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The following event occurred with the unit in a "No Mode" defueled condition. On the 12th of April at 2215 CDT Comanche Peak Nuclear Power Plant Unit 2 had a Train B ESF Actuation (Black Out) reportable per 10CFR50.72(b)(3)(iv) for (the 6.9KV Train B) Emergency AC Electrical power systems including Emergency Diesel Generator (EDG). At the time of the event, Unit 2 was in 'No Mode' - the core was off loaded and post maintenance testing was being performed on the 2-02 Emergency Diesel Generator. The scope of the EDG testing was to verify proper operation of the EDG when it was the sole supply to the Unit 2 Train B 6.9KV safeguards buses. Unit 2 Train B 6.9 KV buses have 3 AC sources - XST1 (138KV) preferred source, XST2 (345KV) alternate source and the 2-02 Emergency Diesel Generator. The 2-02 EDG was started normally and was operating in parallel with XST1 (preferred AC source) just before the event. The event was triggered when the preferred AC supply breaker to the Train B 6.9KV Safeguard bus was opened per plan. The opening of the preferred feeder breaker resulted in the 2-02 EDG being the sole AC source to the Train B 6.9KV bus. Immediately following this breaker operation, the EDG tripped on an Auto Voltage Regulator failure. This caused the Train B 6.9KV safeguard bus to de-energize which resulted in the automatic closure of the alternate AC (XST2) feeder breaker. This in turn triggered the Train B Solid State Sequencer to sequence on Train B Black Out loads - as designed. The Train B Black Out Solid State Sequencer operated as designed. No abnormalities were noted. All available equipment operated as designed. The 2-02 EDG is currently tagged out and the problem associated with the (Auto Voltage Regulator) failure is being investigated. (The) NRC Resident Inspector has been informed and is currently located on site. No fuel movement was in progress during this event. Train A safeguard power was unaffected and available throughout. Non-safety related power was also not affected. The only operational impact was the loss of a Train B spent fuel pool cooling pump for approximately 10 minutes. The spent fuel pool temperature increased approximately 1/2 degree during before cooling was restored.

• * * RETRACTION FROM DAVID BUTLER TO HOWIE CROUCH AT 1830 EDT ON 4/14/11 * * *

Further investigation of this event has determined that the 2-02 Emergency Diesel Generator (EDG) auto voltage regulator functioned as designed. The EDG did not receive an emergency start signal, which was consistent with the design for the plant conditions present. In addition, Unit 2 was defueled and in a 'No Mode' condition, and there was no actuation of any of the systems listed in paragraph 50.72 (b)(3)(iv)(B) by the sequencer operation. Therefore, this event is not reportable because it did not meet the criteria for a system actuation per 50.72(b)(3)(iv)(A) and the guidance in NUREG-1022. The licensee will be notifying the NRC Resident Inspector. Notified R4DO (Miller).