Ansaldo Nucleare at CPExpo 2013: Emergency electrical power supply to nuclear safety systems

Emergency electrical power supply to nuclear safety systems: Design basis and lesson learned from the Fukushima accident

Genova , 30 Ottobre 2013

Corrado Alessandroni Licensing and Safety Technical Advisor Ansaldo Nucleare

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Genova, 30 Ottobre 2013

Emergency electrical power supply to nuclear safety systems: Design Basis and lesson-

learned from Fukushima accident: What’s occurred at Fukushima

FUKUSHIMA DAI-ICHI March 11th, 2011

What’s occurred to the Electrical Power Supply Systems

FUKUSHIMA DAI-ICHI March 11th, 2011

Hour 14.46: massive earthquake (magnitude 9.0) which induced onsite seismic

accelerations above design values

Loss of normal electrical power supply caused by the earthquake

Reactor SCRAM in the three units in operation (of six located onsite)

Electrical power supplied by the emergency diesel generators (EDGs) that started as designed

Hour 15.30: an earthquake induced Tsunami struck the site: waves up to 14 ÷ 15 m

height flooded the major buildings (tsunami barriers where designed for 6 m wave height

protection).

Loss of 12 out of 13 EDGs located onsite – the survived EDG was servicing the Units 6

Massive damage of electrical panel boards

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Genova , 30 Ottobre 2013

Path of inundation Ref. A One-year Review of Fukushima Daiichi Nuclear Power Station “Steps to Achieve Stabilization” March 2012



FUKUSHIMA

DAI-ICHI

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Ref. Special Report on the Nuclear Accident at the Fukushima Daiichi Nuclear Power Station – INPO 11-005 November 2011

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Ref. A One-year Review of Fukushima Daiichi Nuclear Power Station “Steps to Achieve Stabilization” March 2012

Fukushima Daiichi Electrical Distribution Damage

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Ref. Special Report on the Nuclear Accident at the Fukushima Daiichi Nuclear Power Station – INPO 11-005 November 2011

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Ref. Fukushima Nuclear Accident Analysis Report (Interim Report) – Tokyo Electric Power Company, Inc. December, 02nd 2012

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Ref. A One-year Review of Fukushima Daiichi Nuclear Power Station “Steps to Achieve Stabilization” March 2012

Vehicles batteries secured on-site were collected to power monitoring instrumentation

about 6 hours after the initiating event, batteries were connected to power reactor water level indicator at Unit 1

Post-accident recovery actions to restore the electrical power supply

No written emergency procedures to face the severe accident situation Restoration of external power supply impossible due to the massive damage

induced on transmission lines by the earthquake

Acquisition of power supply cars and further batteries from outside

power supply cars started to arrive onsite on 11th March evening-night;

about 50 mobile electric-generators were onsite on 12th March morning;

connection difficulties due to the massive damaged of the electrical panels;

first connection was made in the afternoon of 12th March (15.30) about 25 hours after the earthquake and about 18 hours from the arrival of the first power supply car;

it was decided to use fire engines and motor driven pumps to cool reactors trough injection of sea water due to the impossibility to restore power to plant safety systems.

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Ref. Fukushima Nuclear Accident Analysis Report – Tokyo Electric Power Company, Inc. June, 20th 2012



EMERCENCY POWER SYSTEM (EPS) Design Bases

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learned from Fukushima accident: EPS Design Bases

System definition

EPS includes:

1. Electric power supply sources: Electric Generators (typically Diesel-Generator) and Batteries

2. Electric power distribution system

Functions

Provide power supply to plant components assuring safety functions under all design basis conditions where the normal power supply is not available. The fundamental safety functions are:

1. Shutdown of the nuclear chain reaction 2. Nuclear fuel cooling and decay heat removal 3. Containment of radioactive releases

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System Design Bases

Inspection and Testing Program during operation – Program surveilled by the Safety Authority (Individual Plant Technical Specification)

Separation from the normal power supply system of the plant: Defense-in-Depth Principle

Component redundancy : Application of the Single Failure Criterion

Redundant components separation : protection against from common cause failure (fire, flooding, internal missiles etc.)

Environmental qualification: P,T, humidity and radiation associated also to accidental conditions

Protection / Qualification against design basis External Natural Phenomena (Earthquake, Flooding, Tornado etc.) and Man-made External Events (Aircraft Impact, Explosion etc...)

Component safety classification commensurate with the importance of the safety functions to be performed

Application of Codes and Standard for the design, fabrication, erection, testing and operation commensurate with the importance of the safety functions to be performed

Quality assurance

Reliability requirements for emergency diesel generators (0,95-0,975 per demand for each EDG - US NRC Reg.Guide 1.155 «Station Blackout»)

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REGULATIONS and STANDARDS

International (IAEA, IEC, ISO)

IAEA Safety Standard Series NS-G-1.8 Design of Emergency Power Systems for Nuclear Power Plants

National (Regulatory Guides,IEEE, RCC-E, KTA, …)

EUR Requirements Document (Europe)

Generation III Light Water Reactors: Utilities Requirements

EPRI Utility Requirements Document (USA and other countries)

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Power, Instrumentation and Control Portion of Safety Systems

IEEE Std 603 Criteria for Safety Systems for NPGSs

--------------

US NRC Reg. Guide 1.153

Power Systems

IEEE Std 308

US NRC Reg.Guide 1.32

Protection System

IEEE Std 279

US 10CFR50 and SRP Ch.7

Post-Accident Monitoring System

IEEE Std 497


Digital Computer Systems

ANSI/ANS IEEE Std 7-4.3.2

Periodic Testing

IEEE Std 338

US NRC Reg. Guide 1.118

Environmental and Seismic Qualification

IEEE Std 323 – IEEE Std 344 – IEEE Std 627

US NRC Reg Guide 1.89 and 1.100

Independence and Separation

IEEE Std 384


Single Failure Criterion

IEEE Std 379


U.S. STANDARDS - IEEE Standards for Safety Systems and Components (IEEE Class 1E Items)

• System Design Basis • System Design Criteria • Sense and Command Features

Requirements • Execute Features Requirements • Power Sources Requirements

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U.S. STANDARDS -IEEE Standards for Safety Systems and Components (IEEE Class 1E Items)

• System Design Basis • System Design Criteria • Sense and Command Features

Requirements • Execute Features Requirements • Power Sources Requirements

IEEE Std. 387 – Criteria for Diesel-Generator Units Applied as Standby Power Supplies for Nuclear Power Generating Stations (US NRC Reg. Guide 1.9);

IEEE Std. 485 - Recommended Practice for Sizing of Lead-Acid Batteries for Stationary Applications (US NRC Reg. Guide 1.212);

IEEE Std. 572 - Qualification of Class 1E Connection Assemblies for Nuclear Power Generating Stations (US NRC Reg. Guide 1.156);

……

Examples of Specific Standards for equipment-apparatus

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AFTER FUKUSHIMA

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learned from Fukushima accident: After Fukushima

Stress Tests: European Union

EU Council of 24/25 March, 2011, requested that the safety of all EU nuclear plants should be reviewed

31st May 2011: ENSREG issued EU Stress Tests Specifications

June 2011: all the operators of nuclear power plants in the EU started safety reassessments on the basis of ENSREG Specifications

November 2011: NPP Operators Final Reports to National Safety Authorities

December 2011: National Reports by Safety Authorities

April 2012: Peer Review Report by the Task Force setup by ENSREG and EU Council with experts from all EU countries.

August 2012: ENSREG Action Plan to follow-up the implementation of the peer review recommendations

Post-Fukushima Task Force and Action Plan: USA

INTERNATIONAL INITIATIVES

July 2011 Document with Near-Term Task Force (NTTF) recommendations (SECY-11-0093): NTTF conducted a review of the NRC regulations and processes in light of the events at Fukushima determining additional improvements to be made

September 2011 Selected recommendations to be taken without delay (US NRC SECY-11-0124)

October 2011 NTTF Recommendations Prioritization and Action List (US NRC SECY-11-0137)

March 2012 Approval of the first three orders to Plant Operators (SECY-12-0025)

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Reassessment of the Design Basis External Events with updated methodologies and data

ACTIONS

Definition of Beyond Design-Basis External Events Mitigation Strategies.

Scenario: loss of electrical power and ultimate heat sink affecting all units at a site simultaneously

Goal: prevent fuel damage in the reactor and spent fuel pool with simultaneous actions on all units at site

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Mitigation Strategies– Phased Approach

(rif. NEI 12-06 Maggio 2012 – Diverse Flexible Coping Strategies –FLEX- Implementation Guide)

Phase 1- Initial Phase: use of installed equipment and resources to maintain or restore

nuclear safety functions

most of the existing NPPs are already equipped to manage a total loss

of AC power (external grids + diesel generators) for few hours (typically

4-6 hours) - Station-Blackout scenario

disconnect not-vital electrical loads to extend power sources duration

Phase 2-Transition Phase: provide sufficient, portable, onsite equipment and consumables to

maintain or restore the nuclear safety functions until they can be

accomplished with resources brought off site (for roughly 24 hours)

Phase 3 - Final phase : obtain sufficient offsite resources to sustain those functions

indefinitely

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Portable equipment that provides means of obtaining power and water to maintain or restore key safety functions for all reactors at a site

This could include equipment such as portable pumps, generators, batteries and battery chargers, compressors, hoses, etc.

Elements of the Mitigation Strategy

Reasonable staging and protection of portable equipment and associated transportation routes

The equipment used for mitigation would be staged and reasonably protected from applicable site-specific severe external events to provide reasonable assurance that N sets of equipment will remain deployable following such an event (N is the number of NPP Units on site).

Portable equipment connections qualified-protected to timely supply power to vital loads and provide water to cool down the nuclear fuel in reactors and spent fuel pools

Procedures and guidance to implement mitigations strategies to be integrated in the Plant Operating Procedures (AOP)

Programmatic controls that assure the continued viability and reliability of the mitigation strategies.

These controls would establish standards for quality, maintenance, testing of FLEX equipment, configuration management and periodic training of personnel..

Personnel Training

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ELECTRICAL FLEXIBLE CONNECTIONS for off-site mobile equipment

DIVERSE AND PROTECTED STAND-BY DIESEL GENERATOR STATIONS Diverse and protected (external events) diesel generator stations to be installed at NPP site

FULLY QUALIFIED/PROTECTED 6KV SWITCHBOARDS Possible cross-links for safety electrical power loads 6 kV from any operable emergency diesel generator

Post-Fukushima – ANN Service Products

“Plug and Socket” to the Electrical Distribution System, at different voltage levels (medium up to 11

kV, low up to 400 V)

• identification of emergency electrical loads; • installation of circuit breakers allowing for fast load transfer, isolation

of not emergency loads and connection of emergency power source; • update of the existing distribution network, to respect all the

duplication and segregation requirements ; • electrical cabinet design and qualification process;

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Thanks for your

attention

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Ansaldo Nucleare at CPExpo 2013: Emergency electrical power supply to nuclear safety systems

Technology

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