Application of the revised requirements of IAEA St-1 to the transport of natural uranium hexafluoride

The 1996 issue of IAEA ST1 (formerly SS6) will, from january 2001, call for transport cylinders containing natural uranium hexafluoride ('hex') to be considered against the requirements of the IAEA thermal tests. In parallel with the revision of SS6, the IAEA established a coordinated research program (CRP) to consider the behaviour of hex cylinders in fires. BNFL has contributed to that CRP principally in the areas of computer modelling of the thermophysical processes occurring within the cylinder and of the mechanical behaviour of the cylinder itself. Confidence in the various computer simulations has grown steadily as the CRP progressed. With the benefit of results emerging from the Tenerife test program, our calculations now predict that an unprotected, fully loaded, 48Y hex cylinder would be expected to survive the thermal test. Since the expectation is that the unprotected fully loaded 48Y cylinders should (marginally) survive the fire test, and also taking account both of the excellent safety record to date of hex transports, and the rarity of intense long-lived fires in transport situations, there is clearly very little safety benefit to be gained from the use of fire protection covers on natural hex cylinders. However, the physical handling of such covers, necessarily large and cumbersome, and their relevant fastening, particularly during packaging, loading, unloading and un-packaging in transport depots would, for every cylinder movement, impose additional risks of injury to transport system operators, which we believe could outweigh the small and more hypothetical benefit of improved fire resistance while in transit. We therefore maintain that it would be counterproductive, in both overall safety and in cost terms, to require protective coverings on these cylinders for transport. Based on the estimates made to date, we believe that the unprotected 48 inch cylinders should continue to be the normal package in commercial use, under the relevant multilateral approvals. (authors)