Engineering a defence against corrosion28 July 2004
Years of scientific study have been devoted to designing a proposed underground nuclear waste repository at Yucca Mountain in the Nevada desert. Researchers at Lawrence Livermore are working on engineered barrier systems. By Arnie Heller
Few assignments have been as demanding for Lawrence Livermore researchers as designing a waste system to keep high-level waste packages essentially intact for at least 10,000 years. A team of Livermore researchers – engineers, metallurgists, chemists, microbiologists, and computer scientists – are testing and refining the design and materials for what will eventually be 12,000 waste packages. These efforts are an integral part of a national programme to design, license, and build an underground nuclear waste repository in Yucca Mountain, in the US state of Nevada.
Yucca Mountain was selected by the Department of Energy (DoE) as a highly promising repository site. In 1987, Congress directed the DoE to focus on Yucca Mountain as the candidate location to safely store about 70,000t of waste from civilian nuclear power plants and highly radioactive waste from defence-related activities at DoE facilities. As part of the DoE’s Yucca Mountain project, Livermore scientists have made major contributions in characterising the proposed underground site, determining the effects on the site of storing high-temperature radioactive wastes, and selecting and characterising corrosion-resistant materials.
Livermore’s largest effort is developing Yucca Mountain’s engineered barrier system, which consists of a waste package, drip shield, and supporting structures. The engineered barrier system is designed to work with the natural barriers of Yucca Mountain to contain the repository’s radioactive wastes and prevent them from seeping into the water table which lies about 300 metres below the planned repository.
“We need to show that our design will substantially contain the waste inside the canisters for at least 10,000 years under extreme and varying
conditions of temperature, radiation, and corrosion,” said Dan McCright, Livermore metallurgist and Yucca Mountain programme senior scientist. According to McCright, extensive analyses have shown that even if waste were to eventually leak from the canisters, additional barriers, both natural and engineered, are expected to keep the waste far from the water table – and people.
No direct information exists about how modern materials will behave over thousands of years under a range of conditions. The Livermore research is based on accelerated ageing tests of materials and computer models that simulate how a repository built at Yucca Mountain would perform over such timescales.
The current repository design calls for waste to be stored in a package consisting of a set of two nested canisters – an outer canister made of a highly corrosion resistant metal (Alloy 22) and an inner canister made of a tough, nuclear-grade stainless steel (316NG). An overhanging drip shield made from titanium should provide additional protection to the waste package from dripping water and any falling rocks from the repository ceiling. “Because the waste package and the drip shield are made of different corrosion-resistant materials, they form corrosion defence in depth,” said McCright.
Storing the waste packages horizontally and mingling the different kinds of waste packages will create a relatively uniform temperature in each underground drift, or tunnel, carved inside the mountain. The waste packages have a common diameter (1.8 metres), but their lengths vary according to the type of waste — from about 3.6 metres for defence waste to 5.7 metres for spent nuclear fuel.
The most critical element of the engineered barrier system is the 20mm-thick outer canister made of Alloy 22, which consists of about 60% nickel, 22% chromium, 13% molybdenum, and 3% tungsten. Alloy 22 is highly resistant to fractures and is easier to weld than alternative materials such as titanium. It is also extremely corrosion resistant under the conditions of high temperature and low humidity expected to prevail for hundreds to thousands of years in a repository. In addition, it is resistant under conditions of either low or high humidity at the lower temperatures expected in the repository when radiation levels decrease. Hence, the selection of Alloy 22 should provide containment over a range of environmental conditions.
Nuclear-grade stainless steel was chosen for the 50mm-thick inner canister to add strength and bulk to the waste package. It is corrosion resistant, more compatible with Alloy 22 than carbon steel, and more economical than more complex steel alloys.
The titanium drip shield, which McCright compares to a sturdy awning, would be fabricated from grade 7 titanium. This material contains a small amount of palladium to provide greater corrosion resistance. The drip shield, however, is not considered essential to containing the wastes. Earlier projections of Alloy 22’s corrosion performance assumed that there would be no drip shields and that drips from the repository walls would fall directly on the canisters.
The waste packages will rest on a pallet fabricated from Alloy 22 clad onto steel. The pallet, in turn, will sit on a steel frame and crushed gravel. The waste packages will be placed close together (about a metre apart) so that by design their surfaces would reach a maximum surface temperature of 160°C (caused by radiation levels of up to 180 rads per hour) once the repository is sealed. “It may take hundreds of years before surface temperatures cool below boiling because of the slow decay of radioactive components in the waste,” said McCright. Keeping the canister surfaces above the boiling point will ensure they are dry, with the intention to prevent corrosion.
GETTING A CLOSE-UP VIEW
A major effort is underway to understand and characterise the environments closest to the drip shield and the waste package because these environments will determine the potential for corrosion and how fast it could proceed. Surface conditions will be characterised by the temperature, humidity, and composition of gases in the repository; the contaminants in the dripping water from repository walls; and the mixture of minerals and salts that may eventually be deposited on the drip shield and canisters. As temperatures drop, for example, moisture and dust in the atmosphere would settle on the canisters’ surfaces despite the presence of the drip shield. If a drip shield is eventually breached, water seeping through rock fissures could contact the canisters directly and cause more minerals or salts to precipitate on their surfaces, thereby increasing the potential for corrosion.
Limiting corrosion is the paramount objective. Corrosion can be general, occurring more or less uniformly over the entire surface, or localised, occurring in specific areas such as in pits or crevices on a metal’s surface. Corrosion can also be assisted by cracking from stresses in a metal or weld – stress corrosion cracking.
Both titanium and Alloy 22 gain their corrosion protection from the natural, extremely fast growth of thin films (about 3.5 nanometres or 10 atomic layers thick) of metal oxides caused by oxygen in the environment. When these stable, chemically unreactive films consolidate, the corrosion rate decreases. One Livermore research effort is studying the growth of metal-oxide thin films on Alloy 22 and titanium under the expected environmental scenarios at Yucca Mountain. The observed compositions and structures of the films are compared with model predictions of film growth.
It is essential to demonstrate that Alloy 22 will survive all anticipated repository conditions. In particular, scientists must show that corrosion rates, both general and localised, are extremely low and that welds will not crack over time. Materials performance tests are conducted at Livermore’s Long-Term Corrosion Test Facility (LTCTF) to provide assurance that the waste packages would maintain their integrity and corrosion resistance for thousands of years.
AGEING 18,00 MENTAL COUPONS
The corrosion tests at the LTCTF are designed to rapidly ‘age’ metal samples, called coupons, by subjecting them to much harsher conditions than would be expected in the repository. More than 18,000 alloy coupons are being tested, each of which measures about 5cm2 or less. Fourteen alloys are being tested to compare the corrosion resistance of Alloy 22, stainless steel, and titanium with other materials.
Four kinds of coupons are used to test the various forms of corrosion. Crevice coupons consist of metals tightly pressed against teflon washers to determine the extent of corrosion from liquid trapped between the metal and washer. Weight-loss coupons measure general corrosion. Galvanic coupons measure corrosion that occurs when two dissimilar alloys are pressed against each other. Finally, U-bend coupons are metals bent under continuous stress to try to induce stress corrosion cracking. Many of the coupons are welded to determine the effects, if any, of welds on corrosion.
The coupons are kept in 24 tanks, each filled with about 1000 litres of one of three different solutions derived from those likely to be found in the Yucca Mountain environment. One solution is a concentrated mixture of salts and minerals common to Yucca Mountain. The second solution is a diluted version of this mixture. The third solution is an acidified version of the concentrated mixture. Solutions are heated to either 60°C or 90°C. The coupons are mounted on vertical racks and are either submerged in solution, suspended over the solution, or partially submerged.
Coupons were removed from the tanks six months, one year, two years, and five years after mounting and most of the coupons are still in the tanks awaiting longer-term tests. When a coupon is removed, it is analysed to determine whether corrosion has occurred, and if so, where it is, and how much damage it caused. Corrosion activity is evaluated by weighing the coupons after they are cleaned of compounds that have precipitated on their surfaces and by using an electron microscope and an atomic force microscope to scrutinise their surfaces.
LTCTF manager Dave Fix said that the corrosion detected in the coupons in the various solutions is generally so slight that it resides at the limit of what is measurable. The average corrosion rate is about 20 nanometres per year. At this rate, a 20mm-thick barrier of Alloy 22 would be effective for more than 100,000 years before general corrosion would provide a means for water to contact the underlying stainless-steel layer. In addition, the extremely low corrosion rates appear to be nearly the same for all the water chemistries and temperatures tested.
Significant corrosion is measured only when coupons are subjected to extreme, unrealistic conditions. For example, the basic metallurgical structure of Alloy 22 is transformed over long periods of time at temperatures of more than 500°C. Several hundred millivolts of electrochemical potential are necessary to make the test solution extremely corrosive. “These extreme testing conditions are totally unrealistic for the Yucca Mountain repository setting,” said McCright, “but our models consider them.”
ASSESSING MICROBES' EFFECTS
Livermore microbiologist Joanne Horn leads a team assessing the potential damage microbes can cause to the engineered barrier system. Some bacteria and fungi, both those indigenous to Yucca Mountain and those introduced by construction activities, could cause corrosion of the engineered barriers. Horn notes that an abundance of microbes exists in the Yucca Mountain repository setting. Microbial activity is expected on the canisters when adequate moisture is present. Some bacteria, for example, are expected to form patchy, thin films over the metal-oxide films covering the waste packages and drip shields.
Horn pointed out that microbes have been found in the most inhospitable environments on Earth, such as the scalding-hot vents on the ocean floor. Some bacteria have very efficient DNA repair mechanisms that might enable them to survive high radiation levels.
Horn’s team has identified more than 65 species and subspecies of bacteria living in the Yucca Mountain rock. The team has also identified the different growth requirements for these bacteria. One set of laboratory experiments analyses the extent of corrosion on metal coupons caused by bacteria contained in crushed Yucca Mountain rock and fed with simulated groundwater. Another set of experiments determines the extent of corrosion caused by specific species of bacteria that have potentially corrosive activities, such as a species that oxidises sulfur compounds. The results of these experiments are compared with the corrosion that occurs under identical conditions but in environments that have been presterilised to kill all microbes. The team also analyses the solutions to determine if bacterial metabolic products could change the repository chemistry.
“The findings of the bacterial experiments parallel those of the long-term corrosion facility,” said Horn. “To date, our results show that, over 10,000 years, the corrosion rate from bacteria would not penetrate beyond 1mm. Alloy 22 is a very tough metal.”
OVERCOMING WELD STRESS
How the waste packages are manufactured, especially the required welds, can affect their resistance to corrosion as well as their structural integrity. Residual stresses are a common by-product of manufacturing, especially welding, and if left untreated could lead to stress corrosion cracking. Livermore metallurgists plan to treat the canister welds in the repository with an annealing process that reduces the residual tensile stress and produces instead a compressive stress on the canisters’ surface. Stress corrosion cracking does not occur under compressive stress.
The annealing process involves subjecting the welds to 1100°C and then quenching the metal in a water bath to produce a small overall compressive stress on the exterior surface. The canisters would then be shipped from the factory to the repository for storage until they are ready to be filled and sealed.
The canister lids would be fabricated off-site and then welded on after the canister was filled at the repository. Annealing would not work as a technique to lessen the stresses that will unavoidably occur in the final closure welds because the high heat generated during the process would damage the contents. “The nuclear waste will be in a relatively inert form when it is placed in the canisters,” said McCright. “Subjecting the waste to the temperatures required by annealing might compromise that inertness.”
One promising alternative to annealing is laser peening, a process developed at Livermore, in which a laser produces a shockwave on a weld to form a compressive stress. Laser peening can produce a compressive layer about 3mm deep in the metal to strengthen the final closure welds. Livermore experts are characterising structural changes of peened Alloy 22 samples using transmission electron microscopy, X-ray diffraction and other techniques.
FAVOURABLE RESTING RESULTS
Attempts to accelerate corrosion with solutions representing the waters that could eventually contact the metal canisters have thus far indicated an extremely low general corrosion rate. The tests also have shown that the canister metals have extremely high resistance to all forms of localised corrosion and stress corrosion cracking in environments relevant to the repository. Also, no appreciable differences have been noted in corrosion rates obtained from the various water compositions and temperatures. The testing results support Livermore’s models for long-term prediction of the waste packages’ performance and strongly confirm the selection of Alloy 22 for the outer canister.
Testing and modelling at Livermore will proceed for several more years. In the meantime, McCright and others are refining the engineered barrier design to make components more efficient and economical to manufacture. “The plan is to manufacture 12,000 waste packages over 25 years, the equivalent of manufacturing more than one canister a day,” he said. “We want every one to be as corrosion resistant as we can practically make them.”
Arnie Heller, Science & Technology Review, National Technical Information Service, US Department of Commerce, 5285 Port Royal Road, Springfield, Virginia 22161, USA