ML22339A213
| ML22339A213 | |
| Person / Time | |
|---|---|
| Issue date: | 10/12/2022 |
| From: | Hudson J Office of Nuclear Reactor Regulation |
| To: | |
| References | |
| Download: ML22339A213 (1) | |
Text
TRTR 2022 Annual Conference Risk-Informed Decision Making for NPUFs Ching Ng, Reliability & Risk Analyst, Division of Risk Assessment Justin Hudson, Project Manager, UNPL October 12, 2022
2 Topics What is risk-informed decision making An example of how to calculate risk How to use risk-informed decision making How risk-informed decision making applies to the TRTR community
3 What is Risk-Informed Decision Making?
- 1. Meets Current Regulations
- 2. Consistent with Defense-in-Depth Philosophy
- 3. Maintains Sufficient Safety Margins
- 4. Presents Acceptably Small Increase in Risk
- 5. Develops Performance Measurement Strategies Integrated Decision Making
4 What is Risk-Informed Decision Making?
- Utilizing risk information when thinking about issues
- Obtaining diverse input
- Conducting risk conversations
- Performing either qualitative or quantitative analysis An integrated decision-making approach that incorporates risk insights.
Someone using this approach would consider:
Risk-based would imply using only the numerical results of a risk assessment Risk-based evaluations use models, incorporating uncertainties, assumptions, etc.
5 Examining the Risk of a LOCA An RTR example of how to calculate risk for a Loss of Coolant Accident (LOCA)
6 Background for LOCA Example Loss of Coolant Accident Thermal Hydraulic (planned mitigation)
Heat Removal (planned mitigation)
Water Makeup & Other (planned mitigation)
CONCLUSION:
Fuel Cladding Temperature remains below the Safety Limit Licensing Basis of an RTR
7 RTR Operations Experience Loss of Coolant Accident Thermal Hydraulic (not applicable)
Heat Removal (not applicable)
Water Makeup (becomes the chief mitigation strategy)
CONCLUSION:
Fuel Cladding Temperature May Possibly Exceed the Safety Limit Background for LOCA Example Continued
8 What is the Risk?
Risk = Likelihood x Consequence
= Frequency of an event x Probability of failure of the mitigation strategies For the issue being examined, are there unanalyzed or new Initiating Events (IEs)?
For the issue being examined, what are the frequencies of Initiating Events?
Does the issue being examined affect any mitigation strategies?
9 What is the Risk of the Limiting (worst case) LOCA?
RiskLOCA = (Frequency x Probability)
What is the frequency for the initiating event?
Does it affect any mitigation functions?
In ASME/ANS RA-Sa-2009, Standard for Level 1 / Large Early Release Frequency Probabilistic Risk Assessment for Nuclear Power Plant Applications IE-C6 - If the frequency is less and 1E-6/yr, the IE can be screened out Factors that could affect the probability of failure of the mitigation strategies:
- Only Manual Isolation valves
- No automatic injection system
- All of the coolant drains quickly Probability of Failure = 1.0
10 Frequency of an Initiating Event - Example What does the pipe break frequency depend on?
Design The double-end guillotine break (pipe rupture) of the 10-in coolant loop Stainless steel piping with an approximately length of 300 ft.
Operating Characteristics The piping is designed to a certain ASME Code (for instance, ASME B-31.1)
Demineralized water, atmospheric temperature and pressure and heated up to 120 °F
11 Calculating the Pipe Break Frequency Sources:
1.
NRC Risk Assessment of Operational Events Handbook, Vol 2, Section 3, Internal Flood Modeling and Risk Quantification, has non-primary coolant system rupture frequency 2.
Idaho National Lab report EGG-SSRE-9639 (1991) that covered 1960-1990 3.
NRC Office of Nuclear Regulatory Research: Component Operational Experience Degradation and Ageing Program (Database)
External leakage frequency, FL = (2n+1)/2T Where n is number of external leakages and ruptures and T is operating time External rupture frequency = 0.04 x FL Assumptions: Stainless steel, non-primary coolant system, 1990-2020 Result: External rupture frequency = 0.04 x FL = 4.5E-11/ft/hr
12 What is the frequency of the LOCA?
Initiating Event frequency of the LOCA
= 4.5E-11 x length of pipe
= 4.5E-11 x 300 ft x 4,360 hr/yr 6E-05 per year Assumptions:
The reactor typically operates 8 hours9.259259e-5 days <br />0.00222 hours <br />1.322751e-5 weeks <br />3.044e-6 months <br /> a day plus some occasional overnight operations therefore it is assumed that the reactor at-power for 50% (or 4360 hr/yr) of the time.
No credit was given for operator actions no alarm annunciating the manual isolation valves cannot be operated in control room The resulting calculation indicated that the initiating event frequency is not negligible.
(If the frequency is less than 1x10-6, it may be considered negligible)
External rupture frequency = 0.04 x FL = 4.5E-11/ft/hr (stainless steel, non-primary coolant system)
13 What can go wrong?
(Coolant could drain quickly!)
What are the consequences?
(Fuel cladding probability of failure)
What is the risk? How likely?
(Examine pipe rupture frequency)
Share it here!
Reduce operating time?
Reduce pipe length?
Automatic actuation of Isolation valves?
Automatic water injection system?
14 Level of Risk Dictates Depth of Analysis Necessary What if the calculated LOCA frequency is a lot lower than 1E-06/yr?
We may not need as much margin to cover uncertainty and variability.
Level of Risk Effort If very low risk 5 Hrs and 10 pages of analysis If moderate risk 25 Hrs and 50 pages of analysis If high risk 50 Hrs and 100 pages of analysis
15 Data to Collect for Risk-Informed Decision making
- Record equipment failures, human error, maintenance schedules
- Establish a repository for facility equipment specifications, and drawings Risk evaluation relies upon deterministic calculations
- Success criteria (i.e., number of pumps) is informed by engineering analyses
- Reliability and initiating event frequencies are informed by operating experience
- Calculations of human reliability are informed by training and procedures
16 What Programs can be Used to Help with Risk-Informed Decision Making
- Licensee-initiated Corrective Action Programs These programs gather data on all kinds of subject areas:
- Equipment breakdowns
- Human Error
- Causal Factors
- Licensee-initiated Material/Part History Programs
- Maintaining a material/part history program provides data for trending analyses and the projection of reoccurring maintenance or problems
- Helps keep record of changes in the facility that could be lost to time
17 How Can the TRTR Community Use Risk-Informed Decision Making?
- Licensing actions: use risk-informed decision making when doing analyses for license amendments
- Not all risk analysis has to be quantitative and detailed
- Risk calculation results can inform the workload
- Sharing risk insights between facilities can be useful to the community
18 Questions?