ML20149E540
| ML20149E540 | |
| Person / Time | |
|---|---|
| Site: | Rancho Seco |
| Issue date: | 01/08/1988 |
| From: | Firlit J SACRAMENTO MUNICIPAL UTILITY DISTRICT |
| To: | Miraglia F Office of Nuclear Reactor Regulation |
| References | |
| AGM-NPP-87-301, TAC-63080, NUDOCS 8801130377 | |
| Download: ML20149E540 (5) | |
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SMUD g/ SACRAMENTO MUNICIPAL UTILITY DISTRICT O P. O. Box 15830, Sa 95852-183o,(916) 452 3211 w
AN ELECTRIC SYSTEM SERVING THE HEART OF CALIFORNIA AGM/NPP 87-301 JAN 0 81988 U.S. Nuclear Regulatory Commission Attn: Frank J. Miraglia, Jr.
Associate Director for Projects Philips Building 7920 Norfolk Avenue Bethesda, MD 20014 Docket No. 50-312 Rancho Seco Nuclear Generating Station License No. DPR-54 REACTOR COOLANT PUMP UNDERCURRENT RELAY QUESTIONS
Dear Mr. Miraglia:
During dscussions with the staff several questions with respect to the Reactor Coolant Pump (RCP) undercurrent relay have arisen. The attachment to 1
this letter provides our understanding of the staffs questions and the associated District response.
Based on our understanding of the NRC questions the responses provided should address ti:2 remaining issues.
Please contact me if you have any questions. Members of your staff with questions requiring additional information or clarification may contact Mr.
John Atwell at (209) 333-2935, extension 4917.
Sincerely, k
oseph F. Firlit Assistant General Manager, Nuclear Power Production Attachment cc:
G. Kalman, NRC, Bethesda A. D'Angelo, NRC, Rancho Seco J. B. Martin, NRC, Walnut Creek
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PDR NCHO SEc0 NUCLEAR GENERATING STATION O 1444o Twin Cities Road, Herald, CA 95633-9799;(209) 333 2935
l RE!P0NSE TO NRC QUESTIONS ATTACHMENT Question 1 How does timing with respect to the accident analysis compare to the old relays (prompt initiation of RPS)?
Response 1 The urdercurrent relays are set to trip in 120 + 10 milliseconds as compared to 115 + 3 milliseconds for the former underpower relays.
The difference in tolerance reflect the relays characteristics. The additional 5 milliseconds delay has been reviewed by B & W Document No. 32-1164534-01.
B & W indicated that the increase in delay time for the pump motor sensor is acceptable and does not compromise the assumed total trip delay time of 620 milliseconds stated in USAR Table 14.1-11 for the loss-of-coolant flow analysis.
(The total trip delay time includes RPS and sensor delays, breaker delay, Control Rod Drive Mechanism release delay, and RCP monitor sensor delay).
Questicn 2 Given the new undercurrent relays, is there a failure mode that will push DNBR outside the DNBR curve in the USAR.
Response 2 The functional requirements for the Reactor Coolant Pumg Monitors (RCPM) are given in the Technical Specifications Section 2.3.1B:
The pump monitors prevent the minimum core DNBR from decreasing below the CHR correlation limit by tripping the reactor due to (a) the loss of two reactor pumps in one reactor coolant loop and (b) loss of one or two reactor coolant pumps during two rump operation.
The pump monitors also restrict power level to 55 percent for s.:,eactor coolant pump operation in each loop".
Per USAR Section 14.1.2.6.1, a pump failure can occur from lots or electrical power, or a mechanical failure which is considered to be a locked rotor and is used as a bounding accident which covers also a separation of the pump impeller from the motor shaft.
The locked rotcr analysis referred to in USAR Section 14.1.2.6.3 assumes that the RPS trips on overpower based on flow and imbalance.
Therefore, because the trip due to loss of an RCP due to mechanical failures does not require pump status input, the RCPM is only required to monitor a loss of electrical power to the RCPs and not overcurrent.
No adverse safety impacts or new failure modes are introduced by this modification.
Failure of either a single undercurrent relay or a single auxiliary relay can result in either an inadvertent RCP trip indication or a failure to indicate an actual RCP trip.
In either case, the RPS design criteria of single failure proof protection, redundancy, independency, and physical / electrical separation assure no inadvertent or failed actuation of safety functions. The undercurrent relays should provide a more reliable indication af RCP status than the present combination of underpower and phase imbaiance relays. The new auxiliary relays are also expected to improve RCPM reliability.
Attachment Continued Question 3 Why are the overcurrent relays not-needed?
Response 3 As indicated in the response to question #2, the RCP monitor is only required to monitor a loss of electrical power to the RCPs.
Question 4 Are motor curves available (torque / current /etc. vs. flow)?
Response 4 Motor curves which correlate to flow parameters are not available; however, speed vs. torque / current motor curves with pump torque curve are available to show that the motor has sufficient torque when the voltage is at 70, 75 and 100 percent of rated voltage.
Question 5 Are flow trips fast enough for all flow anomalies?
Response 5 The RPS trip on power to flow imbalance is independent of and does not require the RCP motor monitor sensors.
USAR Section 14.1.2.6 provides analysis of the loss of coolant flow abnormalities (also see the response to question #2) and shows that the flux to flow trip is rapid enough to protect against flow anomal ies.
Question 6 What is the basis for the undercurrent relay setpoints ?
Response 6 The undercurrent relay setpoint is based on the B & W recommendation. SMUD Calculation No. Z-PRL-E0043, Revision 2, documents the relay settings.
The recommended setpoint is discussed in the B & W study included with the Proposed Amendment 150 submittal (letter dated 09/22/86, JEW 86-358).,
Reference 2.
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Attachment Continued p:estion 7 What. fraction of spurious trips are caused by the underpower relays?
Response 7 A specific fraction of spurious trips was not readily available however, the former relays had several component failures and calibration problems dating back to 1977. LERs 83-7 and 83-17 reported the earlier problems. The relay problems continued until mid-1986 and involved numerous defective components within the Wilmar relays.
Question 8 What is affect of voltage changes on relay performance?
Response 8 The undercurrent relay is not affected by system voltage changes. The relay setting does account for the expected increased motor current when the system i
voltage is 90% of nominal during normal operating conditions.
Bus undervoltage relays are provided to automatically initiate transfer of bus sources from the normal supply to alternate supply upon degraded voltage conditions.
Question 9 Broken shaft and locked rotor will the undercurrent relays pick it up?
Response 9 The undercurrent relays are not required to detect the conditions of a broken shaft or locked rotor. The analysis provided in USAR Section 14.1.2.6 bounds the two events and the reactor is protected by the RPS flux to flow trip (se)
Response #2). The USAR further states that an instantaneous locking of a reactor coolant pum) motor resulting in a stalled motor is not credible. The broken shaft or locced rotor (resulting in a broken shaft) events would result in the motor running at no load.
It is expected that the undercurrent relays would dettet this condition although they are not assumed to function to protect against it. For these events, the undercurrent and underpower relays provide comparable capabilites.
Attachment Continued Question 10 Will reducing false trip rate affect the demand trip rate?
Response 10 The relays have been purchased, qualified and installed as quality Class 1.
They are expected to be more reliable than the previous Wilmar relays to trip on demand as well as reduce false trips. The periodic testing required by the Technical Specifications provides additional assurance that the relays will trip upon demand.
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