05000301/LER-2004-002
| Event date: | 05-15-2004 |
|---|---|
| Report date: | 07-12-2004 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| Initial Reporting | |
| ENS 40754 | 10 CFR 50.72(b)(2)(iv)(B), RPS System Actuation, 10 CFR 50.72(b)(3)(iv)(A), System Actuation |
| 3012004002R00 - NRC Website | |
FACILITY NAME (1) DOCKET NUMBER (2) LER NUMBER (6 PAGE (3) Point Beach Nuclear Plant Unit 2 05000301 : YEAR SEQUENTIAL � REVISION
Event Description:
On May 15, 2004, at approximately 11:54 AM (all times are CDT) a manual trip of the Point Beach Nuclear Plant (PBNP) Unit 2 reactor [EIIS System Code: AB] was initiated when the control room [EIIS System Code: NA] was notified that a diver had become entangled in the intake structure [EIIS System Code: MD]. At the time, a diver was inspecting the circulating water (CW) [EIIS System Code: NN] intake structure for winter damage. PBNP Unit 2 was operating at full load and Unit 1 was in a refueling outage. While inspecting the intake structure, the diver's tether, air and communication line became snagged. The diver's line tender, together with the assistance of a rescue diver, were unable to clear the lines. When the communications with the diver were lost, the NMC diving liaison on-board the dive boat requested that the Unit 2 CW system be secured in order to facilitate removing the diver from the water. Following the reactor trip, the Unit 2 CW pumps [EIIS Component Code: P] were secured at 11:55 AM. Both divers were subsequently removed from the water uninjured.
Plant systems functioned as required and designed during the reactor trip transient. The reactor protection [EIIS System Code: JC] and auxiliary feedwater [EIIS System Code: BA] systems initiated as expected. There was no Emergency Core Cooling [EIIS System Code: BQ] System actuation. Since the circulating water system was secured, the main condensers [EllS System Code: SG] were unavailable for decay heat removal as a result of the loss of condenser vacuum.
The atmospheric steam dump valves [EIIS Component Code: RV] were used to remove the decay heat from the steam generators [EIIS Component Code: SG]. This event was determined to be reportable pursuant to 10 CFR 50.72(b)(2)(iv)(B) as any event or condition that results in actuation of the reactor protection system (RPS) when the reactor is critical and 10 CFR 50.72(b)(3)(iv)(A) for a valid actuation of the auxiliary feedwater system. An event notification (EN 40754) was completed to the NRC at 1412 CDT on May 15, 2004. PNBP Unit 2 was returned to full load operations on May 20, 2004.
Event Analysis:
The PBNP CW system circulates water from Lake Michigan through the main condensers. The PBNP CW intake system is common to both units, and is designed to provide a reliable supply of water to the suction of the four CW pumps, six service water pumps, and two fire water pumps.
The intake structure, or crib, is located 1750 ft. from the shore in a water depth of about 22 ft. The structure consists of two annular rings of structural steel H piles. The annulus is filled with large limestone blocks. The structure has an outside diameter of 110 ft., an inside diameter of 60 ft. and a top elevation at approximately 11 ft. below the lake surface. Water enters the intake crib primarily through the 60 ft. opening above the intake cones. Water flows from the intake crib to the on shore pumphouse through two 14 ft. diameter, pipes buried below the lake bed. With two CW pumps in operation the total intake flow is approximately 365,000 gallons per minute. At the time of this event, Unit 2 was operating at 100% power with full flow through the Unit 2 intake pipe. There was no flow through the Unit 1 intake pipe. Estimated flow at the north intake cone edge was between FACILITY NAME (1) DOCKET NUMBER (2) LER NUMBER (6 PAGE (3) 6-7 ft./sec.
The purpose of the dive on May 15, 2004, was to inspect in detail the damage to the trash racks that cover the intake area. The damage had initially been identified during an earlier dive on May 13. The diving crew on May 15 included two boats, five divers, and a NMC diving liaison engineer. The first diver entered the water at about 1004 and began his inspection on the south half of the intake structure. At 1120 this diver began to inspect the north side of the intake from outside of the structure.
At 1125, the diver notified the boat that he could not see the top anchors on the north half of the intake and he would be entering the north side of the structure. The dive team member that was monitoring the video and diver communications paused, waiting for a comment or reaction from the NMC diving liaison. When the engineer did not make any comment or physical motion, the dive team member assumed that entering the north half of the intake structure was acceptable and he acknowledged to the diver that it was understood that the diver was entering the north half of the structure. Just prior to the time of this discussion between the boat crew and the diver, the NMC diving liaison received a return call from Site Engineering to discuss a message he had left earlier in the dive about the damage that had been observed at this point of the inspection. It is believed that he was on the phone or otherwise distracted when the diver entered the north side, as he does not remember hearing any conversation between the boat and the diver about entering the north side of the crib.
After entering the north side of the intake crib, the diver's air and communication lines were twice drawn into the intake bell of the Unit 2 intake pipe and then pulled free by the diver. Upon being drawn in a third time, the lines snagged on a pipe support for a chlorine injection line. Neither the diver nor the tender on the boat were able to free the line. A rescue diver then entered the water to help the first diver to free the line; however, he also was unable to untangle the line. The first diver ended up flattening himself on the lake bottom against the approximate 12" lip of the operating intake bell to avoid being drawn into the intake pipe. When the diver's communication line was lost due to fretting against the pipe support, the NMC diving liaison requested that the circulating water pumps be stopped. As noted previously, the Unit 2 reactor was manually tripped and the CW pumps secured. Once the pumps were stopped, the rescue diver was able to free the snagged line and both divers left the water under their own power. Neither diver required medical attention.
Safety Significance
This CW intake structure diving event was a matter of both industrial safety, for which a diver was placed in grave danger, and nuclear safety, as the plant was challenged by a manual trip of the reactor with a loss of normal heat removal. The increase in risk due to the plant trip was evaluated by considering the conditional core damage probability of a plant trip without the condenser but with main feedwater still available. Based on the Point Beach PRA model, the core damage FACILITY NAME (1) DOCKET NUMBER (2) LER NUMBER (6� I PAGE (3) probability for this scenario demonstrates that this plant trip event was of very low risk significance.
NMC further concludes that at no time during this reactor trip and shutdown transient was there a loss of any system, structure or component safety function; therefore, this event did not constitute a safety system functional failure.
Cause:
An incident response team from the NMC fleet was formed on the afternoon of May 15 to provide an initial investigation into the issues that occurred during the diving activities, recommend immediate actions to implement barriers that would prevent such an event, and to create a time line of the event. These activities were concluded on May 17, 2004. Subsequently a Root Cause Evaluation (RCE) team was appointed to determine the Organizational and/or Programmatic issues as well as the human performance issues that lead or contributed to this significant near miss. The RCE team was also tasked with making recommendations for correcting the problems identified and preventing recurrence. The RCE report concluded that the root and contributing causes of the event included the following:
Complacency/Overconfidence: Plant personnel and the dive crew treat diving operations as a routine job. No heightened or special awareness had been accorded diving operations.
Communication: Communications were unclear and inconsistent throughout the entire diving evolution leading to the plant personnel, the NMC diving liaison, and the dive crew not being aligned on expectations and requirements and consistent understanding of the dive scope and specifically where the divers were located.
Procedure Use and Content: The procedure directing the intake crib inspection was not followed at all times and was determined to be unclear. The pre-dive checklist from this procedure was not completed for this dive.
Management and Supervisory Oversight: There was little or no Management oversight for the diving operation. Supervisory oversight was not adequate.
Dive Crew Line Tending: Although it is understood that boat movement impacts line tending, the video tape of the dive clearly shows several instances of excessive slack in the diver's tending lines.
Corrective Action Less Than Adequate: Some elements of this event were present in the October, 2000 diving event at PBNP (LER 266/2000-010-00) which also involved a reactor trip due to concerns for diver safety while inspecting the pump house structure.
FACILITY NAME (1) DOCKET NUMBER (2) LER NUMBER (6 PAGE (3) Corrective Action:
Corrective actions to address this event and actions to prevent reoccurrence of diving related events include:
- Diving operations were immediately suspended. Procedures were revised to require the authorization of the PBNP plant manager to conduct a diving evolution.
- Supervisory oversight issues were addressed through application of the disciplinary action process.
- An interim diving program will be developed to establish a dive approval protocol involving the appropriate levels of management. This program will require that a NMC "dive liaison" is continuously present at the dive with a clear pilority for the safety of the divers. A diving program owner will be assigned from the plant maintenance group. Existing diving operation procedures and the diving services contract will be revised to include the elements of this interim program.
- In the longer term, PBNP will be working with the NMC Fleet to develop and implement an industry standard diving program.
Corrective actions have been entered in the PBNP corrective action program (CAP) and will be tracked to completion in accordance with the CAP process and procedures.
Previous Similar Events:
A review of recent LERs identified the following event which resulted in a manual reactor trip due to concerns with diver safety.
LER NUMBER� Title 266/2000-010-00�Manual Reactor Trip Due to Concerns For Diver Safety