TECHNICAL FIELD

The present invention relates to a method for long-term storage of at least one canister containing spent nuclear fuel, said method comprising the steps of:

a) placing the canister in a storage space located below the ground water level; b) embedding the canister in the storage space in a buffer material such as clay, for example bentonite clay, and/or sand, for example quartz sand.

STATE OF THE ART

Today, the handling of spent nuclear fuel is based on methods where the fuel is buried in the primary rock for long-term storage that will have to last several hundreds of thousands of years into the future before it is certain that the radioactivity has abated to a harmless level. During this period, it is of outmost importance that no radioactive species or particles escape and spread through the ground water, since this would have devastating consequences for all life coming in contact with leaked material. How to design a safe model for the depth repository is subject to discussion. The Swedish Nuclear Fuel and Waste Management Co (SKB) has developed the KBS-3 concept that is based on encapsulation of spent nuclear fuel in a protective copper casing that is thereafter embedded in bentonite clay in deposition holes situated at a few meters distance from each other in a system of horizontal tunnels at a depth of 400-700 meters below the ground surface. Bentonite clay has properties causing it to form an efficient buffer between rock and canister, and will, inter alia, protect the canister against smaller rock movements and form a barrier against radioactive species spreading from a canister that is not impermeable.

After having arranged the canisters in the deposition holes, the tunnels are filled with backfill material such as, for example, clay and crushed rock. The ground water then slowly seeps into the deposition holes. Initially, the voids between the clay mineral particles are filled with water and, subsequently, the water is drawn in between the mineral flakes. When the bentonite clay gets wet, it swells and fills the cavities and cracks surrounding the deposition hole. The natural saturation process can take up to a hundred years, or even longer, depending on the properties of the rock. According to a recent decision, the planned final repository in Sweden will be placed at a location with bedrock considered to have few cracks and very slow rate of water movement. The canister with the nuclear fuel is intended to last for very long periods of time, at least 100 000 years, and it is therefore important that the corrosion is kept as low as possible. BRIEF ACCOUNT OF THE INVENTION

Contrary to the prevailing opinion, the applicant has identified a number of problems with the method according to KBS-3. When the ground water, as time goes on, slowly seeps into a deposition hole containing a copper canister, the situation may arise where water partially fills the storage space and the canister so that a waterline is formed. This can give rise to so called boundary layer corrosion on the canister casing in the boundary zone between water and gas phase (herein air), a very aggressive local corrosion process where oxygen is consumed locally in the boundary layer. This corrosion is aggravated further since the canister is heated due to activity in the spent nuclear fuel - the canister is estimated to have a temperature of between 50-90 °C for up to 1000 years.

Another corrosion process that may affect the heated metal canister is evaporation- induced corrosion. Ground water entering from cracks in the rock reaches the hot canister surface and evaporates, which can result in an accumulation of aggressive salts (chlorides, sulphides, bromides, carbonates, etc.) from the ground water on the metal surface of the canister.

As a result of the elevated temperature of the canister in an environment with salts and high moisture content, also atmospheric corrosion will arise, even if the oxygen gas content in the air is non-existent. Moisture will also be present in the air and contribute to said atmospheric corrosion. The rate of this process can be very high, up to 100-300 microns/year.

Corrosion damages caused by said processes could lead to destroyed copper canisters, and thus, in the worst case, to leakage of radioactive particles within a very near future, long before the radioactivity has abated to a harmless level.

Furthermore, the, properties of the clay can be destroyed if it is heated and dried for prolonged periods of time. Bentonite clay with degraded properties would not constitute a sufficiently efficient barrier against radioactive leakage from a canister. The object of the present invention is therefore to achieve an improved method for the long-term storage that results in reduced corrosion of the canisters with nuclear fuel.

In the method for long-term storage mentioned in the first paragraph, this object is achieved in that the method further comprises the step of:

c) supplying water to the storage space containing the canister embedded in the buffer material, so that the storage space is actively saturated with water at a rate of saturation exceeding the natural saturation process. By actively saturating the storage space with water, at a rate of saturation exceeding the natural saturation process, the exposure of the canister to boundary layer corrosion, evaporation- induced corrosion and atmospheric corrosion is reduced, and thereby a safer long-term storage can be obtained, which is of course advantageous. By active saturation with water we mean that water is added, in addition to the natural inflow, for example by supplying water from a tank truck.

Preferably, the canister comprises an insert containing said spent nuclear fuel, and a copper casing that encloses the insert. Examples of such a canister can be the one developed in accordance with the KBS-3 method of SKB. In one embodiment, the canister comprises at least one additional casing/layer that encloses the copper casing and that consists of a passive film-forming metal or alloy, wherein the passive film on the casing is constituted by an essentially oxidic film that is rich in one or more of the metals belonging to the group of metals consisting of the metals zirconium, chromium and titanium. This additional casing/layer is primarily intended to protect the canister during the period when the temperature of the canister is elevated in accordance with what has been described in SE 531261 C2.

Preferably, the storage place is located at least 50 meters below the ground water surface, preferably at least 100 meters below the ground surface, more preferably at least 300 meters below the ground surface.

Preferably, in step c), the storage space is actively saturated with water to a level such that the entire canister is covered. Since the entire canister is "flooded", the corrosion problems of the canister caused by boundary layer corrosion, evaporation-induced corrosion, and atmospheric corrosion are minimized. According to one aspect, the storage spaces are arranged as substantially vertical deposition holes at the bottom of a tunnel or a rock chamber, wherein the respective deposition holes are adapted in size to allow a canister to be arranged vertically therein, surrounded by the buffer material. Such a method, with the concept name KBS-3V, has been described by SKB.

Alternatively, the storage space can be constituted of one or several substantially horizontal deposition holes in the side wall of a tunnel or a rock chamber, wherein the respective deposition hole is adapted in size to allow at least one canister to be arranged horizontally therein, surrounded by the buffer material. If horizontal deposition holes are used, it is conceivable that several canisters are placed in one deposition hole, so that they lie on a line. Such a method, with the concept name KBS-3H, has been described by SKB. It is also conceivable to mix horizontal and vertical storage spaces.

Preferably, the respective storage space with an embedded canister is actively saturated with water so that a level of water saturation exceeding the highest point of the canister is reached after a period of time shorter than 5 years, more preferably after a period of time shorter than 1 year, even more preferably within a period of time shorter than 1 week. A more rapid process of water saturation reduces the period during which the canister risks being subjected to boundary layer corrosion, evaporation-induced corrosion, and atmospheric corrosion, which is of course advantageous.

The invention also pertains to a device for long-term storage, containing at least one canister that contains spent nuclear fuel, arranged in a storage space located below the ground water level, embedded in a buffer material, and actively saturated with water to such a level that the entire canister has been covered in accordance with the method of any one of the foregoing claims.

BRIEF DESCRIPTION OF THE FIGURES

In the following, the invention will be described in greater detail with reference to attached drawing figures, in which:

Fig. 1 shows a perspective view of a canister for the storage of a radioactive material;

Fig. 2 shows a schematic view of an underground tunnel system for the storage of radioactive material;

Fig. 3 shows a perspective close-up view of a tunnel with storage spaces, as taken from the circle I in Fig. 2; and Fig. 4 is a cross-sectional view of a storage space containing a canister and backfill material in accordance with the invention.

DETAILED DESCRIPTION

Fig. 1 shows a preferred embodiment of a canister 1 intended for containment of a radioactive material, such as a spent nuclear fuel, for long-term storage. The canister 1 comprises a casing 2 that is substantially cylindrical with an opening at an upper short end 2a, and has an insert 3 that preferably has a number of longitudinal cavities intended for spent fuel rods, and a cover 4 that can be attached to the upper short end 2a of the casing 2 in order to seal the canister.

The casing 2 and the cover 4 are made of metal. For example, the casing 2 can consist of five centimetres thick copper in accordance with the KBS-3 method of SKB

(www.skb.se), but it can also be provided with an additional protective layer of a passive film- forming metal or alloy, wherein the passive layer on the casing is constituted by an essentially oxidic film that is rich in one or more metals from a group consisting of the metals zirconium, chromium and titanium, as has been described in SE 531261 C2, which is hereby included by reference. The insert 3 can, for example, be made of cast iron, as is suggested in the KBS-3 method, or cast steel. The length of the canister 1 is preferably in the range of 3-7 m and its diameter in the range of 0.5-2 m, but it will be appreciated that also other dimensions can be used and that the invention is not limited thereto.

Fig. 2 shows a schematic representation of the appearance of an underground storage 10 intended for long-term storage of the radioactive material, comprising a plurality of tunnels 1 1 which have been formed in the bedrock at a depth below the ground water level 20, at least 200 m, but preferably at least 400 m below the ground surface 22.

Each of the tunnels 1 1 contain a plurality of storage spaces 12 intended for a canister 1 , and these tunnels 11 are connected through a connecting passage 14 to each other and to a connecting passage 13 to the ground surface which, at the ground surface 22, opens into a facility 21, from where the transport of canisters 1 containing nuclear fuel to a storage space 12 can be initiated via channels and tunnels 13, 14, 1 1. Fig. 3 shows an enlargement of the circle I in Fig. 2, wherein a perspective view of a tunnel 1 1 having a plurality of storage spaces 12 can be seen. These storage spaces 12 are designed as vertical deposition holes at the bottom of the tunnels 1 1 , and are each so large that a canister 1 and a surrounding backfill material 5 can be placed in the storage space 12 such as, for example, proposed by SKB in the KBS-3V method. Alternatively, the canisters can be stored in horizontal deposition holes such as, for example, proposed by SKB in the KBS-3H method.

A cross-section of such a storage space 12 and tunnel 1 1 , with a canister 1 placed in the storage space 12, can be seen in Fig. 4. A backfill material in the form of a buffer material 5, such as clay and/or sand, preferably a bentonite clay or quartz sand, or a mixture thereof, is placed in the gap remaining between the canister 1 and the limiting surfaces of the storage space 12. After having placed the canister 1 and the buffer material 5 in the storage space 12, a backfill material 6 is used to fill up the rest of the tunnel 11 so that it is sealed.

The method for storing a spent nuclear fuel according to the present invention will now be described in greater detail

In a first step, a suitable quantity of a nuclear fuel is placed in the insert 3 of a casing 2, whereupon the casing is sealed by a cover 4 that is attached to the upper surface 2a, so that a canister 1 is formed. If suitable, also other substances or materials can be contained in the canister, such as additional backfill material or other types of radioactive material.

In order to bring the canister 1 to a storage space 12 intended therefore, in one of the tunnels 1 1 , the canister is transported from the ground surface 22 down through the connecting passage 13, along the connecting passage 14 and out into the selected tunnel 1 1. Now, the canister 1 is positioned in a storage space 12 that is either completely empty or already contains a quantity of the buffer material 5. After the positioning, an additional amount of the buffer material 5 is added, if necessary, until the canister 1 is enclosed by this material to a desired degree. Preferably, the canister 1 is completely covered by this material 5, so that no part of the surface of the canister 1 is in direct contact with the limiting surfaces of the storage space 12. Thereupon, a liquid, such as water, is added to the buffer material 5, so that it is actively saturated with water. This active saturation occurs more rapidly than if ground water could seep into the storage space 12 through cracks in the bedrock, and it is advantageous to avoid a situation where some part of the canister 1 is in contact with a liquid surface, such as a water surface, so that one part of the canister is above the liquid and another part is submerged therein for any prolonged periods of time. The fact is that such a situation may result in a relatively strong degradation of the casing 2 at the point where the liquid surface is, so that there is a risk of such serious damages being caused that a connection is formed between the contained nuclear fuel and the surrounding environment as time goes on. Such a process also may be accelerated in that the casing 2 is heated by reactions in the contained nuclear fuel, wherein the activity of the spent nuclear fuel may imply that the casing 2 has a temperature in the range of 50-90 °C for hundreds of years. It is therefore very advantageous to accelerate the water saturation of the storage space 12 so that the entire canister 1 embedded into the buffer material 5 is covered more rapidly than what would have occurred with only natural water saturation. After having saturated the storage space 12 with the embedded canister 1, it can optionally be sealed up, for example by casting a cover of concrete.

After the water saturation, and as the required number of the different storage spaces 12 have received a canister 1 , a backfill material 6 can then be used to fill up the tunnel 1 1 so that the canisters 1 can be additionally protected against influence from the outside. It is of course conceivable that a tunnel 11 is left unfilled, in order to make it easier to access the storage spaces. The backfill material 6 can be constituted of clay (for example bentonite clay), sand (for example quartz sand), gravel, rock fill, excavated material, etc. and, the tunnel 1 1 can be sealed up, if desired. When a tunnel 11, or a system of tunnels 1 1 , has been filled with backfill material 6, it is conceivable that it/they also is/are saturated with water to further expel oxygen. Alternatively, it is conceivable to fill the tunnel 1 1 with the backfill material before the respective storage space 12 with embedded canister 1 has been actively saturated with water, to instead saturate the entire system (tunnel/tunnels 11 and storage spaces 12) with water in a single step.

The invention is not limited by what has been described above, but can be varied within the scope of the following claims. For instance, it will be appreciated that, of course, also other designs of storage spaces and systems of tunnels therebetween can be used for storing canisters with nuclear fuel, and it should also be noted that the design and constituent materials of a canister are not a focus of the invention, but they can be varied depending on what is suitable for the case in question. For example, SE425707 and US4834917, SE 509177, US4562001 ^■ show further examples of canister configurations. The buffer material is, of course, not limited to bentonite clay, but different materials can be used for the containment of a canister.