ML18274A116
| ML18274A116 | |
| Person / Time | |
|---|---|
| Issue date: | 10/04/2018 |
| From: | Tina Ghosh NRC/RES/DSA |
| To: | |
| Ghosh, Tina 301-415-2038 | |
| Shared Package | |
| ML18274A112 | List: |
| References | |
| Download: ML18274A116 (11) | |
Text
State-of-the-Art Reactor Consequence Analyses (SOARCA) Project: Sequoyah Uncertainty Analysis Methods and Insights PSAM 14 September 19, 2018 Tina Ghosh Office of Nuclear Regulatory Research U.S. Nuclear Regulatory Commission
- Objectives and Overview of Short-Term Station Blackout (STSBO) Uncertainty Analysis (UA) for Sequoyah Nuclear Plant
- Uncertain Parameters Included in Study
- Severe Accident Progression and Release Results and Observations
- Summary and Next Steps Outline 2
- SOARCA goals and objectives
- Develop body of knowledge on the realistic outcomes of severe reactor accidents
- Incorporate state of the art modeling (MELCOR/MACCS)
Objectives and Scope 3
- Scope of Sequoyah analyses
Limited to station blackouts (SBOs)
Focused on issues unique to ice condenser containment and hydrogen challenges Relatively low design pressure and smaller volume leads to potential susceptibility to early failure from hydrogen combustion in a station blackout
- Integrated UA focused on unmitigated STSBO scenario
- Input parameter uncertainty propagated in a two-step Monte Carlo process, using 567 (out of 600) samples
- The UA results were examined in detail using both quantitative and qualitative approaches, such as phenomenological analysis of individual realizations.
Overview of Sequoyah STSBO UA 4
The case with earliest containment rupture - RLZ 554 0
20 40 60 80 100 120 140 160 180 200 500 250 0
250 500 750 1000 0
6 12 18 24 ContainmentPressure[psig]
Mass [kg]
time [hr]
invesselgen exvesselgen burnedindome burnedoutsidedome pressure Rlz:554
Overview (continued) 5
- Four regression techniques were used to identify input parameters contributing to key accident progression characteristics and public health impacts.
Cesium Regression Table
- A stability analysis was performed for the result metrics of interest (e.g., cesium and iodine release to the environment) using a bootstrapping method, to gain an understanding of the level of convergence in the statistical results
MELCOR Model Parameters 6
Sequence Related Parameters Primary safety valve stochastic number of cycles until failure-to-close Primary safety valve open area fraction after failure Secondary safety valve stochastic number of cycles until failure-to-close Secondary safety valve open area fraction after failure In-Vessel Accident Progression Melting temperature of the eutectic formed from fuel and zirconium oxides Oxidation kinetics model Ex-Vessel Accident Progression Lower flammability limit hydrogen ignition criterion for an ignition source in lower containment Containment rupture pressure Barrier seal open area Barrier seal failure pressure Ice chest door open fraction Particle dynamic shape factor Time within the Fuel Cycle Time-in-cycle (Beginning, Middle, End-of-Cycle: BOC, MOC, EOC)
Barrier seal
7 MACCS Uncertain Parameter Groups Deposition Wet Deposition Dry Deposition Velocities Dispersion Crosswind Dispersion Linear Coefficient Vertical Dispersion Linear Coefficient Time-Based Crosswind Dispersion Coefficient Latent Health Effects Dose and Dose Rate Effectiveness Factor Lifetime Cancer Fatality Risk Factors Long Term Inhalation Dose Coefficients Early Health Effects Threshold Dose Lethal Dose to 50% of population Hazard Function Shape Factor Shielding Factors Groundshine Shielding Factors*
Inhalation Protection Factors*
Emergency Response Evacuation Delay*
Evacuation Speed*
Hotspot Relocation Time and Dose Criteria Normal Relocation Time and Dose Criteria Keyhole Forecast Time Aleatory Uncertainty Weather Trials
Containment Failure Outcomes 8
Long-tem containment over-pressurization failure due to prolonged steam production and non-condensable gas generation Early containment overpressure failures due to sufficiently large burns in containment No BOC cases exhibit long-term overpressure failure before 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br />
Cesium (left) and Iodine (right)
Environmental Release Fraction Horsetails from STSBO UA 9
Severe Accident Progression STSBO High Level General Observations 10 Consequences strongly (and intuitively) affected by early vs. late containment failure. Early containment failure dominated by hydrogen combustion, and late containment failure results mainly from ex-vessel phenomena (e.g., CCI)
Early containment failures occur only on the first hydrogen burn (subsequent burns do not challenge containment integrity)
Protracted safety valve (SV) cycling produces lower in-vessel hydrogen by the time of first burn Pressurizer SV failure to close (with large open area) results in greater hydrogen production and transport to the containment prior to the first burn, which increases the potential for early containment failure Late containment failures generally have reduced source term release benefiting from gravitational settling
Summary and Next Steps
- SOARCA Sequoyah analysis confirmed insights from previous analyses and yielded some new insights too
- MACCS offsite consequence analyses will be discussed in session Th14, Paper 211
- Will be published as NUREG/CR-7245 within the next month, and available through NRCs website www.nrc.gov