A A A

Publications

The Swedish Program for Long Term Isolation of High Level Nuclear Waste in Copper Canisters

Copper Applications in Health & Environment

July 1999

By Lars O. Werme
Swedish Nuclear Fuel and Waste Management Company (SKB)

Sweden's 12 nuclear reactors, which produce about 50% of Sweden's electrical power, will have produced approximately 8800 tons (8000 metric tons) of spent fuel by 2010. In 1976, Sweden instituted a program for disposing of its nuclear waste. That program has grown into a comprehensive system for nuclear waste isolation. The waste disposal system is owned and managed by the Swedish Nuclear Fuel and Waste Management Co. (SKB).

Swedish Program Relies on Copper

Like the U.S. waste isolation program, the Swedish program calls for disposing of its spent fuel without reprocessing the fuel to recycle its uranium and plutonium. The fuel will therefore be disposed of while still sealed inside in its stainless steel and zirconium-alloy fuel assemblies.

The fuel will first be transferred to the interim storage facility (Swedish acronym: CLAB), where it will cool for 30 to 40 so that when packed the surface temperature will not exceed 100 C in the repository. The temperature of the fuel itself will of course depend on the storage conditions. The residual power in the fuel after CLAB storage will be 100 - 150 W for a boiling water reactor (BWR) fuel element. Storage will also reduce the fuel's radioactivity slightly. Next, the fuel will be encapsulated in corrosion-resistant copper isolation canisters. Encapsulation will take place in a plant built as an extension to the CLAB in order to reduce the amount of handling needed.

After encapsulation, the fuel will be transported to a geologic repository where the canisters will be deposited in granite at a depth of 1640 to 2300 ft (500 to 700 m) beneath the surface. The repository's location has not been selected yet, but several sites have been studied extensively for several years. All of the sites are compatible with copper.

The Swedish program relies on a series of barriers to prevent the release of radioactive substances (called radionuclides) into the environment or at least delay their re-lease until radioactivity has decayed to safe levels. What is a safe level? The Swed-ish program proposes the level of radioactivity associated with the uranium from which the fuel was originally produced, i.e., its "natural" state.

The multibarrier system consists of the following components:

The fuel itself. Spent fuel contains a number of mineral-like compounds, some of which are only sparingly soluble in groundwater. That "inaccessibility" in itself constitutes a form of barrier. The zirconium-alloy tubes in which the fuel pellets are sealed are highly corrosion-resistant and constitute a second barrier.
A corrosion-resistant copper canister. The outer shell of the canister consists of a thick layer of pure copper. Copper is the primary barrier between the fuel and the environment. It alone will retard release of radionuclides for at least 100,000 years.
A bentonite clay buffer, which completely surrounds the copper canister. Ben-tonite is a clay mineral found in Wyoming. It swells when soaked with water, causing it to squeeze into and seal crevices in the rock surrounding the canisters. Any radionuclides that leak from a canister would have to diffuse through the bentonite, a process that could take more than 10,000 years. In addition, ben-tonite can absorb certain radionuclides and hold them indefinitely. Bentonite also has the ability to control the pH (acidity) of the groundwater. This buffering action helps protects the canister.
The overlying rock mass and the slow movement of groundwater through it is the final and most massive barrier. It will slow the dispersal of radionuclides should a canister start to leak. The multibarrier system is illustrated below.

The Swedish repository's multiple-barrier system.

Left to right: the nuclear fuel itself (including its zirconium-alloy cladding), the copper-walled canister and the bentonite buffer that surrounds it, and the granite bedrock.

How Safe is Safe? The Hazard Index

Spent reactor fuel is extremely radioactive; even momentary contact would be fatal. Fortunately, the fuel becomes progressively less radioactive over time as the ra-dionuclides in it decay. The decay takes a very long time, but it is possible to calcu-late when the fuel will no longer be hazardous.

Scientists have combined nuclear physics with health-effects data to create a number called the hazard index, which describes how toxic the fuel is at any point in time. For example, a hazard index of 1.0 corresponds to the toxicity of the amount of natural uranium needed to make one metric ton of fuel. It represents the hazard the uranium presented in its "natural" state and is therefore a useful baseline number against which spent fuel's toxicity can be compared.¹

Spent fuel leaves the reactor with a hazard index of about 1000, as shown in the fig-ure, below. The figure contains two curves because some radionuclides are more accessible in the environment than others and are usually considered separately. (These species are, for example, more readily transported by groundwater.) Among the relatively inaccessible radionuclides are elements called actinides (americium, curium, plutonium, etc.) and their isotopes, some of which (such as plutonium and neptunium) decay very slowly while others, such as curium, decay in a few decades. Despite their inaccessibility, these species must be very carefully isolated until they decay.

Isolation of the fuel for 1000 years reduces the spent fuel's hazard index by a factor of about 10, but it also ensures that most of the more readily accessible radionu-clides will have decayed. It takes another 50,000 years to attain a further ten-fold reduction. Isolation for 100,000 years reduces the toxicity of the fuel by a factor of more than 100, and after about 180,000 years, radioactivity will have decayed to lev-els comparable with those of the natural uranium from which the fuel was produced.

Hazard index of spent nuclear fuel as a function of time.

Highly accessible radionuclides decay in a few hundred years, inaccessible radionuclides decay very slowly. After about 180,000 years, the spent fuel's radioactivity will be comparable to radia-tion from the natural uranium from which it was made.

The waste system designers' goal is to devise a series of barriers that will isolate the fuel from contact with the habitable environment until the fuel no longer presents a hazard. The conservative approach adopted by SKB requires that the copper canis-ter itself remain intact for more than 100,000 years.

Canisters Will Meet Strict Design Criteria

A canister that can isolate its dangerous contents for 1000 centuries must have the highest attainable integrity when it is built; it must be strong enough to withstand any stresses imposed by the repository, and above all, it must resist corrosion in the re-pository environment. The canister should also not produce any adverse effects on other barriers; heat and radiation in its immediate vicinity should be limited, and the configuration inside the canister should be such that the fuel cannot reach criticality, i.e. that it cannot sustain a nuclear chain reaction.

Why Sweden Chose Copper

Corrosion resistance is the canisters' most important technical requirement. Copper is naturally resistant to corrosion. Moreover, because copper is a pure element and not an alloy, it is possible to calculate conditions under which the metal will remain stable indefinitely. Pure metals like copper also hold fewer surprises from a corro-sion standpoint.

Over a period of more than 15 years, Swedish scientists and engineers have built up an extensive database on the country's groundwater. They have learned that the water entrapped in the granite rock more than about 330 to 660 ft (100 to 200 m) beneath the surface contains no dissolved oxygen. The water is neutral to mildly al-kaline and contains varying amounts of sodium and calcium, plus chloride and min-ute amounts of sulfide. It is, in fact, a rather high-quality groundwater.

Water that soaks into the bentonite buffer will, in addition to the minerals listed above, contain some sulfate, carbonate and bicarbonate. Its pH will be buffered to 9.3, mildly alkaline. These conditions are very benign toward copper.

Copper is stable, i.e., it doesn't corrode, in the oxygen-free water that will exist in the Swedish repository. The small amount of oxygen introduced during construction of the repository and emplacement of the canisters will soon be consumed, mainly by oxidation of minerals in the rock.

Copper can also be corroded by sulfides in the groundwater; in fact, sulfides will account for most of the corrosion that will occur. However, the concentration of sulfide in the groundwater is extremely low and the supply of fresh sulfide to the canister surface can only take place at a very low rate.

After more than 15 years of extensive study, Swedish scientists concluded that less than two-tenths of an inch (5 mm) of copper will be consumed by corrosion during 100,000 years. Still, considering the long time period involved and uncertainties in the development of conditions in the repository, a minimum of six-tenths of an inch (15 mm) of wall thickness should be allowed for corrosion protection.

Strength Needed to Withstand Pressure

Exploded view of a Swedish canister designed to hold 12 spent BWR fuel elements.

The Swedish repository will be located well below the water table and will become flooded soon after it is sealed. The canisters therefore must withstand the hydrostatic stress imposed by the groundwater, which at 2300 ft (700 m) is 1015 pounds per square inch (psi) (7 Mpa). They must also withstand an additional 1015 psi imposed by the swelling bentonite. And, because the advent of an ice age within the life of the repository cannot be ruled out, the canisters are designed to withstand an additional 4350-psi (30-Mpa) stress that would be imposed by a 9840-ft (3000-m) -thick ice cap.

The maximum pressure that might be exerted on the canister will therefore be 6380 psi (44 Mpa). There will be no additional stresses due to vaporization of groundwater since the canisters will have cooled to well below the boiling point before being placed in the repository. Also, because the canisters are cool, there will be no ten-dency for water to evaporate and leave concentrated minerals on the canisters' sur-faces.

Six-tenths of an inch of copper wouldn't withstand the expected or potential stress levels; therefore, the canister was designed so that the required mechanical strength is provided by an inner structure, made from nodular cast iron, that supports the outer copper shell.

An inner cast iron structure prevents crushing while an outer copper shell provides corrosion resistance. The canister can withstand up to nearly three times the stress expected in the repository.

Components of Swedish reference-design waste isolation canister.

Components of the Swedish reference-design waste isolation canister.

The 2-in (50-mm) thick copper shell (left) provides corrosion protection for more than 100,000 years; the nodular cast iron insert (center) provides strength. The other copper items include lower seal and a lid, both of which will be electron-beam welded to the copper shell.

Reference Design

The so-called "reference" Swedish canister has an overall length of 190 in (4833 mm) and an outer diameter of 41 in (1050 mm). The cast nodular iron insert provides for a 2-in (50-mm) spacing between the fuel channels and at least two inches (50 mm) of metal between any fuel element and the insert's periphery. The wall thick-ness of the copper canister is 50 mm; ten times the amount needed to withstand cor-rosion for 100,000 years. There will be a 0.14-in (3.5-mm) gap between the cast iron insert and the copper shell. The gap will close as the ductile copper squeezes inward under the pressures imposed by groundwater and swelling bentonite.

Loaded with spent fuel, the canister weighs about 55,000 lb (25 metric tons) if equipped to hold boiling water reactor (BWR) fuel and about 59,500 lb (27 metric) in the pressurized water reactor (PWR) fuel version. The total copper weight is about 16,500 lb (7.5 metric tons).

Straightforward Fabrication

The copper canister is fabricated either as two tube halves formed from rolled plates that are welded together or as large seamless tubes that are made either by extru-sion or by piercing and drawing. Copper lids and bottoms are machined from for-gings. Forging produces a near-final shape, reducing the amount of material to be machined off. Forging also results in a fine grain structure and increased strength.

All welds, including the final seal, will be made using a process known as electron-beam (EB) welding. EB welding, which is carried out in a vacuum chamber, creates very narrow, high-precision joints while not adding excessive heat to the canister.

The picture below shows two half-tubes being assembled prior to welding. After welding, the tube is annealed to remove residual stresses. After annealing, the tube is machined to its final dimensions, as shown in the subsequent picture.

Two copper half-tubes are assembled for electron-beam welding. Welded canister being machined to finished dimensions.


A 24,000-lb copper ingot used to produce seamless canister shells.		A punched blank ready for extrusion.

Extrusion is a hot-forming process used to produce seamless tubes. The starting material for this form of canister fabrication is a cylindrical copper ingot about 6.5 ft long and 33.5 in (0.85 m) in diameter, weighing about 24,000 lb (11 metric tons). The ingot is heated, forged into a cylinder and pierced into a hollow blank ready for extru-sion. The photos above and below, illustrate these manufacturing stages.

The punched blank is heated to the required temperature and placed in position un-der a press tool. An internal mandrel controls the inner diameter. A large press tool is pushed downward and the tube is formed and pressed upward around the man-drel. The process is similar to that used to produce ordinary copper water tube. Ex-truded copper tubes are shown below.

Pierce-and-Draw Processing of Copper Tubes

Seamless tubes can also be made by the pierce-and-draw process. Like extrusion, pierce-and-draw is a hot-forming method. The starting material is again a cylindrical copper ingot. The ingot is first hot-forged to produce a shorter block with larger di-ameter. Next, it is pierced, then placed in a special tool in which it is drawn length-wise to form what can best be described as a deep cup. The bottom of the cup is kept intact in order to support the large punches that draw the canister to its final length while successively increasing its inside and outside diameters.

Complete full size canister.

More Work to be Done

Spent fuel is currently being accepted into the CLAB interim storage facility, where it will remain for the next 30 to 40 years. A considerable amount of work must be ac-complished during that time, including the selection and thorough characterization of the actual repository site.

Essential baseline work on the isolation canister has been completed. To date (summer 1999), some 20 canisters have been constructed, most of them made us-ing the roll-forming method described above. The method must, of course, be opti-mized and scaled up to accommodate the large number of canisters that will be re-quired in the future. On the other hand, production of seamless tubes by extrusion and pierce-and-draw has yielded such promising results that these methods will be-come the focus of work in the immediate future.

The SKB program will continue to rely on pure copper as the primary "engineered" barrier to the release of radionuclides to the environment. Nothing can be said with absolutely certainty when dealing with a timespan of the size we are dealing with here, but we are confident that copper canisters will remain intact in the Swedish repository environment long enough to ensure that future generations will not be harmed by their contents.

References

1. A hazard index of 1.0 represents the toxicity of eight metric tons of natural uranium (including its radioactive "daughter" elements). That is the quantity needed to produce one metric ton of fuel for a reactor that operates on enriched uranium. Eight metric tons of natural uranium may represent as much as several thousand tons of uranium ore.

Helpful Hints

Innovations was developed to cover technological changes involving copper.

Articles are archived by subject area (see Topics Archives on the left) and by year of publication (see Yearly Archives on the left).

You can also search all of the Copper Development Association's Web site by using Search vox on top right of every page.