ML19308B811
| ML19308B811 | |
| Person / Time | |
|---|---|
| Site: | Crane  |
| Issue date: | 08/10/1976 |
| From: | Rusche B NRC COMMISSION (OCM) |
| To: | Gossick B NRC COMMISSION (OCM) |
| Shared Package | |
| ML19308B807 | List: |
| References | |
| TASK-TF, TASK-TMR NUDOCS 8001170342 | |
| Download: ML19308B811 (13) | |
Text
__
.. '* *g Of UNITED STATES
~
-, q NUCLEAR REGULATORY COMMISSION d
Q/
}
WASHIN?,70N, D. C. 20555 I, (f(/
9M Lee V. Gossick, Executive Director for Operations LIGHTilING PROTECTION FOR flUCLEAR POWER STATIONS This is in response to Chairman Rowden's memorandum dated July 9, 1976, requesting a report that addresses a reactor safety concern relating to the effects of atmospheric lightning on nuclear power plants.
The staff formed a study group after several incidents associated with severe cold weather last winter to evaluate the safety significance of all severe weather phenomena.
Lightning is one phenomena that is included in the study.
If the results of this study indicate safety aspects not presently being considered or being inadequately considered, appropriate modification in the staff review process will be made.
We have prepared responses to the four questions that were stated in Chairman Rowden's memorandum as well as additional detailed discussions on related matters which are included in an enclosure.
The questions and answers are:
1.
To what extent does the staff consider the effects of lightning on nuclear power station design?
In general, the staff does not review in detail the measures taken for lightning protection on nuclear power plants.
However, in one case, during the con-struction permit review of the Farley Plant, the staff did review in iame detail lightning protection and its effects on nuclear power station safety. This review indicated that the utility had been using industry codes and standards to provide acceptable lightning.
protection.
2.
Is lightning'a safety issue that should be included in the Standard Format and Standard Review Plans?
The Standard Format does request that applicants provide data on the seasonal and annual frequencies of severe weather phenomena, including lightning, a
for design and operating bases for the plant site.
l
[ These data are reviewed as set forth in the Standard Review Plan Section 2.3.1, " Regional Climatology" as 1 they affect safe design and siting of the plant.
j Furthermore, for power systems classified as Class IE 1
l 8001170.39 2 s
i 7
o h
Lee V. Gossick,
I which are important for safe operation and shutdown of the plant, the acceptance criteria in Standard Review Plan Section 8.3 references Regulatory Guide 1.32 that endorses Institute of Electrical and Electronic Engi-neers (IEEE) Standard 308. The referenced IEEE Standard requires that Class IE power systems shall be designed such that no design basis event including lightning will cause a loss of electric power to engineered safety features, surveillance devices, or protection system devices sufficient to jeopardize the safety of the plant.
As already noted, the staff does not routinely review the detailed implementation of these criteria.
3.
What is the technical basis for the staff positions with respect to the above questions? This technical basis need not be limited to unclassified information.
Based on our understanding of the codes and standards used by the applicants in designing 1.ightning pro-tection, we believe that proper implementation of these standards provides acceptable protection of nuclear power plants. These codes and standards include the National Fire Protection Association (NFPA) Code, the National Electrical Code (NEC), the Institute of Electrical and Electronic Engineers (IEEE) Standards, American National Standards Insti-tute (ANSI) Standards, the National Electrical Man-ufacturers Association (NEMA) Standards, and the Underwriters Laboratories (UL) Rating Standards.
The staff has recently been made awue of classified material relating to the effects of lightning. We will have the staff uxamine carefully ti:e contents of these reports.
We understand that the protection provided for direct or indirect lightning strikes is accomplished for the total piant by designing the system to meet the cri-teria and requirements stated in NFPA Standard No. 78, the NEC and UL No. 96A. The standards require grounded air terminals (lightning rods) that transmit any electrical discharge directly to the plant ground bus system. Systems inside the plant are therefore not affected.
Additional protection is provided the electrical transmission system and the equipment l
W d
r
- y 4
Lee V. Gossick inside the plant by designing systems to the requirements of the 1((L Slathlard W, SLM St#dAVd LA-\\, AX55 GBl.\\,
C-62.2 and C-2.
We believe that by designing the systems to the above stated requirements, the effects of lightning on safety systems would be essentially eliminated. The inherent design of safety systems that operate at low voltages, provides additional separation from the effects of lightning because of the number of breakers and trans-formers that exist between the path of discharge and the safety equipment.
i Although meteorolcgical towers have been struck and rendered inoperable due to lightning a number of times, and these towers may be needed for post-accident monitoring, the probability of the loss of a mete-orological tower coupled with an accident of such severity that monitoring of meteorological conditions may be needed is very remote indeed.
Because of the design requirements routinely followed by applicants in their designs based on codes and standards and because of the inherent separation of the safety systems from the effects of lightning, the staff has not found a technical basis to made the lightning protection systems a part of its normal audit safety review on individual licensing appli-cations.
4.
What has been the actual experience to date with respect to lightning impacts at nuclear power plants?
The staff has surveyed the Abnormal Occurrence Reports, the Licensee Event Reports, and each of the Regional Offices of the Office of Inspection and Enforcement.
Over the past decade, there has not been a single incident wherein a lightning strike resulted in:
a) an accident or transient condition in the plant or degradation of the plant safety systems that uould preclude the safe shutdown of the plant, or b) release of radioactive material to the plant environs exceeding the Technical Specifications limits.
Several occurrences have been reported where lightning strikes on concrete off-gas stacks have caused explosion i
e
!'3 Lee V. Gossick and fire in the off-gas system but the plant safety was not jeopardized in any of these occurrences. Other minor occurrences have been reported where radiation monitors located near the top of the off-gas stacks have been put out of service by lightning, however, other monitors were available to maintain the monitoring function.
Or. the basis of the considerations given to the matter of lightning phenomena and the safe operation of nuclear power plants, we have reached the following conclusions:
1.
Based upon our understanding, it is standard industry practice to protect nuclear power plants against direct strikes, primarily by the use of a lightning conduction system. The reinforcing steel of contain-nent structures is not relied upon for grounding the structure.
2.
Based upon our understanding, it is standard industry practice to protect outdoor electrical components; e.g., main transformers and switchyard equipment with lightning arrestors.
3.
Traveling surges induced by lightr.ing strikes on outdoor equipment do not affect the plant's indoor system; however, particular cases have not been evaluated in detail.
4.
The acceptance criteria given in the Standard Review Plan require conformance to industry standards that require lightning protection of plant safety systems, but the staff review does not audit the detailed implementation of these standards.
5.
Current operating experience reveals that there have not been any incidents wherein a lightning strike has:
a) affected plant safety or caused damage that would i
preclude safe plant shutdown, or b) caused a significant
, release of radioactivity to the plant environs.
Furthe information would be needed to confirm that items 1. and 2. are l
valid for all operating plants. Although we could obtain this confir-mationLin about six months, it is the judgment of the staff that the effectsof the lightning on the safe operations of nuclear power plants are not a significant source of concern; however, a detailed review and
~*
4
,, _. _ _ _ _. ~, _ _. _... _
... _ ~.. _,
,m.,
5
.o Lee V. Gossick ?
data base do not presently exist to translate a finding of reasonable assurance into a finding of a particular numerical probability.
We will consider the results from the study on severe weather phenomena to determine if our reviews of plant protective systems need to be altered.
j,{l(,..
E i,".',: '_.
men C. Rusche, Director '
Office of Nuclear Reactor Regulation
Enclosure:
Detailed Discussion e
e e
e e
W '
W l
r l
'~h
~
1 EllCt.0SURE f
~
t A.
QUESTI0ft:
Direct Strikes One would assume that electrical and other corrponents within a nuclear reactor would be shielded from the effects of a direct strike by the conductive steel reinforcing bar, liners, etc., acting as a faraday cage.
Several considerations make this a less tenable assumption.
Constructors will not guarantee the electrical continuity nor complete grounding of the "rebar."
A strike could travel along a conductive portion of the containment, reach an area of high resistance, the resultant heat generation could produce steam from the water of hydration in the surrounding concrete and either spall or penetrate the containment. This might not be noticed until this engineered safety feature was called upon to fulfill its function, at which point it might fail.
Auxiliary generators and other critical structures are not always within contain-ment and these could be disabled by direct strikes.
The massive ground system required by an electrical generating station, plus a system of lightning rods, would seem to greatly minimize this aspect of concern if we had assurance that the system was designed and constructed on sound principles.
Can one feel confident of this in present plants?
SUMMARY
QUESTION: With respect to direct strikes of electricity eithbr to the containment structure or other critical equipment and structures outside containment, can one feel confident the i
grounding system has been designed and con-structed on sound principles?
RESPONSE
Protection against direct strikes is primarily provided by sound engineering design of a lightning conductor system consisting of rods projectino into the air above the uppermost parts of these structures with interconnecting and grounding conductors.
Each projecting rod above the structure normally has at s
op
2 i
least two conn:cting paths to earth or more f
if practicable. The grounding conductor is grounded with as low a resistance as practicable to obtain at each of these structures and also connected to the plarff ground bus system, while assuring ample distribution of metallic contacts in the earth.
The total system is normally designed according to standard industrial practice to meet the criteria and requirements stated in Underwriters Laboratories UL #96A on Master Labeled Lightning Protection System, National Fire Protection Association (flFPA) 78 on Lightning Protection Code, and National Electric Code (NEC).
For containment structures, the electrical continuity of the reinforcing steel is not relied upon for grounding the lightning discharge. These structures are normally protected by grounded air terminals in accordance with the above referred UL #96A as Master Labeled Lightning Protection System and NFPA Number 78. This protection virtually eliminates the possibility of depositing large amounts of energy in the containment wall. Furthermore, periodic leak testing andinspectionofthecontainment(AppendixJ) should uncover any violation or damage to' the containment structure.
Items of outdoor electrical equipment, like the main transformer of the generator and the switchyard equipment, are provided with lightning arrestors to protect them from'the effects of lightning. The insulation of this outdoor equipment'has insulatior strength -
in excess of the voltage level tnat can be protected by these lightning arrestors.
These outdoor lightning protection devices are designed to eliminate any traveling surges that.might be generated in the outdoor power circuits from entering into the plant systems.
The main generator, because of its low impulse strength desigr1 and direct connection to the outdoor equipment, is usually provided with surge arrestors.
In addition, the medium and high voltage motors are sometimes installed with surge protectors primarily against switching surges induced locally within plant electrical systems. The application of the above lightning arrestors are governed by AflSI standards ANSI C-62.1 and ANSI C-62.2, National Electrical Manufacturers Association (NEMA)StandardsLA-1.
m.m.m w
--s
^N
~
n t
3 i
l l
The grounding of the plant ground-bus system, l
the lightning arrestors, and other equipment and structures are designed to IEEE Std 142-1972, I
"IEEE Recommended Practice for Grounding of t
Industrial and Commercial Power Systems" and National Electrical Safety Code ANSI C2.
In l
addition, NELPIA requires that Nuclear Power Plants be adequately protected against lightning in accor-dance with established industry standards.
In summary, based upon our understanding and current information, we are confident that the grounding system has been designed on sound principles ar.d is adequate to protect the plant against the effects of a direct strike.
l B.
QUESTION:
Indirect Strikes Strikes on transmission lines, auxiliary generating facilities, etc., could cause large surges to enter the plant, possibly damaging critical equipment i
or electrical circuitry. All electrical generating plants have interruptors installed to prevent damage from just such an event. Unfortunately, 2
these mechanisms are installed to protect the generators, transformers and other large pieces of expensive gear from' damage from matsive charges. They are not set sensitively enough to protect the type of electronic equipment now being used in engineered. safeguard controls and j
instrumentation.
In addition, " reaction time" of the interruption is much too slow to prevent d
shor't' duration pulses from entering the syst'em. The above insensitive settings are understandable as the operators of the plant do not wish it to be.
shut down ~ unnecessarily or frequently.
Has.a thorough analysis of the effects of surges on the control instruments of a nuclear plant been-conducted?
SUMMARY
QUESTION: With. respect to indirect strikes of electricity ~
to plant equipment; e.g., transmission lines and' auxiliary generating facilities, has a thorough analysis of the effects of surges on the control instruments of a nuclear plant been conducted?
l
RESPONSE
A literature search of over 150 papers published id!
on lightning protection for switchyards, sub-stations and transmission systems in the last five decades does not indicate that traveling surges induced by lightning strikes on outdoor equipment affect a plant's indoor syttems. This is due to the lightning protection provided for the over-head power lines terminating at the plant and the surge arrestors installed at the outdoor electrical
~-
,oe
.J
c 4
L i
equipment limiting the surge voltages reaching the plant's indoor systems, if any, to a value r
well below the impulse strength of the plant system apparatus.
The incoming lines to nuclear plants are nonnally shielded by overhead ground wires. This feature reduces the possibility of direct strikes to the line in the vicinity of the plant and thus -
limits both the magnitude and rate of rise of any voltage surges which do reach the station. This ground wire is in turn grounded through a least resistance path at each structure and is also connected to the ground bus system at the plant.
In summary, the plant is designed and protected against indirect lightning strikes which could produce surges affecting the plant's control instruments. Accordingly, an analysis of the effects of surges is not performed.
C.
QUESTION:
Magnetic Fields Electronic circuitry, particularly integrated circuitry, is susceptible to magnetic influence.
Lightning strikes produce significant pertur-bations in the earth's magnetic field. Many modern designs of engineered safeguard systems are using integrated circuits.
The effc :ts of lightning strikes near a nuclear power plant on these circuits is unknown but probably_signi-ficant.
SUMMARY
QUESTION:
Have the perturbations from lightning to the earth's magnetic field been considered in the design of integrated circuitry?.
RESPONSE
Integrated electronic circuitries/ components are adequately shielded from any external magnetic influences by their housing in totally enclosed metallic cabinets which are in turn connected to the plant grounding system. Also, low energy level circuits are run within grounded metal l
conduits or raceways to shield these circuits from external magnetic effects.
The above installation criteria preclude the effects of external magnetic fields on integrated circuitry.
D.
QUESTION:
Are the following alleged events possible at nuclear power stations?
If so, please indicate the impact of the event,or, if not,deredplease indicate why the event is not consi possible.
.m
,,ene
m y
t.
1.
A strike could put the emergency generating equipment out of action.
RESPONSE
It is a normal design feature to have the emergency generators in a separate building abutting the reactor auxiliary building or in separate rooms within the auxiliary building. This ensures the structural continuity between the two buildings while ensuring a good, continuous grounding of the equipment and the structures in the diesel building.
In rare instances, the diesel generators are housed in a separate building from the auxiliary building without sacrificing any of the above design features.
Because of the lower height of the diesel building and its proximity to the reactor auxiliary building, though separate lightning protection is not essential, it is normal to provide a lightning rod system for the diesel building also to avert any direct strikes affect'ing the diesel building and its equipment.
All the associated emergency power system buses, breakers, and feeders are located indoors within the reactor auxiliary building. The diesel generator breakers will normally be in open condition thus isolating the diesel units and their associated cabling from the plant's power system. The feeders from the diesel generators to the switchgear in the reactor auxiliary building are installed in underground duct banks, thereby eliminating the exposure of these cables to direct lightning strikes. The above design features give reasonable assurance that lightning strikes will not adversely affect the emergency generating equipment.
2.
Control instrumentation could be damaged so that reactivity is added at the same time that "ccram" systems are disabled.
RESPONSE
The safety-related instrumentation and controls are operated at 120V and below, with a majority of the instrumentation circuits ~ ! ling well below this voltage level. These circuits are isolated from any external traveling surges by
' a number of " discontinuities" in the form of transformers. Also these circuits are run within grounded metal conduits or raceways. Most of the low energy level circuits are also adequately shielded from any external magnetic influence and voltage spikes.
J L
[
'[ )
l m
With a g ed plant grounding system and adequate external lightning protection, the critical i
instrumentation and control systems required for l
safety in the current operating power plants have not experienced any adverse effects due to any, i
surge phenomena occurring outside the plant.
1 With the adoption of sound engineering practice as referenced above, utilization of quality co.mponents and the implementation of periodic i
inspection and maintenance of the plant's lightning protection and grounding systems, as required by the industry codes and practices, the nuclear power plant can be operated safely..
We are reasonably confident that such practice i
has been employed in the design of nuclear power plants since these are aspects that can be readily reviewed and inspected.
3.
If a lightning-induced power surge could cause a turbine trip with simultaneous loss of auxiliary power, we might be well on our way toward an event which could lead to a reacitvity surge resulting in steam generator tube failure with resultant release of radioactivity.
RESPONSE
A loss of auxiliary power in a pWR is analyzed by all applicants as required by the Standard Format. Under such circumstances, a typical PWR would experience a simultaneous loss of load, feedwater flow, forced reactor coolant flow, and reactor trip, without a resultant surge.
The loss of station AC power' would be followed by automatic startup of standby emergency diesels which would supply electrical power to all essential engineered safety systems. Pressure protection and decay heat removal is provided by primary coolant natural circulation, primary and secondary safety valves, auxiliary feedwater flow, and the decay heat removal system.
A mechanism for failure of a steam generator tube is not apparent from the foregoing scenario; however, the concern is not consequential to the public health and safety. The staff does, on a routine basis, analyze the consequences of i
a postulated rupture of a steam generator tube to establish allowable coolant activity limits for the Technical Specifications.
l 1
w, w
.. ~
?
I l
4.
If surges or magnetic field fluctuation disable the overload or overspeed controls on the turbine, a runaway turbine condition could develop.
RESPONSE
Modern turbine generators are provided with an-Electrohydraulic Speed Control (EHC) system comprising a normal overspeed protection system and emergency overspeed protection system.
If an abnormal surge or a magnetic field fluctuation occurs and disables the turbine speed control the following consequences may result:
(i) The normal overspeed protective system is an electrical system using two speed signals (pulses) generated by magnetic pickups.
This system cannot be relied upon due to loss of the electrical signal. However, modern turbines are provided with a fail safe design that upon loss of electrical power the turbine control valve and re-heater intercept valves are closed to prevent turbine overspeed.
(ii) The emergency overspeed protective system is a completely mechanical system (hydraulic) independent of electrical power. This system is set at a slightly higher speed than the normal speed control system and upon actuation it can be relied upon (by depressurizing the hydraulic sjstem pressure) to fast close the main stop valve and reheater stop valve.
(iii) The emergency cverspeed protective system also includes a backup overspeed control (electrical / mechanical) system set slightly higher than the mechanical overspeed control system.
On loss of electrical power this
, system may not be relied upon to function.
However, modern turbines are provided with a fail safe design that upon loss of electrical power or loss of any component, the turbine will automatically trip by depressurization of the hydraulic system which will fast close the inlet stop valves, control valves,. reheater intercept valves, reheater stop valves, and extraction steam valves.
Therefore, in the event of an abnormal electrical surge, a turbine runaway condition would not occur through the action of the mechanical overspeed control system and the inherent fail safe design of the EHC turbine speed control system.
[
p 8
i i
5.
Surges, etc., could cause instrumentation to falsely represent operating conditions causing plant operators to take inappropriate or dangerous actions.
RESP 0flSE:
This matter has been discussed in response to -
Question D.2 above.
6.
Alarms or automatic scram controls might be disabled in a manner which leaves the operator unaware of impending problems.
Standard Review Plon criteria include conformance to Institute of Electrical and Electronic Engineers (IEEE)Std308(alsoendorsedby Regulatory Guide T.32) which requires that 1
Class lE power systems shall be designed such that no design basis event including lightning will cause a loss of electric power to engineered i
safety features, surveillance devices, or protection system devices sufficient to jeopardize the safety of the plant.
Most of the safety systems and their components are not normally in an operating state, but they i
are kept in a ready state to go into operation should an event occur in the plant requiring their functions to mitigate the consequences of that event and for safe shutdown of the plant.
The voltage level at which the safety systems and their components are operated is 416C volts and below. These systems are shielded by
" discontinuities" such as an open circuit breaker, or a transformer from the other higher voltage level systems and components that are directly connected to the outdoor power system which are exposed to lightning strikes. As pointed out in the earlier paragraphs, all the~ outdoor equipment are adequately protected against lightning by suitable protection devices, and thus any perturbation induced at the outdoor system does not travel into the plant to a level in the system at which the safety system components are connected.
Based on the protective devices that are normally
, ;1 provided, the redundancy in instrumentation for o
monitoring safety systems, alarms and automatic scram co7trols are not expected to be disabled from a 1ightning strike in a way that would have the operator unaware of existing or impending problem!.
\\
k
,. - -.