PV Hot Leg New, first cycle exposure 4PP1 Hot Leg Second cycle exposure 309L AR Cold Leg Second cycle exposure 309L EP/PV Cold Leg Second cycle exposure 309L Cr/PV Cold Leg New, first cycle exposure CF8M AR Cold Leg Second cycle exposure CF8M EP/PV Cold Leg Second cycle exposure CF8M Cr/PV Cold Leg New, first cycle exposure 316L Cr/PV Cold Leg New, first cycle exposure PD A-304 Cold Leg Second cycle exposure ______________________________________ *AR = As received, EP = electropolished, P/V = RCT Passivation, Cr = Chromium deposition layer applied, PP and PD = Palladium coated. TABLE 4 __________________________________________________________________________ DOEL-2 COUPON ANALYSIS, PRE-DECONTAMINATION, APPROXIMATELY 2500 HOURS EXPOSURE, JAN 1990. MEASURED ACTIVATION CONTACT PRODUCTS DOSERATE (DECAY CORRECTED) IDENTIFICATION (mR/hr) Co-58 Co-60 Mn-54 __________________________________________________________________________ HOT LEG 309L AR 180 5.11E + 5 6.43E + 4 1.27E + 4 309L EP/PV 120 3.89E + 5 4.59E + 4 9.42E + 3 309L Cr/PV 30 4.62E + 4 5.23E + 3 1.24E + 3 CF8M AR 310 9.87E + 5 1.26E + 5 2.47E + 4 CF8M EP/PV 160 5.29E + 5 6.64E + 4 1.49E + 4 CF8M Cr/PV 26 5.11E + 4 5.71E + 3 1.24E + 3 316L Cr/PV 34 5.41E + 4 5.32E + 3 9.99E + 2 4PP 1 210 1.78E + 5 2.60E + 4 5.46E + 3 COLD LEG 309L AR 200 5.12E + 5 8.59E + 4 1.05E + 4 309L EP/PV 135 4.25E + 5 5.02E + 4 7.72E + 3 309L Cr/PV 24 5.21E + 4 5.08E + 3 6.68E + 2 CF8M AR 300 6.36E + 5 1.07E + 5 1.18E + 4 CF8M EP/PV 200 5.20E + 5 6.94E + 4 7.86E + 3 CF8M Cr/PV 180* 3.94E + 4 7.19E + 3 1.80E + 3 316L Cr/PV 28 2.74E + 4 3.68E + 3 4.04E + 2 PD A - 304 240 2.65E + 5 5.51E + 4 6.25E + 4 __________________________________________________________________________ *APPARENTLY MISREADING, SHOULD BE 18 mR/hr TABLE 5 __________________________________________________________________________ DOEL-2 COUPON ANALYSIS, PRE-DECONTAMINATION, APPROXIMATELY 2500 HOURS EXPOSURE, FEB 1990. MEASURED ACTIVATION CONTACT PRODUCTS DOSERATE (DECAY CORRECTED) IDENTIFICATION (mR/hr) Co-58 Co-60 Mn-54 __________________________________________________________________________ HOT LEG 309L AR 140 2.77E + 5 6.27E + 4 1.23E + 4 309L EP/PV 90 1.95E + 5 4.47E + 4 6.78E + 3 309L Cr/PV 17 1.76E + 4 3.71E + 3 6.43E + 2 CF8M AR 210 5.43E + 5 1.22E + 5 1.86E + 4 CF8M EP/PV 110 2.07E + 5 4.77E + 4 8.36E + 3 CF8M Cr/PV 15 1.85E + 4 3.97E + 3 6.39E + 2 316L Cr/PV 14 2.28E + 4 4.27E + 3 9.58E + 2 4PP 1 210 9.70E + 4 2.49E + 4 3.14E + 3 COLD LEG 309L AR 200 2.99E + 5 8.05E + 4 8.50E + 3 309L EP/PV 90 2.00E + 5 4.42E + 4 5.33E + 3 309L Cr/PV 15 3.11E + 4 4.44E + 3 6.77E + 2 CF8M AR 180 3.25E + 5 1.02E + 5 8.46E + 3 CF8M EP/PV 140 2.70E + 5 6.28E + 4 6.32E + 3 CF8M Cr/PV 15 2.51E + 4 3.91E + 3 5.47E + 2 316L Cr/PV 10 1.47E + 4 2.72E + 3 4.27E + 2 PD A - 304 220 1.39E + 5 4.53E + 4 3.90E + 3 __________________________________________________________________________ Table 4 shows the pre-decontamination data normalized to the appropriate material. It should be noted that in this table, that the 316L stainless steel and palladium coated coupons were normalized to the 309L coupon data. Table 5 shows the effectiveness of the decontamination. All coupons were re-installed for further exposure in the plant. TABLE 6 __________________________________________________________________________ DOEL-2 COUPON ANALYSIS, PRE-DECONTAMINATION, JANUARY 1990 DATA-NORMALIZED. CONTACT NORMALIZED ACTIVATION PRODUCTS COUPON DOSERATE (TO AR FOR SPECIFIC MATERIAL) IDENTIFICATION (mR/hr) Co-58 Co-60 Mn-54 __________________________________________________________________________ HOT LEG 309L EP/PV 0.67 0.76 0.71 0.74 309L Cr/PV 0.17 0.09 0.08 0.10 F8M EP/PV 0.54 0.53 0.60 CF8M Cr/PV 0.08 0.05 0.05 0.05 316L Cr/PV* 0.19 0.11 0.08 0.08 4PP 1* 1.17 0.35 0.40 0.43 COLD LEG 309L EP/PV 0.68 0.83 0.58 0.74 309L Cr/PV 0.12 0.10 0.06 0.07 CF8M EP/PV 0.67 0.82 0.65 0.67 CF8M Cr/PV 0.60 0.06 0.07 0.15 316L Cr/PV* 0.14 0.05 0.05 0.04 PD A - 304* 1.20 0.52 0.64 5.95 __________________________________________________________________________ 316L & Pd DATA NORMALIZED TO 309L TABLE 7 __________________________________________________________________________ DOEL-2 COUPON ANALYSIS, PRE-DECONTAMINATION, JANUARY 1990 DATA-NORMALIZED. CONTACT MEASURED ACTIVATION PRODUCTS COUPON DOSERATE (TO AR FOR SPECIFIC MATERIAL) IDENTIFICATION (mR/hr) Co-58 Co-60 Mn-54 __________________________________________________________________________ HOT LEG 309L EP/PV 1.3 2.0 1.0 1.4 309L Cr/PV 1.8 2.6 1.4 1.9 CF8M AR 1.5 1.8 1.0 1.3 CF8M EP/PV 1.5 2.5 1.4 1.8 CF8M Cr/PV 1.7 2.8 1.4 1.9 316L Cr/PV* 2.4 2.4 1.2 1.0 4PP 1* 1.0 1.8 1.0 1.7 COLD LEG 309L AR 1.0 1.7 1.1 1.2 309L EP/PV 1.5 2.1 1.1 1.4 309L Cr/PV 1.6 1.7 1.1 1.0 CF8M AR 1.7 2.0 1.1 1.4 CF8M EP/PV 1.4 1.9 1.1 1.2 CF8M Cr/PV 1.2 1.6 1.8 3.3 316L Cr/PV 2.8 0.2 1.4 0.9 PD A - 304 1.1 1.9 1.2 16.0 AVERAGE, HOT LEG 1.6 2.3 1.2 1.6 AVERAGE, COLD LEG 1.5 1.6 1.2 3.3 __________________________________________________________________________ As shown graphically in FIGS. 3 and 4, the coupons coated with a chromium film and then passivated show very low activity deposition after a few months of exposure. The combination of electropolishing and RCT passivation show a 25 to 50% benefit in activity buildup over the long term. The palladium coated specimen have high doserates and more radionuclides observed than any of the other coupons. The 9300 hour data show that chromium coating followed by passivation represents a four to five times increase in effectiveness over steel passivation without chromium based upon corrosion product deposition. This effectiveness is reduced to two to three times when based on doserate.