Owners of the Oyster Creek nuclear power plant say they will shut it down rather than build cooling towers mandated by state environmental regulators. Exelon Corp. says the $800 million it would cost to build the towers is more than the 40-year-old Nuclear Generating Station is worth. But environmentalists say the job could be done for about $200 million. Earlier this year, the state Environmental Protection Department required the Lacey Township plant to build one or more cooling towers instead of relying on water drawn to cool the reactor. The state says that process kills billions of shrimp and tens of thousands of fish, crabs and clams each year.