05000249/FIN-2008005-04
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Finding | |
|---|---|
| Title | Inadvertent Control Rod Withdrawal |
| Description | On November 3, 2008, Unit 3 was in Day 1 of the D3R020 refueling outage and the operations department was performing multiple tasks to support removing systems from service. The plant was shutdown with all control rods fully inserted into the core. One of the scheduled tasks was alignment of the control rod drive system in preparation for hydro-lazing the Unit 3 scram discharge volume. Non-licensed operators (NLOs) were in the process of isolating the control rod drive (CRD) mechanisms per a clearance order that directed using Procedure DOP 0500-05, Discharging of CRD Accumulators with Mode Switch in Shutdown or Refuel, when a control rod drift alarm was received in the control room at 10:42 a.m. Over the next few minutes, multiple rod position indication system (RPIS) indications went to green double-dashes (- -), indicating the control rod had slightly inserted beyond the full-in position. The reactor operators notified the control room supervisor and the shift manager. Ensuing discussion between these individuals and the operations staff supervisor came to the decision that instrument maintenance individuals in the auxiliary electrical equipment room (AEER) had probably caused the interruption in the RPIS indications. The operations staff supervisor was dispatched to the AEER to determine if the work there had disrupted the RPIS indication. Over about 17 minutes, seven control rod indications sequentially went from full-in to over-travel in. Four of the indications settled back to indicated full-in; however, three control rod indications drifted out from the full-in position (D-7 to position 06, E-7 to position 18, and E-6 to position 16). Until the three control rods drifted out, the reactor operators had not recognized that the seven affected control rods were actually moving and had not taken any action to prevent possible outward rod motion. The control room operators then entered Technical Specification (TS) 3.1.1, Condition D, Procedure DOA 0300-12, Mispositioned Control Rod, and referenced DGA 7, Unexpected Reactivity Addition, stopped multiple clearance orders involving the CRDs, verified no work was in progress on RPIS and notified the qualified nuclear engineer. The operators subsequently discovered the control rods had drifted due to increasing differential pressure between the CRDs and the reactor when NLOs had sequentially shut the insert riser isolation valve (101) and the withdraw riser isolation valve (102) to each CRD, isolating the related CRD. Operations department staff returned the three control rods to full-in by opening the related 101 valve until the control rod moved fully in to the overtravel position. When the related 101 valve was re-shut, each of the control rods settled to the full-in position. Inspector interviews revealed that the control room operators were not in communication with the NLOs who were isolating CRDs and did not try to establish communication via the plant announcing system when the indications started to change; did not believe the indication that control rods were actually moving; did not take actions to prevent outward motion (SCRAM the plant or go to shutdown on the Mode switch) before the three rods started to drift out; and had not discussed the possibility of a reactivity addition or control rod motion during the pre-job briefing. The licensee completed a Prompt Investigation Report on this event (AR 839678) and determined that industry operating experience (OE) existed specific to this event. Institute of Nuclear Power Operations (INPO) Significant Event Notice (SEN) 264, Unplanned BWR Control Rod Withdrawals While Shutdown, dated April 10, 2007, detailed historical events at several BWRs between 1978 and 2000 where single or multiple control rods unexpectedly moved out of the core without a deliberate withdrawal signal. The reactor had become critical at two plants, one of which had the reactor vessel head removed. The key lessons from SEN 264 were: 1. The isolation of multiple hydraulic control units (HCUs) with the control rod drive pumps in operation can cause higher-than-normal cooling and exhaust header pressures that may be a precursor to inadvertent rod motion (insert or withdraw) if a sufficient number of HCUs are not in service or if alternate system flow paths are not established. 2. Station Procedures should specify the minimum number of HCUs to be kept in service while the control rod drive pump is in service, to prevent inadvertent control rod movement when HCUs are being isolated and restored, particularly during outage conditions. 3. Reactor operators should monitor control rod drive system pressures, rod positions, and alarms during outages when the system is being manipulated to identify changing conditions that could lead to inadvertent control rod movement. 4. Personnel who operate valves to isolate and restore HCUs should be aware that their actions directly affect control rod drive system pressures that can lead to inadvertent control rod movement. When SEN 264 was originally received at Dresden, the HCU system manager and an operations technical superintendent performed the subject matter expert review of the SEN under Action Tracking Item (ATI) 616696-04. A qualified nuclear engineer also reviewed SEN 264; however, he stated that it was unlikely that he would have reviewed any operations procedures independent of the one procedure identified in ATI 616696-04 because he is not an operations procedures expert. The licensee incorporated the SEN 264 information into the 300 Series procedure on the control rod drive system, specifically, to monitor the cooling and exhaust header pressures every 10 HCUs after 50 HCUs had been isolated. This change was intended to alert operators to the potential increase in pressure in the CRD system so that operators could take actions to reduce pressure and avoid an unplanned control rod withdrawal event similar to SEN 264. However, the inspectors determined the licensee had not reviewed all procedures that isolated the HCUs. Specifically, the information was not entered into the 500 Series procedures that applied to the reactor protection system. During the performance of the clearance order to isolate the control rod drive mechanisms on November 3, 2008, the non-licensed operators were using a 500 series procedure, Procedure DOP 500-05, Discharging CRD Accumulators with Mode Switch in Shutdown or Refuel, Revision 5, when the last three control rods, (D-7, E-7, and E-6) drifted out of the core to positions 06, 18, and 16. The inspectors reviewed the event and determined that more than three control rods could have moved out and that the control rods would have continued moving out continuously until the 102 valve to the related HCU was closed. Therefore, the inspectors concluded it was possible that the three (or more) control rods could have moved to full out position 48. The licensee analyzed the shutdown margin for the reactor for the following possible conditions: • The actual position of the three rods at the actual temperature and xenon conditions -- the reactor was 4.5 percent subcritical. • Three drifted rods at 48, actual temperature and xenon conditions 3.1 percent subcritical. • Cold conditions (actual rod positions, 68F, and zero xenon) 1 percent subcritical. • Design shutdown margin (actual rod pattern plus 1 rod full out, 68F, and zero xenon) critical. However, the licensee had not analyzed the shutdown margin for the three drifted rods if they were full out at cold conditions. After the inspectors requested the results of those conditions, the licensees analysis showed that the reactor would have been critical under those conditions. The temperature of the reactor coolant, the amount of xenon in the core, the order in which the control rod mechanisms were isolated, the pressure in the control rod drive system, and the time between when the inset valve was shut and the withdraw valve was shut were key parameters for this event. The procedure in use, DOP 500-05, did not appear to control any of these parameters. The inspectors were concerned that under different conditions the inadvertent, unplanned control rod movement could have caused the reactor to go critical. Additionally, the inspectors were concerned that the licensee had not reported the event to the NRC in accordance with 10 CFR 50.72. In discussions with the licensee on this topic, the inspectors learned that the licensee interpreted the guidance in NUREG 10-22, Event Reporting Guidelines 10 CFR 50.72 and 50.73, to not require immediate notification because the reactor remained sub-critical at the time of discovery. Subsequent to these discussions, the licensee made a 50.72 report to the NRC on November 18, 2008. In order to resolve a difference of opinion regarding whether this event should have been promptly reported under 50.72, the inspectors planned to request assistance from the Office of Nuclear Reactor Regulation (NRR). Pending additional clarification from NRR, the inspectors considered the reporting of this event to be an URI Subsequent to a preliminary exit meeting on site on November 18, 2008, the inspectors received additional information regarding the circumstances and activities surrounding the unplanned control rod withdrawal event in the form of the licensees root cause investigation report. This information was provided to the inspectors on January 7, 2009. The issues associated with this event are considered an URI (URI 05000249/2008005-04) pending inspector review and evaluation of the new information in the root cause report. |
| Site: | Dresden  |
|---|---|
| Report | IR 05000249/2008005 Section 4OA2 |
| Date counted | Dec 31, 2008 (2008Q4) |
| Type: | URI: |
| cornerstone | Mitigating Systems |
| Identified by: | NRC identified |
| Inspection Procedure: | IP 71152 |
| Inspectors (proximate) | T Bilik D Melendez-Colon W Slawinski C Phillips C Brown M Ring C Acosta Acevedoj Bozga J Draper R Murray E Sanchez-Santiago R Winter F Ramirezc Brownm Ring |
| INPO aspect | |
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Finding - Dresden - IR 05000249/2008005 | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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Finding List (Dresden) @ 2008Q4
Self-Identified List (Dresden)
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