William H. Zimmer Power Station

The William H. Zimmer Power Station, located near Moscow, Ohio, was a 1.35 gigawatt (1,351 MW) coal power plant. Planned by Cincinnati Gas and Electric (CG&E) (a forerunner of Duke Energy), with Columbus & Southern Ohio Electric (a forerunner of American Electric Power (AEP)) and Dayton Power & Light (DP&L) as its partners, it was originally intended to be a nuclear power plant. Although once estimated to be 97% complete, poor construction and quality assurance (QA) led to the plant being converted to coal fired generation. The plant began operations in 1991. Today, the plant is owned and operated by Vistra Corp.

The construction of Zimmer was a consortium of CG&E, Columbus & Southern Ohio Electric, and DP&L. The project was announced in 1969 with a cost of $420 million to construct two 840 MW nuclear power units with completion dates set for 1975 and 1976. Construction of the plant did not start until 1972 because the Atomic Energy Commission (AEC) demanded stricter standards to approve of nuclear plants. The second proposed unit was upgraded from 840 MW to 1,170 MW in 1974. General Electric's BWRs were chosen for the plant's nuclear reactors. Ultimately, the second unit was never constructed due to regulatory issues and costs. An investigation into the construction of Zimmer began in 1978. A worker charged that a welding contractor did defective work below nuclear safety standards. In 1980 the Chicago Sun Times reported accusations of a cover up by CG&E of time card falsifications and the installation of defective piping in one of the plant's safety systems. As a result, the Nuclear Regulatory Commission (NRC) fined CG&E $200,000 for a faulty quality assurance program in 1981. The constructor was Kaiser Engineering whereas the primary owner, CG&E, did its own procurement, awarding contracts for equipment, and quality assurance requirements. The Federal Bureau of Investigation (FBI) launched an investigation into the plant's safety records and harassment of inspectors. Retired Navy Admiral Joe Williams Jr. was hired to bring the plant on line, and Bechtel was retained to nuclear qualify the plant. Bechtel came in with an estimate of more than $1.5 billion—beyond the $1.6 billion already spent—to adequately complete the plant. The lawsuit was settled in November 1987 with Sargent & Lundy agreeing to pay $27.4 million to the utilities. The utilities agreed to pay $12.7 million to Sargent & Lundy. The conversion process of Zimmer into a coal power plant began in March 1987. During the conversion of the plant from nuclear to coal, the Little Indian Creek was routed through the plant to protect fish populations. The plant now has several unique environmental protection features to keep the creek at its natural state. In addition, mussels along the riverbed at Zimmer were relocated upstream. After $3.3 billion in expenditures, the world's first nuclear to coal power plant was completed in 1991. Duke Energy sold its stake in Zimmer to Dynegy in 2014. AEP sold its 25% stake in Zimmer to Dynegy for a swap of Dynegy's stake in Conesville Power Plant in February 2017. In April 2017, Dynegy was given sole ownership of Zimmer when DP&L announced it would sell its 28% stake to Dynegy. Zimmer would become owned by Vistra Energy after Vistra's merger with Dynegy in 2018. In March 2020, Vistra Energy would settle with the Environmental Protection Agency (EPA) over a period of two decades of exceeding its emission operating limits. Vistra would pay a $600,000 fine to the Department of Justice (DOJ), retrofit $45,000 in energy efficient lighting at a nearby school, and replace three school buses that are cleaner burning.

In September 2020, Vistra announced it would be retiring the Zimmer plant by year end 2027. On July 19, 2021, Vistra announced the plant will shut down five years earlier, on May 31, 2022, because they failed to secure any capacity revenues in the May 2021 wholesale electricity market auction held by the grid operator, PJM Interconnection.

Zimmer utilizes equipment from the old nuclear plant in conjunction with the coal conversion equipment. The original Westinghouse high pressure/low pressure turbine was modified and the high pressure section was removed. The General Electric reactor was to generate 840 MW per the original nuclear design. The HP/IP generator produces 900 MW and the Westinghouse LP generator produces 497 MW. The plant's feedwater pump is powered by a turbine. The feedwater turbine was designed by ABB and generates
The boiler, designed by Babcock & Wilcox, is a supercritical steam generator with a maximum pressure of about and temperature of about The reheat section of the boiler operates at about The gypsum byproduct of the FGD system is sold to make drywall. The plant's cooling tower was designed to handle the cooling for the original nuclear plant. This creates generating efficiency issues during very hot summer days, as the cooling tower must cool much more equipment than it was originally designed for. A 573 feet (175 m) smokestack was constructed for discharging the flue gas.