U.S. Nuclear Regulatory Commission

Meaning of the Expression "Dose Equivalent Xe-133" in the Technical Specifications

HPPOS-102 PDR-9111210230

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

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

Applicability: Reactors