A Methodology for Determining the Dose Rate for Bounding Mass Limits in a 9977 Packaging

摘要

The Small Gram Quantity (SGQ) concept is based on the understanding that the hazardsassociated with the shipment of a radioactive material are directly proportional to its mass. Thisstudy describes a methodology that estimates the acceptable masses for several neutron andgamma emitting isotopes that can be shipped in a 9977 Package compliant with the Title 10 ofthe Code of Federal Regulations, Part 71 (10CFR71) external radiation level limits. 10CFR71.33states that a shipping application identifies the radioactive and fissile materials at their maximumquantity and provides an evaluation demonstrating compliance with the external radiationstandards. Since rather small amounts of some isotopes emit sufficiently strong radiation toproduce a large external dose rate, quantifying of the dose rate for a proposed content is achallenging issue for the SGQ approach. It is essential to quantify external radiation levels fromseveral common gamma and neutron sources that can be safely placed in a specific packaging, toensure compliance with federal regulations. A methodology was established for determining thedose rate for bounding mass limits for a set of isotopes in the Model 9977 Shipping Package.Calculations were performed to estimate external radiation levels using the MCNP radiationtransport code to develop a set of response multipliers (Green’s functions) for “dose per sourceparticle” for each neutron and photon spectral group. The source spectrum from one gram ofeach isotope was folded with the response multipliers to generate the dose rate per gram of eachisotope in the 9977 shipping package and its associated shielded containers. The maximumamount of a single isotope that could be shipped within the regulatory limits for dose rate at thesurface was determined. For a package containing a mixture of isotopes, the acceptability forshipment can be determined by a sum of fractions approach. Furthermore, the results of thisanalysis can be easily extended to additional radioisotopes by simply evaluating the neutronand/or photon spectra of those isotopes and folding the spectral data with the Green’s functionsprovided.