Assessment of Tank 18 Corrosion During Waste Removal

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High-level waste in Tank 18 is to be suspended and transferred to Tank 7, and Tank 18 is to be isolated and closed. The process for the removal of the majority of the waste is specified in the Tank 18 Waste Removal Operating Plan [1]. Tank 18 contains concentrated (> 1M nitrate) waste, and is therefore subject to limits on hydroxide and nitrite concentrations to prevent stress corrosion cracking. Analysis of the Tank 18 supernate shows that the tank currently exceeds the minimum inhibitor concentrations for corrosion control. This condition will continue until the planned addition of inhibited water to slurry the last several inches of waste. The inhibited water additions will take the tank out of compliance with chemistry limits. However, sufficient inhibitor concentrations will remain so that the residual waste removal can be conducted without the risk of significant corrosion to the tank liner. Under the proposed waste removal process, it is acceptable to remove Tank 18 from the area of process applicability of PCO (Process Condition for Operation) 2.6.2 of the Corrosion Control Program. Sampling of the tank may safely be limited to process samples necessary to support closure.

High-level waste in Tank 18 is to be suspended and transferred to Tank 7, and Tank 18 is then to be isolated and closed. Tank 18 presently contains sufficient inhibitor concentrations to meet the chemistry limits in PCO (Process Condition for Operation) 2.6.2 of the Corrosion Control Program. The process for the removal of the majority of the waste is specified in the Tank 18 Waste Removal Operating Plan [1]. Tank 18 will remain in compliance with the corrosion limits until the potential addition of three inhibited water (IW) ‘fill-backs’ to recover the last several inches of sludge waste. The residual waste would be suspended by slurrying with inhibited water (0.01 M hydroxide, 0.011 M nitrite).

Tank 18 presently contains 1.01 M nitrate, 0.99 M nitrite, and 0.86 M hydroxide (sampled July 6, 2002), and thus meets the minimum hydroxide and nitrite concentrations in limit 3 of PCO 2.6.2 to prevent stress corrosion cracking (SCC) [2]. SCC is the operative corrosion mode in carbon steel waste tanks that contain concentrated (> 1 M nitrate) waste. For tanks with dilute (< 1 M nitrate) waste the corrosion concern changes from SCC to pitting, based on historical tank experience. An October 2001 chemical analysis actually showed Tank 18 to be a dilute waste tank and subject to pitting corrosion control limits. Regardless of its present composition, Tank 18 will contain dilute waste as a result of the IW fill-backs during final waste removal, and pitting will be the corrosion mode under assessment in this report.

As expressed in the corrosion control program PCO 2.6.2, pitting corrosion is prevented by maintaining either (l) a minimum hydroxide concentration of 1 M or (2) a minimum nitrite concentration of 1.65 times the nitrate concentration along with a minimum pH of 10.3. The two inhibitor limits carry different maximum allowed supernate temperatures. For hydroxide inhibition with [OH^-] > 1.0 M, the maximum allowed temperature is 100°C; for nitrite inhibition, the temperature limit is 40°C. These limits apply to non-slurried waste tanks. In the special circumstance of a dilute waste tank that is (1) nitrite inhibited, (2) has an intermediate hydroxide concentration between 0.01 and 1.0 M, and (3) is classified as a slurried tank by virtue of the operation of mixing pumps, the allowed supernate temperature rises 75°C [Reference 2, PCO 2.6.1]. Thermal analysis of Tank 18 concluded that the supernate temperature will exceed 40°C and rise to no higher than 52°C during the operation of mixing pumps [3]. The supernate temperature is expected to fall below 40°C in the absence of mixing.

The major anion concentrations in Tank 18 during the final waste removal are shown in Table 1 below. Tank 18 will fall out of compliance with the corrosion program limits beginning with the first IW fill-back. The decrease in the nitrate concentration below 1 M causes the tank to become subject to the concentration limits to inhibit pitting rather than SCC. Neither the nitrite nor the hydroxide concentration meets those limits. The nitrite concentrations required at 40°C for the given nitrate concentrations during the fill-backs are shown in the fourth row of the table. The duration of the out-of-compliance condition is expected to be less than two months, after which the waste will be pumped down to a height of less than 3 inches and the tank readied for grouting and closure. Significant corrosion is not expected to develop in Tank 18 during the period of the IW fill-backs and final pump-down.

Anion	After Last Slurry Transfer to Tank 7	After 1^st IW Fill-Back	After 2^nd IW Fill-Back	After 3^rd IW Fill-Back
Nitrate	1.01	0.10	0.072	0.052
Nitrite	0.99	0.11	0.081	0.061
Hydroxide	0.86	0.10	0.071	0.054
Nitrite Required at 40°C (PCO 2.6.2)		0.16	0.13	0.086
Nitrite Required at 52°C		0.52	0.37	0.27

The lowest row of entries in Table 1 shows the nitrite concentrations that would be required to prevent pitting under the highest supernate temperature of 52°C. (The experimental database for pitting in waste tanks covers temperatures from 23 to 60°C. The minimum nitrite concentration follows an exponential dependence on temperature in °C, 10^0.041*T.) The nitrite concentrations expected in Tank 18 after the IW fill-backs are about one fifth of the 52°C levels. However, these nitrite concentrations can yet provide some protection for the carbon steel, as was observed in pitting growth studies in intentionally corrosive simulated waste [4]. In those studies, coupons exposed to a test solution with a nitrite concentration that was 20% of the required minimum concentration experienced half the depth of pitting compared to coupons exposed to a solution with no nitrite. In addition, the hydroxide concentrations in Tank 18 will be more than 200 times greater than the equivalent of pH 10.3 (i.e., 0.0002 M), the minimum pH required in association with nitrite inhibition. Because of the synergy between nitrite and hydroxide in preventing corrosion [5], this hydroxide concentration is expected to compensate to some degree for the low nitrite concentration. An additional factor opposing significant corrosion is the fact that the waste level in Tank 18 will be raised and lowered repeatedly as IW is added and slurried waste is removed. Significant pitting corrosion occurs in steel exposed to a stable liquid-air interface of the order of 30 days’ duration, which is much longer than the steps of 2 to 10 days’ duration proposed in the final waste removal from Tank 18.

The worst-case pitting corrosion rate at 50°C for dilute radioactive waste solutions that contain no nitrite has been calculated to be 0.05 inches per year [4]. The depth of penetration at this bounding rate in four months, or twice the planned time of residual waste removal, would be only about 0.017 inches, or less than 5% of the thickness of the Tank 18 steel liner. The actual extent of corrosion under the prospective Tank 18 conditions should be much smaller because of the chemistry and tank mixing factors cited above and temperatures much lower than 50°C in the absence of mixing pump operation. Therefore the planned waste removal from Tank 18 can be conducted without the risk of significant corrosion of the tank liner. In the event that waste removal operations were suspended for more than six months under the uninhibited conditions associated with the IW fill-backs, the tank status should be re-evaluated and the addition of inhibitor considered.

The schedule for Tank 18 waste removal calls for completion of waste transfers by early 2003. Grouting and closure of the tank are scheduled to be completed in the period March to June 2004. Under the waste removal plan, it is acceptable to remove Tank 18 from the area of process applicability of PCO 2.6.2, and suspend sampling for corrosion control purposes. Sampling may be safely limited to process samples necessary to support closure of the tank.