05000267/LER-1985-025, :on 851108,plant Protective Sys Rod Withdrawal Prohibit Function Actuated.Caused by Spurious Noise Spikes on Startup Channel I.Startup Channel Cable Routing Sys & High Rate of Neutron Flux Trip Setting Evaluated
| ML20138E087 | |
| Person / Time | |
|---|---|
| Site: | Fort Saint Vrain  |
| Issue date: | 12/08/1985 |
| From: | Eggebroten J PUBLIC SERVICE CO. OF COLORADO |
| To: | |
| Shared Package | |
| ML20138D851 | List:
|
| References | |
| LER-85-025, LER-85-25, NUDOCS 8512130332 | |
| Download: ML20138E087 (6) | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv), System Actuation |
| 2671985025R00 - NRC Website | |
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While performing a reactor training start on November 8, 1985, the Plant Protective System (PPS) Rod Withdrawal Prohibit (RWP) function was actuated on two occasions by spurious noise spikes on startup channel I.
Both incidents occurred prior to achieving reactor criticality.
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).
110weve r, due to recent concerns expressed by the Senior Resident Inspector and Region IV, RWP actuations will be reported until this issue can be reviewed further with Regional and NRR offices.
Similiar incidents were reported in LER #85-019.
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BACKGROUND:
The RWP circuitry is part of the overall Plant Protective System (PPS) that monitors various plant parameters and automatically initiates corrective action upon the onset of abnormal or unsafe plant conditions. Actuation of the RWP system automatically terminates any withdrawal of all thirty-seven control rod pairs.
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 circuitry will result upon occurrence of the following:
Neutron Countrate.
Neutron ccuntrate 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% rated power.
Neutron Flux Level Interlocks with Interlock Sequence Switch. A Rod Withdrawal Prohibit (RTP) interlock is enabled when:
(1) indicated
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reactor power is above 5% with the Interlock Sequence Switch (ISS) in the Startup position, or (2) indicated reactor power is less than 5% 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 power levels without advancing the ISS accordingly.
Rod Control Circuit load.
Rod control load sensor 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.
Pcwer 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 atte pted following a reactor scram, we. c.M 2...
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EVENT DESCRIPTION
reactor training start on November 8, 1985, the RWP function of the During a PPS was actuated on two occasions by spurious noise spikes on startup channel I.
These actuations occurred at 0359 and 0405 hours0.00469 days <br />0.113 hours <br />6.696429e-4 weeks <br />1.541025e-4 months <br />, prior to achieving reactor criticality.
In both instances, the spurious actuations were on "High Rate of Rise" (setpoint 1.5 DPM).
At the time of these RWP actuations, the reactor was pressur.ized to approximately 300 psia with "A" and "D" helium circulators operating on their steam drives, both loop Economizer-Evaporator-Superheater sections operating on feedwater and both loop reheater sections operating on steam. Average core fuel temperatures were approximately 110 F During reactor startup, control rods are individually withdrawn in a preplanned sequence to bring the reactor critical.
Rod withdrawal is controlled by hand switches in the control room on panel I-03.
RWP actuation is alarmed in the main control room by both an audible alarm and a visual alarm window located on I-03.
Following RWP actuation, control room operators verified that core neutron flux levels had not changed utilizing the available nuclear instrumentation.
If RWP actuation is caused by an actual increase in core neutron flux levels, improper ISS switch positioning, or actual withdrawal of more than one control rod pair, corrective action is performed in accordance with plant procedures.
If the actuation is determined to have resulted from erroneous noise spikes, as was the case, the RWP circuitry is reset as required, and startup operations continue.
Both during and following these spurious actuations, there were no abnormal core neutron flux levels identified on control room nuclear instrumentation.
ANALYSIS:
Fort St. Vrain FSAR Section 14.2 addresses postulated reactivity accidents and transient response.
Pertinent analyses and corrective action include:
Multiple control rod pair withdrawal accidents are terminated by the 140T, reactor pcwer scram function (FSAR Section 14.2.2.6).
Various control interlocks decrease the likelihoed of such an incident.
Maximum worth control rod pair withdrawal accident at full power terminated at 140% rated power by an automatic scram, or 60 seconds after accident initiation by operators manually inserting a scram or 105 seconds af ter accident initsiation by an automatic scram signal on high reheat steam temperature, 1075 F.
Maximum worth rod pair withdrawal at source power, terminated at 140%
rated power by an automatic reactor scram (FSAR Section 14.2.2.7).
The five decade per minute rate of power change scram decreases the likelihood of such an incident.
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As stated in the basis of Fort St. Vrain Technical Specification LSSS 3.3,
" Based on complete evaluation of the reactor dynamic performance during normal operation and expected maneuvers and during and following various assumed mechanical failures, it was concluded that sufficient protection is provided by the single fixed point scram setting." This "High Neutron Flux" scram settirg of $140% rated thermal power is initiated by the six power channels, independent of either startup channel.
The operability of the PPS reactor scram function was not affected by these actuations of the RWP systein. Noise induced spiking on the power channels is not a problem, as the power channel's DC output signal is less. susceptible to noise and can be more easily filtered of spurious AC noise.
Therefore, on November 8,
1985, the PPS automatic and manual reactor scram function was operable to perform as designed upon the onset of,an unsafe condition.
Following RWP actuation, control room operators verified neutron flux levels on control rocm nuclear instrumentation to determine the necessary course of acticn required.
Since no abnormal flux levels were identified, the RWP circuitry was reset, and startup operations continued.
Public Service Ccmpany recognizes the importance of reducing the frequency of spurious RWP actuation as to prevent operator desensitizing towards the RWP function. Therefore, v'arlous'cor~rective actions are being pursued.
CAUSE
Startup Channel Noise:
The startup channels operate on a one of two logic.
Startup channel RWP actuations will occur if neutron flux rate exceeds 1.5 DPM or if startup is attempted without adequate neutron flux indication.
Investigations into this noise problem have identified plar.t equipment as a primary source of noise induced spiking on the startup channels.
Plant Equipment Noise:
Each startup channel has three cables associated with it, a high voltage detector supply, a 15VOC preamp supply and an output cable. These cables are routed in cabir trays from the detector wells to the control room.
It is believed that cable routing is such that plant equipment noise is picked up on the channels.
During reactor startup with low core neutron flux levels, startup channel noise frequently causes signal spikes that exceed the "High Rate of Rise" trip setting (1.5 OPM) and initiate an RWP. As core neutron flux levels increase, these noise-induced spikes are not significant enough to cause a 1.5 OPM signal.
Therefore, spurious RWP actuations on "High Rate of Rise" are not normally experienced at higher core flux levels.
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CORRECTIVE ACTION
The startup channel connector pins and cables in control room panel 1-03 will be inspected, to determine whether they can be shielded to reduce noise pickup from operation of the rod withdrawal hand switches.
The startup channel cable routing system will be evaluated to determine whether cable routing should be changed to reduce plant equipment electrical noise pickup.
The startup channel "High Rate of Neutron Flux" trip setting will be evaluated to determine whether the setpoint should be increased, to reduce the likelihood of spurious trips.
NOTE:
Action requests (0 CAR's) have been initiated to shield the startup channel connector pins on I-03, and to reroute startup channel cables.
These DCAR's are presently being reviewed by Nuclear Engir.ee ri ng.
Results Engineering is presently evaluating whether the "High Rate of Neutron Flux" trip setting should be increased.
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