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 Xe-133.
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
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