ML19262A921

Summarizes Fluctuation Test Program & Plans for Future Testing.Significant Similarities in Fluctuation Data Observed During Cycles 1 & 2.Rise-to-power Program Should Begin 791210

ML19262A921
Person / Time
Site:	Fort Saint Vrain
Issue date:	12/04/1979
From:	Warembourg D PUBLIC SERVICE CO. OF COLORADO
To:	Kuzmycz G Office of Nuclear Reactor Regulation
References
P-79293, NUDOCS 7912110316
Download: ML19262A921 (7)

Text

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pubue service cornpany e cchwede 16805 Road 19 1/2, Platteville, Colorado 80651 December 4, 1979 Fort St. Vrain Unit No. 1 1-79293 Mr. George Kuzmycz, Proj ect Manager U. S. Nuclear Regulatory Commission Division Proj ect Management Special Projects Washington, D. C. 20555 Docket No. 50-267

Subject:

Fort St. Vrain Unit No. 1 Fluctuation Testing

Dear Mr. Kuzmycz:

As a result of our recent on-site meeting (November 19-20) we agreed to summarize 21uctuation test program to date and our plans for future testing. As we indicated to you, the recent fluctuation data is still undergoing considerable analysis, and we have been unable to reach final and detailed conclusions. On November 19 and 20 we presented the data from the recent fluctuation tests to you and members of the staff, and the following represents our preliminary conclusions.

Cvele 2 Fluctuation Summarv Fluctuations have been initiated and observed on seven (7) occasions during cycle 2 at power levels between 38% and 63% of full power. Four (4) of these fluctuations were initiated by power increases and three (3) commenced during orifice valve adjustments. The data from the in-pile test program has demonstrated that while the fluctuations observed during cycle 2 appear to be somewhat more regular and widespread throughout the core, it is basically the same phenomenon experienced during cycle 1.

There are many significant similarities in the fluctuation data observed during cycles 1 and 2, and only a few differences.

In cycle 2, as in cycle 1, the core pressure drop is an important para-meter intluencing the fluctuation threshold, and power reductions are an effective means of stopping fluctuations. While the cycle 2 fluctua-tion threshold does exhibit a dependence on core pressure drop similar to that of cycle 1, fluctuations were observed during cycle 2 at condi-tions below the cycle 1 threshold line. The reason for this observed difference is under investigation. Initial indications are that it may be due in part, to the different orificing patterns used in cycles 1 and 2. (The different orificing patterns result in different lateral pressure force fields in the upper portion of the core.) We are'also I

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Mr. George Kuzmycz December 4, 1979 Page Two continu'ag to analyze the core resistance parameter and are analyzing the ef'.ect of core resistance as it relates to the fluctuation thresh-hold. Although core pressure drop appears to be the most dominant parar.eter, we feel that there are some relationships of core resistance that may have an effect on the fluctuation threshold line. The fluctua-t 4 or. phenomenon is however, cumplex in nature and these initial observa-tions must be regarded as tentative.

Similar to cycle 1, the fluctuations during cyc e 2 exhibited a period of about 10 minutes and the temperature fluctuation amplitudes (from all instruments) were about the same for cycles 1 and 2. Although the cycle 2 fluctuatians appeared to be more of a core wide phenomenon as compared with cycle 1, the region exit temperatures were very similar in accure. As in cycle 1, the onset of fluctuations during cycle 2 involved simultaneous changes in the various instrumentation signals (e.g., nuclear channels, calibration tube thermocouples, region exit gas temperatures). Furthermore, the same correspondence of region and steam generator module temperatures is seen during cycle 2 as was seen during cycle 1, i.e., the steam temperatures lag inlet helium temperatures to the steam generator modules. The rapid changes in steam generator module inlet temperature, (with some adjacent modules being out-of-phase) which were seen during cycle 1 were also observed during cycle 2.

Ihere were some differences in the phase relationships of various region core outlet temperatures between cycles 1 and 2, but this is not un-expected due to the random nature of the fluctuations.

In the case of the nuclear channels, there were some differences noted in their behavior during cycle 2. Specifically, the fluctuations on the

.aclear channels exhibited smaller amplitudes during cycle 2. This may be due to the fact that fluctuations occurred at lower core pressure crops and therefore with smaller driving forces during cycle 2. Further-more, during cycle 2 no single nuclear channel was clearly more active than the others. All six (6) of the nuclear channels displayed about the same degree of activity during cycle 2 fluctuations, as opposed to the several fluctuations during cycle A in which a single nuclear channel (e.g., channel VI) exhibited significantly more activity than the others.

Af ter reviewing the data obtained during the initial cycle 2 fluctuation testing snd observing that the fluctuation amplitudes on the nuclear channels were smaller during cycle ., the definition of a fluctuation (as given in RT-50rF) was revised. Under the revised definition (in-corporated into RT-500G), the plant is defined to be in a fluctuation operating mode when individual nuclear channels exhibit cyclic deviations from the average power equal to or greater than 0.5% peak to-peak of full power not exceeding a 30 minute period. These " deviations" proved to be an excellent means for monitoring for fluctuation 1 during testing.

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Mr. George Kuzmyc:

December 4, 1979 Page Three The instrumentation on the instrumented control rod drives (ICRD's) in-stalled in regions 5 and 35 durin; the refueling outage has provided useful new information. The data acruired from those ICRD's is currently being analyzed. However, from an initial review of the data, a couple of observations can be made. First, the in-core fission chamber response appears to support the contention that the fluctua-tions seen on the ex-core nuclear channels are primarily due to neutron streaming through narrow gaps in tSe permanent side reflector which a e opened (or siderf.d) and closed by block motion during fluctuations.

The response of the in-core fission chamber follows the core average behavior, whereas thi ex-core nuclear channels show deviations from the average. Second, during fluctuation testing, these deviations on the ex-core nuclear chant.els are an excellent means for monitoring the beginning and end of fluctuations.

The fundamental aspects of the current theory of FSV core fluctuations have not changed from earlier statements. The mechanism continues to be characterized as a coupled fluid mechanics / thermal phenomenon where the measured effects i.e., region outlet temperature changes, steam generator inlet temperatures, gap temperatures at the core support level, and external nuclear flux deviations are still explainable by small movements of reactor core-elements. The resulting phenomena 1.e.,

changing gaps and gap flow, " jaws," and type II flow still appear appropriate to explain the observed data as has been discussed pre-vicusly for cycle 1 fluctuations.

Based upon the above observations, it is concluded that the fluctuations experienced during cycle 2 are a manifestation of the same phenomenon observed during cycle 1.

Proposed Future Fluctuation Testing As presented at the November 19, 20 meeting the Region Constraint Device (RCD's) have been installed and we are preparing a test plan for further fluctuation testing with the RCD's installed. Our operating and testing plans upon completion of the present outage are ar follows:

1. Verify the Expected Operating Characteristics of Selected Orifice Valve Assemblies As the plant is returned to power, testing will be performed (RT-490) wherein the orifices will be calibrated and results compared with previous calibrations to determine the effect, if any, of the RCD's on the orific_s.

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Mr. George Kuzmycz December 4,1979 Page Four

2. Verify Power and Flow Distribution and Temperature Profile During the rise-to-power, testing will be performed to measure core region power and flow distribution (T-119) and the outlet temperature profile (RT-524) again with the primary purpose of determining if the RCD's have had any ef fect on these parameters.

3. Verify Calibration of ICRD's and Recalibrate All Other Fluctuation Data Gathering Systems The instrumentation performance and calibration for the ICRD's installed in Regions 5 and 35 (RT-486) will be verified and functional tests will be performed on other data gathering systems.

4. Take the Reactor to Approximatelv 68% - 69% in the Non-Fluctuating >bde Following completion of this outage it is our intent to operate the plant in the non-fluctuating mode to meet our winter peak demand. In this respect we intend to increase reactor power in a controlled rise-to-power program keeping core pressure drop and core resistance as low as possible based on data obtained from past fluctuation testing. As indicated in P-79094, while operating in the non-fluctuating mode control room instrumentation will be closely monitored for indications of fluctuations. If inadvertent fluctuations are observed in normal operation corrective action will be taken to terminate the fluctuation, and PSC management authorization will be required prior to returning to a power level that would approach that level in which inadvertent fluctuation were observed.

5. Demonstrate That Temperature Fluctuations No Longer Exist at Power Levels Up To 100%_and/or Define the Fluctuation Threshold With RCD's For fluctuation testing post RCD installation, we intend to to essentially continue the testing as previously performed under RT-500. Testing will be performed in two (2) parts, with part 1 invciving testing below 70% power and part 2 in-volving testing above 70% power. Testing under part 2, of course, is predicated on completion of part 1 testing and a review by the NRC staff before proceeding. We are presently revising RT-500G and plan to isoue RT-500H to control fluctua-tion testing with RCD's installed. A copy of RT-500H will be forwarded to you as soon as necessary internal approvals h ve been obtained.

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Mr. George Kuzmycz December 4, 1979 Page Five The essence of part 1 of RT-500 (version H), testing bclow 70% power, consists of first establishing the core configura-tion from which fluctuations were initiated at the lowest power during previous cycle 2 testing. Then the reactor power is in-creased in 3% increments up to about 70% power attempting to initiate fluctuations. If no fluctuations occur, power is to be decreased to approximately 40% and the core pressure drop and resistance are increased by closing the orifice valves.

At this higher initial core pressure drop, attempts to initiate fluctuations by incremental power increases are again made.

These operations are repeated until stable operation at about 10% power with a core LP of about 4.5 is demonstrated or until the fluctuation threshold is determined. (This testing mode is shown graphically in Attachment 1.)

Part 2 of RT-500 (version H), testing above 70% power, is essentially an extension of the procedure followed in part 1.

First, a stable core configuration is established at about 70% power. Then reactor power is increased in 3% increments up to 100% power attempting to initiate fluctuations. If no fluccuations occur, power is decreased to approximately 70%

and the core pressure drop and resistance are increased by closing the orifice valves. At this higher initial core pres-sure drop, attempts are again made to initiate fluctuations by incremental power increases. These operations are repeated ur.til stable operation at approximately 1007 power with the highest reasonable core pressu:e drop is demonstrated or until the fluctuation threshold is determined.

Other than the method of determining system operating lines as described above the conditions and controls of RT-500G remain unchanged for RT-500H. Test prerequisite, administra-tive controls, system and operating limits, and reporting re-q2irements as established by RT-500G remain unchanged. It is still our intent to keep operations in the fluctuating mode r.inimized and to this end fluctuations will be terminated as soon as practicable after data collection is compleu 4 We trust that the above information is adequate to fulfill your request resulting from the November 19, 20 site meeting, and we are proceeding on this basis.

Our present pLins indicate that we should be in a position to begin the rise-to-power program on or about December 10, 1979. If

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Mr. George Kuztr/cz December 4, 1979 Page Six you have any questions concerning our plans it is requested that you contact us as soon as possible. It is essential that we get Fo rt S t .

Vrain on line as soon as possible to meet our system demands, and it should be noted that your SER and final approval of the RCD safety evaluation are needed promptly to maintain our schedule.

Very truly yours,

}Y W Don '4. Warembourg Manager, Nuclear Pro uction DWW:dkm 1528 087

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