05000267/LER-1985-024, :on 851107,25 Incorrect Rod Sequence Rod Withdrawal Prohibits Received During Reactor Shutdown Operations.Caused by Incorrect Positioning of Rod Select Switch HS-1221.Switch Placed in Off Position
| ML20138D899 | |
| Person / Time | |
|---|---|
| Site: | Fort Saint Vrain  |
| Issue date: | 12/09/1985 |
| From: | Eggebroten J PUBLIC SERVICE CO. OF COLORADO |
| To: | |
| Shared Package | |
| ML20138D851 | List:
|
| References | |
| LER-85-024, LER-85-24, NUDOCS 8512130321 | |
| Download: ML20138D899 (7) | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv), System Actuation |
| 2671985024R00 - NRC Website | |
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U S. NUCLE AR REGUL A102V COMutSSION N
Form 386 O
APPCOVED OWS NO. 3150 4104
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LICENSEE EVENT REPORT (LER)
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DOCKET NUUSER W FACILITY NAME DI Fort St. Vrain, Unit No. 1 o;5loloIol21617 1 lOFl 0l 7 TITLE (44 Received Incorrect Rod Sequence RWP's During Reactor Shutdown Operations EVENT DATE (Si LER NUM82 (6)
REPORT DATE (7)
OTHER F ActOTIES INVOLVED 187 MONTH DAY YEAR VEAR MONTH DAY YEAR 7 AC4LII Y h AMES DOCALT NUwaERa5)
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On November 7,
- 1985, following several weeks of reactor operation below 8%
power, reactor shutdown operations were initiated.
At 1800 hours0.0208 days <br />0.5 hours <br />0.00298 weeks <br />6.849e-4 months <br />, control room operators began inserting the group 4E control rods. At 2124 hours0.0246 days <br />0.59 hours <br />0.00351 weeks <br />8.08182e-4 months <br />, with the group 4E rods fully inserted, and reactor power at approximately 2%, the Interlock Sequence Switch (ISS) was placed in the startup position. At this time, control room operators began inserting the next control rod group in sequence, group 28. During insertion of the group 2B control rods from 2124 hours0.0246 days <br />0.59 hours <br />0.00351 weeks <br />8.08182e-4 months <br /> to 2230 hours0.0258 days <br />0.619 hours <br />0.00369 weeks <br />8.48515e-4 months <br />, a total of twenty five " Incorrect Rod Sequence" Rod Withdrawal Prohibits (RWP's) were received.
Upon investigaton, it was discovered that control rod select switch, HS-1221, was in the incorrect position.
The switch was placed in the correct position and the problem was resolved. Control rod insertion was performed in correct sequence throughout this event.
The RWP function of the PPS is not classified as an Engineered Safety Feature nor is it considered part of the Reactor Protective System, i.e.,
reactor scram system.
Therefore, spurious / unplanned actuations of the RWP circuitry are not considered to require Nuclear Regulatory Commission notification nor submittal of a
Licensee Event Report per the requirements of 10CFR50.72(b)(2)(ii) and 50.73(a)(2)(iv). However, due to recent concerns expressed by the Senior Resident Inspector and Region IV, unplanned RWP actuations will be reported until this issue can be reviewed further with the Regional and NRR offices.
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BACKGROUND:
The RWP circuitry is part of the overall Plant Protective System (PPS) that monitors various plant parameters and initiates automatic corrective action upon the onset of abnormal or unsafe plant conditions. Actuation of the RWP system automatically terminates withdrawal of all thirty-seven control rod pairs by preventing the rod "OUT" contactors from being energized, but does not prevent energizing any of the thirty-seven red "IN" contactors and hence driving control rods into the core.
There are no automatic corrective actions initiated by the RWP system directed towards reducing reactor power or actuating engineered safety features.
In detail, actuation of the RWP function of the PPS will result upon occurrence of the following:
Neutron Countrate.
Neutron countrate indication from startup channels I or II (FSAR Section 7.3.1) is below a set minimum, 5 cps.
This prevents control rod withdrawal without adequate neutron flux indication.
Rate of Neutron Flux Change.
High rate of neutron flux rise (short reactor period) from startup channels I or II and/or two of the three wide range logarithmic channels III, IV, V; setpoint 1.5 DPM.
Neutron Flux Level.
High neutron flux level from power range channels III, IV, and.V (2 of 3) or power range channels VI, VII, and VIII (2 of 3); setpoint s 120*4 rated power.
Neutron Flux level Interlocks with Interlock Sequence Switch. A Rod Withdrawal Prohibit (RWP) interlock is enabled when:
(1) indicated reactor power is above 5% with the Interlock Sequence Switch (ISS) in the Startup position, or (2) indicated reactor power is less than 5*4 or greater than 30% with the. ISS in the Low Power position, or (3)-indicated reactor power is less than 30% with the ISS in the Power position.
These ensure that the operator does not increase or decrease power beyond these levels without advancing the ISS accordingly.
Rod Control Circuit Load.
Rod control load sentor senses withdrawal of the equivalent of more than one rod; setpoint 3.85 volts.
Rod Withdrawal Sequencing.
ISS in startup position and rod withdrawal is attempted out of the correct prearranged group withdrawal sequence.
Power Range Downscale Failure.
Downscale failure of a power range channel III, IV, V, VI, VII, and VIII.
The rod withdrawal prohibit may be manually bypassed by placing the failed channel in a tripped condition.
Rod Control Circuit.
Momentary restoration of power to the rod drive motor control centers via the bypass contactors, automatically initiates a rod withdrawal prohibit if manual rod insertion is attempted following a reactor scram.
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EVENT DESCRIPTION
On November 7, 1985, with the reactor operating at approximately 6.2% power, control rod insertion was commenced to shut the reactor down. Core cooling at the time was provided by the "A" and "C" helium circulators operating at 3200 RPM via their steam drives, and both loop Economizer-Evaporator-Superheater sections operating on feedwater. The PCRV was pressurized to approximately 350 PSIA and core _ fuel temperatures were approximately 310 F.
At 1800 hours0.0208 days <br />0.5 hours <br />0.00298 weeks <br />6.849e-4 months <br />, with the ISS in " Low Power", control room operators began reducing reactor power by inserting the control rods of rod group 4E.
While inserting these rods, a RWP on " Incorrect ISS Position" was automatically actuated at approximately 5% power, as designed and expected. With this RWP function actuated, control room operators continued to insert the group 4E control rods to the fully inserted position. At approximately 2124 hours0.0246 days <br />0.59 hours <br />0.00351 weeks <br />8.08182e-4 months <br />, placed in "startup" (which cleared the " Incorrect ISS Position" the ISS was RWP), and the next group in sequence, 2B, was readied for insertion. However, upon operating the "A",
"B",
or "C"
rod select hand switches, a RWP on
" Incorrect Rod Sequence" was received.
Operators verified correct rod sequence (see Figure 2),
and began to investigate.
Since the cause of the RWP could not be immediately determined, Resolts (Instruments and Controls)
Department personnel were called to the site,-the RWP was reset, and group 2B rod insertion was continued. At 2215 hours0.0256 days <br />0.615 hours <br />0.00366 weeks <br />8.428075e-4 months <br />, v fle inserting rod group 28, the reactor was taken subcritical. At 2230 hou n, all of the group 28 rods were fully inserted.
During group 2B rod insertion utilizing the "A", B", and "C" rod select hand switches, a total of twenty-five RWP's on " Incorrect Rod Sequence" were received.
Actuation of the RWP circuitry occurred simultaneously with cperation of the "A", "B", or "C" rod select hand switches to drive in one of the individual group 28 control rods (see Figure 1). The Results Department investigation, determined that control rod select switch, HS-1221, was in position "0-9".
This hand switch is normally in the "0FF" position during control rod withdrawal or insertion at low reactor power levels. HS-1221 was placed in the "0FF" position and the problem was resolved.
ANALYSIS:
The RWP function of the PPS prevents the "OUT" contactors for all thirty-seven control rods from becoming energized, thereby precluding any control rod withdrawal. This automatic action will occur upon onset of various conditions (see Background section of this report) preventing the furtherance of abnormal or undesirable neutron flux levels inside the reactor core.
By design, the RWP function will not, under any circumstances, prevent any of the thirty-seven control rods from being driven into the core by their individual control rod drive shim motors. This allows control room operators to manually insert any of the thirty-seven control rod pairs and hence reduce reactor power regardless of RWP status.
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- 'N Fort St. Vrain, Unit No. I 01510 l o l o l 2l 617 65 01 2l 4 01 0 01 4 0F 0l7 TEXT is mare spece a w ese esenwie MC # arm JteLa is (171 During this event, control rods were being inserted into the core to take the reactor subcritical from 6.2*,; power. Actuation of the RWP system did not
'e f fect the capability of control' room operators to reduce reactor power or follow the proper rod insertion sequence.
In addition, RWP actuation had no effect on the operability of the PPS scram system, automatic or manual, to function as designed upon the onset of an unsafe condition. Therefore, these RWP actuations did not cause the plant's principle safety barriers to be in a degraded condition.
The RWP system functioned as designed, with no effect.on the operability of the remaining PPS system.
All control rod position instrumentation indicated no abnormal or unexpected control rod movement.
Control room operators confirmed proper control rod insertion sequence, and continued with the reactor shutdown while attempts to identify the cause of the RWP actuations continued.
CAUSE
Incorrect Rod Select Switch Position / Operator Error:
Following ISS placement to the "startcp" position, in conjunction with HS-1221 in position "0-9" and HS-1220 in position "1",
actuation of the RWP system
" Incorrect Rod Sequence" function was initiated by operation of the "A", "B",
or "C" rod select hand switches (see Figure 1). After attempts to identify the cause of these actuations failed, rod insertion was continued.
Hcw centrol rod select switch HS-1221 became incorrectly positioned is not exactly known.
It is suspected that during maintenance work performed earlier that week on rod position indication ZI-1203, HS-1221 was placed in position "0-9" to allow post rnaintenance testing. Upon completion of the work, HS-1221 was left in position "0-9" until it was discovered on November 7, 1985, at approximately 2230 hours0.0258 days <br />0.619 hours <br />0.00369 weeks <br />8.48515e-4 months <br />. HS-1221 is normally left in the "0FF" position and not operated during control rod insertion at low reactor power levels.
CORRECTIVE ACTION
HS-1221 was placed in the "0FF" position and the problem was resolved.
All operations personnel involved in the incident were verbally reprimanded by the Superintendent of Operations.
Operators will receive additional training with the " Incorrect Rod Sequence" function of the RdP system.
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