Browse by author
Lookup NU author(s): Emeritus Professor Mike Reeks
The Nuclear Decommissioning Authority (NDA) is developing a family of Standard Waste Transport Containers (SWTCs) for the transport of unshielded intermediate level radioactive waste packages. The SWTCs are shielded transport containers designed to carry different types of waste packages. The combination of the SWTC and the waste package is required to meet the regulatory requirements for Type B packages. One such requirement relates to the containment of the radioactive contents, with the IAEA Transport Regulations specifying release limits for normal and accident conditions of transport. In the impact tests representing accident conditions of transport, the waste package will experience significant damage and radioactive material will be released into the SWTC cavity. It is therefore necessary to determine how much of this material will be released from the cavity to the external environment past the SWTC seals. Typical assessments use the approach of assuming that the material will be evenly distributed within the cavity volume and then determining the rate at which gas will be released from the cavity, with the volume of radioactive material released with the gas based on the concentration of the material within the cavity gas. This is a pessimistic approach as various deposition processes would reduce the concentration of gas-borne particulate material and hence reduce their release rate from the SWTC. This paper assesses these physical processes that control the release rate and develops a conservative methodology for calculating the particulate releases from the SWTC lid and valve seals under normal and accident conditions of transport, in particular: a) the flows within the SWTC cavity, especially those near the cavity walls; b) the aerodynamic forces necessary to detach small particles from the cavity surface and suspend them into the cavity volume; c) the adhesive forces holding contaminant particles on the surface of a waste package; d) the breakup of waste material upon impact that will determines the volume fraction and size distribution of fine particulate released into the cavity. Three mechanisms are specifically modelled, namely Brownian agglomeration, Brownian diffusion and gravitational settling, since they are the dominant processes that lead to deposition within the cavity and the easiest to calculate with much less uncertainty than the other deposition processes. Calculations of releases under normal conditions of transport concentrate on estimating the detachment of any waste package surface contamination by inertial and aerodynamic forces and show that very little of any contamination removed from the waste package surface would be released from the SWTC. Under accident conditions of transport, results are presented for the fraction released from the SWTC to the environment as a function of the volume fraction of the waste package contents released as fine particulate matter into the SWTC cavity. These show that for typical release fractions of 10-6 to 10-8 for the release of radioactive material from waste packages into the SWTC cavity, the release fraction of the waste package inventory from the SWTC of typically 10-9 to 10-10. Hence, the effective decontamination factor provided by the SWTC is 102 to 103. Whilst this analysis has been carried out specifically for the SWTC carrying waste packages, it is applicable to other arrangements and its use would reduce the high degree of pessimism used in typical containment assessments, whilst still giving conservative results.
Author(s): Gray I, Sievwight B, Reeks M
Publication type: Article
Publication status: Published
Journal: Packaging, Transport, Storage & Security of Radioactive Material
Year: 2010
Volume: 21
Issue: 1
Pages: 13-18
Print publication date: 01/01/2010
Date deposited: 08/06/2010
ISSN (print): 1746-5095
ISSN (electronic): 1746-5109
Publisher: Maney Publishing
URL: http://dx.doi.org/10.1179/174650909X12511278683302
DOI: 10.1179/174650909X12511278683302
Altmetrics provided by Altmetric
