An error was discovered when calculating the overall site atmospheric dispersion factor (χ/Q) using the ARCON computer code for offsite χ/Qs in accordance with the methodology defined in Regulatory Guide 1.249 Revision 0.
The Reg Guide states “To determine the overall site χ/Q, analysts should apply a “wind direction window” width of 360 degrees. This will effectively include all the hours in the data set and will therefore represent each wind direction, regardless of sector.” When applying a wind direction window of 360 degrees, and a wind direction window width of 360 degrees, only a small number of hours were included in the calculation performed by the code.
No guidance is given for the sector direction that should be used for the overall site case. Multiple sector directions were tested, and it was found only the wind direction window of 180 degrees, with a wind direction window width of 360 degrees, includes all hours in the data set during calculation.