ML19340C957
| ML19340C957 | |
| Person / Time | |
|---|---|
| Site: | Beaver Valley |
| Issue date: | 11/18/1980 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML19340C955 | List: |
| References | |
| NUDOCS 8012180336 | |
| Download: ML19340C957 (4) | |
Text
ENCLOSURE SAFETY EVALUATION REPORT BEAVER VALLEY UNIT #1 R00 POSITION INDICATOR CHANNELS INTRODUCTION By letter dated 11/14/80, Duquesne Light Company (the licensee) has requested a change in the license for Beaver Valley Unit #1. The request involves a change to Technical Specification 3.1.3.2 to allow the use of alternate means of determining rod positions, under certain conditions.
BACKGROUND The present Technical Specification requires that the installed rod position indicator channels (RPI) (one per shutdown and control rod) be operable and capable of indicating the positions within +12 steps. An Action Statement pravides that when one RPI channel per rod group is inoperable, reactor operation may continue if other methods are used to periodically determine the rod's position, specifically by use of moveable in-core detectors. A letter dated October 27, 1979 to all licensees from the NRC states that voltage measurements at the rod position detector is also an acceptable alternate method.
The rod position detector is a linear variable transformer consisting of primary and secondary coils alternately stacked on a stainless steel cylindrical tuce. An extension shaft from the rod drive mechanism extends up into the tL'ae and serves as the variable " core" for the transformer. With an a.c. current source (200 mA)' applied to the primary windings, the position of the rod drive extension shaft changes the primary to secondary coupling and produces a secondary voltage that is directly related to rod position.
The secondary voltage (8.C-12.5 VAC) is sent to an electronic module which converts the a.c. signal into an appropriate d.c. voltage to drive the indicator mounted on the main control board. This module contains "Zero" and " Span" adjustments plus an output voltage test point (0-3.45 Vdc). A built-in set of test points facilitates l
measurements of the primary voltage of the detector transformers. A test signal generator is provided to adjust the " rod bottom" bistables.
EVALUATION During testing in November 1980 in preparation for Cycle 2 operations, the i
licensee found that a significant number of rod position indicator channels l
(RPI's) were not indicating within the specified accuracy of +12 steps.
The licensee has conducted an investigation of this matter and has obtained technical assistance from Westinghouse (W). The NRC Senior Resident Inspector, the regional office, and NRR/Uivision of Licensing have closely monitored the actions taken and proposed by the licensee.
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-2 Technically, the RPI's are not in a state of malfunction or failure but rather are manifesting characteristics innerent in the instrumentation.
Ine characteristics of interest are of two general types.
First, the channels have a degree of non-linearity in the steady-state response.
Second, the channels display a time-dependent (transient) response which is due to ther al effects in the detector assembly.
A typical steady-state calibration curve is an arc-shaped curve, with the indicated position low at the near full-in and near full-out extremities and the indicated position high in the mid-travel region. The steady-state response also depends to some degree upon whether the last rod motion was a withdrawal or insertion.
For most rods, but not all, the Zero and Soan adjustments allow the steady-state calibration curve to ce fitted witnin the +12 step acceptance band. The Zero and Scan adjustments are '.ntersependent.
Changes in either of these adjust:nnts invalidates any previous output voltage-to-position calibration, necessitating a re-calibration by rod full-stroke,ovement. Once calibrated, hcwever, voltage measurements can be used to determine rod position.
The transient response for the RPI's is typically of the "over shoot" type. That is, if the rod is being pulled out, the RPI indication will show a greater withdrawal and later settle (at thermal equilibrium) back to the steady-state value; if the rod is being inserted the initial indicaticn is greater insertien than actual.
The macnitude of tnis ther al transient response appears empirically to be insignificant in j,
the region of the lowest one-third of rod travel. However, near the fully withdrawn positions, this transient response can be of a magnitude as great as 25 steps. The time constat of the thermal recovery toward tne steady-state value varies with rod location radially across the core and has values between 10 and 15 minutes. " Setting Times" of 20 to 45 minutes have been observed cefore steady-state is reached.
It has been found that a measure of the primary voltage of the detector transformer provides a better indicaticr of rod position than does the secondary (output) voltage. While all the same characteristics are evident, the magnitude is substantially less. Our on-site review of the data (curves) found tnat, including all the variations discussed above, the primary voltage is accurate to values of i (3-6) steps.
Therefore, the primary voltage is technically an acceptable point to determine rod position.
From a human factors engineering view;oint, the use of primary voltage measurements has certain limitations. Unlike the installed RPI's, the voltage measurements are presently not available in the control room, the measurements require operator use of a calibration curve (voltage-to-position), and the measurements are not continuously di spl a."e d. To accomodate these and other considerations, voltage measurc ents, as an alternate indication, should be limited to a small number of rods (such as 1-3 rods).
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. The licensce has proposed a revised Technical Specification that has the following characteristics:
1.
When an RPI is found to deviate by more than 12 steps from the position demand step counter, a primary voltage measure-ment will be made immediately '(10-20 minutes) to determine the rod position. This initial measurement will be repeated imediately following movement of 6 or more steps for the affected rod.
2.
If the voltage indicates the rod more than 12 steps different from the demand counter, the red will be declared " mis-aligned" and appropriate Technical Specification Action Statements will be entered.
3.
If the voltage measureinent indicates that rod is within 12 steps and the RPI channel stabilizes within one hour of the rod movement, no further special action is required.
4.
If up to three RPI's per bank do not stabilize to within
+12 steps, voltage measurements will be made periodically Ti.e.,everyfourhoursforrodsnotfullywithdrawn)as a substitute method to confirm rod position.
5.
If more than three RPI's per bank are not stabilized within one hour and are for rods that are not fully withdrawn, the instrumentation system will be declared INOPERABLE. A shutdown will be initiated imediately and the reactor will be in Hot Standby within the next hour.
To assure an overall + 12 step instrumentation accuracy, the 6-step accuracy of the voltage method directly infers an immediate remeasure-ment after ar :ntentional change of another 6 steps. The accident analysis is r :ed on a maximum rod position deviation of 24 steps, i.e.,12 steps actual known position deviation (T.S. 3.1.3.1) plus 12 steps instrumentation inaccuracy in rod position indication (T.S. 3.1.3.2).
The licensee has proposed that, if the position of each rod in bank has been shown by voltage measurement to be within 6 steps of each other within the past 4 hours4.62963e-5 days <br />0.00111 hours <br />6.613757e-6 weeks <br />1.522e-6 months <br />, then immediate confirmation of rod position by voltage measurements would be made following intentional rod movements of 12 steps or greater. The ;,roposal maintains the overall 24-step limit (i.e., 6-step instrumentation inaccuracy, re-check after 12-step movement, and 6-step actual measured deviation). The present Technical Specification allows that when an RPI channel is totally inoperable, operation may continue if the actual position is confirmed to be within 24 steps by moveable in-core detectors every 9.jours.
In that the overall 24-step limit is being maintained by voltage measurement and that such measurements will be made more often than the 8-hour in-core measurement, this proposal is acceptable for Cycle 2 operation.
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i 4-i In consideration of our understanding of the technical aspects of this matter and being aware that W has concurred with the method, we conclude that the voltage measurement method is an acceptable backup alternate i
means for rod position indication for this reactor operating cycle. We plan to discuss this matter further with W to detemine what long-term fixes may be needed.
CONCLUSIONS Based on infomation provided by the licensee, 'our site visit, discussions with W, the Senior Resident Inspector, et al, we conclude that W had previously recognized and has partially quantified the observed 1
phenomena and the licensee understands this also. The licensee has developed calibration curves of primary detector voltage vs. rod position and understands the technical limits of their validity.
The measurement of primary detector voltage is therefore an acceptable backup alternative method for determining rod position for this reactor operating cycle. A long term solution will be developed later.
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