Corrosion and environmental-mechanical characterization of iron-base nuclear waste package structural barrier materials. Annual report, FY 1984

Abstract

The disposal of high-level nuclear waste in deep underground repositories may require the development of waste packages that will keep the radioisotopes contained for time periods up to 1000 years. The primary geologic media currently being considered in the United States for repository siting are salt, basalt, tuff, and granite. sidered for the structrual A number of iron-base materials are being conbarrier members of waste packages. Their uniform and nonuniform (pitting and intergranular) corrosion behavior and their resistance to stress-corrosion cracking in aqueous environments relevant to salt media are under study at Pacific Northwest Laboratory {PNL). The purpose of the work is to provide data for a materials degradat1on model that can ultimately he used to predict the effective lifetime of a waste package overpack in the actual repository environment. This report summarizes the results of the studies conrlucted at PNL during the FY 1983-FY 1984 time period in support of the Salt Repository Project of the Department of Energy. The corrosion behavior of the candidate materials was investigated in simulated intrusion brine (essentially NaCl) in flowing autoclave tests at 150°C, and in combinations of intrusion/inclusion (high-Mg) brine environments in moist salt tests, also at 150°C. Studies utilizing a 6°co irradiation facility were performed to determine tne corrosion resistance of the candidate materials to products of brine radiolysis at dose rates of 2 x 103 and 1 x 105 rad/h and a temperature of 150°C. These irradiation-corrosion tests were "overtests," as the irradiation intensities employed were 10 to 1000 times as high as those expected at the surface of a thick-walled waste package. Slow-strain-rate (SSR) tests and corrosion fatigue tests conducted in intrusion brine environments at 150°C and, in the case of some SSR tests, with a superimposed radiation field of 3 x 10 5 rad/h, were used to determine the resistance of the candidate alloys to environmentally enhanced crack propagation. With the exception of the high general corrosion rates found in the tests using moist salt containing high-Mg brines, the ferrous materials exhibited a degree of corrosion resistance that indicates a potentially satisfactory application to waste package structural barrier members in a salt repository environment.

The Analytical Repository Source-Term (AREST) model: Description and documentation

“…At 150°C (423 K), for example, Equation (A.2) predicts a corrosion rate equal to 15.6 mpy. This value is significantly larger than corrosion rates determined for carbon steel in similar brine environments by Westerman et al (1986). At 150°C (423 K), Westerman et al (1986) found the corrosion rate of A216 steel in anoxic PBB-2 brine(a) equal to 0.575 mpy.…”

“…This value is significantly larger than corrosion rates determined for carbon steel in similar brine environments by Westerman et al (1986). At 150°C (423 K), Westerman et al (1986) found the corrosion rate of A216 steel in anoxic PBB-2 brine(a) equal to 0.575 mpy. The reason for this disparity is probably that the corrosion rates were determined at different times after the experiments were initiated.…”

“…In general, corrosion rate is a function of time, typically exhibiting large values shortly after the experiment is initiated and decreasing to smaller values over longer time periods. Oata collected by Westerman et al (1986) were taken after 4480 h (0.511 yr), while the time interval used by Posey and Palko (1979), although unspecified 9 seems to have been much shorter (less than 8 h). Clearly 9 a more complete corrosion rate equation should include a functional dependence on time.…”

“…While being far from complete, the data base was adequate for generating an approximate corrosion equation which was a function of temperature {between 25° and 250°C), chloride concentration (between 70 and 169,416 ppm), oxygen concentration (between 0.03 and 3.0 ppm}, and time (between 0.083 and 16 yr). This equation, which has been differentiated with respect to time to give an expression for corrosion rate, is shown below: Using the corrosion rate found by Westerman et al (1986) for A216 steel in PBR-2, and taking T = 432 K, (Cl) = 191,000 ppm, (0) = 0.1 ppm for ''anoxic" brine, and t = 0.511 yr, d is calculated to be 0.0368. This value is within the range specified by the BNL model.…”

SUMO, System performance assessment for a high-level nuclear waste repository: Mathematical models

“…For example, at 150°C (423 K), Equation ( Under the conditionsused by Westerman et al (1986) During formationof a pit, a local environmentis produced and sustainedthat may be very different from that of the bulk solution. The concentrationsof reactantsand products that contributeto pitting produce a situationdifferent from that of uniform corrosion.…”