Depleted irradiated magnox fuel is processed by British Nuclear Fuels. The chemical process separates the radioactise isotopes produced during irradiation from the depleted natural uranium and in doing so con­centrates them for safe storage. The transport of this fuel is accomplished by its containment in packages known as road transport flasks. The flasks are moved by road and/or rail from the CEGB’s sites to BNFL at Sella field.

The flasks for the transport of magnox fuel ele­ments {Fig 3.45) consist of the following principal items:

• Flask body with integral base heat shield.

• Flask lid with integral heat shield.

• Skip contained within the water-filled flask and to hold the fuel.

The body consists of a forging of box form which is machined internally and externally to the required dimensions. Cooling fins are welded on to the sides, with full penetration welds to give an adequate heat flow path. A laminated heat shield is seal-welded on to the base to reduce the temperature rise of the con­tents during a fire accident condition. A single water level valve is mounted in the side of the flask and is used to determine the required water level in the flask. The valve is protected from accident damage by a padlocked cover plate.

The lid is manufactured from a single forging. It is located on the flask body with a tapered spigot and retained by bolts. Face sealing is incorporated at the joint between the lid and body by double seals re­tained in a single groove provided in the lid. The lid is shaped to provide shock-absorbing character­istics and also incorporates a laminated heat shield

• Code of Practice for the Carriage of Radioac­tive Materials by Road issued by HM Stationery — Office.

The regulations require that conditions are met both for normal transportation and under postulated accident conditions. A safety case is made for each type of flask used and is submitted via the CEGB’s Nuclear Health and Safety Department to the De­partment of Transport for approval.

The safety case provides material specifications to­gether with manufacturing details including quality — assurance procedures. It further defines the flask contents and the handling procedures to be used. Theoretical assessments are made of thermal and stress analysis for normal and accident conditions. The ana­lyses are supported, where possible, by model or full scale tests. With regard to accident conditions, the requirements of the IAEA regulations are met by — considering:

• A free drop from a height of 9 m onto an un­yielding target.

• A thermal test at 800°C for 30 minutes.

• Immersion in water to a depth of 15 m.

Arising from the regulations, certain conditions have to be met to allow the fuel to be transported in a flask of approved design:

• In the case of magnox fuel, this must be stored and cooled on site for a minimum period of 90 days following its discharge from the reactor. This en­sures an adequate reduction of the residual heat and the decay of the short lived isotopes 1-131, thereby limiting the possible release of that isotope under the postulated accident conditions.

• For magnox fuel the water contained in the flask is chemically controlled to provide a pH greater than 11.5. This is achieved by a sodium hydroxide content of 200 ppm.

• To reduce magnox corrosion at elevated tempera­tures following a postulated accident, a sodium tluoride solution is injected into the flask after it has been loaded with fuel such that the resultant fluoride concentration is 1000 ppm. The injection is etfected by a pressurised nitrogen system or by a gravity feed system,

• Before dispatch of the flask, all watertight seals are pressure tested to 20.68 bar and the ullage space above the water is purged with nitrogen, which in the magnox case avoids the possibility of an ex­plosive hydroeen/air mixture. [38]

with the flask design but is typically 5 kW.

• The possibility of exposed uranium enabling fission products to be leached into the water is monitored by caesium release rate measurements. The mea­sured rate is temperature and irradiation dependent and no skip load of fuel may be dispatched if the rate is above clearly defined limits.

Utilisation of the CEGB’s tlasks is determined cen­trally and their movement is planned in collaboration with British Rail. Emergency plans are available to provide for the unlikely event of an accident to a loaded flask in transit. These have been drawn up by the CEGB, British Rail and BNFL. The plans enable a health physics team to be called out from the nearest nuclear power station or BNFL establishment to deal with such an emergency and ensures national coverage and a rapid response.