UK REGULATORY EXPERIENCE WITH THE CRITICALITY SAFETY OF TRANSPORT PACKAGE APPLICATIONS

摘要

The UK Government has created the Office for Nuclear Regulation (ONR) as an independent public body to ensure effective and efficient regulation of the nuclear industry in the UK. ONR brings together for the first time in the UK the regulation of nuclear safety, civil nuclear security, nuclear safeguards and the transport of radioactive substances within a single organization. Applications requesting approval to transport packages containing fissile material through the UK have been received for many decades from numerous organisations across the world. Each application is treated equally with the judgement of safety based on the information provided in the design safety report. In recent years, the ONR has questioned a number of criticality safety related assumptions; some of which have been held for many decades. The outcome from these discussions will be examined. Uranium hexafluoride (UF_6) has been transported across the world for many years, carried in 30B steel cylinders within a variety of overpacks. For simplicity, some criticality safety cases have not claimed for the presence of the overpack. However, it shall be shown that these safety cases will have difficulty in justifying the criticality safety of transporting uranium hexafluoride if credit for the overpack is not claimed. In addition, the applications that have been submitted for Competent Authority approval have made various assumptions regarding the shell thickness of the 30B cylinder, the distribution of the hydrogen fluoride (HF) impurity and the potential for the uranium hexafluoride residues to hydrolyse. The adequacy of these assumptions will be discussed in turn along with the need for the applicant to demonstrate compliance with any certificate restriction on the mass of heel (residue) allowed within the package. Uranium oxide (UO_2) powder is regularly transported around the world. For most designs, it is assumed that water will enter the package under accident conditions of transport. Under these conditions, a common assumption is that the grains of uranium oxide powder will evenly distribute within the water and internal volume of the package. The validity of this assumption from a realistic and pessimistic perspective will be challenged. It will then be shown that if instead the powder grains of uranium oxide are agglomerated together into larger sized groups, an increase in the k_(eff) of the system of up to 2,000pcm can occur. As packages are commonly loaded up to the criticality safety limit, an increase in k_(eff) such as this is likely to be problematic.