CONTROL OF STRESS CORROSION CRACKING IN STORAGE TANKS CONTAINING RADIOACTIVE WASTE

SLn:ss c.orro::Jion of carbon stAP.l storage ta.nks containing alkaline nitrate radioactive waste, at the Savannah River Plant is controlled by specification of limits on waste composition and temperature. Actual cases of cracking have occurred in the primary steel shell of tanks designed and built before 1960 and were attributed to a combination of high residual stresses from fabrication welding and aggressiveness of fresh wastes from the reactor fuel reprocessing plants. The fresh wastes have the highest concentration of nitrate, which has been shown to be the cracking agent. Also as the wast.e soluLiuns are reduced nitrite and hydroxide ions more concentrated and Thus, by providing a heel aged tanks that receive


DISCLAIMER
Portions of this document may be illegible in electronic image products. Images are produced from the best available original document. The carbon steel in the tanks is susceptible to stress corrosinn ~rackin~ in nitrate and caustic solutions, and stress corrosion cracks have been observed in the primary steel shells of sonie tanks. This paper reviews the performance of tanks used for interim storage of high-heat waste (HHW) at SRP with respect to nitrate stress corrosion cracking. The technical investigations to identify the cause of cracking are summarized, and modifications in the construction of the tanks and in waste management that resulted from these studies are described. These modifications helped to maintain the iuL8grity of the tank~, providing morA secure containment.

Waste Storage Operations
Radioactive high-he.at waste from the reprocessing plant is trau!>ferred to a tank containing cooling coils that remove decay heat ( Figure 1). Fresh waste is aged from one to two years to permit solid material to settle and short-lived fission products to decay. During this aging period insoluble materials form a layer of sludge at the bottom of the tank. The sludge consists of oxides and hydroxides of manganese, iron, and aluminum, small amounts of uranium, plutonium, and mercury, and most of the longerlived fission products originally in the irradiated fuel except cesium. The aged supernate solution containing dissolved salts, including radioactive cesium, is transferred to a continuous evaporator. The concentrate from the evaporator is transferred to a waste t'ank that is equipped with cooling coils. In the tank the salts crystallize and settle as the liquid cools. The remaining supernate is returned to the evaporator for further concentration. This process continues until the liquid has been converted to a damp salt cake. Aging and processing causes changes in the chemical and radionuclide composition. The major changes are 1) radiolytic conversion of nitrate to nitrite, 2) absorption of C02 from air, converting NaOH to Na2C03, 3) separation of radionuclides into soluble and insoluble fractions, and 4) decay of the radionuclides. The composition of the wastes in individual tanks varies widelydepending on the detailed history of its contents.
For this reason, the composition of the 'waste contained in the tanks is analyzed periodically. Concentration ranges of the major components in the high heat storage tanks are given in Table 1. 3 Nitrate is the major anion, but nitrite from radiolysis of nitrate is a major component after aging and evaporation. The relative amount of hydroxide has increased because of the difficulty in cryst.alliz.ing sodium hydroxide.
The only radionuclides present at high concentration in the supernate (Table 2) are the isotopes of Cs which were relatively minor originally. Some Ru is. also present. 3  The results of these inspections for the cracked tanks are shown in Table 3.

Nitrate Stress Corrosion Cracking
Investigation of the cause of leakage has continued since the first leak was observed. The first major conclusion drawn from the investigation was that the tanks cracked from nitrate stress corrosion.
Mild steels are susceptible to stress corrosion cracking in a number of environments, 5 including nitrate and caustic solutions.
Measurements indicated that the corrosion potential, the emf at zero current, for several waste tanks was in the range of -0.44 to -0.064 V vs. a saturated calomel electrode (SCE). This emf was within the range for nitrate stress corrosion (-0.3 to 1.1 V vs. SCE), but outside that required for caustic cracking (-0.8 to For stress corrosion cracking to occur, both stress and a corrosive environment are required.

Stresses
In waste tanks, stress corrosion cracks have been predominantly associated with welds. Cracks form at right angles to the weld bead and propagate a short distance into the base metal, then stop. The largest observed crack in a waste tank was six inches long. Crack propagation decreases with distance from the weld. Figure 5 shows the residual stresses associated with a butt weld, the most common type of weld made in fabricating the tanks. 6 A weld with the properties shown in Figure 5 might be expected to generate a crack 1 to 2 inches long.
Residual stresses can be relieved by uniformly heating a structure to a sufficiently high temperature (approximately 1100°F in mild steels) for annealing to take place. Such heat treatment should eliminate SCC by removing the stress. This effect was demonstrated in the laboratory 7 ' 8 by welding large, 3-foot square plates and exposing as-welded and stress-relieved plates to synthetic waste solutions or to SO% NaN0 3 , which is a particularly aggressive solution in causing SCC. Plates that were fully stress relieved by heat treatment did not crack in 167 days; as-welded plates consistently cracked after a short exposure (about 3 to 10 days). "Flame-washed" plates gave inconsistent results because of the difficulty in controlling the temperature in this procedure.
On the basis of this work and other supporting laboratory 9 and industrial 10 experience on the role of residual stresses, the new Type III waste tanks have been heat treated after fabrication to relieve the welding stresses. Any long-range reaction stresses would also be relieved. This heat treatment is a signifi~ant advance in minimizing the probability of stress corrosion cracks in waste tanks.

Corrosion
In addition to the studies on reducing stresses to prevent stress corrosion cracking, corrosion studies were also performed.

Tensile Tests with Impressed Current
A test to evaluate the influence of waste solution composition on stress corrosion initiation was also developed. 11 The test determines the loss of ductility when a tensile specimen is_ strained to failure. The specimen is extended at a uniform rate while acting as the anode in a controlled-current electrochemical cell with the waste solution being tested as the electrolyte. The current level was chosen on the basis of potentiodynamic polarization curves and was the same for all tests in a given series. The effects of synthetic waste solutions of various compositions compared with tests in air are shown in Figure 7. The test data show that aged evaporated waste (Tanks 1 and 9) were less aggressive than fresh (newly generated) wastes (Tanks 8 and 14).
Tensile strength and all of the common measures of ductility, uniform elongation, total elongation, or reduction in area, were similarly affected by the test solutions. Figure 8  In addition to NO~, NO;, and OH-, to simulate waste solutions more closely, the test solutions also contained six ionic constituents in constant concentrations. Surface cracking was observed in all specimens when total elongation in the test was less than the uniform elongation observed in air (approximately 13%). This value was used as a figure of merit to estimate whether stress corrosion cracking of A 285-B steel would be a risk if the steel were exposed to equivalent solutions and temperature in service.
Like other corrosion phenomena, the severity of stress corrosion usually increases with temperature. Tensile tests with constant applied current support this generalization, as shown in Table 4.

Mechanical Studies
Modified wedge opening load (WOL) specimens 13 with a fatigue crack ( Figure 9) were used for this laboratory study to determine the influence of waste composition on crack growth. Specimens were stressed by tightening the bolt, and, by relating the load to crack opening and crack length, the state of stress can be calculated.
Specimens were immersed in the test solution and the crack length measured as a function of time. For thi~ t)~C of specimen, stress intensity K, a parameter that characterizes the state of stress in the crack tip region, decreases with crack length. Therefore, the threshold stress intensity, Kscc (the minimum intensity necessary to cause stress corrosion cracking), is the K value at which the crack ceases to propagate.
In solutions that caused crack growth, the rate of growth and Kscc were independent of solution composition. This observation is evidence of the independence of crack tip chemistry from bulk solution chemistry. The initiation time, however, was related to the composition of the bulk solution.
From these data, a map dividing the compositional variables into cracking vs. noncracking regions was drawn 14 as shown in The comparison is good for sodium hydroxide concentrations to about 0.3M. The difference between the two may be due to the six anionic constituents added to the electrochemical tensile test to more closely simulate the waste. In waste management, the NaUH concentration is maintained above 0.3M when nitrate is at its maximum of S.SM (Table 5).
As the nitrate concentration decreases, the amount of inhibitor required also decreases. In SRP wastes these values, chosen as conservative ones, are shown in Table 5. At low nitrate concentrations hydroxide is not added to prevent stress corrosion, but rather to avoid general corrosion and pitting attack. During waste storage operations, technical standards require waste composition to be controlled. A maximum N03 concentration is specified to limit the aggressiveness of the supernate. The concentration of inhibitors, OH-and N02, is maintained at specific minimum levels depending on the N03 concentration. These levels of OH-and N02 have been shown to prevent crack initiation in highly stressed specimens.

Stress Corrosion Control. Methods
Temperature of fresh supernate is maintained at less than 70°C. Since stress corrosion is a thermally activated process, the lower temperature will also inhibit the initiation and growth of cracks. Because aged and evaporated waste contain sufficient OH-and N02 to inhibit SCC by themselves, the above temperature limit specifically applies only to fresh waste.