ML18026B097

Mitigating Strategies for Beyond-Design-Basis External Events

ML18026B097
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Issue date:	01/26/2018
From:	Eric Bowman Beyond-Design-Basis Management Branch
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Andrukat D
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ML18026B126	List: ML18026B088 ML18026B097 ML18026B110
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Download: ML18026B097 (18)

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Mitigating Strategies for Beyond-Design-Basis External Events Eric Bowman Special Advisor, Beyond-Design-Basis Management

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Background===

On March 11, 2011, a major earthquake struck off the coast of Honshu, resulting in a large tsunami that caused widespread devastation and significantly affected the infrastructure and industry in the northeastern coastal areas of Japan. This led to damage to the nuclear fuel at Fukushima Dai-ichi Units 1, 2 and 3.

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NTTF Tier 1 Actions

• Order EA-12-049 on Mitigating Strategies

• Order EA-12-050 (now EA-13-109) on BWR Vents

• Order EA-12-051 on Spent Fuel Pool Instruments

• Seismic and Flooding Walkdowns

• Seismic and Flooding Re-evaluations

• Staffing and Communications Assessments 3

Principles for Mitigating Strategies In examining the operating experience at Fukushima Dai-ichi, the NRC noted that many of the post-9/11 strategies in the U.S. may have helped lessen the effects of the event. The NRC also noted that strategies similar to these were attempted by the operators at Fukushima Dai-ichi during the event.

Major differences exist between the localized damage assumptions of the post-9/11 strategies and the widespread damage in the operating experience in Japan.

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Order EA-12-049 March 12, 2012 Licensees or construction permit holders shall develop, implement, and maintain guidance and strategies to maintain or restore core cooling, containment and spent fuel pool cooling capabilities following a beyond-design-basis external event.

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Order EA-12-049 Continued

• Simultaneous loss of all ac power and loss of normal access to the ultimate heat sink

• Adequate capacity to address challenges to core cooling, containment, and spent fuel pool cooling capabilities at all units on a site

• Reasonable protection of associated equipment

• Capable of implementation in all modes

• Procedures, guidance, training, and acquisition, staging or installing of equipment 6

NEI 12-06 Site Assessment Process 7

BWR FLEX Baseline Capability Summary Safety Function Method Baseline Capability Reactor Core Cooling

• RCIC/HPCI/IC

• Use of installed equipment for initial coping

• Depressurize RPV for

• Primary and alternate connection points for Injection with Portable portable pump Injection Source

• Means to depressurize RPV

• Sustained Source of Water

• Use of alternate water supply to support core Core Cooling heat removal makeup Key Reactor Parameters

• RPV Level * (Re-)Powered instruments

• RPV Pressure

• Other instruments for plant-specific strategies Containment Pressure Control

• Containment Venting or

• Reliable, hardened vent (per EA-12-050 for Mk I

/Heat Removal Alternative Containment and Heat Removal II) or other capability.

Containment Integrity

• Hydrogen igniters

• Re-powering of hydrogen igniters with a Containment (BWR Mark III Containments portable power supply.

Only)

Key Containment Parameters

• Containment Pressure * (Re-)Powered instruments

• Suppression Pool Temperature

• Suppression Pool Level Spent Fuel Cooling

• Makeup with Portable

• Makeup via hoses direct to pool SFP Cooling Injection

• Makeup via connection to SFP makeup piping or other suitable means Source

• Spray via portable nozzles 8

SFP Parameters

• SFP Level

• Per EA 12-051

Example BWR Licensee Flow Diagram 9

PWR FLEX Baseline Capability Summary Safety Function Method Baseline Capability Reactor Core Cooling &

• AFW/EFW

• Use of installed equipment for initial coping Heat Removal (steam

• Depressurize SG for

• Connection for portable pump to feed required SGs Makeup with Portable

• Use of alternate water supply to support core heat generators available) Injection Source

• Sustained Source of Water removal RCS Inventory Control and

• Low Leak RCP Seals

• Low-leak RCP seals and/or providing on-site high and/or RCS high pressure Core Cooling Core Heat Removal makeup pressure RCS makeup capability (shutdown modes with steam

• All Plants Provide Means to

• Diverse makeup connections to RCS for long-term Provide Borated RCS Makeup RCS makeup and shutdown mode heat removal generators not available)

• Source of borated water

• Letdown path if required Key Reactor Parameters

• SG Level * (Re-)Powered instruments

• SG Pressure

• RCS Pressure

• RCS Temperature Containment Pressure

• Containment Spray

• Connection point on containment spray header for Control/Heat use with portable pump or alternate capability or Removal analysis demonstrating that containment pressure Containment control is not challenged, e.g., MAAP analysis.

Containment Integrity

• Hydrogen igniters

• Re-powering of hydrogen igniters with a portable (Ice Condenser Containments power supply.

Only)

Key Containment Parameters

• Containment Pressure * (Re-)Powered instruments consistent Spent Fuel Cooling

• Makeup with Portable

• Makeup via hoses direct to pool SFP Cooling Injection Source

• Makeup via connection to SFP makeup piping or other suitable means

• Spray via portable nozzles 10 SFP Parameters

• SFP Level

• Per EA 12-051

Example PWR Licensee Flow Diagram 11

Additional Mitigating Strategies Characteristics

• Strategies based on plant-specific analyses

• Time constraints identified with basis to show they can be met

• Ability to use portable pumps for RPV/RCS/SG makeup

• Spare equipment

• Maintenance and Testing

• Training 12

Off-site Resources

• Licensees have set up off-site resource centers to provide additional equipment and supplies

• 24 hour2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> delivery time

• Transportation by road or by air 13

National SAFER Center Locations 14

Alternative Approaches NEI 12-06 provided one acceptable approach. Others that have been proposed include:

• Additional hardened generators

• Repowering installed pumps

• Other minor differences from NEI 12-06 15

MBDBE Rulemaking

• Integrated Response Capability in new 10 CFR 50.155

• Adds requirements for reevaluated seismic and flooding hazard mitigation

• Specifies removal of requirements during decomissioning 16

Risk Credit for Mitigating Strategies

• U.S. Power Reactor Licensees are in the process of incorporating mitigating strategies in their Probabilistic Risk Assessments of as built, as operated plants

• Degree of risk reduction varies based upon site and plant configurations 17

Bibliography Document Accession No.

SECY-11-0093 ML11186A950 Order EA-12-049 ML12054A736 NEI 12-06, Revision 0 ML12242A378 JLD-ISG-2012-01, Revision 0 ML12229A174 NEI 12-06, Revision 2 ML15348A015 JLD-ISG-2012-01, Revision 1 ML15357A163 MBDBE Rulemaking SECY-16-0142 18