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AmerenUE Releases Consultant’s Report on Taum Sauk Failure Company Creates Dam Safety Program, Establishes Quality Assurance Team
Ameren Corporation (NYSE: AEE) today announced that its utility subsidiary AmerenUE is acting decisively to respond to a report from an independent expert the company commissioned to examine the cause of the Dec. 14, 2005 breach of AmerenUE's Taum Sauk Plant Upper Reservoir.

The report is an analysis of the root cause and contributing factors behind the failure and was filed today with the Federal Energy Regulatory Commission (FERC), which is investigating the cause of the breach. Its author is a veteran dam expert AmerenUE hired -- Paul C. Rizzo, PhD, P.E., who has 40 years experience as a professional engineer, and whose firm -- Paul C. Rizzo Associates, Inc. -- specializes in dam design, analysis and repair. The full report can be found today on the Ameren Web site at www.ameren.com/TaumSauk and will be available soon on the FERC Web site. This report becomes part of the public record, and it is expected that it will be considered in the final report issued by the FERC independent panel early this summer.

"Rizzo Associates was charged by Ameren to investigate the breach and uncover the facts surrounding it," said Gary L. Rainwater, Ameren chairman, president and chief executive officer. "They have performed that task thoroughly and with the company's full cooperation, and Ameren is taking their conclusions very seriously. In the aftermath of the Taum Sauk breach, we have already begun a review of operational safety procedures throughout the Ameren system. Guided further by the Rizzo report, that review will continue, and all necessary corrective measures will be taken."

As part of these safety initiatives, the company is:

• Creating a dam safety program with a highly experienced civil engineer as its chief dam safety engineer. The program will include development of an updated dam inspection plan and implementation of site-specific safety and instrumentation training. The chief dam safety engineer has the authority to conduct unannounced facility inspections and to take necessary corrective action when dam safety is threatened\-including the issuance of a stop work order for operational activities and plant shutdowns. Acting as a single point of contact for state and federal regulatory officials, the chief dam safety engineer will work with a group of highly experienced engineering professionals\- including consultants--all with expertise in dam safety.

In addition, after considering the "lessons learned" from Ameren's own investigation of the Taum Sauk incident and from the Rizzo report, the company plans to:

• Establish an independent quality assurance team to review and strengthen, if necessary, operational processes and procedures throughout our generation system. The team will be analyzing engineering plans, looking at best practices and processes and implementing recommended changes.

• Reexamine the safety procedures at all of our facilities, including our emergency action plans; and

• Evaluate the training of all employees in safety and operations to insure they fully understand the signs of possible plant failure.

"We are taking multiple steps to identify potential risks at all plants, and we've established action plans to minimize those risks," said Thomas R. Voss, executive vice president and chief operating officer of Ameren Corporation. "Clearly our aim in hiring an independent expert was to seek a complete understanding of the factors behind the failure of the Upper Reservoir at Taum Sauk Plant. We're releasing the report voluntarily to advance public understanding of the event and its causes. We have found this report to be extremely constructive. The Taum Sauk breach has caused a great deal of soul- searching at our company, particularly because our investigation shows that everyone involved in this incident was well-intentioned. At every step of the way our employees took actions they believed were sufficient to protect the facility's safety. In hindsight, those steps clearly proved inadequate. It is incumbent upon us to do everything in our power to guard against something like this ever happening again. If we're going to be good stewards of the facilities placed in our care, and if we're going to be committed to achieving industry leading performance, we can do nothing less."

A summary of the findings of the Rizzo Associates' report follows:

Expert Points to Stability Failure, Poor Design, Specification and Maintenance of Instrumentation and Control Systems as Key Causes of Taum Sauk Failure

In its analysis of the root causes behind the Taum Sauk Plant Upper Reservoir rupture, Rizzo Associates cites stability failure as the root cause of the breach and the resulting uncontrolled rapid release of water. The firm's modeling indicates that the reservoir's failure began in the range of 10 to 13 minutes after overtopping began.

According to the report, the rock dam wall failed so quickly because of the quality of the material used in the dam's original construction -- material that deviated from the original design specifications. Rather than being constructed on bedrock, the original foundation was constructed on areas where up to 18 inches of soft material, such as soil or weathered rock, sat on top of the bedrock. In addition, because of the quantity of fine-grained material used in the dam, water that should have been able to pass through the rockfill dam was instead trapped inside, much like water saturates a sponge. As a result, the Dec. 14 overtopping event produced an excessive amount of water that collected in the dam material and at the foundation interface. This created an unsafe condition which caused the dam to fail in a rapid manner. When planning the Upper Reservoir, the company chose both dam design and contracting firms that were highly regarded at the time. However, Rizzo Associates found that while the original design of the Upper Reservoir was consistent with the practices of the late 1950s and early 1960s, the builders did not follow the best construction practices available at the time. (The dam was completed in 1963.) Rizzo Associates added that the reservoir's construction is not consistent with today's design and construction standards. These conditions were determined only through borings and visual inspections of the breach and thus could not have been uncovered by annual inspections of the plant by the Federal Energy Regulatory Commission, weekly operating inspections, frequent visual inspections or the five-year inspections conducted by independent dam experts.

In addition to the root cause, Rizzo Associates cites several contributing factors to the breach:

• Design and specification of the (water level) instrumentation system was not sufficiently conservative. (The plant had one control system monitoring the water levels that normally shut off the pumps and a back-up system that was designed to shut off the pumps should the normal level control system fail.)

Had the (backup water level) protection probes been maintained at the design elevations, the overtopping event may not have occurred. The failure of the level protection probes to remain at design heights was due to human error.

• Even given the loss of the (back-up) water level protection, overtopping still could have been prevented had the level control instrumentation supports not failed.

• Plant operators and technicians were following operational and inspection procedures as provided by AmerenUE; however, inadequate attention was paid to dike (dam) safety considerations with regard to design, operations and management of the facility.

• Responsibilities for plant operation and dike (dam) safety were combined under a single individual. Anyone with this job description might have been forced to balance dam safety and operational constraints.

The Rizzo Associates report was developed after collecting physical evidence through such initiatives as subsurface drilling and pressure testing; conducting comprehensive discussion with plant employees and others connected to the incident; and reviewing operating, maintenance and construction records and transcripts of all official interviews of plant personnel.

In addition to serving as an analysis of the root cause, the report also offers advice on future rebuild options. The consultants concluded that restoration of the Upper Reservoir to an operating condition cannot be achieved by simply repairing the breach area. Restoration, if undertaken, will require a complete rebuild of the entire dam with a completely different design concept---one that is substantially more robust, they wrote.

No decision will be made about rebuilding the Upper Reservoir until the FERC investigation is complete.

BACKGROUND: On Dec. 14, 2005, the AmerenUE Taum Sauk Plant experienced a breach in its Upper Reservoir that caused significant flooding in the Johnson's Shut- Ins State park. The plant's 1.5 billion gallon Upper Reservoir experienced a rupture in the northwest corner causing water to flow down the mountain. The company implemented its emergency plan and assembled a multi-disciplinary team of experts, company officials and consultants to analyze the event and determine next steps. Built in1963, AmerenUE's Taum Sauk Plant is a "pumped- storage" hydroelectric plant. It stored water from the Black River in the Upper Reservoir, built atop the 1,590-foot-high Proffit Mountain and released the water to generate electricity when power was needed. The plant employs 12. The water flowed down a mile-long tunnel inside the mountain, turning turbine-generators to produce electricity. When power demand was low, the same turbines ran in reverse to pump water back to the upper reservoir. AmerenUE is a subsidiary of St. Louis-based Ameren Corporation. Ameren companies serve 2.4 million electric customers and nearly one million natural gas customers in a 64,000-square-mile area of Missouri and Illinois.

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