Concluding Remarks | AIChE

Concluding Remarks

Department of Energy (DOE) Standard DOE-STD-3009-2014 is an acceptable methodology for meeting the requirements of 10 C.F.R. Part 830 for the preparation of Documented Safety Analyses (DSAs) for both new and existing nonreactor nuclear facilities. Three dispersion options are outlined in the Standard, and include: (1) Use of NRC Regulatory Guide 1.145; (2) Application of a DOE-approved toolbox code and application of conservative input parameters; and (3) Use of site-specific methods and parameters as defined in a site/facility specific DOE-approved modeling protocol. In addition, DOE-STD-3009-2014 defines the general methodology and parameters that may be used in the dispersion analysis, including the dose coefficients, i.e., dose per unit activity from various pathways, which is typically dominated by the inhalation pathway for nonreactor nuclear facilities, and expressed in radionuclide-specific inhalation dose conversion factors, or IDCFs. DOE-STD-3009-2014 specifies use of dose coefficients consistent with International Commission on Radiological Protection (ICRP) Publication 68, Dose Coefficients for Intakes of Radionuclides by Workers, and Publication 72, Age-dependent Doses to Members of the Public from Intake of Radionuclides, for adults. However, within this specificity, there is still uncertainty, as many radionuclides typically found in nuclear waste facilities have multiple physicochemical forms with different subsequent impacts to worker and general public populations. As noted in ICRP Publication 71, the physicochemical form and particle size of a radionuclide are incorporated in a dosimetric model that quantifies retention of deposited activity in the respiratory tract regions and clearance to other organs by representing clearance processes and ultimately absorption of radiological material into body fluids. A radionuclide that is present in many nuclear waste accident analysis scenarios and illustrates this dilemma is Strontium-90. The three observed absorption types of 90Sr are fast (F), moderate (M), and slow (S) and characterize absorption in the sense of overall physiological clearance, and can be broadly thought of in terms of timescales of days, weeks, and years. The corresponding IDCFs increase monotonically from 2.4E-08 sievert/becquerel (Sv/Bq) for F absorption, to 3.6E-08 Sv/Bq for M absorption, and to 1.6E-07 Sv/Bq for S absorption. Experimental and accident evidential data show mostly moderate and fast types, with the slow type appearing mostly for cases where a compound such as strontium titanate is released. In absence of specific information, ICRP 71 recommends that type M be used as a default basis. ICRP 68 and ICRP 119 provide an alternative recommendation of type S for strontium titanate and type F for unspecified compounds. In this study, we apply the dispersion options from DOE-STD-3009-2014 for DSA support to postulated release of different forms of strontium to quantify the individual doses as a function of distance from the point of release. Individual dose estimates are compared by option to illustrate the effects of atmospheric deposition and other inputs and assumptions on the form of 90Sr being released.