Offsite fallout is characterized by an excess of volatile phase, because the smaller particles tend to be deposited farther from ground zero. The calculation of fractionation effects for offsite fallout is straightforward according to the picture above: refractory phase in the calculation. remove some of the The composition of each phase is determined by the volatility of elements at 1500°C (Appendix 1). fission product elements of a given The mass number are either all volatile or all refractory, except the mass chains 91, 140 and 141. The refractory fractions of these latter chains have been determined at 20 sec postshot (Appendix 2, Sec. A2). and these mass chains partitioned accordingly. The computer program allows removal of any fraction of the refractory phase. 3. CALCULATIONS The purpose of these calculations is to relate the ground surface concentration (uCi/m?) of every radionuclide to the external gamma-ray exposure Tevels (mR/hr) from the fallout pattern as a function of time and fractionation behavior. The results of the calculation are the contributions of each of 152 fission products and ‘each of 25 neutron-induced nuclides to the surface deposition and to the external gamma-ray exposure levels. These values and the sums of all values at each decay interval are computed from zero time to 50 years postshot. The entire calculation is outlined in this section and is described in detail in Appendix 2.