Safety in depth
The full ‘Safety in Depth’ system encompasses the material, canisters, assemblies, special packages called casks or flasks and the protection provided by the reinforced double hulls of the PNTL fleet.
Together, these provide a series of independent barriers between the radioactive material and the outside environment.
This safety system provides much greater protection than typically exists for other hazardous cargoes such as chemicals, oil and liquid gases, which are shipped far more frequently. It also removes reliance on specialist emergency assistance being available from countries adjacent to shipping routes.
Professor HW Lewis, a former Chairman of the United States Nuclear Regulatory Commission Risk Assessment Review Group wrote with regard to PNTL shipments of vitrified waste that:
‘An accident of any kind is unlikely to happen… and the environmental damage done by any conceivable accident would be thousands of times less than that done by a single oil spill.
Regulatory Assessment
At the request of the IMO, the IAEA conducted a Coordinated Research Program from 1995-1999 to determine whether the IAEA regulations on packages take adequate account of accidents at sea. This study, “Severity, probability and risk of accidents during maritime transport of radioactive material (PDF)”, evaluated accident scenarios and concluded:
“Since the probabilities of severe ship collisions and severe ship fires are small and the individual radiation doses that might result should such a collision or fire occur are smaller than background doses, the risk of maritime transport in Type B packages of highly radioactive material such as irradiated [used] nuclear fuel, vitrified high level waste and plutonium are very small.”
This “Safety in Depth” transportation system explains why PNTL shipments are regarded by experts as some of the safest on the seas today.
Barriers – the reinforced double hulls
The cargo compartments of the PNTL fleet are protected by a double hull configuration, extending to twenty per cent of the beam (width) on both sides of each ship. In addition, the hulls are reinforced with 400 tonnes of extra steel to withstand collision damage.
The reinforced double hulls of the PNTL ships are designed to ensure that the casks on board would not be directly impacted in a severe collision at sea.
Barriers – Transport Casks
The radioactive material is transported in special transport packages that are designed to withstand severe accident scenarios.
The packages – called “casks” or “flasks” – are massive steel structures made from 25 cm thick forged steel. The ones typically carried by PNTL weigh around 100 tonnes.
The standards for packages of nuclear material are set by the International Atomic Energy Agency (IAEA), which is a United Nations agency. The standards have been established by experts and national regulators from many of the 151 member countries and are systematically reviewed to take account of technical advances.
The IAEA regulations require that nuclear materials are placed in packages that will protect workers, the public and the environment in severe accident situations. This ensures high levels of safety whatever mode of transport is used. Many shipments involve transport by a combination of different modes – road, rail, air and sea.
The IAEA has set standards for packages based on the different characteristics of the nuclear material being transported. PNTL’s cargoes of used fuel, vitrified waste and MOX fuel must be moved in “Type B” packages. These special packages must be able to withstand a series of challenging tests that demonstrate resistance to severe impact, fire and immersion.
Safety material
Working from the radioactive material to the outside environment, the first engineered barrier is the material itself, which is usually used nuclear fuel, vitrified waste or MOX fuel.
Used nuclear fuel and newly-manufactured MOX fuel comprise of solid pellets contained within sealed, corrosion-resistant metal fuel rods. They are designed to withstand the extreme heat and pressures of a nuclear reactor. In turn, the fuel rods are loaded into assemblies.
High-level nuclear waste is transformed from a liquid into a solid by mixing it with borosilicate glass – vitrifying it. The vitrified waste is solid, stable and passive, making it ideal for transportation and long-term storage.
Each material, if exposed to seawater, would maintain its integrity over long periods. None would dissolve readily – they would behave in much the same way as a marble in a glass of water.
In other words, if this material were somehow to become exposed to seawater, even though it is highly radioactive, the environmental impact would be negligible.
Environmental impact assessments have calculated that the maximum radiation exposure to the public in such a scenario would be more than 1,000 times smaller than radiation levels found naturally in the environment.