Functional Bounding Content Envelope for Actinides-Impact of Subcritical Multiplication

 The contents approved for shipment in a Type B radioactive material transportation package have historically been descriptions of discrete items, or groupings of well-defined similar items, in the package safety basis documentation. The need for a comprehensive functional content envelope of both gamma and neutron emitting nuclides compliant with regulatory limits has become necessary as the DOE complex requires shipments of unique mixtures of radionuclides and impurities. Recent publications have presented a calculational model and a corresponding content envelope intended to be compliant with federal regulatory external radiation limits as well as design decay heat limits based on the Model 9977 Packaging. The methodology used to develop this content envelope consisted of determining the external radiation dose rates based on one gram of a given isotope combined, in the case of actinides, with various levels of light element impurities and determining the allowable mass to the regulatory limits based on these dose rates. The method of ratioing up the masses to meet the regulatory limits fails in the case of some combinations of actinides and light element impurities because of the effect of subcritical multiplication. This is particularly true for many actinides combined with beryllium, boron, fluorine, lithium, and sodium. This paper rectifies this deficiency by adjusting the previously determined mass limits so that the resulting dose rates are compliant with regulatory limits. Some amount of iteration was required in some instances to adjust the masses for dose compliance. This paper will present a revised set of mass limits (i.e., content envelope) for several actinides with varying levels of light element impurities compliant with both external radiation and design decay heat limits. The revised content envelope covering a wide range of materials present in the DOE complex, was developed for the Model 9977 Packaging, providing conservative limits for other commonly-used Type B Packagings such as the Models 9975 and 9978.