Each power reactor has a specific DEX value that is their technical requirement limit. These values may vary from about 200 to about 900 μCi/g based upon the height of their plant vent the location of the site boundary, the calculated reactor coolant activity for a condition of 1% fuel defects, and general atmospheric modeling that is ascribed to that particular plant site. Should the DEX measured activity exceed the technical requirement limit the plant enters an LCO requiring action on plant operation by the operators.

The determination of DEX is performed in a similar manner to that used in determining DEI, except that the calculation of DEX is based on the acute dose to the whole body and considers the noble gases ⁸⁵mKr, ⁸⁵Kr, ⁸⁷Kr, ⁸⁸Kr, ^131mXe, ^133mXe, ¹³³Xe, ^135mXe, ¹³⁵Xe, and ¹³⁸Xe which are significant in terms of contribution to whole body dose.

It is important to note that only fission gases are included in this calculation, and only the ones noted in ¹. For example 83mKr is not included even though its half life is 1.86 hours. The reason for this is that this radionuclide cannot be easily determined by gamma spectrometry (low energy x-rays at 32 and 9 keV) and its dose consequence is vanishingly small compared to the other, more prevalent krypton radionuclides.

Activity from ⁴¹Ar, ¹⁹F, ¹⁶N, and ¹¹C, all of which predominantly will be in gaseous forms in the RCS, are not included in this calculation.

If a specific noble-gas radionuclide is not detected, it should be assumed to be present at the minimum-detectable activity. The determination of DOSE-EQUIVALENT XE-133 shall be performed using effective dose-conversion factors for air submersion listed in Table III.1 of EPA Federal Guidance Report No. 12, or the average gamma-disintegration energies as provided in ICRP Publication 38 (′′Radionuclide Transformations'') or similar source.