ML25014A399
| ML25014A399 | |
| Person / Time | |
|---|---|
| Site: | National Bureau of Standards Reactor (TR-005) |
| Issue date: | 12/19/2024 |
| From: | US Dept of Commerce, National Institute of Standards & Technology (NIST) |
| To: | Office of Nuclear Reactor Regulation |
| Shared Package | |
| ML25014A396 | List: |
| References | |
| Download: ML25014A399 (1) | |
Text
10 CFR 50.59 EVALUATION ECR No.: 1399 ECR/Experiment
Title:
Reactor Plenum Flow Meters License Amendment System: Primary Coolant System (Reactor Vessel &
internals, drains, etc)
Date: 12/5/2024 NCNR-0003-CM-06 1 of 9 Does the proposed ECN:
YES NO A. Require a change to the Technical Specifications (Enter justification below).
Yes. The justification for requiring a change to the Technical Specifications (TS) is as follows:
The NBSR utilizes a double plenum coolant inlet at the bottom of the vessel to provide optimized cooling to the core. The inner six fuel positions and the central thimble are fed by the inner plenum, while the outer plenum feeds the remaining 24 fuel elements and thimbles. The inner plenum is located within and is concentric to the outer plenum. Coolant exits the vessel through two outlet pipes which are welded to the vessel bottom on either side of the outer plenum pipe. The lower grid plate is bolted to both the inner and the outer plenums, forming a watertight seal. The primary coolant flow is distributed between these two plenums by the inherent flow resistance of the two different paths. After the recent replacement of primary pumps (ECN 834, 2018), the total flow is measured (with one (standard deviation) less than 1 %) up to 8700 GPM, where approximately 2300 GPM for the inner plenum and 6400 GPM for the outer plenum, respectively. Approximately 4% of the flow bypasses the core (National Bureau of Standards, 1966A). Until Amendment 11 (May 1984), NBSR Technical Specifications did require both plenum channels. The changes in the technical specifications (May 1984) for the operability requirements of the inner and outer plenum flow channels do not have any technical basis and lack the necessary safety analysis. Nevertheless, in all current safety analysis scenarios, it is assumed that the scram occurs 0.4 s after the flow has reached the trip value taking into account instrumentation delays. There is no technical basis or corresponding safety analysis that would allow operation of the reactor if any of the primary flow channels is bypassed. The existing technical specification for the inner and outer plenum flow is Forced coolant flow shall not be less than 60 GPM/MW for the inner plenum and not less than 235 GPM/MW for the outer plenum (TS 2.2 specification 3). The technical basis for the listed LSSS flow rates is Even in the extremely unlikely event that reactor power, coolant flow, and outlet temperature simultaneously reach their Limiting Safety System Settings, the critical heat flux ratio (CHFR) is at least 2. For all other conditions, the CHFR is considerably higher.
NCNR TS (as updated with License Amendment No. 15 (March 2023)), uses the low LSSS trip points of 1200 GPM and 4700 GPM (4% of outer plenum flow is assumed for bypass) for the inner and outer plenum flow channels, respectively, and references updated FSAR thermal hydraulic analysis for normal operation and safety analyses.
Technical Specifications (Amendment 15, March 2, 2023) Table 3.2.2 and Table 4.2.2 does allow the NBSR to operate when either one of the inner or outer plenum flow channels is bypassed. Hence, the existing text is not in accordance with the available technical basis and safety analysis.
To eliminate the possibility of such an incident, the NCNR is requesting a License Amendment to modify Technical Specifications 3.2.2 and 4.2.2 to require both inner and outer plenum flow channels be operable during reactor operations. This change will ensure that the Technical Specifications and existing safety analysis are in agreement. As part of the review, accident scenarios and thermal hydraulic analysis models used in the Amendment 15 are verified to be in synchronization with TS 2.2, TS Table 3.2.2 and Table 4.2.2 as proposed in this ECN and listed scram setpoints in the updated FSAR Table 7.1.
Consistency with Safety Analysis:
The current TS allows the reactor to operate when either the inner or outer plenum flow channels are bypassed.
However, the safety analysis assumes that both plena channels are operable to ensure adequate cooling and prevent fuel overheating. Ensuring both plena flow channels are operable aligns the TS with the safety analysis, thus maintaining the designed safety margins.
10 CFR 50.59 EVALUATION ECR No.: 1399 ECR/Experiment
Title:
Reactor Plenum Flow Meters License Amendment System: Primary Coolant System (Reactor Vessel &
internals, drains, etc)
Date: 12/5/2024 NCNR-0003-CM-06 2 of 9 Technical Basis for Flow Rates:
The existing TS specifies flow rates to maintain a critical heat flux ratio (CHFR) of at least 2, even in the unlikely event that reactor power, coolant flow, and outlet temperature simultaneously reach their Limiting Safety System Settings (LSSS). By requiring the flow channels for both plena to be operable, the amendment ensures these flow rates are maintained, preserving the safety margins.
Historical Precedent and Regulatory Compliance:
Both plenum channels were required to be operable until Amendment 11 (May 1984). The removal of this requirement lacked a technical basis and necessary safety analysis. Reinstating the operability requirement aligns with historical safety practices and ensures regulatory compliance.
The NCNR is requesting a License Amendment to modify Technical Specifications 3.2.2 and 4.2.2 to require that the inner plenum flow be operable during reactor operations. This change will ensure Technical Specifications and existing safety analysis are in agreement. Corresponding operating Instructions and Reactor Procedures will also be updated to clearly require the primary flow channels.
Proposed changes to TS Table 3.2.2 and Table 4.2.2 are shown below.
\
10 CFR 50.59 EVALUATION ECR No.:
1399 ECR/Experiment
Title:
Reactor Plenum Flow Meters License Amendment System:
Primary Coolant System (Reactor Vessel &
internals, drains, etc)
Date:
12/5/2024 NCNR-0003-CM-06 3 of 9 Therefore, the proposed amendment to require that the flow channels for both inner and outer plena be operable ensures consistency with the safety analysis, maintains necessary CHFR, and follows NRC and NEI guidance, ultimately preserving the margin of safety.
Does the proposed ECN:
YES NO B.
Result in more than a minimal increase in the frequency of occurrence of an accident previously evaluated in the updated FSAR (Enter justification below).
The proposed change does not result in more than a minimal increase in the frequency of occurrence of an accident previously evaluated in the updated FSAR. The justification for this conclusion is as follows:
Consistency with Safety Analysis:
The current safety analysis assumes that the inner and outer plenum flow channels are operable to ensure adequate cooling and prevent fuel overheating. Requiring both flow channels to be operable aligns the Technical Specifications with this safety analysis, thus maintaining the designed safety margins and not introducing any new failure modes or accident scenarios.
No New Accidents Introduced:
The proposed change does not create a possibility for an accident of a different type than any previously evaluated in the FSAR. It ensures that the cooling system operates as intended, which is already accounted for in the existing safety analyses.
Engineering Judgement
10 CFR 50.59 EVALUATION ECR No.: 1399 ECR/Experiment
Title:
Reactor Plenum Flow Meters License Amendment System: Primary Coolant System (Reactor Vessel &
internals, drains, etc)
Date: 12/5/2024 NCNR-0003-CM-06 4 of 9 Qualitative engineering judgment suggests that maintaining the operability of both flow channels does not increase the likelihood of failure of critical safety systems or components. The proposed change ensures that the reactor operates within its designed safety parameters, thereby not increasing the frequency of accidents.
Therefore, based on the evaluation criteria and the justification provided, the proposed change does not result in more than a minimal increase in the frequency of occurrence of an accident previously evaluated in the updated FSAR.
Does the proposed ECN:
YES NO C. Result in more than a minimal increase in the likelihood of occurrence of a malfunction of a structure, system, or component important to safety previously evaluated in the updated FSAR (Enter justification below).
The change requested by the NCNR to modify Technical Specifications 3.2.2 and 4.2.2 to require that the inner plenum flow be operable during reactor operations will ensure consistency with the existing safety analysis and technical basis. The current safety analyses and technical specifications are based on the assumption that both the inner and outer plenum flow channels are functional and provide the necessary coolant flow to ensure safe reactor operation.
The justification for this change can be derived from the following points:
Current Safety Analysis Assumptions:
Existing safety analyses assume that scram occurs 0.4 seconds after the flow reaches the trip value, taking into account instrumentation delays, and both inner and outer plenum flows are functional during normal and accident conditions.
Technical Specifications Alignment:
The technical specifications for the limiting safety system settings (LSSS) flow rates for the inner and outer plenum (60 GPM/MW and 235 GPM/MW respectively) ensure that the critical heat flux ratio (CHFR) remains at least 2, even under extreme conditions, thereby preventing the possibility of burnout.
Operational Safety:
Requiring both plena flow channels to be operable will prevent potential malfunctions and ensure that the reactor operates within the parameters set by the safety analysis, thereby eliminating any discrepancies between the technical specifications and the safety analysis.
Regulatory Compliance:
Aligning technical specifications with safety analysis prevents unauthorized operation conditions that could lead to unsafe scenarios.
Therefore, modifying the technical specifications to explicitly require the operability of the inner plenum and outer plenum flow channels during reactor operations is a necessary step to maintain safety and regulatory compliance, ensuring that the likelihood of malfunctions does not increase beyond what has been previously evaluated in the updated FSAR.
Does the proposed ECN:
YES NO
10 CFR 50.59 EVALUATION ECR No.: 1399 ECR/Experiment
Title:
Reactor Plenum Flow Meters License Amendment System: Primary Coolant System (Reactor Vessel &
internals, drains, etc)
Date: 12/5/2024 NCNR-0003-CM-06 5 of 9 D. Result in more than a minimal increase in the consequences of an accident previously evaluated in the updated FSAR (Enter justification below).
The proposed change to require that the inner plenum and outer plenum flow channels be operable during reactor operations does not result in more than a minimal increase in the consequences of an accident previously evaluated in the updated FSAR. Here are the key points supporting this conclusion:
No Significant Increase in Accident Consequences:
According to the evaluation under 10 CFR 50.92(c)(1), the proposed amendment does not involve a significant increase in the probability or consequences of an accident previously evaluated. The existing safety analyses take credit for both inner and outer plenum flow channels, and ensuring operability of both aligns with these analyses.
Accident Scenarios Remain Unchanged:
The proposed change does not introduce new accident scenarios or modify existing ones in a manner that would increase their consequences. The FSAR includes thorough analyses of various accident scenarios, such as startup accidents, reactivity insertion accidents, and loss of coolant accidents, and the proposed change is within the bounds of these analyses.
Therefore, the proposed change to require operability of the inner plenum flow channel does not result in more than a minimal increase in the consequences of an accident previously evaluated in the updated FSAR.
Does the proposed ECN:
YES NO E. Result in more than a minimal increase in the consequences of a malfunction of a structure, system, or component important to safety previously evaluated in the updated FSAR (Enter justification below).
The proposed change to require that the inner plenum and outer plenum flow channels be operable during reactor operations does not result in more than a minimal increase in the consequences of a malfunction of a structure, system, or component important to safety previously evaluated in the updated FSAR. Here are the key points supporting this conclusion:
Consistency with Safety Limits:
The technical specifications ensure that the safety limits are not violated. The limiting safety system settings (LSSS) for coolant flow are designed to maintain a significant margin from the safety limits, ensuring that any abnormal reactor condition caused by equipment malfunction or operator error will be terminated well before the safety limits are reached.
Adequate Safety Margins:
The safety analysis demonstrates that the system provides adequate control of reactivity, containment of coolant, and barriers to the release of radioactive material, ensuring that the reactor can be safely operated and shut down from any operating condition or accident assumed in the safety analysis. The proposed change aligns with these safety margins.
No Increase in Accident Consequences:
The safety analysis includes conservative calculations that account for uncertainties in process instrumentation, ensuring that the critical heat flux ratio remains well above safety limits even under extreme conditions. The proposed change does not alter these assumptions or the resulting safety margins.
10 CFR 50.59 EVALUATION ECR No.: 1399 ECR/Experiment
Title:
Reactor Plenum Flow Meters License Amendment System: Primary Coolant System (Reactor Vessel &
internals, drains, etc)
Date: 12/5/2024 NCNR-0003-CM-06 6 of 9 Therefore, the proposed change does not result in more than a minimal increase in the consequences of a malfunction of a structure, system, or component important to safety previously evaluated in the updated FSAR.
Does the proposed ECN:
YES NO F. Create a possibility for an accident of a different type than any previously evaluated in the updated FSAR (Enter justification below).
The proposed change to require that the inner plenum and outer plenum flow channels be operable during reactor operations does not create a possibility for an accident of a different type than any previously evaluated in the updated FSAR. Here are the key points supporting this conclusion:
No New Accident Types:
The proposed change does not introduce new types of accidents. It ensures that both plenas flow channels are operable, which is consistent with the existing safety analysis and assumptions. The safety analysis has already considered various accident scenarios, and ensuring operability aligns with these existing evaluations without introducing new types of accidents.
Existing Accident Scenarios:
The updated FSAR evaluates a broad spectrum of anticipated transients and potential accidents, including loss of primary coolant, maximum hypothetical accidents, and experiment malfunctions. The proposed change does not modify the nature of these scenarios or create new initiating events that could lead to different types of accidents.
Accident Analysis Consistency:
The proposed change ensures consistency with the technical specifications and safety margins described in the FSAR. It does not alter the fundamental design or operational principles that would lead to a different type of accident. The FSAR's accident scenarios remain applicable and bounded by the existing safety analyses.
The proposed change does not create the possibility of a new or different kind of accident from those previously evaluated. The change falls within the scope of existing regulations and safety evaluations, ensuring that the reactor operations remain within the analyzed and approved safety envelope.
Therefore, the proposed change does not create a possibility for an accident of a different type than any previously evaluated in the updated FSAR.
Does the proposed ECN:
YES NO G. Create a possibility for a malfunction of a structure, system, or component important to safety with a different result than any previously evaluated in the updated FSAR (Enter justification below).
The proposed change to require that the inner plenum and outer plenum flow channels be operable during reactor operations does not create a possibility for a malfunction of a structure, system, or component (SSC) important to safety with a different result than any previously evaluated in the updated FSAR. Here are the key points supporting this conclusion:
Evaluation of Malfunctions:
Malfunctions of SSCs are generally postulated as potential single failures to evaluate plant performance with the focus being on the result of the malfunction rather than the cause or type of malfunction. The proposed
10 CFR 50.59 EVALUATION ECR No.: 1399 ECR/Experiment
Title:
Reactor Plenum Flow Meters License Amendment System: Primary Coolant System (Reactor Vessel &
internals, drains, etc)
Date: 12/5/2024 NCNR-0003-CM-06 7 of 9 change does not introduce new failure mechanisms or alter the results of existing failure modes in a manner that would produce different outcomes from those already evaluated.
Bounding Analysis:
The existing safety analyses are designed to be bounding and assume that all applicable acceptance criteria are met. The proposed change does not invalidate these assumptions or the results of the analyses. The safety analyses already assume the operability of the inner and outer plenum flows, and ensuring their operability aligns with these analyses.
No New Failure Modes:
The change does not create new failure modes that would result in different outcomes. The proposed change maintains existing design functions and does not introduce new failure mechanisms.
Therefore, the proposed change does not create a possibility for a malfunction of an SSC important to safety with a different result than any previously evaluated in the updated FSAR.
Does the proposed ECN:
YES NO H. Result in exceeding or altering a design basis limit for a fission product barrier as described in the updated FSAR (Enter justification below).
The proposed change to require that the inner plenum and outer plenum flow channels be operable during reactor operations does not result in exceeding or altering a design basis limit for a fission product barrier as described in the updated FSAR. Here are the key points supporting this conclusion:
Definition of Exceeding or Altering:
A specific proposed activity requires a license amendment if the design basis limit for a fission product barrier is "exceeded or altered." The term "exceeded" means that as a result of the proposed activity, the facility's predicted response would be less conservative than the numerical design basis limit. The term "altered" means the design basis limit itself is changed.
No Direct Impact on Design Basis Limits:
The proposed change ensures operability within the existing design and operational framework without altering the fundamental design basis limits for fission product barriers. The safety analyses and evaluations confirm that the inner plenum flow operability requirement does not impact the numerical limits that ensure the integrity of the fission product barriers.
Therefore, the proposed change does not result in exceeding or altering a design basis limit for a fission product barrier as described in the updated FSAR.
Does the proposed ECN:
YES NO I.
Result in a departure from a method of evaluation described in the updated FSAR used in establishing the design bases or in the safety analysis (Enter justification below).
The proposed change to require that the inner plenum and outer plenum flow channels be operable during reactor operations does not result in a departure from a method of evaluation described in the updated FSAR used in establishing the design bases or in the safety analysis. Here are the key points supporting this conclusion:
10 CFR 50.59 EVALUATION ECR No.: 1399 ECR/Experiment
Title:
Reactor Plenum Flow Meters License Amendment System: Primary Coolant System (Reactor Vessel &
internals, drains, etc)
Date: 12/5/2024 NCNR-0003-CM-06 8 of 9 The proposed change does not alter the methods used in the FSAR in a way that would make the results non-conservative or different from the original intent. All evaluations are completed using existing methods in the current licensing basis and safety analyses.
Therefore, the proposed change does not constitute a departure from a method of evaluation described in the updated FSAR used in establishing the design bases or in the safety analysis.
No Change in Methodology:
The proposed change to ensure operability of the inner plenum and outer plenum flow channels does not alter the analytical methods or the underlying assumptions used in the existing safety analyses. The existing methods of evaluation remain valid and applicable, and the proposed change does not introduce new methods or elements that would require NRC approval.
10 CFR 50.59 EVALUATION ECR No.: 1399 ECR/Experiment
Title:
Reactor Plenum Flow Meters License Amendment System: Primary Coolant System (Reactor Vessel &
internals, drains, etc)
Date: 12/5/2024 NCNR-0003-CM-06 9 of 9 Conclusion (Check one)
Based on the evaluation conducted in the above table, it is concluded that the proposed action does not meet any of the 10 CFR 50.59 criteria; therefore, the activity does not require a license amendment or prior NRC approval to perform the proposed action.
Based on the evaluation conducted on the above table, it is concluded that the proposed action does meet one or more of the 10 CFR 50.59 criteria; therefore, the activity does require a license amendment to be obtained from the NRC under 10 CFR 50.90 to perform the proposed action.
12/11/2024 X
Dagistan Sahin 50.59 Evaluator Signed by: National Institute of Standards and Technology NOTE Consistent with the intent of 10 CFR 50.59, the justifications should be complete in the sense that another knowledgeable reviewer could draw the same conclusion. Restatement of the criteria in a negative sense or making simple statements of conclusion is NOT sufficient and should be avoided. The basis and logic used for engineering judgment and the determination should be documented to the extent practicable and to a degree commensurate with the safety significance and complexity of the activity.