The Iowa Utilities Board today voted unanimously to allow Interstate Power and Light Co. (IPL) to amend and extend its long-term contract to purchase electricity generated by the Duane Arnold Energy Center in Palo, the state’s sole nuclear plant.

The amended 12-year contract is expected to lower the overall cost to IPL customers for electricity produced by the plant, which is owned by NextEra Energy Inc.

In a press release, the utilities board said the resulting rate adjustments will be included in IPL’s energy adjustment clause beginning on Feb. 22, 2014. Rob Hillesland, an Iowa Utilities Board spokesman, said the amount of the rate decrease is confidential because it was part of the contract negotiations between the companies.

The Duane Arnold Energy Center is estimated to supply 32 percent of Iowa’s emission-free power.

Until the new contract becomes effective, electricity rates will be maintained at current levels for all IPL customers. IPL, a subsidiary of Alliant Energy Corp., served more than 483,000 customers in Iowa as of the end of 2011.