U.S. Nuclear Regulatory Commission
Meaning of the Expression "Dose Equivalent Xe-133" in the Technical Specifications
Title: Meaning of the Expression "Dose Equivalent Xe-133"
in the Technical Specifications
See the memorandum from C. A. Willis to D. M. Montgomery
dated March 4, 1985. "Dose equivalent Xe-133" means
equivalent in ability to deliver gamma-ray doses to the
whole body. Either 0.018 Ci of Kr-89 or 18 Ci of Kr-85 is
equivalent to 1 Ci of Xe-133.
Historically, the activity inventory limits for waste gas
storage tanks have been expressed in curies (Ci) of dose
equivalent Xe-133, specifically "curies noble gas
(considered as Xe-133)." In the RETS implementation
program, it was suggested that this be clarified by adding
a definition to the RETS. This suggestion was rejected on
the grounds that the intent was manifest from the "basis"
statement. The "basis" statement says that this limit is
to ensure the release of a tank's contents will not cause a
whole body dose to any individual at the exclusion area
boundary of more than 0.5 rem. Questions have indicated
that further clarification may be appropriate.
The intent of the LCO is to ensure that the inadvertent
release of the contents of a waste storage tank does not
cause a gamma-ray dose to the whole body of over 0.5 rem
offsite. Thus, the LCO whole body was given in terms of
Xe-133 equivalent curies to facilitate implementation.
That is, the licensee need never determine the actual
radioactivity contents of a tank; instead it may simply
determine the dose rate from gamma rays and convert to
equivalent curies of Xe-133 based on a calibration with
This approach seems more accurate than the alternative.
The alternative is to determine the quantity present of
each nuclide and calculate the potential gamma-ray dose to
the whole body using the various dose conversion factors.
The problem is more difficult if the detector responds to
beta-particles. The dose rate from beta particles is not
the quantity of interest and so cannot be used directly.
It is necessary to determine the nuclide composition of the
gas and relate this to the total activity. The quantities
of the various nuclides can be converted to Xe-133
equivalent curies using the dose conversion factors (DFB1)
of Regulatory Guide 1.109, the values for gamma radiation
of DOE / TIC-11026, the energy specific values for gamma rays
from the "Table of Isotopes" (7th Edition), or other
convenient reference. The slight differences in results
obtained with the different references is unimportant.
Where this approach is used the "dose equivalent Xe-133"
concept offers no practical advantages; it is simply
another way of saying "potential for delivering a gamma-ray
dose to the whole body."
If the inventory is determined by sampling and isotopic
measurement by gamma-ray spectrometry, the problem is much
the same as with the beta-particle measurements, and
involves the weighting by various dose-conversion factors.
Regulatory references: Technical Specifications
Subject codes: 7.3, 9.1