Nuclear materials may be highly radioactive. The goal is to prevent any risk of an accident and ensure maximum safety. To achieve these goals, the principles of "defence in depth" are applied. Maximum safety is achieved through robust packages and casks, reliable transport and effective accident prevention measures.
Due to the radioactivity that it contains, irradiated fuel must be transported in specially-designed containers known as "casks". In France, PWR fuel assemblies are transported in casks designed to hold 12 fuel assemblies, in a dry air atmosphere. Casks with similar specifications are used to carry MOX and vitrified waste. They have thick walls that protect against radiation and are sealed to prevent any dissemination of radioactive materials.
The transport cask forms a containment barrier, enabling fuel elements to be transported by road, rail or sea.
These very strong steel containers weigh more than 100 tonnes and have walls up to 30 cm thick. Transport casks are designed to withstand fire, rocket strike or crushing, as might occur in the event of an accident involving the truck or train on which they were being carried. They are tested to assess their resistance to impacts (at 50 km/h), puncture, fire (exposure to an 800°C fire for 30 minutes) and immersion (200 m depth of water).
Rail, acknowledged to be a very safe means of transport for large convoys, is the preferred option for carrying large or heavy packages whenever a rail link is available.
Nuclear fuel is never transported by air, for obvious safety reasons.
Sea transport accounts for 3% of all movements of radioactive material relating to the nuclear fuel cycle.
The ships used to carry MOX fuel to Japan are specially equipped with redundant features and systems, including a double hull, fire detection and extinguishing systems and anti-collision radars. The ship is accompanied by an armed escort, and its position is tracked in real-time by a satellite positioning system.
In the event of a shipwreck, the walls of transport casks and containers, as well as the zirconium alloy cladding (in the case of irradiated fuel) or the glass matrix (in the case of vitrified waste) would impede the dissemination of radioactive materials. If and when corrosion breaches the containers, after a few decades at the least, the uranium oxide fuel pellets and insoluble vitrified matrices would also resist dissemination. We all remember the tragic loss of the Kursk submarine, which was eventually raised to the surface in October 2001. Contrary to many people's fears, no radioactive leaks were detected, due to the relatively short time spent in the sea.
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