ML20209G722

Ro:On 870401,reactor Experienced Safety Channel 1 Trip. Caused by Operator Error.Rapid Recovery Restart Performed by Operator.Memorandum Issued Placing Restrictions on Automatic Operation & Limiting Removal of Regulating Blade

ML20209G722
Person / Time
Site:	05000083
Issue date:	04/13/1987
From:	Vernetson W FLORIDA, UNIV. OF, GAINESVILLE, FL
To:	Grace J NRC OFFICE OF INSPECTION & ENFORCEMENT (IE REGION II)
References
NUDOCS 8705010046
Download: ML20209G722 (7)

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NUCLEAR ENGINEERING SCIENCES DEPARTMENT Nuclear ReactorFacility University of Florido i j c .v., , mes., .

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< a u 21 P 2 : ,6 April 13, 1987 Nuclear Regulatory Commission Suite 2900 101 Marietta Street, N.W.

Atlanta, GA 30323 Attention: J. Nelson Grace Regional Administrator, Region II Re: University of Florida Training Reactor (UFTR)

Facility License R-56; Docket No. 50-83 Gentlemen:

As reported by telephone and by telecopy on April 2, 1987, the University of Florida Training Reactor experienced a Safety Channel 1 (125% overpower) full trip (blade drop plus water dump) at 1416 hours0.0164 days <br />0.393 hours <br />0.00234 weeks <br />5.38788e-4 months <br /> on April 1, 1987. The Reactor Safety Review Subcommittee (RSRS) reviewed this event and concluded it should be promptly reported since it involved a challenge of the overpower trip sys-tem on the UFTR due primarily to operator error. The event was, however, not considered to meet any specific prompt reportability requirements listed in the UFTR Technical Specifications under Section 6.6.2. Following implementa-tion of corrective / preventive actions and notification of the NRC, the RSRS authorized restart and resumption of nonnal operations. This event was re-viewed by telephone with Mr. Larry Mellen and Mr. Paul Burnett (twice) on April 2, 1987 as to the details of the occurrence. The event was further dis-cussed with Mr. Paul Frederickson on April 7, 1987. Based on the RSRS recom-mendation and NRC inspector advice, this event has been considered a promptly reportable event as specified in the reporting requirements of paragraph 6.6.2 ( 3 ) of the UFTR Technical Specifications. Therefore, this report is being transmitted to meet the requirement for a final report closing out the occur-rence.

SCENARIO At 1416 hours0.0164 days <br />0.393 hours <br />0.00234 weeks <br />5.38788e-4 months <br /> on April 1,1987, the UFTR experienced a Safety Channel 1 (125%

overpower) Trip. There had been approximately 6.8 hours9.259259e-5 days <br />0.00222 hours <br />1.322751e-5 weeks <br />3.044e-6 months <br /> full power operation until early evening on March 31 resulting in significant xenon buildup in the UFTR core on April 1, 1987. The xenon present plus a small-worth experiment resulted in a normal control blade position of 424 units versus the normal value of ~375 units at the beginning of March,1987 for a clean core and a value of ~383 units on April 1 with the experiment in place.

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1 Nuclear Regulatory Commission April 13,1987 Page Two On April 1, there was a brief operation of just over an hour including step increases in power level (10 kw, 50 kw, 80 kw) and ending with three minutes at full power for radiation surveys for Training Class for Radiation Protec-tion Technology students from Central Florida Community College. At this point a normal shutdown was performed by the operator (G.W. Fogle) so he could at-tend to other activities before continuing full power operation for a full re-stricted area survey and concurrent build up of Argon-41 inventory for ac-tivity measurement and concentration calculations for this same group of ra-diation protection technology students. Permission to restart by Dr. Vernetson (Facility Director) was interpreted as approving restart without . determination of a new critical position since restart would be performed after only a few minutes shutdown (actual shutdown time was 4 minutes) and since the critical position was already determined. The operator was aware that there was still significant xenon buildup in the core from the previous day's extended run at full power. The system was also in a hot condition from the previous hour's run.

A rapid recovery restart was performed by the operator implying no stop at 1 watt (in this case only about 100 watts or above would have been possible) to record a steady state condition. Although not a procedural violation, the op-erator normally would have delayed for a critical position determination in the vicinity of 100 watts. This fact is not considered to have directly af-fected the resulting trip except that, the longer the delay in restart, the less likely the occurrence of the trip as temperatures are reduced. The opera-tor tested the servo system at ~80 Kw and then allowed it to control the re-turn to 100 kw on the 30 second limiting period. Although aware of the xenon built up in the core, the operator failed to monitor the power increase suffi-ciently. With both temperature and xenon negative reactivity effects acting plus a small negative reactivity for a sample present in one port, the regula-ting blade was sufficiently removed (800-900 units from the bottom) that its insertion rate following a startup at a 30 second period was inadequate to avoid reaching the 125 kw transient overpower condition resulting in a Safety 1 (125% power) trip at 1416 hours0.0164 days <br />0.393 hours <br />0.00234 weeks <br />5.38788e-4 months <br />. A contributor in this trip event was that the operator began to take a set of log readings upon reaching 100 Kw and was also distracted by a question from the class for which the operation was being performed. The time period during which the operator was distracted between reading 100 kw and tripping on overpower at 125 kw is estimated to have been about 20 seconds. Basically, the operator failed to assure the automatic con-troller was bringing the reactor to a steady state power level with a near infinite period before recording log readings. As a result, the power reached the limiting safety system setting (trip point) of 125 kw at which point all safety systems responded properly for a full trip: Safety 1 trip with full drop insertion of all control blades plus dumping of primary coolant resulting in derived LSss trip indicators on the Primary Coolant Pump, Coolant Level and Coolant Flow.

r Nuclear Regulator Commission April 13,1987 Page Three EVALUATION The evaluation of this occurrence was first directed to determine whether it was promptly reportable under UFTR Tech Specs items in Section 6.6.2, "Special Reports." Although the trip represents an unanticipated change in reactivity greater than one dollar, the trip was from a known cause and hence considered to be not necessarily promptly reportable under Tech Specs Section 6.6.2(3)(d).

Therefore, item (3)(d) was eliminated from requiring prompt reporting.

Similarly, although the operator would normally have established a critical position prior to resuming power operations, SOP-A.:2, " Reactor Startup," does not preclude a restart as performed here. Finally, consideration was given to item (3)(f) for inadequacy in implementation of administrative or procedural controls. Item (3)(f) is also not considered to apply for this occurrence in that operator error was the primary cause of the occurrence and the procedure involved actually has served well, though some clarifications are recommended later in this report.

The second part of the evaluation was of the UFTR systems themselves. First, the RSRS Executive Committee agreed that this event is considered to have no impact on the health and safety of the public. Similarly, the event is con-sidered to have no impact on UFTR safety. All safety systems performed as de-signed for a full trip (blade drop to full insertion and dumping of primary coolant) with Safety Channel (125% overpower) Trip indicating and with derived limiting safety system setting (LSSS) trip indications on the primary coolant pump, primary coolant level, and primary coolant flow limiting safety system settings. The f act that Safety Channel #2 did not indicate a trip supports the evaluation that power was stopped promptly at the 125 kw level as both chan-nels are nominally set at 125 kw but one is expected to yield a trip signal slightly prior to the other due to uncertainty in the LSSS. All system indica-tions supported a trip at the 125 kw level. Since this was a transient, not a steady state, power level, no safety limit was exceeded. Finally, from the bases for the Limiting safety System Settings presented in the UFTR Tech Specs Section 2.2, the 125 kw LSSS value is specifically set at the 125% overpower value for the protection of the fuel, fuel cladding and the reactor core.

Therefore, it was considered by UFTR Management and agreed by the RSRS Execu-tive Committee on April 2, 1987 and supported by all members at the regular RSRS meeting on April 9, 1987 that the UFTR system is well designed for such an overpower trip with no safety impact expected or resulting.

Appendix 15 of the UFTR Safety Analysis Report supports this evaluation in Ap-pendix 15B where the effects of large reactivity additions are considered and in Appendix 15D where decay heat effects are considered following a long run at full power. Here the total energy generation for the hour preceding the accident was less than 35 kw-hrs with a much smaller reactivity addition of only ~0.18% Ak/k. Certainly Section 14.1, " Excess Reactivity Insertion" of the UPTR Safety Evaluation Report (NUREG-0913) also supports this evaluation.

Finally, Section 15.2, " Loss of Coolant Accident" in the UFTR Safety Analysis Report considers a scram with coolant drop from 625 kw equilibrium operation, again without excessive fuel temperatures.

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Nuclear Regulatory Commission April 13,1987 Page Four Since this event was an overpower trip with potential operator error involved in that the operator did not properly assure the steady state operation at full power and did not stabilize power and take a complete set of readings prior to raising power into the power range this event was required to be re-ported by the RSRS in agreement with UFTR management. In addition, it was recommended to and required by the RSRS that the operations in automatic con-trol should be restricted to assure such a condition does not challenge the operator in the future and that operator training be conducted to assure fol-lowing such restrictions to prevent recurrence of such a trip.

CONSEQUENCES Based on the evaluation above, this overpower trip occurrence is considered to have no impact on the health and safety of the public. In addition, the event is not considered to impact on the safe operation of the UFTR. Reporting is required by the RSRS with agreement by UFTR management because of the over-power trip challenging the trip system due to operator error.

CORRECTIVE / PREVENTIVE ACTIONS NRC Region II was notified of the trip on April 2, 1987; the trip event was discussed once with Mr. Mellen and twice with Mr. Burnett prior to restart. In addition, prior to authorizing restart, the following conditions were required by the RSRS to be implemented:

1. A memorandum was issued (see Attachment I) placing restrictions on auto-matic operation and limiting removal of the regulating blade to no greater than 750 units above 50 Kw power levels. The memorandum also in-dicated the following to allow for condi tions which would otherwise re-quire excessive regulating blade removal:

If the zero power critical position on the regulating blade is >30 units above the normal critical position as indicated in the opera-tions log (first day of month), then an alternate authorized bank position will be used for the safety blades.

Note that the memorandum implementing the current alternate authorized safety blade bank positions is enclosed as Attachment II.

2. Training was conducted for all available operators prior to restart (or participation in operations) to include:

a. Reminder on how to take logs (verbal and written).

b. Reminder on observing the console and noting transient versus steady-state conditions especially for a high regulating blade cri-tical position (verbal and written).

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Nuclear Regulatory Commission April 13,1987 Page Five

c. Training on the memorandum restricting use of automatic control and limiting removal of the regulating blade.

Finally, . a commitment is made to revise SOP's A.2 (Reactor Startup) and A.3 (Operation At Power) to reflect these requirements and conduct training on them within 60 days. The special memorandum will serve as the preventive ac-tion in the interim.

FINAL NOTE These corrective / preventive actions were further reviewed at the April 9,1987 full RSRS meeting with concurrence by all members. Although this event is con-sidered to have no impact on the health and safety of the public or on reactor safety, the actions delineated above will assure such an overpower trip does not recur. With this report this incident is considered closed with corrective actions committed to be implemented as noted above.

Sincerely, uk)h Ub William G. Vernetson Director of Nuclear Facilities WGV/ps cc: Reactor Safety Review Subcommittee P.M. Whaley, Acting Reactor Manager l

NUCLEAR ENGINEERIN 3 SCIENCES DEPARTMENT NuclearReactor Facility University of Florido '

l l tnvenes a.o eee .

mamsuanees ATTACHMENT I Gainewte,Flodde 33611 Phone (804)3921429.Teesa M330 April 2, 1987 MEMORANDUM TO: UFTR Operations Staff FROM: W.G. Vernetsor50

SUBJECT:

Corrective / Preventive Actions for Safety 1 Overpower Trip Due to the Safety 1 ( overpower) trip on , April 1,1987, certain restrictions are being implemented as follows; these restrictions carry the weight of SOP requirements and will be commitments to the NRC:

1. If the zero power critical position on the regulating blade is more than 30 units above the normal critical position as indicated in the operations log (first day of the month),

then an alternate authorized bank position must be used for the safety blades. Specific authorization for use of this alternate bank position must be obtained each time it is used from Level 3 (Reactor Manager or designated alternate) or above.

2. Automatic (Servo) Control operations are restricted so there is to be no automatic operation unless:

a. Power is within 20% of demand setting prior to going into automatic servo control and period is near in-finity yielding a near steady-state condition of reac-tor power.

b. Regulating blade position is less than 750 units.

3. The regulating blade should not be withdrawn to greater than 750 units for power operation above 50 Kw.

These commitments will be incorporated into the next revision of UFTR SOP-A.2 and A.3 within 60 days.

WGV/ps cc: P.M. Whaley RSRS Emriopporks*y/Atkm2W Action fanployer I

NUCLEAR ENGINEERING SCIENCES DEPARTMENT Nuclear Reactor Facility Universityof Florida I

w.avemeeson.oi,.ceer NUCLEAR REACTOR BUILDN6G ATTACHMENT II Gainevue,Florido 32411 Phone (904) 392-1429.Teles 56330 April 2, 1987 MOMORANDUM TO: UFTR Staff FROM: W.G. Vernetson

SUBJECT:

Alternate Authorized Control Blade Positions

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Based on the 1 watt critical position determination on' April 1, 1987 (540, 540, 540, 424) following the 6.8 hour9.259259e-5 days <br />0.00222 hours <br />1.322751e-5 weeks <br />3.044e-6 months <br /> full power run on March 31, 1987 which produced considerable xenon, the alter-nate authorized control blade positions as of April 2, 1987 shall be:

Safety-1: 550 units safety-2: 550 units Safety-3: 550 units with the Regulating Blade up to where required for criticality.

Note that this set is an alternate authorized control blade bank position, the use of which must be approved by Level 3 (Reactor t

Manager or designated alternate) or above each time it is used.

Documented verbal permission is acceptable.

This alternate authorized set of control Blade Positions is to be implemented per the Corrective / Preventive Actions delineated in the attached memo - item number 1 to prevent occurrences such as the overpower trip on April 1,1987 due to operations with the regulating blade removed beyond 750 units such as for xenon, ex-periment worth, etc.

Any settings other than these must also be authorized by the Reactor Manager or Facility Director.

WGV/ps cc: P.M. Whaley RSRS S-3 F.k tw opsxwtuney/Amirncswo Action tervoyer