ML19312C940
| ML19312C940 | |
| Person / Time | |
|---|---|
| Site: | Oconee  |
| Issue date: | 05/06/1976 |
| From: | DUKE POWER CO. |
| To: | |
| Shared Package | |
| ML19312C936 | List: |
| References | |
| RO-287-76-04, RO-287-76-4, NUDOCS 8001140596 | |
| Download: ML19312C940 (1) | |
Text
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O nUxt 10wtR COMriuT Oconsa Unit 3 Report No.:
R0-287/76-4 Report Date: May 6, 1976 Occurrence Date: April 22, 1976 Facility: Oconee Unit 3, Seneca, South Carolina Identification of Occurrence: Rod overlap in excess of Technical Specification limit due to a dropped control rod group Conditions Prior to Occurrence: Unit at 95% full power Description of Occurrence:
On April 22, 1976, while Oconee Unit 3 was operating at 95% full power, group 7 control rods dropped into the core from a 15% withdrawn position. As a result, the reactor power level reduced to 80% full power, and the Integrated Control System, in order to maintain the power level at 95%, withdrew group 6 control rods from 90% to 100% withdrawn. This resulted in a condition of 0% control rod overlap between rod groups 6 and 7.
Oconee Technical Specification 3.5.2.5 specifies an overlap of 25 + 5% between two adjacent operating contro? rod groups.
To re-establish a 25% overlap, the Control Operator immediately placed the Control Rod Drive System in manual control and inserted group 6 to a 75%
withdrawn position. Control rod group 7 was immediately placed on the auxiliary power supply, and the power level was established at 80% full power.
Apparent Cause of Occurrence:
This incident occurred when one of the two parallel power supplies to the group 7 control rod drive stators was de-energized to perform electrical testing.
Either of the two power supplies will control group 7 stators, but the supply which remained energized had one phase (AA) with an inoperable gate drive which prevented it from supplying power to the group 7 stators.
Therefore, when an attempt to move the group 7 rods was made by the ICS and the programmer called for AA phase to be energized, the group 7 control rods dropped into the core.
Analysis of Occurrence:
Following the control rod group 7 rod drop, the condition of 0% overlap existed for approximately ten minutes until the required overlap was re-established.
No unsafe conditions were created by this incident, and no core protection limits were approached.
It is considered that the transient experienced is in accordance with design bases.
It is concluded, therefore, that this incident did not affect the health and safety of the public.
Corrective Action:
Within six hours following the incident, the faulty gate drive was replaced, and control rod group 7 was transferred back to its normal power supply.
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