Waste transport in Sweden3 August 2003
Sweden's SKB has been assigned the task of finding a safe way of disposing of the country's radioactive waste. Since 1985, waste facilities and a transportation system have been in operation.
Radioactive waste should not be passed on to future generations, but rather be managed and disposed of today. The Swedish Nuclear Fuel and Waste Management Company (Svensk Kärnbränslehantering, SKB) has developed a system for disposing of the waste safely and permanently.
All Swedish nuclear power plants are situated on the coast. SKB can therefore transport the radioactive waste by sea. This is done by m/s Sigyn SKB's specially built ship.
Sigyn was built in 1982. For added seaworthiness, the ship has a double bottom and a double hull, which gives her very high floatability. The double hull also protects the cargo in the event of grounding or collision. A modern passenger vessel can float even if two of its watertight sections below the waterline refilled with water. Sigyn can float with four sections water-filled. The cargo is driven on board using the stern ramp according to the roll-on/roll-off principle. The cargo hatches can also be opened to allow the cargo to be lifted on board. The cargo hold has robust lashing fittings to which the cargo is secured. Sigyn is classified as an INF 3 ship according to IMO regulations.
The walls around the cargo hold are radiation-shielded, and there are instruments on board for measuring radiation. The measurements have shown that the crew is not exposed to any extra radiation doses at sea beyond the natural background radiation, which is lower at sea than ashore. To operate the ship, SKB has engaged the shipping line Rederiaktiebolaget Gotland. Sigyn normally makes 30-40 trips per year between the nuclear power plants and CLAB/SFR. The ship is also chartered out for special shipments of other heavy goods.
Exactly how shipments to the deep repository will take place will depend on where the repository is located. If it is built on one of the sites where site investigations are currently being conducted Forsmark and Oskarshamn long overland shipments will not be necessary. Just like today, the future shipments will be regulated by a number of laws and international agreements. They cover such aspects as radiation protection, training of personnel, safety procedures and documentation. The authorities are notified in advance of all shipments, which are furthermore insured against third party injuries.
Safety is primarily guaranteed by the transport casks and containers that are used. The casks will remain intact even if something should happen to Sigyn for example, if she should sink. Special terminal vehicles are used for transportation to and from the harbours, at the nuclear power plants and at the facilities. They only travel a few kilometres at the most and at very low speeds.
The transport casks and containers are specially designed for the type of waste they are meant to carry. They provide the ultimate assurance of safety in transport. The casks and containers comply with international requirements and are built to withstand extreme stresses.
Low-level waste needs no radiation shielding. It can therefore be transported in ordinary steel freight containers. Intermediate-level waste, however, requires radiation shielding and is solidified in moulds at the nuclear power plants. The waste is then transported in containers with 7-20cm thick walls of steel, depending on the type of waste in question.
Spent nuclear fuel requires both radiation shielding and cooling during transport. The requirements on the casks used to ship the waste are therefore extremely tough. The fuel is placed in transport casks with walls of about 3cm thick steel. External cooling fins dissipate the heat. The casks are designed to withstand much greater stresses than can reasonably be expected to occur during the waste shipments. They must be able to withstand a free drop from a height of 9m onto an unyielding surface, fire for 30 minutes at a temperature of 800°C, and an external pressure equivalent to a water depth of 200m all without the integrity of the casks being affected. The casks meet these requirements with ample margins. For example, they can withstand an external pressure equivalent to a water depth of 4000m. (The water along the Swedish coast is never that deep.) The casks/containers are placed on load carriers or frames, which are lashed to the deck of the hold in Sigyn. The load frames are also fitted with signal equipment so the containers can be located and salvaged if Sigyn should founder.
When the time comes to transport spent fuel to the deep repository, it will be less radioactive than today, after interim storage in CLAB.
Furthermore, it will be encapsulated in copper canisters with cast iron inserts. The future transport casks will nevertheless meet the same tough international requirements as today's casks.