Year
2010
File Attachment
Abstract
Paragraph 417(a) of the Regulations for the Safe Transport of Radioactive Material (2009 Edition) imposes mass limits for packages and consignments where the dispositions of the fissile material under Normal Conditions of Transport (NCT) or Accident Conditions of Transport (ACT) cannot be guaranteed. Therefore, it is proposed to revise these exceptions in paragraph 672 to strengthen these mass limits by requiring the use of the Criticality Safety Index (CSI). To be consistent with the safety level required for certified packages, the calculated CSI must ensure that two groups of packages, each having a total CSI of 50, contain a subcritical mass. Under NCT, five groups of these packages must be safe. Thus, proposed subparagraph 672(a) requires that a group of packages with a total CSI of 50 contains less than 1/5th of a given subcritical mass but the release of fissile material from the package is not limited. Proposed subparagraphs 672(b) and (c) require that a group of packages with a total CSI of 50 contains half a given subcritical mass and that the packages do not release their content under NCT. The values of the subcritical masses are provided in a table (Table M) for two cases depending on the moderator material: with restrictions (based on H2O moderation) and without restrictions (based on CH2 moderation). This paper discusses the criticality issues related to the use of CSI calculated in case of restrictions. Indeed, since packages do not necessarily withstand the ACT or even NCT, ruined packages configurations cannot be excluded. Then, considering the fact that material used for transport may contain high density polyethylene (HDPE), retention trays, containers, bottles, pallets for example, or that packages could be transported alongside any other package carrying any hydrogenated material (CH2…), a fissile material from packages whose CSI has been calculated with restrictions can actually become moderated by a material whose hydrogen density exceeds the hydrogen density in water. Different scenarios of consignments mixing packages approved using different cases of Table M have been studied. Results show that the mass of CH2 per consignment has to be lower than 500 g. Moreover if materials used for transport are taken into account, only the case without restrictions should be considered.