The design of lid seal system of any package transporting irradiated nuclear fuel is a major aspect of the containment safety case under normal and accident conditions. Consequently, when BNFL, now known as British Nuclear Group Sellafield, BN-GS, decided on a change in lid seal material, year long proving trials on the new material were conducted, these simulating actual service and accident temperature conditions. Several years ago, BN-GS with the Rubber and Plastic Research Association (RAPRA) developed an elastomer called EPDM 30H which could perform at temperatures below −40°C and had excellent resistance to radiation, as later proven by long term testing. EPDM 30H material has been used in many site applications, with its first transport package application being on the Excellox 6 type, commissioned in 1991. Towards the end of the 1990s, BN-GS became aware that the fluorocarbon grade of lid seal material, used on several other packages, may be discontinued by the manufacturers and so they took the decision to replace the seals on the NTL 11 type packages with EPDM 30H. Consequently, BN-GS embarked on an extensive programme of seal testing which went beyond any it had previously carried out. Comprehensive data were available on radiation resistance and performance at low temperatures, but additional data were needed on its behaviour at elevated temperature over periods of about one year. A number of test sets were assembled comprising seals in representative lid seal grooves. Most of the test sets were fitted with EPDM 30H seals but another grade of EPDM seal was also included. Two test sets were continuously maintained at temperatures of 120 and 150°C, for one year. Other test sets were maintained at 60, 90, 120, 140, 160 and 180°C, but periodic inspection and compression set measurements were taken. At the end of one year the continuously heated seals were subjected to a thermal cycle corresponding to the thermal accident safety cases studies. These tests demonstrated that the EPDM 30H material had lower compression set characteristics than the other grade of EPDM and that EPDM 30H was a suitable seal material for all irradiated LWR fuel transport packages currently operated BN-GS.