05000530/LER-1995-002

LER-1995-002,

Event date:	9-5-1030
Report date:	0-0-0530
5301995002R00 - NRC Website

1. REPORTING REQUIREMENT:

This LER 530/95-002-00 is being written to report a condition where reactor core power exceeded 3800 megawatts thermal (MWT) (100 percent power, Facility Operating License Section 2.C) as specified in the Facility Operating License Section 2.G.

Specifically, at approximately 0700 MST on September 24, 1995, Palo Verde Unit 3 was in Mode 1 (POWER OPERATION), operating at approximately 100 percent power when APS Operations personnel (utility, licensed) identified that the previous night shift Operations crew had isolated the abnormal blowdown valves to the blowdown flash tank (BFT) while both steam generators (SB) were in normal blowdown. Since the blowdown flow rate constants used in calculating reactor core (AC) power (secondary calorimetric) were determined with both the normal and abnormal blowdown isolation valves open, this condition resulted in indicated power being less than actual power. Therefore, Unit 3 operated for approximately 9 hours with a non-conservative error of 0.11 percent which resulted in Unit 3 exceeding 100 percent power. The highest rolling 12 hour average was 100.07 percent power.

2. EVENT DESCRIPTION:

On September 23, 1995, APS Chemistry personnel (utility, nonlicensed) requested that both steam generators (SGs) be placed in normal blowdown to the BFT in preparation for chemistry (tracer injection) testing. The dayshift operations crew ensured that both SGs were aligned with normal blowdown to the BFT and that the abnormal blowdown isolation valves were open as required by procedure 43OP-3SG03, "Operating the Steam Generator System." During turnover, the dayshift informed the oncoming nightshirt of the chemistry request and blowdown lineup to the BFT.

On September 23, 1995, at approximately 2300 MST, the Control Room Supervisor (CRS, utility licensed), after reviewing procedure 43OP-3SG03, determined that the abnormal blowdown isolation valves should be closed and directed that the valves be isolated. On September 24, 1995, during shift turnover, the nightshirt CRS informed the dayshift CRS that the abnormal blowdown isolation valves were closed during the nightshirt.

On September 24, 1995, at approximately 0740 MST, the dayshift CRS determined that abnormal blowdown isolation valves should have been left open and reopened the abnormal blowdown isolation valves. At approximately 1027 MST, reactor power was reduced to 98.8 percent for an hour to ensure that the 12 hour rolling average was again less than 100 percent power.

EVENT:

The highest hourly and 12 hour rolling average calculated was 100.14 percent and 100.07 percent power (respectively). The Update Final Safety Analysis (UFSAR) assumes that reactor thermal power can range from zero to 102 percent power for accident analysis. Therefore, the event on September 24, 1995, was bounded by the assumptions made in the UFSAR. This event did not adversely affect the safe operation of the plant or the health and safety of the public.

APS does not have real time blowdown flow rate measurement capability. Consequently, the blowdown flow rate is measured during a special test. Constants are developed from this test which are then used in the secondary calorimetric in place of real time blowdown flow measurements.

The secondary calorimetric power is based on measurements of feedwater flow, feedwater temperature, steam flow, and steam pressure and blowdown flow rate constants determined from blowdown flow rate measurement testing. A detailed energy balance is performed for each steam generator. The energy output of the two steam generators is summed and allowances made for reactor coolant pump (AB) heat, pressurizer heaters (AB), and primary and secondary system energy losses.

The blowdown flow rate for normal blowdown to the BFT was measured with the abnormal blowdown isolation valves to the BFT open.

Therefore, the leakage past the abnormal control valve to the BFT was included in the blowdown flow rate constants. When the abnormal blowdown isolation valves were closed, the actual blowdown flow rate became less than the calculated blowdown flow rate constant by approximately 22000 lbm/hr. This resulted in a non-conservative condition with the algorithm calculating a power lower than actual power (indicated versus actual).

4. CAUSE OF THE EVENT:

An independent evaluation of this event is being conducted in accordance with the APS Corrective Action Program. Based on the current results of the evaluation, the cause of the event was determined to be personnel error on the part of the CRS (SALP Cause Code A: Personnel Error). Based on past operating experience, the CRS questioned if the blowdown lineup was correct. His previous experience was reinforced by an incorrect precautions and limitations procedure step. In addition when the CRS was reviewing the main body of the procedure, the CRS went to the section discussing lineup of blowdown to the main condenser instead of blowdown to the BFT. The main condenser section of the procedure properly stated that unused flowpaths could be isolated if they would not be used in a period of less than eight hours.

The CRS, believing that he was in the correct section of the procedure and that SG blowdown was expected to remain in the normal lineup for greater than eight hours, ordered the abnormal blowdown isolation valves be closed.

An operating philosophy change was made in April 1995 in the way blowdown constants were calculated to provide a more accurate calculated power for plant conditions. Currently, the blowdown constants are calculated with both the normal and abnormal blowdown isolation valves open. This change in philosophy was communicated to Units 1 and 2; however, there is no documented evidence that this change was effectively communicated to Unit 3 Operations personnel.

No unusual characteristics of the work location (e.g., noise, heat, poor lighting) directly contributed to this event.

5. STRUCTURE, SYSTEM, OR COMPONENT INFORMATION:

No structures, systems, or components were inoperable at the start of the event which contributed to this event. There were no component or system failures involved in this event. No failures of components with multiple functions were involved. No failures that rendered a train of safety system inoperable were involved. There were no safety system responses and none were required.

6. CORRECTIVE ACTIONS TO PREVENT RECURRENCE:

On September 24, 1995, at approximately 1027 MST, reactor power was reduced to 98.8 percent to ensure that the current 12 hour rolling average was less than 100 percent power.

On September 24, 1995, a common night order was written (all three units) to explain the reason for the over power event in Unit 3.

The night order also stated that step 3.10 in the limits and precautions of procedure 43OP-3SG03 was incorrect and that a temporary procedure change was made to delete this step. On October 13, 1995, this temporary procedure change was incorporated, and the revised procedure was effective in all three units.

Actions required from the above evaluation will be tracked by APS' Commitment Action Tracking System (CATS). If the evaluation results differ from this determination or if information is developed which would affect the readers understanding or perception of this event, a supplement to this report will be submitted.

7. PREVIOUS SIMILAR EVENTS:

There have been no previous similar events reported pursuant to 10CFR50-73 in the last three years.

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