ML19011A434

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Lecture 6-3 Low Power Shutdown 2019-01-18
ML19011A434
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Issue date: 01/16/2019
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Other Operational Modes Lecture 6-3 1

Overview Key Topics

  • Operational experience
  • Early studies
  • LPSD conditions and effects on modeling
  • Current situation 2

Overview Resources

  • U.S. Nuclear Regulatory Commission, Shutdown and Low-Power Operation at Commercial Nuclear Power Plants in the United States, NUREG-1449, September 1993.
  • F.E. Haskin, A.L. Camp, S.A. Hodge, and D.A. Powers, Perspectives on Reactor Safety, NUREG/CR-6042, Revision 2, March 2002.
  • M. Barriere, et al., An Analysis of Operational Experience During Low Power and Shutdown and a Plan for Addressing Human Reliability Assessment Issues, NUREG/CR-6093, June 1994.
  • D.W. Whitehead, et al., "Evaluation of Potential Severe Accidents During Low Power and Shutdown Operations at Grand Gulf Unit 1: Analysis of Core Damage Frequency from Internal Events for Plant Operational State 5 During a Refueling Outage," NUREG/CR-6143, 1994.
  • T.L. Chu and W.T. Pratt, Evaluation of Potential Severe Accidents During Low Power and Shutdown Operations at Surry, Unit 1: Summary of Results, NUREG/CR-6144, Vol. 1, October 1995.

3

Overview Other References

  • Nuclear Energy Agency, Shutdown and Low Power Safety Assessment NEA/CSNI/R(93)19, Boulogne-Billancourt, France, November 1993. (Available from:

http://www.oecd-nea.org/nsd/docs/indexcsni.html).

  • Nuclear Energy Agency, Proceedings of the Workshop on Precursor Analysis, NEA/CSNI/R(2003)11, Boulogne-Billancourt, France, 2003, (Available from:

http://www.oecd-nea.org/nsd/docs/indexcsni.html).

  • Nuclear Energy Agency, Improving Low Power and Shutdown PSA Methods and Data to Permit Better Risk Comparison and Trade-Off Decision-Making, Vols. 1-3 NEA/CSNI/R(2005)11/VOL1, NEA/CSNI/R(2005)11/VOL2, and NEA/CSNI/R(2005)11/VOL3, Boulogne-Billancourt, France, September 2005, (Available from: http://www.oecd-nea.org/nsd/docs/indexcsni.html).
  • Nuclear Energy Agency, Low Power and Shutdown Operations Risk: Development of Structure for Information Base and Assessment of Modelling Issues, NEA/CSNI/R(2009)17, Boulogne-Billancourt, France, December 2009, (Available from:

http://www.oecd-nea.org/nsd/docs/indexcsni.html).

  • T.L. Chu, et al., Evaluation of Potential Severe Accidents During Low Power and Shutdown Operations at Surry, Unit 1: Analysis of Core Damage Frequency from Internal Events During Mid-Loop Operations, NUREG/CR-6144, Vol. 2, June 1994.
  • Z. Musicki, et al., "Evaluation of Potential Severe Accidents During Low Power and Shutdown Operations at Surry, Unit 1: Analysis of Core Damage Frequency from Internal Fires During Mid-Loop Operations," NUREG/CR-6144, Vol. 3, July 1994.

4

Overview Other References

  • Nuclear Energy Agency, Use and Development of Probabilistic Safety Assessment: An Overview of the Situation at the End of 2010, NEA/CSNI/R (2012)11, Boulogne-Billancourt, France, 2012. (Available from: http://www.oecd-nea.org/nsd/docs/indexcsni.html)
  • International Atomic Energy Agency, The Fukushima Daiichi Accident:

Report by the IAEA Director General, STI/PUB 1710, Vienna, Austria, 2015.

  • U.S. Nuclear Regulatory Commission, Loss of Vital AC Power and the Residual Heat Removal System During Mid-Loop Operations at Vogtle Unit 1 on March 20, 1990, NUREG-1410, June 1990.

5

Operational Experience Who Am I?

  • During a maintenance outage, my operators started to perform a safety test (which had been successfully done on some of my sister plants) to verify that during a loss of offsite power (LOOP), my main generator (coasting down) would provide sufficient power until my EDGs kicked in.
  • Early in the test, my operators disabled my Emergency Core Cooling System (ECCS) to prevent it interfering with the test. At this point, outside dispatchers requested that the test be delayed so I could provide power to the grid for several hours. Late in the evening the test resumed.
  • Complications following the delay led to xenon buildup, which caused difficulties in reaching needed test conditions and put me in an unstable operating regime. My operators decided to proceed with the test.
  • In the early morning, my operators partially disabled my automatic scram system (to enable a rapid test repeat, should the first attempt fail) and started the test, ignoring a computer printout indicating that I should be shut down immediately. My power started to rise rapidly.
  • 36 seconds after the start of the test, the shift manager ordered an emergency scram, but it was too late. My core disassembled a few seconds later.

6

Operational Experience Who Am I?

  • During an outage for routine inspections, I was in cold shutdown and undergoing a pressure leak test. My safety relief valves were disabled to prevent inadvertent opening during the test.
  • With considerable effort under trying circumstances, my operators were able to restore power (laying cables to an air-cooled diesel at a neighboring unit that had escaped flooding damage) in 16.5 hours5.787037e-5 days <br />0.00139 hours <br />8.267196e-6 weeks <br />1.9025e-6 months <br /> and establish cooling (using a low-pressure non-safety system) in 2.5 days.
  • I reached a safe and stable condition 9 days after the earthquake.

7

Operational Experience Who Am I?

  • During a scheduled refueling outage, I had been refueled and was in mid-loop condition,* waiting for final tensioning of my reactor vessel head studs. One of my two emergency diesel generators (EDGs) was undergoing maintenance.
  • In mid-morning, a service truck backed into a power pole and caused a loss of offsite power. My remaining EDG started but tripped and I had no power to my safety buses.
  • After unsuccessful attempts to find the cause of my EDG trip, my operators manually restarted my EDG 36 minutes into the incident and RHR cooling was restored.
  • Mid-loop: reactor coolant system water level is lower than the top of the junction of the hot leg with the reactor vessel. 8

Historical Analyses Risk During Low Power and Shutdown Operations

  • Originally discounted because low decay heat => long times available for recovery
  • Concerns raised about possibility of a slug of unborated water causing a reactivity accident
  • More careful consideration

- Operational experience with loss of decay heat removal, loss of inventory, reactivity additions

- Early PRAs indicating potential for high (conditional) CDFs 9

Conditions and Modeling POS Mode* Description 0 1 Full power A Combinatorial Problem 1 2

1,2 3

Low power, rx shutdown Cooldown w/SGs to 177C (350F) 3 4 Cooldown w/RHR to 94C (200F)

  • Multiple combinations of outage types, 4 5 Cooldown to ambient and plant operational states (POS) 5 5,6 Draining to midloop
  • Every outage is different (there is no 6 5,6 Midloop such thing as an average outage) 7 6 Filling refueling cavity 8 6 Refueling (old core)

=> Potentially large number LPSD PRA Defueled models to develop/analyze 8 6 Refueling (new core) 9 6 Draining to midloop 10 5,6 Midloop Outage Type 11 5,6 Refilling RCS Non-Drained Maintenance w/o RHR 12 5 RCS heatup Non-Drained Maintenance w/RHR 13 4 RCS heatup to 177C (350F)

Drained Maintenance w/RHR 14 3 Startup w/SGs to Hot Stdby Refueling 15 1,2 Rx startup

Conditions and Modeling Thought Exercise Outages can involve periods of:

  • Equipment testing and/or maintenance
  • Open containment How might these conditions lead to differences with an at-power PRA model?
  • Hazards
  • Initiators
  • Event Trees
  • Fault Trees
  • Basic Events 11

Historical Analyses Early U.S. LPSD PRAs Date Study CDF (/ry) Notes Zion Extension of Zion PRA. Reduced inventory scenarios = 61% total CDF.

1985 1.8E-5 (NSAC-84) LORHR = 56%, LOCA = 6%. Operator errors dominant.

NUREG/ Modified NSAC-84. LORHR = 82%, LOOP = 10%, LOCA = 8%. Operator 1988 5.2E-5 CR-5015 errors dominant.

Supplement to at-power Level 3 PRA. Analyzed Modes 4 (hot shutdown), 5 (cold shutdown), and 6 (refueling). Included fires and floods. LORHR = 82%,

Seabrook 1983 4.5E-5 LOCA = 18% (but dominates health risk - equipment hatch cant be closed in (PLG) time; overpressure => stuck open relief valves or ruptured RHR pump seals).

Reduced inventory = 71%.

Brunswick Loss of RHR only. Dominant contributors: RHR/RHRSW maintenance; 1985 7.0E-8 (NSAC-83) RHR/RHRSW pump failures; RHRHX CCF LOCA during Mode 5 only. Initiators: safe shutdown earthquake, operator Sequoyah 8-5E-7 to error. Dominant contributors: operator-induced LOCA, availability of power, (SAIC) 7.5E-5 maintenance, operator errors during response, RHR pump air binding, RHR suction failure Adapted from: U.S. Nuclear Regulatory Commission, Shutdown and Low-Power Operation at Commercial Nuclear Power Plants in the United States, NUREG-1449, September 1993 12

Historical Analyses Early International LPSD PRAs CDF Study Dominant Notes

(% Total)

Doel 3 Full PSA including LPSD; Level 1 + containment Tihange 2 response; internal events

1. Leakage below core LPSD; Level 1; internal events; improvements reduce TVO-SEPRA >50%
2. Loss of RHR CDF ~10%

Full PSA including LPSD; Level 1; internal events; EPS 900 32% LOCA + loss of SI human error dominant EPS 1300 70%

Phase 1: LPSD screening; Level 1; internal events GRS-BWR 72 Phase 2: Full PSA including LPSD, fire, flood; Level 2

1. Deboration GRS-PWR 1300 LPSD screening; Level 1; internal events
2. Loss of RHR at midloop Vattenfall - LPSD; Level 1; internal events; pessimistic Loss of RHR at midloop Ringhals 2 assumptions Sizewell B >60% Fires Full PSA including LPSD, fire, flood; Level 3 Borssele LPSD (planned)

Adapted from: Nuclear Energy Agency, Shutdown and Low Power Safety Assessment NEA/CSNI/R(93)19, Boulogne-Billancourt, France, November 1993.

13

Historical Analyses NUREG-1150 follow-on studies

- Internal events

- Mode 5 (cold shutdown)

- Full scope (internal events, fire, flood, seismic)

- Screening Level 1 analysis for all plant operating states, four outage types (refueling, drained maintenance, non-drained maintenance with RHR, non-drained maintenance without RHR)

- Detailed analysis for mid-loop (Level 3) 14

Historical Analyses Example Results - Surry Level 1 Screening:

T.L. Chu and W.T. Pratt, Evaluation of Potential Severe Accidents During Low Power and Shutdown Operations Detailed: at Surry, Unit 1: Summary of Results, NUREG/CR-6144, Vol. 1, October 1995 CDF (/ry) Mid-Loop At Mean 5th 95th Power Internal Events 5E-6 5E-7 2E-5 4E-5 Internal Fires 2E-5 1E-6 8E-5 1E-5 Internal Floods 5E-6 2E-7 2E-5 Seismic 9E-8 3E-10 4E-7 3E-5 Total 3E-5 8E-5 Notes:

1) Sources: NUREG-1150 and NUREG/CR-6144
2) LPSD results include fraction of time in mid-loop (0.066)
3) Seismic results are based on EPRI hazard curves 15

Current Situation Current Situation

  • U.S.

- Not required for risk-informed applications

- Included in ongoing Level 3 PRA project

- Trial Use PRA standard

  • International

- Full scope (internal events, internal hazards, external hazards) required in many countries 16

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