ML19011A435

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Lecture 6-4 Level 2-3 PRA 2019-01-18
ML19011A435
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Issue date: 01/16/2019
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Office of Nuclear Regulatory Research
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Nathan Siu 415-0744
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Level 2/3 PRA:

Beyond Core Damage Lecture 6-4 1

Overview Key Topics

  • Accident Mitigation and Emergency Response

Overview Resources

  • American Nuclear Society and the Institute of Electrical and Electronics Engineers, PRA Procedures Guide, NUREG/CR-2300, January 1983.
  • F.E. Haskin, A.L. Camp, S.A. Hodge, and D.A. Powers, Perspectives on Reactor Safety, NUREG/CR-6042, Revision 2, March 2002.
  • U.S. Nuclear Regulatory Commission, Severe Accident Risks: An Assessment for Five U.S. Nuclear Power Plants, NUREG-1150, December 1990.

3

Overview Other References

  • D. Helton, Scoping Study on Advanced Modeling Techniques for Level 2/3 PRA, U.S.

Nuclear Regulatory Commission, May 2009. (ADAMS ML091320447)

  • N. Bixler, et al., MACCS Best Practices as Applied in the State-of-the-Art Reactor Consequence Analyses (SOARCA) Project, NUREG/CR-7009, August 2014.
  • U.S. Nuclear Regulatory Commission, Technical Study of Spent Fuel Pool Accident Risk at Decommissioning Nuclear Power Plants, NUREG-1738, February 2001.
  • D. Algama, et al., Consequence Study of a Beyond-Design-Basis Earthquake Affecting the Spent Fuel Pool for a U.S. Mark I Boiling Water Reactor, draft report, U.S. Nuclear Regulatory Commission, June 2013. (ADAMS ML13133A132) 4

Overview Terminology

  • Level 2 commonly used in two different ways

- Analysis starting with initiating event and ending with radiological release

- Analysis starting with plant damage (Level 1) and ending with radiological release

  • Similarly, for Level 3

- Analysis starting with initiating event and ending with offsite consequences

- Analysis starting with radiological release and ending with offsite consequences

  • This lecture uses latter, narrower definitions 5

Context Level 2 and Level 3 PRA Hazards Initiating Events Level 1 Plant Damage States Source Level 2 Term Groups Release Categories Level 3 Offsite Consequences 6

Context A More Detailed, Historical View NUREG-1150 7

Accident Mitigation and Emergency Response Overview: Accident Mitigation Mitigation Aims

  • Arrest core damage Before trip 3300 MWt (cooling) After trip 260 MWt
  • Reduce source term 1 hour1.157407e-5 days <br />2.777778e-4 hours <br />1.653439e-6 weeks <br />3.805e-7 months <br /> 50 MWt 1 day 15 MWt (scrubbing, deposition, 1 week 7 MWt filtration)
  • Prevent/delay release (isolation, venting)

Active and Passive Systems/Features

  • Injection/recirculation, containment sump
  • Spray, fan coolers
  • Isolation, vent

Accident Mitigation and Emergency Response Overview: Emergency Preparedness and

Response

  • Emergency Planning Zone (EPZ)

- Plume exposure pathway (~10 mile radius)

- Ingestion pathway (~50 mile radius)

  • Emergency Classifications

- Notification of Unusual Event

- Alert

- Site Area Emergency

- General Emergency

  • Protective Actions

- Sheltering

- Evacuation

- Potassium iodide

- Interdiction

- Relocation 9

Accident Mitigation and Emergency Response EPA Protective Action Guides (PAGs)

PAG = projected dose to an individual from a release of radioactive material at which a specific protective action to reduce or avoid that dose is recommended Environmental Protection Agency, PAG Manual: Protective Action Guides and Planning Guidance for Radiological Incidents, EPA-400/R-16/001, November 2016 10

Level 2 PRA Level 2 PRA

  • Interfaces

- Level 1: plant damage states include information beyond core damage, e.g., status of RCS (temperature, pressure, integrity) and support systems

- Level 3: Source terms and other characteristics (e.g., release location, energy) relevant to consequence analysis

  • Key processes

- Mitigating system response

- Severe accident progression

- Containment response

- Human and organizational response 11

Level 2 PRA Mitigating Systems

  • Active Systems

- Containment spray

- Fan coolers

- Hydrogen igniters

- Isolation

- Vents

  • Analogous to Level 1 models

- Bridge trees

- Consider support, environmental conditions 12

Level 2 PRA Severe Accident Progression

  • Stages

- Core uncovery and heatup

- Cladding oxidation

- Fuel liquefaction and holdup

- Core slumping/relocation

- Lower head failure

- Core-coolant and core-concrete interactions

- Selection of representative scenarios for system codes (e.g., MELCOR, MAAP)

- Selection of simulation end time

- Treatment of uncertainties (model and parameter)

NUREG/CR-6042 13

Level 2 PRA Containment Response

  • Severe-accident failure mechanisms

- Direct containment heating

- Fuel-coolant interactions

- Liner meltthrough

- Hydrogen explosion

- Long-term overpressure

  • Other mechanisms

- External missiles

- Isolation failure

- Bypass NUREG/CR-6042 14

Level 2 PRA Human Reliability Analysis

  • Complications for an already difficult analysis

- Performance for an extreme scenario that overwhelmed protection systems and caused core damage

- Guidance rather than procedures - adherence to prioritization or selection of lower-priority options?

- Uncertain information; dont necessarily know what PRA scenario is occurring

- Need for field actions; potential effect from severe accident progression

- Increased challenges from multi-unit events TEPCO photo from The Yoshida Testimony, Asahi Shinbun, 2014.

- Ex-control room organizations (Technical Support Center, offsite emergency response)

  • No established standard approach; important to interview emergency response staff, observe exercises 15

Level 3 PRA Level 3 PRA (aka Probabilistic Consequence Assessment)

Interface with Level 2 - map source term groups to release categories 16

Level 3 PRA Severe Accident Consequence Analysis Codes Early Health Lagrangian Currently Trajectory Gaussian Met. Exposure Counter-Effects Latent Code Origin Health Supported Plume Sampling /Dose measures Effects Economic Impacts CRAC/

USA X X X X X X X CRAC2 CRACIT USA X X X X X X X ARANO Finland X X X X X X X CONDOR UK X X X X X X X COSYMA EU X X X X X X X X LENA Sweden X X X X X MACCS USA X X X X X X X X OSCAAR Japan X X X X X X X PACE UK X X X X X X  ? X X 17

Level 3 PRA Atmospheric Transport

  • Gaussian plume model based on averaging process
  • More accurate modeling might make a difference for threshold phenomena (acute fatalities, EPA PAGs)
  • HYSPLIT: Gaussian puff
  • Other considerations

- Weather sampling

- Correlation with plant conditions for Level 1 and 2 analyses 18

Level 3 PRA MACCS Transport Illustration (Video)

  • Plume segments move with wind shifting from northwest to northeast
  • Segment width depends on dispersion that has occurred due MACCS Video to varying weather conditions
  • Segment length depends on wind speed 19

Level 3 PRA Other Considerations

  • Protective Actions

- Timing

- Compliance

- Vulnerable cohorts

- Correlation with initiator

- Disruptive events What can go wrong?

- Non-radiological impacts

- Long-term effects

  • Dose and Effects

- LNT

- Compliance 20

Spent Fuel Pools

  • Features

- Low decay heat levels, large water inventories

- Strong structures

  • Concerns

- Outside containment

- Zirconium oxidation (fires)

- Combined core + SFP accident

- Hazardous environment prior to fuel damage

  • Initiators

- Loss of inventory

- Loss of SFP cooling

  • Level 1 metric: fuel damage frequency
  • U.S. studies include:

- NUREG-1738 (2001)

- Algama et al. (2013)

  • International interest 21

Comments

  • Changing view on the nature of accidents

- Past emphasis

  • Large, early releases => acute fatalities
  • Large, late releases => cancer fatalities, other health effects

- Improved analyses + empirical experience

  • Low likelihood of large early doses, avoidability of late doses
  • Increased importance of: a) non-radiological effects, and b) land contamination and associated effects (psycho-social, economic)
  • Increased importance of non-atmospheric pathways
  • Current Level 3 analyses are inductive; deductive approaches might be needed to confirm the above 22

Thought Exercise Following the 2011 earthquake and tsunami in Japan, the Grand Duchy of Fenwick decides to hold an earthquake/flooding emergency preparedness exercise.

This an expensive and disruptive undertaking and so will be done only one time. The Exercise Coordinator says she will design the scenario to ensure that all parts of the Duchys Emergency Plan are exercised, and will develop the specific scenario elements by asking the heads of key departments (police, fire, building & safety, etc.) what they think might happen. Do you have any suggestions for her?

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