Document: NRC Regulatory Guide
Document ID: c9ebcbb0-96c4-4d29-be51-5acae9cc858a
Document Type: regulatory_guide
Title: Estimating Aquatic Dispersion of Effluents from Accidental and Routine Reactor Releases for the Purpose of Implementing Appendix I (Rev. 1)
Source: NRC Regulatory Guide Division 1
Source URL: https://www.nrc.gov/docs/ML0037/ML003740390.pdf
Revision Date: 2023-06
Chapter: 
Section ID: RG-1.113
CFR Part: 
CFR Title: 

Content:
uitable for estimating concentrations in reservoirs, ponds, and lakes that are downstream of the radionuclide discharge and consequently already par tially mixed. In such a case, the hydraulics of the pond are less important and simple methods may suffice. More elaborate models may be required, however, for a direct radionuclide discharge to the reservoir. In this case, the hydraulics of the reservoir may strongly affect the way in which the radionuclide releases2disperse. (5) Buildup of Isotopes Using Simple Models For large cooling ponds with relatively small blowdown rates, the concentrations of the longer-lived isotopes may build up over a period of several years (exclusive of sediment uptake effects). It may be desirable to know the rate of this buildup, since the flushing time would be a significant fraction of the useful life of the plant. 1.113-37 Since the only concern is long-lived isotopes, the completely mixed model is analyzed for transient conditions. If the concentration in the pond is initially zero, then o .-- ' 0(1 - exp(-[T + ln2Jt/t½}) (44) 0 Figure 14 shows the buildup to steady-state concentration CSS as a function of dimen sionless time e (time/flushing time) for several values of T. Notice that Equation (44) becomes fairly insensitive to T for large T. This fact is illustrated more clearly by observing the time to reach some arbitrary fraction, say 99% of steady state: 1 - exp [-(T + n2) t] 0.99 and (52) S1(tqb\ ) 9 - nO.OT *gg "\vT / g9 *r- +-T 1n Figure 15 illustrates the 99% buildup time as a function of T, clearly showing how, for very long half-lives compared with pond flushing times, the time depends only on the flushing time of the pond. (6) Hydraulics of Ponds Using Simple Models The simple models presented here must be used with caution for several reasons. Large portions of a pond may be unused for dilution if the pond is unstratified and irregularly shaped. If the same pond becomes stratified during certain times of the year,