Transfer function method in estimating external radiation levels

The methodology of transfer functions (TF) is based on calculating the contribution to the dose rates generated by a radiation source from each of the discrete energy groups that comprise the spectrum. This set of functions generated can be folded into any source spectrum with the same discrete energy group structure for similar geometry and material properties to estimate the final dose rates of interest, thus avoiding the need for repeated calculations. This approach was used to estimate allowed mass limits for several actinides in pure form or combined with light elements, as well as a set of gamma sources, valid for a set of modern transportation packages used by the US Department of Energy. However, in several instances this approach failed since the estimated masses were non-conservative because subcritical multiplication that enhanced the neutron contribution to the dose rate, was not accounted for in many of the actinides. In other instances, self-shielding was more dominant than subcritical multiplication thus making the estimates too conservative. In other applications the TF method works well. In the case of dose rates outside spent fuel assemblies the TF can be useful in eliminating the need for repeated full radiation transport calculations. This paper will address the issue of the applicability of the TF method with examples and provide a cautionary note on the use of this method. (author)