ML20032D681
| ML20032D681 | |
| Person / Time | |
|---|---|
| Site: | Crystal River  |
| Issue date: | 04/29/1981 |
| From: | Joseph Kelly, David Williams BABCOCK & WILCOX CO. |
| To: | |
| Shared Package | |
| ML20032D660 | List: |
| References | |
| BR-1182, NUDOCS 8111170384 | |
| Download: ML20032D681 (8) | |
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Abnormal transient operating procedures for nuclear power plants J. J. Kelly, Jr., Supervisory Engineer Engineering Department Nuclear Power Generation Division Babcock & Wilcox Company Lynchburg, Virginia D. H. Williams Special Products Coordinator Arkansas Power & Light Little Rock, Arkansas Presented to American Power Conference Chicago, Illinois April 27-29,1981
/
The aims, objectives and methodology involved in symptoms and immediate actions. If a loss of producing abncrmal transient operating procedures feedwater occurs, he is expected to recognize it, for nuclear power plants were discussed in detail at perform the appropriate immediate actions, and the American Power Conference in April,1980.*
then use the event-oriented loss-of-feedwater Abnormnl Transient Operating Guidelines now procedure for determining follow-up actions. This exist in draft form for Arkansas Power and Light, approach has several inherent drawbacks:
and it is possible to detail a truly symptom-oriented
- 1. At time zero, the operator must correctly approach to transient management. Using this diagnose the initiating event. Ile does this approach, the operator does not have t mentally, based on training or prior experience.
immediately diagnose the iriitiating casualty and After taking several actions, depending c,n this locate the event-specific procedure for that casualty.
instant evaluation, be then refers to the event-Instead, the operator can pick up and use one.
oriented procedure that fits his diagnosis. If he simple procedure for all transients startmg with a were to treat a small steam line break inside the reactor trip. This paper describes the approach and reactor building, but actually had a small loss of provides several examples of the simplified decision coolant accident (LOCA) inside the building, he making process now availaole to the operator. Als would be tracking through the wrong procedure.
discussed is one possible approach to implementing He would eventually recognize this these new guidelines into the existing procedure misinterpretation; however, by then he would be structure.
well into the transient and possibly confused.
- 2. Procedures must be written to cover every
Background
conceivable initiating event. If the operator
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The traditional approach to transient and accident corrcctly diagnoses a loss of nonnuclear control has been to develop many " emergency" instrumentation power and no procedure covers procedures, each based on a postulated event such that event, his actions will be based only on as loss of main feedwater. The operator is then experience.
required to study this event and memorize its
- 3. If more than one event contributes to the
- " Engineering basis for operator control of nuclear power transient, the operator will find himself working stations in abnormal operations - closing the loop."
two or more procedures at the same time. For E. A. Womack J. J. Kelly. and N. S. Elliott. American Power instance, if a main steam safety valve failed to Conference. Chicago. Illinois. Apnl 2123.1%0 (llabcock rescat following the loss of main feedwater, the
& Wilcox IIIt 1151).
operator would have to use the loss of feedwater 1
. procedure and small-steam-line-break procedure identify a transient. Similarly, some parameters (if available). These procedures may conflict and were common to all transients. One event found he would have to decide a priority between them throughout the study was a reactor trip.
- with no convenient method of shifting Consequently, it was used as the key for L-
~n the two procedures. Writing a procedure entering the abnormal guidelines procedure.
, mmbme these two events is possible:
- 2. Event trees for various initiating events
- and however, if just a few more failures are consequential failures were developed. These considered (e.g., the power-operated relief valve included various multiple failures (including or spray valve remains open), the number.of operator error), and therefore covered a large combinations of failures, along with possible number of possible scenarios. Event trees were initiating events, quickly increases. Even if ctudied to find repetitive patterns and common writing the appropriate procedures was end points. The study showed that although attempted, the operator's ability to pick the many failures can occur, the symptoms of correct procedure would certainly diminish.
unbalanced heat transfer that result from these
- 4. Because of these limitations, most operators are failures followed a few common patterns or likely to use no specific procedure. They will use trends.
training, experience, intuition, etc., to bring the
- 3. Actual operating transients were investigated, plant under control. They will then choose what again looking for patterns. This ti ne the they think is the closest procadure to what is emphasis was on parameter trends and the time happening and confirm their actions or see if available for operator action.
they overlooked anything.
- 4. Where necessary, computer simulations were run t complete the baseline and fill in gaps in To correct these deficiencies, u is necessary to understandm, g plant response. Because the step back from the traditional approach and
" E" was mtended for use in developmg examine what the operator is attempting to do Peratm.g guicehnes, realistic mput was used (as during reactor posttrip abnormal transient control.
PPosed to boundmg safety analysis He can best protect the health and safety of the assumptions).
public by guarding the integrity of tha core. To do this he must ensure the continuous removal of This investigation's conclusion was that the decay heat from the fission products to the ultimate operator can track the removal of decay heat from heat sink. By adjusting the priorities and the core to the ultimate heat sink by monitoring concentrating efforts on maintaining proper heat just a few symptoms which reflect the " health" of transfer along this path, he can protect the core and the thermodynamic process around the reactor minimize radioactive release. To give the ope ~itor coolant system and its coupling to the secondary this capability, a concept of symptom-oriented h side.
opposed to event-oriented) procedures was investigated. The symptoms are based on upsets in Symptoms identified heat transfer from the core to the coolant and from the coolant to the steam generators. The symptom.
The three symptoms of primary interest to the oriented procedures thus focus on core cooling first Pressurized water reactor (PWR) operator are and on event identification second. The result of adequate subcooling of the primary system this investigation is the Abnormal Transient inventory, inadequate primary-to-secondary heat Operating Guidelines (ATOG).
transfer, and excessive primary to-secondary heat transfer. These symptoms are important for the Expected plant response following reasons:
To produce a symptom-oriented procedure, B&W
- 1. Adequate primary inventory subcooling. If the developed a thorough understanding of expected cperator knows the primary fluid is in a liquid plant responses during many varied abnormal state, he is aesured that it is available and transients. These transients included classic capable of removing heat from the core and singular initiating eventu as well as additional transferring it to the steam generators. If single and multiple componeat failures. The subcooling is lost, these issues are in doubt, and procedure was developed through the following he is therefore directed to make every effort to steps (usually in parallel):
regain subcooling.
- 1. Existing phnt casualty procedures were
, included as initiating events were loss of main feedwater, icss investigated for common symptoms. Few single of off ite power. excessive main fnedwater, small steam line alarms or parameters were found to uniquely break. and steam generator tube rupture.
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2., Inadequate primary-to-secondary heat transfer.
j This symptom addresses the heat transfer E 2400 Post Trip
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4 flow of energy moving from the reactor coolant subcooled system to the ultimate heat sink. The operator E
Region monitors the relationship between the reactor 8 1600 J
coolant cold leg temperature and steam h
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- 1200 Limit Following a reactor trip, these two values should
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be nearly equal under good heat transfer N 800 conditions. If this coupling is broken, the y
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subcooled Margin to restore it, p 400 Line
- 3. Excessive primary-to-secondary heat transfer. In a
4do 500 550 600 650 400 this case, the symptom is mdicative of a Reactor coolant and steam outlet teniperature. F secondary side malfunction (e.g., loss of steam pressure control or steam generator overfill). The
, End Point - Post Trip with Forced Circulation (Tm and T )
p heat transfer is again unbalanced and the 4
and for Natural Circulation (T )
operator's attention is directed toward generic
' Normal Operating Point. Power Operation (Tw) actions to restore this balance.
By tracking these basic symptoms the operator
] End Point. Post Trip with Natu al Circulation (T%)
can quickly focus on problems without checking a Figure 1 Basic pressure-temperature (P.T) display large number of parameters. At the same time, by their nature the symptoms allow rapid elimination is also input. The saturation ten.peret.:re for this of problem sources and continue to emphasize core input pressure is displayed as a vertical line. The protection. Additionally, the symptoms are so basic subcooled margin line accounts for potential that the procedure inherently covers many more instrumentation inaccuracies with the objective of initiating events than those first etudied. This assuring subcooling above that line.
happens because imtiating events cause equipment A typical plant response to a reactor trip is to fail, and equipment failures affect these shown in Figure 2. For simplicity, only reactor symptoms. As the operator follows the procedure to
.y treat the symptom he will probably identify and E 2400 correct the cause.
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the symptoms of interest. The solution developed in 8 400 the ATOG is shown in Figure 1, which is basically 2*
400 450 500 550 600 650 a pressure temperature (P-T) display with a saturation curve included. The area above and to Reactor coolant and steam outlet temperature, F the left of this curve is the subcooled region. The Figure 2 Typical posttrip response area below and to the right is the superheated coolant hot leg temperature is plotted. With the region. Reactor coolant system hot leg temperature reactor coolant pumps running (forced circulation)
(T ot) and cold leg temperature (Teold) are input to and the comparatively small amount of energy h
this display and plotted as functions of reactor being added ta the coolant by decay heat, the cold coolant system pressure. Steam generator pressure leg temperature is also expected to settle out close 3
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the.iT across the steam generator tubes is small, remain within a "posttrip window " tt< plant is both of these temperatures should approach the responding norrr. ally.
saturation temperature of the secondary side of the With this type of display, the symptoms of steam generator (SG Tut). The Figure also shows interest are highlighted and brought into focus for steam pressure moving from its pretrip value up to the operator. Consider the example in Figure 3.
the steam safety valve setpoint and back to its Combinations of these symptoms are also easily y 2400 y 2400 1
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n 400 450 500 550 600 650 400 450 500 550 600 650 Reactor coolant and steam outlet temperature, F Reactor coolant and steam outlet temperature, F Figure 3A Inadequate subcooling margin:
Figure 3C Excessive primary-to-secondary heat transfer:
Tnot is not progressing toward its target value; in SG Tsat has decreased below its established limit, 2
fact, it has rapidly dropped through the subcooled That and Tcosa have reached equal values but both margin line. This condition is diagnosed as loss of have gone out of the positrip winoow following SG adequate primary inventory subcooling, or simply Tsat. This condition is diagnosed and treated as
" inadequate subcooling margin." and the excessive primary-to-secondary heat transfer.
r procedure is written with directions to take care of inadequate subcooling margin.
recognized. Consider the example in Figure 4, taken from the first twenty minutes of the TMI-2
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As shown by these examples, the symptoms of
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the entire event. This record allows initial diagnosis, positive feedback on corrective actions, 5
and early detection of subsequent malfunctions.
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,gg Reactor coolant a.id steam outlet temperature. F of parametric change to mdicate an overall plant Figure 38 Loss of primary-to-secondary heat transfer:
Tnot is increasing as SG Tsat is decreasing. A.1T between the two is grow ng larger. The secondary is ATOG organization no longer removing heat and has lost coupling with Once the symptoms are identified and a method of the primary. This t.onditson is diagnosed and treated monitoring those symptoms developed, the next as loss of Onadequate) pnmary.to-secondary heat transfer.
step is to reduce this mformation mto somethm.g 4
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N E 400 Subc led Margin g 400 Une 400 450 500 550 600 650 400 450 500 550 600 650 Reactor coolant and steam outiet temperature. F Reactor coolant and steam outlet temperature, F Figure 4A 0 to 5 minutes:
Figure 4C 8 to 15 minutes:
At time zero the reactor has tripped o.. loss of Steam pressure and temperature have recovered to feedwater. At % minute Tnot and Teoio are essentially their normal posttrip values. A substantial cooling the same temperature. At 2% minutes the ESFAS of the primary is also in progress.
pressure setpoint is reached and high pressure injection (HPI) is automatically started. At 3%
minutes subcooling rnargin is lost, and at 4%
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u aturation Primary-to-secondary heat transfer (coupling) is j
400 now almost completely restored. T and Tem are approaching their normal posttrip values. However.
the inadequate subcooling margin is evident.
400 450 500 550 600 650 Reactor coolant and steam outlet temperature. F design base 5 lor, and the use of, the procedures.
Figure 4B 5 to 8 minutes:
Figure 5 outlines the organization of Part I. The The primary continues to heat up along the immediate actions are common to every reactor trip saturation line while secondary temperature and and must be performed regardless of the cause. The pressure drop. At 8 minutes the primary-to' vital system status verification is a short checklist secondary aT -80 degrees. Also, auxiliary used to determine a baseline for possible operator
'eedwater is first directed to the steam generators.
actions. This checklist considers m.strumentation useful to the operator. The Abnormal Transient power supplies, engineered safety features Operating Guidelines consist of two parts. The first activation system (ESFAS) status, steam line break part is procedural guidance to be used in the protection system status, etc. Includcd in this control room during transients. The second part, a checklist is a requirement to monitor the ATOG much larger volume, is a training aid explaining the display. If everything is normal, the plant has 5
intent as to why various steps are taken in Part I.
Section i It also describes, using many graphic examples, the immediate Actions expected plant response information gathered during the guideline development stage. Part II has V tai s stem status ver.f. cation been written to aid the operator's training and is important to the guidelines because an mtelligent, Section til capable operator is a basic part of the plant A. Treatment of lack of adequate subcoohng margin operating structure in which the guidelines are built B Treatment of lack of primary to-secondary heat transfer (i.e., the guidelines try to optimize the operator's C. Treatment of too mucn primary to-secondary heat transfer d
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Cooldown procedures
. Large LOCA Guideline validation
,' $,'y$a ed RCS Once written, the potential guidelines were tested
. HPl cooling on a PWR simulator by imposing multiple
. Sobd water cootoown casualties and using the guidelines to recover.
Guideline credibility was also established by back Figure 5 ATOG - organization of Part I checking the guidelines against event tree paths and benchmarking event tree paths and computer responded as designed and come to a steady pcst-simulations against actual plant transients. The trip condition. No further action is required.
event trees were also reviewed by the utility However, if the operator diagnoses an imbalance in operators to take advantage of their plant one of the basic symptoms, he is directed to the experience. The draft guidelines were sent to the appropriate section for follow-up actions. These plant sim for walk-through drills to test their sections treat the symptoms and do not require the applicability. Feedback from the eperator to the operator to determine the cause. It is expected, plant designer served to greatly reduce however, that as he treats the symptoms he will communication errors and increase confidence in the find the original problem.
final guidelines.
Treating the symptoms will allow returning the An important final step in validation involves plant to a stable condition. This stable condition implementing the guidelines into the plant could very well be abnormal compared to what the procedures system. This implementation tests the operator normally sees. Accordingly, various guidelines' scope and appropriateness since they cooldown procedures are provided to give him must be a workable part of the overall plant guidance on long-term recovery from these possible procedures system or their worth diminishes.
conditions.
Existing posttrip procedures must be checked Figure 6 outlines the organization of Part II.
against the guidelines to determine the following:
Intended to give the operator a thorough understanding of Part I, it conveys the writer's
- 1. Necessary actions outside the development program scope but needed for a posttrip volume i procedure in the same time frame. This assures Fundamentals of reactor control for abnormal transients that, although everything may not be A. Heat transfer considered, the adoption of ATOG does not B. Use of P.T diagram decrease in any area the adequacy of procedures C. Abnormal transient diagnosis and mitigation g
g gg D. Backup cooling methods E. Best methods of equipment operation previous procedures may be found good but not F. Stabihty determination necessary, and either be deleted or relegated to a lower level of instruction. The goal is to vowme 2 maximize simplicity.
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- 2. Actions that should be included in an instruction A E cess ve fe er
- 8. Loss of feedwater for longer term action. Current posttr:p C. Steam generator tube rupture procedures include many necessary follow up D Loss of off site power actions that are not appropriate for ATOG, but E. Small steam kne break must be included somewhere. Three actions, F. LOCA identification of these items, determination of y skuM h ghn, and
- nw Figure 6 ATOG - organization of Part ll optimtzation of the mterface between the form m, 6
which they are given and the ATOG, are e
mmay necessary to make ATOG a workable part of the overall plant procedure system. Again, the goal By using the Abnormal Transient Operating is to maximize simplicity.
Guidelines, the operator can enhance plant safety
- 3. Any posttrip procedures not accommodated by by monitoring reactor posttrip parameters for only ATOG, but which must remain intact. One goal a small number of symptoms and taking corrective of ATOG is to eliminate these procedures, but action as directed by the procedure. The guidelines that goal has not yet been proven consistently allow him to use one simple procedure for all attainable. Any such procedures identified must transients which start with a reactor trip. The be entered in a manner compatible with ATOG unique feature of this approach is that it provides a implementation.
common starting point, independent of initiating e an 8 8 e perator tkagh a s@y-Although plant procedures vary from plant to step procedure to regain stable plant conditions plant, preliminary work indicates that portions of wi w ng fy ei her phe cause of the all of the procedures, such as the following, may be transient or any additional posttrip malfunctions.
replaced by the ATOG:
- Reactor-turbine trip
- Degraded electrical power
- Loss of coolant /RC pressure a Steam supply system rupture
- Loss of steam generator feedwater
- Steam generator tube rupture
- Loss of reactor cooling flow - RCP trip i
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