ML22300A212
| ML22300A212 | |
| Person / Time | |
|---|---|
| Site: | Vallecitos Nuclear Center, Vallecitos |
| Issue date: | 10/07/2022 |
| From: | Heckman D GE-Hitachi Nuclear Energy Americas |
| To: | Office of Nuclear Material Safety and Safeguards, Office of Nuclear Reactor Regulation |
| Shared Package | |
| ML22300A209 | List: |
| References | |
| M2200143 | |
| Download: ML22300A212 (1) | |
Text
Evaluation of Vallecitos Nuclear Center NTR Effluent Abnormal Radiological Conditions Emergency Action Level Thresholds PROBLEM STATEMENT The Vallecitos Nuclear Center (VNC) Reactor Facilities Radiological Emergency Plan (REP), Revision 1 (June 2021) contains the Emergency Action Levels (EALs) applicable to the reactors at VNC, including the Nuclear Test Reactor (NTR). Section 5.1 of the REP includes abnormal radiological Conditions that would result in an unusual event or alert declaration. For an unusual event, RU3 states:
Radiological effluent level (µCi/cc) meet any of the following for greater than 60 minutes:
2.4E5 NTR Noble Gas Stack Monitor 2.4E5 NTR Particulate Stack Monitor 2.4E1 from any reactor facility effluent Noble Gas sample 4.7E3 from any reactor effluent Halogen sample 2.4E7 from any reactor facility effluent Particulate sample 2.4E5 from any reactor facility effluent Particulate sample For an alert, RA3 states:
Radiological effluent level (µCi/cc) meet any of the following for greater than 60 minutes:
1.2E4 NTR Noble Gas Stack Monitor 1.2E4 NTR Particulate Stack Monitor 1.2E0 from any reactor facility effluent Noble Gas sample 2.4E2 from any reactor effluent Halogen sample 1.2E6 from any reactor facility effluent Particulate sample 1.2E4 from any reactor facility effluent Particulate sample The EAL thresholds notwithstanding, the NTR Noble Gas Stack Monitor reads approximately 2.7E5 to 3.3E5 µCi/cc during normal operations, which exceeds the RU3 limit for the monitor. Procedure NTR SOP 5.2 currently sets the operational alarm setpoint at 9.5E5 µCi/cc based on the stack release action level in SAR Table 61 of License Amendment 25 dated June 29, 2022. This is approximately 4 times the RU3 EAL threshold (but below the RA3 EAL threshold).
BACKGROUND The EAL thresholds for RU3 and RA3 were calculated based on the 10CFR20, Appendix B, Table 2 values for the applicable radionuclides. The assumed radionuclides are as follows:
Table 1 Current REP EAL Threshold Basis Monitor/Sample Radionuclide Noble Gas Unlisted, nonAlpha and nonspontaneous fission, greater than 2 hour2.314815e-5 days <br />5.555556e-4 hours <br />3.306878e-6 weeks <br />7.61e-7 months <br /> halflife Particulate Stack Monitor Unlisted, nonAlpha and nonspontaneous fission, greater than 2 hour2.314815e-5 days <br />5.555556e-4 hours <br />3.306878e-6 weeks <br />7.61e-7 months <br /> halflife Noble Gas Sample Ar41 Halogen Sample I131 Alpha Particulate Sample Np237 Page 1 of 7
Evaluation of Vallecitos Nuclear Center NTR Effluent Abnormal Radiological Conditions Emergency Action Level Thresholds Beta Particulate Sample Unlisted, nonAlpha and nonspontaneous fission, greater than 2 hour2.314815e-5 days <br />5.555556e-4 hours <br />3.306878e-6 weeks <br />7.61e-7 months <br /> halflife The EAL thresholds were determined as the activity concentration that, if released from the NTR stack with the assumed X/Q dispersion coefficient (3.48E11 s/cc), and with a modifier for multiple stack releases, would result in a fraction (either 10% or 20%) of the 10 CFR 20, Appendix B, Table 2 effluent concentrations for members of the public.
For the noble gas monitor, the unlisted, nonalpha, > 2 hour2.314815e-5 days <br />5.555556e-4 hours <br />3.306878e-6 weeks <br />7.61e-7 months <br /> halflife effluent concentration limit of 1E 12 µCi/cc was assumed.
The basis for the RU3 EAL thresholds for RU3 and RA3 is contained in the REP, Att. 3:
Figure 1 RU3 and RA3 Threshold Setpoints (from REP Att. 3)
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Evaluation of Vallecitos Nuclear Center NTR Effluent Abnormal Radiological Conditions Emergency Action Level Thresholds The site boundary release concentrations (above) are determined by adjusting the 10CFR20 Effluent concentration limit (which correspond) to 50 mrem/yr, to a concentration of 4 mR/hr (for the unusual event declaration) or 20 mR/hr (for the alert declaration) The NTR Stack Release Rate (in µCi/s) is that concentration adjusted by the assumed /Q (3.48E11 sec/cc). The NTR Stack Release concentration then accounts for the NTR stack release rate (nominal 1800 cfm).
SYSTEM CONFIGURATION Effluents from the NTR are released via the NTR stack. The noble gas sample is drawn from the stack at a rate of 1 cfm. The sample is pulled through a particulate filter, then a charcoal filter, and finally into the noble gas (Kanne) detector (See figure below). As such, the sample measured by the noble gas monitor should only contain radioactive noble gases, as the particulates and halogens have been stripped out. The NTR stack exhaust is powered by a 3000 cfm motor, which nominally operates at 1800 cfm.
Figure 2 NTR Ventilation System (from SAR Figure 63)
EVALUATION The basis for the noble gas monitor EAL threshold is that the applicable 10CFR20 effluent concentration is based on the Unlisted, nonalpha radionuclides with > 2hour halflife. That value, however, assumes that the unlisted, or unknown radionuclides may include particulates. However, based on the system configuration no particulates nor halogens are present. The effluent concentration limits for particulates and halogens are generally much lower than those for noble gases. A lower effluent concentration limit will result in a correspondingly lower EAL threshold.
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Evaluation of Vallecitos Nuclear Center NTR Effluent Abnormal Radiological Conditions Emergency Action Level Thresholds For an accident (or operating) scenario, the radionuclides that will be seen by the noble gas monitor will be radioactive noble gases. Under normal operations, that is predominantly Ar41. Under accident scenarios, the radionuclides will be the various isotopes of fission noble gases, namely Krypton and Xenon. The 10CFR20 Appendix B, Table 2 Effluent Concentrations for pertinent radionuclides are as follows:
Table 2 10 CFR 20 Appendix B, Table 2 Effluent Concentrations Nuclide 10CFR20 Appendix B Table 2 Effluent Concentration Limit (uCi/cc)
Unlisted 1.00E15 alpha Unlisted > 2 1.00E12 hour Unlisted <2 1.00E09 hour Kr88 9.00E09 Ar41 1.00E08 Kr74 1.00E08 Xe121 1.00E08 Kr77 2.00E08 Kr87 2.00E08 Xe138 2.00E08 Xe123 3.00E08 Kr76 4.00E08 Xe120 4.00E08 Xe135m 4.00E08 Xe127 6.00E08 Kr79 7.00E08 Xe125 7.00E08 Xe135 7.00E08 Kr85m 1.00E07 Xe122 3.00E07 Xe133 5.00E07 Xe133m 6.00E07 Kr85 7.00E07 Xe129m 9.00E07 Xe131m 2.00E06 Kr81 3.00E06 Kr83m 5.00E05 As shown above, the smallest and most conservative effluent concentration that would be seen by the noble gas monitor is 9E09 µCi/cc for Kr88.
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Evaluation of Vallecitos Nuclear Center NTR Effluent Abnormal Radiological Conditions Emergency Action Level Thresholds However, the effluent concentration limits (ECL) in 10CFR20 Appendix B, Table 2 do not have consistent dose basis. Per Reg. Guide 4.20, radionuclides with a stochastic based Annual Limit of Intake (ALI) have an ECL based on a 50 mrem/yr dose basis. For purposes of this evaluation, that is any particulate or halogen radionuclide. Conversely, for radionuclides with a submersion based ALI (i.e., noble gases) the ECL is based on a 100 mrem/yr dose basis. As a result, the noble gas monitor EAL thresholds must be calculated assuming that the 10 CFR 20 ECL results in a 100 mrem/yr dose rate.
The REP Att. 3 calculations for the noble gas monitor EAL thresholds for RU3 and RA3 should be 1.07E 01 µCi/cc and 5.34E01 µCi/cc. respectively.
CONCLUSION AND RECOMMENDATION The RU3 EAL threshold for the noble gas monitor as contained in the Vallecitos Radiological Emergency Plan is incorrect and overly conservative, because it is based on an overly conservative 10 CFR 20 effluent concentration value. The most conservative, but reasonable, effluent concentration value for a radionuclide that will be detected by the noble gas monitor under accident conditions is Kr88.
Conservatively assuming all fission gasses released during an accident are Kr88, the RU3 and RA3 EAL threshold for the noble gas monitor should be revised to 2.13E01 µCi/cc and 1.07E+00 µCi/cc.
While this change results in a nonconservative adjustment in both EALs, the existing basis currently yields a calculated RU3 level of 2.4E5 µCi/cc. This is not discernible from normal Ar41 effluent releases which range from 2.7E5 µCi/cc to 3.3E5 µCi/cc. Further, it is below the effluent alarm setpoint for the NTR stack monitor of 9.5E5 µCi/cc and, as a result, provides no prompt for the operators to suspect they may be entering into an EAL. This change does not constitute a reduction in effectiveness. It does result in an overall improvement to the emergency planning function of classifying emergencies by providing a discernible yet conservative RU3 setpoint. This change does not reduce the capability to perform any emergency function in the event of a radiological emergency as per 10 CFR 50.54(q)(1)(iv).
A revised REP Appendix 3 EAL Technical Bases is shown below:
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Evaluation of Vallecitos Nuclear Center NTR Effluent Abnormal Radiological Conditions Emergency Action Level Thresholds Appendix 3 - EAL Technical Bases for Effluent Releases Method: 10 CFR 20 effluent concentration limits, representative of a 50 mrem/yr dose, are adjusted to an equivalent concentration limit for the EAL site boundary threshold value in mr/hr. The site boundary EAL concentration limit is backcalculated to an effluent stack release rate using the limiting SAR X/Q dispersion factor and stack flow rate.
10 CFR 20 RU3 Site Boundary RU3 NTR Stack RU3 NTR Stack Release Release Concentration Limit Release Rate Concentration Concentration (Ci/sec) (Ci/cc)
(Ci/cc) (Ci/cc)
Noble Gas* Kr88 9.00E09 7.01E10 2.02E+01 1.07E01 Noble Gas** Ar41 1.00E08 7.01E06 2.02E+05 2.37E01 Halogen** I131 2.00E10 1.40E07 4.03E+03 4.74E03 Alpha Particulate** Np237 1.00E14 7.01E12 2.02E01 2.37E07 BetaGamma Particulate** 1.00E12 7.01E10 2.02E+01 2.37E05 RA3 Site 10 CFR 20 RA3 NTR Stack RA3 NTR Stack Boundary Release Release Concentration Release Rate Concentration Concentration Limit (Ci/cc) (Ci/sec) (Ci/cc)
(Ci/cc)
Noble Gas* Kr88 9.00E09 3.51E09 1.01E+02 5.34E01 Noble Gas** Ar41 1.00E08 3.51E05 1.01E+06 1.19E+00 Halogen** I131 2.00E10 7.01E07 2.02E+04 2.37E02 Alpha Particulate** Np237 1.00E14 3.51E11 1.01E00 1.19E06 BetaGamma Particulate** 1.00E12 3.51E09 1.01E+02 1.19E04 Calculations Constants 10 CFR 20 Effluent Concentration Dose Basis - Stochastic (mrem/yr): 50 10 CFR 20 Effluent Concentration Dose Basis Stochastic (mrem/hr): 5.70E03 10 CFR 20 Effluent Concentration Dose Basis - Submersion, noble gas (mrem/yr): 100 10 CFR 20 Effluent Concentration Dose Basis - Submersion, noble gas (mrem/hr): 1.14E02 EAL RU3 Limt (mR/hr): 4 EAL RU3 Limt (mR/hr): 20 Limiting X/Q* (sec/cc): 3.48E11 NTR Bldg 105 Stack Flow Rate* (cfm): 1800 Conversion Factor (cc/sec per cfm): 471.95 NTR Bldg 105 Stack Flow Rate (cc/sec): 8.50E+05 Page 6 of 7
Evaluation of Vallecitos Nuclear Center NTR Effluent Abnormal Radiological Conditions Emergency Action Level Thresholds
- 10 CFR 20 effluent concentration limit for Kr88, the most conservative value for fission noble gases.
-07'00' 10/7/2022 Dave Heckman Date Page 7 of 7