Back End Transport

The transport of back end material, as with all other radioactive material transport, is governed by a stringent regulatory regime, which includes standards, codes and regulations that have been revised and updated over the past four decades. The safety measures have been developed to protect people, property and the environment against the hazards posed by the cargoes.

The solid nature of the products – spent fuel, MOX fuel, and vitrified high-level waste – is one of the most important safety factors. The materials are characterised by longterm stability and low solubility in water and will stay contained in a solid form after accident. Spent fuel and MOX fuel are both made of hard ceramic pellets that are contained in zirconium alloy metal tubes (fuel rods). The difference lies in the content; spent fuel contains uranium (96%), plutonium (1%) and fission products (3%) and is highly radioactive, while MOX fuel is made of uranium and plutonium oxides and has a low level of radioactivity. In the case of vitrified high-level waste, the vitrification process allows the fission products to be incorporated into a molten glass which is then poured into a stainless steel canister, where it solidifies. As a result, the fission products are immobilised and the vitrified product is protected by the stainless steel canister.

The IAEA has set standards for packages based on the characteristics of the different types of nuclear material. Spent fuel, MOX fuel, and vitrified high-level waste are transported in specially designed transport packages known as flasks or casks (termed Type B packages in the Regulations). They are specially designed for the particular radioactive material they contain, give protection to people, property, and the environment against radiation and are designed to withstand severe accidents. Type B packages range in size from drum-size to truck-size, but are always highly resistant and heavily shielded.

The philosophy of the IAEA Regulations is that safety is ensured by the packaging no matter what mode of transport is used. Under these Regulations the packaging design has to meet a series of rigorous impact, fire and immersion tests, notably:

• two drop tests – a 9 metre drop onto an unyielding surface and a 1 metre drop onto a steel punch bar;
• a subsequent fire test in which the package is subjected to a fully engulfing fire of 800°C for 30 minutes;
• immersion test where the cask is then subjected to conditions equivalent to 15 metre submersion for 8 hours. For casks designed for the more highly radioactive materials there is an enhanced immersion test of 200 metres for 1 hour.

These tests ensure that packages can withstand transport accidents involving crashes, fires or submergence which can be realistically envisaged and, in the case of fissile materials, ensure that an unwanted chain reaction cannot occur.

National competent authorities must certify the Type B package. Once the packaging design has been approved, it can be used for surface transport by truck, rail or ship.

Several demonstration tests have been carried out to show the large safety margin and robustness of Type B Packages. For example, engineers and scientists at Sandia National Laboratories conducted a wide range of demonstration tests in the 1970s and 1980s on Type B packages. These tests included truck impact tests at 98 and 138 km/h in which truck trailers carrying packages were impacted into 3 metre thick concrete barriers, and a diesel locomotive crashed into a Type B package at 131 km/h at a simulated rail crossing. Post-crash assessments showed that packages suffered only superficial damage and would not have released their contents. Although spectacular, these demonstration tests were not as severe as the IAEA series of tests summarised above. This shows the IAEA series of tests are conservatively representative of real-world accidents.

In 1993, the IMO introduced the voluntary Code for the Safe Carriage of Irradiated Nuclear Fuel, Plutonium and High-Level Radioactive Wastes in Flasks on Board Ships (INF Code), complementing the IAEA Regulations. These complementary provisions mainly cover ship design, construction and equipment. The INF Code was adopted in 1999 and made mandatory in January 2001. It has introduced advanced safety features for ships carrying spent fuel, MOX or vitrified high-level waste.