Nuclear Power Plant Onsite Electric Systems - Safety Assessment

1. Defense in Depth of
Electrical Systems and
Electrical Grid Interactions
Preliminary Findings and Recommendations
to OECD Nuclear Energy Agency
Committee for Safety of Nuclear Installations
December 2008
Dr. John H. Bickel, Chairman
DIDELSYS Working Group
jhbickel@esrt-llc.com
jhbickel@esrt-

2. This presentation summarizes preliminary
DIDELSYS findings and recommendations
OECD Nuclear Energy Agency created DIDELSYS working
group in January 2008
Purpose: evaluate current defense in depth of nuclear power
plant electrical systems to cope with plant and grid caused
transients
DIDELSYS working group consists of:
John H. Bickel, ESRT, LLC (Sweden) - Chairman
Alejandro Huerta, OECD/NEA – Secretary
Per Bystedt, SSM (Sweden)
Tage Eriksson, SSM (Sweden)
Andre Vandewalle, Nuclear Safety Support Services (Belgium)
Franz Altkind, HSK (Switzerland)
Thomas Koshy, USNRC (United States)
David M. Ward, Magnox Electric Co. (United Kingdom)
Brigitte Soubies, IRSN (France)
Kim Walhstrom, STUK, (Finland)
Alexander Duchac, EC Joint Research Centre Petten (European Commission)
Commission)
Robert Grinzinger, GRS (Germany)
Ken Kawaguchi, JNES (Japan)

3. Simplified NPP Electrical System
Internationally Recognized
design standards:
IAEA NS-G-1.8
IEEE Std. 308
KTA-3701
IEC
Common to all standards is:
Concept of interfacing safety
related control and motive
power to non-safety AC
sources via Aux. and
Startup Transformers.

4. All countries recognize need for
lightning surge protection
Unprotected:
Lightning surge can pass
back through Aux.
Transformers to onsite
equipment
Design standards assure:
Lightning surge diverted
to ground
Ground is not “floating”
Example standards:
IEC-60071-1
KTA-2206
IEEE Std. C62.23

5. Other Power/Voltage Surges
Require Protection
Standards recognize: Overvoltage vs. Duration spectrum and
have inherent assumptions about “Withstand Voltage” capability
Circuit breakers can’t address lightning surges because they
operate too slowly
“Surge arrestors” can divert short duration Overvoltage
Problem is sizing for different areas of concern

6. Experience indicates gap in protection
NPP electrical systems
nominally designed for
operation with +/-10%
Voltage
Voltage above 120% but
below lightning protection
features operate is a
“terra incognita”
2006 Forsmark-1 and
2008 Olkiluoto-1 events
indicate:
Previously assumed
“Withstand Voltage” may
be as low as: ~130%

7. Preliminary identification of events
causing voltage surges in this range:
Courtesy of the German Reactor Operators Association, VGB

8. Surge Stress Tests per IEC 60071-1

9. Surge Arrestors are
Very Complex Nonlinear Devices
Surge arrestors have highly non-linear Voltage-Current
relationships
Surge arrestors also have significant dynamic characteristics
as well

10. Role of Testing and Simulation Codes
Testing has been performed for lightning arrestors, medium,
and lower voltage surge arrestors
Above photos from actual destructive tests by Siemens
Key issue:
What is design basis voltage surge for qualification tests ?

11. Role of Testing and Simulation Codes
Simulation tools exist for modeling 3-phase in plant response
to symmetric, asymmetric faults, generator excitation failures
MATLAB/Simulink “Power System Toolbox” has been used
by Helsinki Univ. of Technology (Preliminary 2007 results)
To use in supporting single failure analyses Simulink models
would require V&V commensurate with T-H analysis codes

12. Role of Testing and Simulation Codes
Dynamic simulation of effect of local surges input to local
equipment can also be done using SPICE models
SPICE models are well established technology for
telecommunications industry
Similar to MATLAB/Simulink, SPICE models would require
significant V&V for use in NPP single failure analyses

13. DRAFT Report on DIDELSYS Project
Issued in DRAFT working group report for comment in
December 2008
External Peer Review comments sought from EdF,
Vattenfall AB
Intent is to solicit further Peer Review comments from
IEEE, IEC
Convene technical information exchange meeting at
OECD in May 2009
Focus of future safety improvements will be
responsibility of industry standards working groups and
national regulators.

14. To improve Defense in Depth:
Combined actions are required to improve Robustness at each
Defense in Depth level, including:
Preventing electrical grid - NPP interaction challenges to NPP
electrical power systems (Preventing Grid Challenges)
Improving Robustness of NPP electrical power systems to
cope with electrical grid, and internal NPP electrical faults
should they occur (Electrical System Coping)
Improving NPP training, procedures, information capabilities to
deal with degraded electrical systems (Procedures)
Improving Coping Capability of NPP to deal with NPP
electrical power system failures (NPP Coping)
Improving capability to recover offsite grid to support NPP
electrical power systems (Electrical System Recovery)

15. Preventing Grid Challenges
Recommendations:
WANO SOER 99-1 and 2004 Addendum offer practical
approaches to reduce electrical grid challenges, including:
Have Binding Agreements for communication, coordination of
planned activities
Jointly planning, coordinating electrical circuit test &
maintenance activities
Grid operators: provide NPPs early warning of on-going grid
problems
NPP operators: provide grid early warning of operational
limitations that might impact NPP power output
Grid procedures must recognize NPP as priority load center
requiring efforts to avoid shedding circuits to NPP

16. Electrical System Coping
Recommendations:
Identify range of possible voltage surge transients between
nominal operation and existing lightning surge protection.
Include consideration of combinations of events, such as:
Large load rejection → attempted runback to house load AND
failure of main generator excitation system
Conduct equipment review to determine current Voltage
Withstand capability for power frequency over-voltage
transients (including: asymmetric cases)
Give special emphasis to recently upgraded solid state
equipment that may have least Voltage Withstand capability
This includes: UPS units, rectifier circuits, chargers, I&C cabinet
power supplies

17. Procedure Recommendations:
WANO SOER 99-1 and 2004 Addendum recommend NPP
to have procedures for addressing :
Degraded voltage
Degraded grid frequency
How well these recommendations have been implemented,
information systems to monitor such events, thoroughness
of procedures – should be evaluated in each country

18. NPP Coping Recommendations :
Recognizing that defense in depth requires improving ability to
cope with losses of “uninterruptible” in-plant electrical buses:
Review RPS and ESFAS logic circuits to identify any
undesirable effects given loss of “uninterruptible” in-plant
electrical buses
Examples would include: generation of ADS signal in BWRs or
AUTO Switchover to Recirculation in PWRs
USNRC (1993) issued Information Notice 93-11 describing
concern and requiring evaluations & modifications for US NPPs
Concern is not unique to US

19. NPP Coping Recommendations :
For BWRs with All-electric Core Cooling:
Evaluate providing a diverse means for promptly
supplying power to core cooling systems
This could include:
Direct diesel driven pump
Dedicated fast starting gas turbine

20. Electrical System Recovery
Recommendations:
WANO SOER 99-1 and 2004 Addendum offer practical
approaches to improve electrical system recovery:
Grid procedures must recognize NPP as priority load
center requiring highest priority for restoration