ISSUES AND CLARIFICATION ON USE OF GAMMA RAY FLUENCE-TO-DOSE FACTORS FOR PACKAGE SHIELDING EVALUATIONS

NRC Standard Review Plan for Transportation Packages for Radioactive Material, NUREG- 1609, suggests that the flux-to-dose conversion factors in ANSI/ANS 6.1.1-1977, “Neutron and Gamma-Ray Fluence-to-Dose Factors”, should be used when estimating the dose rates for compliance with regulations. This standard presents data recommended for computing the biologically relevant dosimetric quantity in neutron and gamma-ray radiation fields. Specifically, this standard is intended for use by shield designers to calculate effective dose equivalent. ANSI/ANS-6.1.1-1991, which is the latest version of the standard, is designated as a “historical” standard because it was officially withdrawn by ANSI as an American National Standard in 2001. ANS was unable to develop a revision to the standard within the ten-year period specified by ANSI for all standards, but intends to issue a revision in the future. The dose equivalents in the two versions of the standard (i.e., 1977 and 1991) are based on different data. The 1977 standard ceased to be recognized as authoritative in 1991 with the issuance of the later version. The NRC guidance recognizes that there are significant differences in the dosimetric models and the dosimetric quantities that form the basis of the factors in the two versions of the standard. The current NRC position remains that the 1977 version of ANSI/ANS 6.1.1 should be used for 10 CFR Part 71 applications. Codes commonly used by shield designers for estimating dose rates, for example MicroShield®, use recent ICRP data that is consistent with ANSI/ANS- 6.1.1-1991. A review of the recent data has been done to consider what conversion factors may be applicable to and acceptable for use in radioactive material package shielding evaluations.