they contributed about one-third of the neutron activation at 1500 m (ground range) at Nagasaki and 8% at Hiroshima according to the original DS86 calculations (Roesch 1987). The revised delayed neutron contribution to activation should be considered in comparing calculated and measured thermal activation.

• The prompt-neutron output from the Hiroshima device is also estimated to have an uncertainty (CV) of about 10%. However, both the number and energy distribution of the neutrons from the Hiroshima source might be considerably more uncertain, as discussed below. The new calculation of this source being carried out at LANL (see Chapter 4) should provide an improved estimate of uncertainty for the total radiation output and for the energy and angular source spectra.

• Errors in air and soil density and moisture content can affect the transport of low-energy neutrons in particular but would probably have only a small impact on the kerma estimates—CV about 5%, according to Kaul and Egbert (1989). However, these errors might have a substantial impact on the calculation of thermal-neutron activation for some locations. The DS86-calculated thermal and epithermal neutron fluences vary by as much as about 25–50% as the altitude increases from 1 m to 25 m (see Chapter 3). A sensitivity analysis of the effect of the uncertainty in these values on the calculated low-energy component of the fluence at various distances and heights should be included in the uncertainty assessment. The variations in low-energy fluence and their possible impact on the comparison between measured and calculated activation are discussed in more detail in Chapter 3.

• Errors and limitations in the shielding and organ-dose methodology (forward-adjoint fluence coupling) could have had a relatively small impact on the estimated kerma (around 5–10% CV), as discussed by Roesch (1987).

• The uncertainty in kerma and activation due to uncertainty in air cross-section values increases with distance, particularly for the neutron component (Lillie and others 1988). The effect on the neutron-kerma uncertainty was estimated to be about 15% (CV) at 1500 m; the prompt and secondary gamma CVs were estimated to be about 3% and 6%, respectively. The uncertainty in nitrogen and oxygen cross sections and improvements in the transport code energy bin structure have been extensively investigated since 1986 and appear to have a substantial impact on the calculated kerma in air. Kaul and Egbert (1989) estimate a CV of about 15% for both Hiroshima and Nagasaki for the uncertainty in the neutron kerma due to uncertainty in the new cross sections (see Chapter 4).

Components that can result in random error are uncertainties in the assumed survivor-shielding and organ-dose model.

• The shielding assignment was estimated to contribute substantially to total uncertainty (Roesch 1987). Estimates of terrain shielding at Nagasaki and the model