Document: NRC Regulatory Guide
Document ID: 4d46a966-d280-43da-9b03-8b0abe7b29ce
Document Type: regulatory_guide
Title: Alternative Radiological Source Terms for Evaluating Design Basis Accidents at Nuclear Power Reactors (Rev. 1)
Source: NRC Regulatory Guide Division 1
Source URL: https://www.nrc.gov/docs/ML2120/ML21204A065.pdf
Revision Date: 2023-05
Chapter: 
Section ID: RG-1.183
CFR Part: 
CFR Title: 

Content:
ies (Ci) must be converted to becquerel (1 Ci = 3.7x1010 Bq). The radioactive iodine concentration can be found using radiological decay formulas that account for time before fuel movement. If this is done, the activity of the other iodine isotopes at the time before fuel movement should be added to the I-131 activity. Step 2—Calculate volatile iodine fraction in pool. Next, determine the fraction of iodine atoms in the pool that are in I2 (volatile) form: DG-1389, Appendix B, Page B-4 • Calculate the radioactive and total concentrations in the pool: Cr = concentration (M) (moles I atoms /L) of radioactive I atoms = NI-131gap / Vpool (Equation B-7) Ct = total I concentration (M) (moles I atoms / L) = ( NI-129gap + NI-131gap) / Vpool (Equation B-8) Note: Vpool must be converted to liters to calculate concentrations in moles/liter. • Calculate the H+ concentration: Ch = [H+] = 10-pH (Equation B-9) • Calculate the [I2]/[I-]2 concentration ratio, Ri (Ref. B-2) 2: Ri = [I2]/[I-]2 = Ch2 / (6.0603E-14 + 1.4708E-09 Ch) (Equation B-10) • Calculate the fraction of I atoms in I2 form: o First evaluate Bm (negative B for quadratic equation below): Bm = 4 Ct + 1 / Ri (Equation B-11) o Then evaluate the volatile fraction, Xe (fraction of I atoms in I2 form): Xe = ( Bm – ඥܤ௠ଶ−16 ܥ௧ ଶ ) / ( 4 Ct) (Equation B-12) Step 3—Calculate applicable removal coefficients. The evolution removal coefficient, λe, is calculated using the mass transfer coefficient, the pool surface-to-volume ratio, and the fraction of I that is in I2 form: λe = KL Xe Spool / Vpool (Equation B-13) The removal rate is reduced to account for the fraction of iodine that is volatile and thus available to evolve to the gas space. This evolution rate applies to both nonradioactive and radioactive iodine. Step 4—Evaluate release as an overall release. The removal coefficient is used to model the time-dependent concentration of radionuclides released from the pool as follows: Qe = λe Vpool