ML23086C025
| ML23086C025 | |
| Person / Time | |
|---|---|
| Site: | Vallecitos Nuclear Center |
| Issue date: | 03/24/2023 |
| From: | GE-Hitachi Nuclear Energy Americas |
| To: | Office of Nuclear Reactor Regulation |
| Shared Package | |
| ML23086C023 | List: |
| References | |
| M230046 | |
| Download: ML23086C025 (1) | |
Text
Vallecitos Enclosure 1 NRC NTR License Renewal Audit Questions QUESTION 001 GEH modified TS 3.7.4, table 33, Stack Release Action Levels in revision 5 of the technical specifications and indicated in discussion that this would be explained in Enclosure 1, to submittal of 2/23/23 (ML23054A321). The reason for the change is explained but the alarm setpoints are not explained such that it can be understood where they came from or if they will support the weekly release limit.
The audit team requires a better understanding of the alarm setpoints. Please clean up verbiage in Table 33.
Possibly remove the explanation of effluent calcs from the TS and refer to explanation in SAR 11.2.5.
The VNC staff has reviewed sections 11.2.4 and 11.2.5 of the SAR and agrees with the NRC audit team that it is confusing. Attempts to explain it as currently written are also confusing. Therefore, the following changes have been made to the SAR and the basis for Technical Specification 3.7.4 has been updated to align with the correspond to the SAR.
Sections 11.2.4 and 11.2.5 of the SAR have been rewritten as follows:
11.2.4 Radioactive Gaseous Waste Management Release of routine gaseous effluents is dominated by Ar41, which is generated by neutron activation of Ar40 in air. Airborne radioactive waste exiting through the NTR stack is monitored as radioactive effluent and is well within the Technical Specification and 10 CFR 20 requirements. Monitoring and alarms associated with the NTR stack have been discussed in this Chapters 7 and Section 3.5 of this SAR.
The reactor cell and stack ventilation system were originally required to mitigate an analyzed fueled experiment failure of a type that has not been performed at the NTR for more than 30 years. Nevertheless, the reactor cell will contain any radioactive release while it is exhausted through the ventilation system and out the stack.
11.2.5 Stack Release Action Levels The Stack Release Action Levels are defined as the release rates for each radionuclide group (noble gas, I131, beta particulate, or alpha particulate) at which action should be taken to reduce the release rate. Ongoing operation below these Stack Release Action Levels ensures doses to members of the public due to airborne release are at or below the 10 CFR 20.1101(d) limit of 10 mrem per year. The method for establishing the Stack Release Action Levels is described below.
The VNC is a multilicensed site that performs processes not associated with the NTR. These processes take place in other buildings apart from Building 105 that also emit radionuclides from stacks. This is relevant for two reasons.
First, contributions from all stacks to site boundary exposure must be accounted for in establishing the stack action levels for each stack. This is conservatively and simply done by assuming each stack produces half of the dose at the site boundary and applying an other stack reduction factor of 2.
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Vallecitos Enclosure 1 NRC NTR License Renewal Audit Questions QUESTION 001 Second, while all site stack action levels discussed in this section are tracked by the sites effluent program, only gases and particulates are included in Technical Specification 3.7.4. This is because the realtime NTR effluent monitors are only capable of detecting gaseous and particulate (, ) releases, making these the only Stack Release Action Levels that are actionable by the NTR operators. Alpha particulate and halogen (I131) releases are evaluated via samples collected and counted on a weekly basis and are monitored according to the sites effluent control program.
11.2.5.1 Basis for Stack Release Action Levels The Stack Release Action Levels (weekly total or specific concentrations) for noble gas releases from the NTR stack ensure that the activity released will not exceed an annual average concentration of Ar41 at the site boundary of 10% of the annual effluent concentration limit (ECL) in 10 CFR 20, Appendix B, Table 2 Column
- 1. Ar41 has been shown to be the predominant noble gas in the stack effluent (Climent, 1969). Fission produced noble gases are a minor fraction unless fuel material is exposed to the effluent air. Ar41 is produced by the neutron irradiation of the air passing through the reactor.
The Stack Release Action Levels for all other isotope groups ensure releases from each stack (including NTR) at the VNC will not yield an annual average concentration at the site boundary of 20% of the 10 CFR 20 effluent concentration limit for the restrictive, credible isotopes of each of the isotope groups: I131, unidentified beta radionuclide, and Np237.
Once reduced by a factor of 2 to account for other stacks, the Stack Release Action Levels ensure that any single stack release will not exceed an annual average concentration that is effectively 5% of the ECL for Ar41 and 10% of the ECL for other credible isotopes.
11.2.5.2 Computing Stack Release Action Levels To derive the Stack Release Action Level, a bounding release concentration rate (µCi/sec) must be determined that will not exceed the effective ECL (5% for gas and 10% for other). To do this, the effective site boundary ECL is divided by the atmospheric dispersion (/Q) factor (sec/ml) to determine the allowable release rate
(µCi/sec) at the stack (point of release). The allowable release rate is then adjusted to a weekly release rate
(µCi/week), which has been determined to be an actionable time period in which operator actions can be taken to ensure the stack release doesnt challenge the effective ECL. The allowable release rate is then adjusted for the average site building ventilation flow rate (1800 ft3/min) to determine the specific concentration Stack Release Action Level. For noble gas (Ar41) and beta/gamma particulates, the specific concentration Stack Release Action Levels are applied in Technical Specification 3.7.4 as the Alarm Setpoints for the effluent monitors and are then further adjusted to provide weekly release Stack Release Action Levels based on a 30hr operational week accounting for the fact that these concentrations are monitored in real time.
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Vallecitos Enclosure 1 NRC NTR License Renewal Audit Questions QUESTION 001 Table 11-4 STACK RELEASE ACTION LEVELS Nominal Noble Gas Halogen Alpha Beta Stack Flow Rate, Ci/wk mCi/wk µCi/wk µCi/wk cfm µCi/cc µCi/cc µCi/cc µCi/cc 9 3.48E+02 1.74E+01 1.74+E03 105, NTR 1.80E+03
- 9.5E05 6.8E07 3.4E11 1.9E08
- Access to the reactor cell when the reactor is not operating is procedurally controlled by observation of CAM (see Table 113) activity readings.
11.2.5.3 Assumptions Stack flow rates fluctuate. For example, the NTR flow depends on the position of the cell door. The flow in all filtered systems varies as the dust loading on filters increases and as containment systems are changed. The 1,800cfm average stack flow rate for Building 105 is used for limiting concentrations and calculating measured releases.
The applicable effluent concentration limit values from Appendix B, Table 2, Column 1 of 10 CFR 20 are given below:
10 CFR 20 Effluent Limiting Release Category Concentration Limit, Isotope
µCi/ml Noble Gas* Ar41 1.00E08 Halogen I131 2.00E10 Alpha Particulate Np237 1.00E14**
BetaGamma Particulate *** 1.00E12 The dilutiondispersion (/Q) factor and reduction factor to account for releases from other stacks on site are given below:
Other stack reduction Stack Location /Q, sec/ml factor Building 105, NTR 3.48E11 2
- The NTR noble gas inventory available to the boundary has been found to be primarily Ar41, which is an activation product of air. Fission products would be of concern in the event of fuel failure, an abnormal condition.
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Vallecitos Enclosure 1 NRC NTR License Renewal Audit Questions QUESTION 001
- There are several isotopes with more restrictive limits, but they can be shown to be insignificant fractions of the typical mix of alpha emitters found at VNC.
- Unidentified isotopes, where several natural, transuranic, and other rare elements are known to be absent. These are mainly alpha emitters which would be accounted for in the alpha analysis.
The annual average dilutiondispersion factor for the NTR, and the other stacks at VNC, was calculated from valid hourly records of measured meteorological conditions for a twoyear period in 1976 and 1977. The sector average /Q factors were conservatively computer calculated for each of 16 sectors (22.5 degrees each) using:
Scaled distances from a site layout map to determine the distances from the reactor to the center of the sector at the site boundary.
A building crosssection of 281 square meters, for wake effects.
A ground level release elevation.
No credit taken for plume depletion.
The single maximum calculated annual average /Q value of 3.48E11 sec/ml was selected from the 16 sector average values. This value, which happens to occur in the eastsoutheast sector at 622 meters from the stack, is used to determine the NTR stack release limits.
The ECL release rate, i.e., the continuous release rate which would produce an annual average boundary concentration equivalent to the ECL, would be calculated by division of the ECL by the /Q value. The Action Level rates are calculated as 5% of the ECL release rates for noble gas (10% of the ECL release rate, and a reduction factor of 2 for releases from other stacks). The Action Level Release Rates are calculated as 10% of the ECL release rates for other isotope groups (20% of the ECL release rate and a reduction factor of 2 for other stacks).
Isotope Group Action Level Release Rates µCi/sec Noble Gas 1.44E+01 Halogen 5.75E01 Alpha 2.87E05 Beta 2.87E03 These conservative release rate limits are converted and presented as action levels based on cumulative weekly releases in Table 114Table 114 and the Technical Specification weekly release rate limits of Table 33 in Chapter 14, Technical Specifications.
A normal maximum operating time for the NTR typically would not exceed 30 hours3.472222e-4 days <br />0.00833 hours <br />4.960317e-5 weeks <br />1.1415e-5 months <br /> in a week. Therefore, this partial operating time is used to calculate the operating stack effluent concentration limits.
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Vallecitos Enclosure 1 NRC NTR License Renewal Audit Questions QUESTION 001 Technical Specification 3.7.4 Basis is rewritten as follows:
The basis statement has been changed to read:
The stack release action levels are based on the annual average dilution factor from the NTR stack to the SITE boundary.
A nominal stack flow rate of 1800 ft3/min and 30 hours3.472222e-4 days <br />0.00833 hours <br />4.960317e-5 weeks <br />1.1415e-5 months <br /> per week NTR operation time are assumed. This information, along with other conservative assumptions, ensures that effluent concentrations at the site boundary will not exceed those listed in 10 CFR 20, Appendix B, Table 2, Column 1, nor will the dose from air emissions exceed the 10 mrem/yr constraint from 10 CFR 20.1101(d). A detailed description of the weekly release and alarm setpoints can be found in SAR sections 11.2.4 and 11.2.5.
Technical Specification 3.7.4 table 33 has been updated as follows:
Table 33 STACK RELEASE ACTION LEVELS Gaseous Activity Particulate Activity (Ar41) (Beta)
Weekly release 9 Ci/wk 1.7E+03 µCi/wk Alarm setpoint 9.5E05 µCi/cc 1.9E08 µCi/cc Due to slight variations in assumptions used in calculation, some values in Table 11-4 have been changed. The spread sheet below is provided to demonstrate corrections made to some values. The real time operating limits are used in Technical Specification 3.7.4 as the Alarm Setpoint Stack Release Action Levels for the effluent monitor. The calculated weekly stack limits are adjusted for a 30-hr week and results for Ar-41 and
+ particles are used in Technical Specification 3.7.4 as weekly release Stack Release Action Levels. Weekly stack limits for halogens (I-131) and alpha particulate (Np-237) are tracked according to site procedure VSS 7.2, Radioactive Effluent Control.
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Vallecitos Enclosure 1 NRC NTR License Renewal Audit Questions QUESTION 001 Limiting 10CFR20, ECL Dose Reduction NTR Design NTR Action Nominal NTR Stack RealTime Isotope in Appendix Rate Factor for Basis Weekly Level NTR Stack Action Level How monitored? Operation Each B, Table 2, Reduction Other Dilution Stack Limit Release Rate Flow Rate Concentration a Limit b Category Column 1 Factors Stacks Factor
(µCi/mL) (sec/mL) (µCi/sec) (cc/s) (µCi/cc) (µCi/cc) (µCi/week)
Ar-41 1.0E-08 10% 50% 3.48E-11 1.44E+01 849505 1.69E-05 RealTime 9.5E-05 8.69E+06 Weekly Charcoal I-131 2.0E-10 20% 50% 3.48E-11 5.75E-01 849505 6.77E-07 N/A c 3.48E+05 on Gamma Np-237 Weekly
( 1.00E-14 20% 50% 3.48E-11 2.87E-05 849505 3.38E-11 particulate on N/A c 1.74E+01 particles) Tennelec RealTime and
+ Weekly 1.0E-12 20% 50% 3.48E-11 2.87E-03 849505 3.38E-09 1.89E-08 1.74E+03 particles Particulate on Tennelec 6
Vallecitos NRC NTR License Renewal Audit Questions QUESTION 002 TS 3.8.11 and 3.8.12 in revision 5 of the NTR Technical Specifications seek to bound experiments for fissionable materials. TS 3.8.11 is good as written; however, in combination they dont preclude experiments on non encapsulated plutonium or otherwise comprehensively explain the bounding for such fueled experiments. Also, the Enclosure 1, to submittal of 2/23/23 (ML23054A321) was not updated to reflect the addition of 3.8.12. It does seem that these TSs could be combined and reworded to clearly define the bounding for fueled experiments.
Tech Specs 3.8.11 and 3.8.12 have been combined and reworded to align with bounding assumptions made for the new proposed design basis accident in SAR 13.6.4.
3.8.11 FISSILE MATERIAL EXPERIMENTAL LIMITATIONS Experiments containing fissile material SHALL be encapsulated and limited to a U235 inventory of 50 mg.
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Vallecitos NRC NTR License Renewal Audit Questions QUESTION 003 The NUREG1537, app 14.1, 3.3(8) Secondary and Primary Coolant Radioactivity Limits, recommends a TS for performing sampling for radiological constituents in the primary coolant. The NTR has no such TS. The audit team requests the addition of such a TS or an explanation as to why one is not needed.
NUREG1537, App 14.1, 3.3(8) recommends a TS requiring (for example) quarterly isotopic analysis and weekly sampling for gross activity trending or an output fission product monitor or other primary coolant continuous radiation monitor. The given intended purpose of this TS is to detect deterioration of components in the primary coolant loop, such as a control element, and leakage into the secondary coolant loop. Because the NTR primary operates at atmospheric pressure and has minimal flow through the tank core, deterioration of the control elements is not a significant concern and since the NTR secondary operates at about 30 psi (higher than the primary), a primaryto secondary leak is not a feasible event. More important to the ongoing operation of the NTR is monitoring for fuel degradation, which is continuously and effectively done by the stack monitors.
This question was largely addressed in the docketed response to SAR audit Question 023 (ADAMS ML21265A247, ML21265A248, ML21265A249, ML21265A250). As per that response:
Stack particulate and gaseous activities would provide the first indication of escaping fission products. Stack alarm action levels are very close to the normal operating activity levelsStack activity that exceeds the alarm level limits would prompt an immediate reactor shutdown according to procedure SOP 8.3, Abnormal Operation, to evaluate the cause, which would include drawing a primary sample for analysis.
Periodic strontium 91 and 92 activity monitoring of the primary coolant provides an indication of a less serious breach condition than that which would be detected by continuous monitoring of the stack alarms. Strontium 91 and 92 are measured in accordance with NTR planned maintenance procedure SOP 12.15, Primary Chemistry. Starting in 2021, samples are drawn and analyzed 3 times per year, while in past years, an annual sample was taken. There are no action levels on primary sample activity results; however, strontium 91 and 92 trends are monitored to look for unexpected/unfavorable trends over time. Actions taken based on unfavorable strontium 91 and 92 trends would be dependent on the nature and severity of the trend but would include actions up to and including suspending reactor operation to evaluate the condition and to identify appropriate corrective actions.
Because the NTR primary is, by design, continuously vented to the atmosphere in the reactor cell, the air in the reactor cell is essentially an extension of the primary system for any gaseous and volatile constituents in the coolant. Since the reactor cell atmosphere is continually exhausted through the stack, the stack gas monitor is extremely reflective of changes in noble (or fission product) gasses in the coolant. This renders an additional tech spec for coolant sample analysis unactionable and valueless since TS Rev 5, 3.7.4 would place the reactor in shutdown mode long before a marked increase in coolant gross activity would be detected - regardless of the frequency of sampling. The only true value of sampling then, is as confirmatory analysis or as an investigatory tool following a stack monitor alarm (as noted in the above Question 023 response.). Aside from approved maintenancerelated work, there is no scenario that would result in a marked acute increase in the stack monitor readings apart from fuel failure.
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Vallecitos NRC NTR License Renewal Audit Questions QUESTION 003 As stated in the SAR chapter 16.1, GEH does monitor for chronic fuel degradation by periodically sampling for Sr91 and Sr92 - incremental increases of which would be less likely than noble gases to escape from the coolant to the reactor cell atmosphere, and therefore less likely detected by the stack monitor. Internal 2020 GE report DBR0052607 (which was made available to the audit team during the SAR audit) concluded that about half of the original fuel cladding thickness remains after over 60 years of operation. Nevertheless, that report conservatively recommended that SR 91/92 sampling frequency be increased to quarterly or triannually (3 times per year). It also suggested that tighter conductivity limits would serve to limit the possibility of fuel degradation.
Based on the recommendations of DBR0052607, sampling frequency has been increased to triannually and the coolant conductivity limit lowered from 10 µS/cm to 5 µS/cm (as was also suggested by the NRC audit team (see TS 3.3.3)).
Other guidance:
ANSI 15.1 has no specific requirement for a tech spec related to radioisotopes in the coolant and generally says that specifications are to be assigned only to those parameters and equipment requirements directly related to verifying and preserving the safe operating envelope. The ability to detect incremental fuel failure is not a safe operating envelope limiting issue and it is unclear at what level of increase in Sr91/92 an action would be established or what that action would be.
It is notable that power reactors routinely operate with failed fuel elements and that the required action for exceeding a water chemistry LCO limit is generally to increase sampling frequency to monitor the condition until the reactor is forced to shut down by another LCO. This LCO is most likely one associated with containment atmosphere or ventilation exhaust radiation monitors since coolant in such reactors is typically at higher pressures than the containment atmosphere and fission products inevitably leak. In the case of the NTR, gases leak continually from the coolant to the reactor cell by design. This makes the stack monitor the ultimate method for detecting incremental fuel failure and LCO 3.7.4 provides the ultimate assurance that the reactor is placed in shutdown long before airborne radiation levels become impactful to staff or members of the public.
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Vallecitos NRC NTR License Renewal Audit Questions QUESTION 004 Technical Specification 3.5.3.13 (shown below) is in the currently approved Technical Specifications but has apparently been removed in revision 5; however, no justification has been provided.
TS 3.5.3.13 - The radioactive material content, including fission products, of any doubly encapsulated or vented experiment to be utilized in the experimental facilities shall be limited so that the materials at risk from the encapsulation or confining boundary of the experiment could not result in a dose to any person occupying an unrestricted area continuously for a period of 2 hours2.314815e-5 days <br />5.555556e-4 hours <br />3.306878e-6 weeks <br />7.61e-7 months <br /> starting at the time of release in excess of 0.5 rem to the whole body or 1.5 rem to the thyroid or a dose to any person occupying a restricted area during the length of time required to evacuate the restricted area in excess of 5 rem to the whole body or 30 rem to the thyroid.
TS 3.5.3.13 is necessary to bound the existing SAR Chapter 13 experiment design basis accident (DBA) which includes variable exposure results based on double/single encapsulated, powder/solid, Pu239/U235.
The new proposed DBA in SAR 13.6.4 narrowly specifies 50 mg of powdered U235 in a single capsule and concludes 2hr exposure at the SAB to be 66 mrem and 5min escape exposure to be 470 mrem. Per SAR 13.6.2.1, Dose consequences for doubly cladded or pellet forms of U235 </= 50 mg are bounded by the results of this analysis.
Therefore, under the new SAR, the conditions that warranted the need for this tech spec cannot exist because they are outside analysis. The new proposed TS ensures experiments stay within the new analysis.
New TS 3.8.11 (Refer to Question 002.) provides appropriate bounding for this updated analysis.
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