SNUPPS is an acronym standing for Standardized Nuclear Unit Power Plant System. It refers to a 4-loop PWR

Pressurized water reactor

Pressurized water reactors constitute a large majority of all western nuclear power plants and are one of three types of light water reactor , the other types being boiling water reactors and supercritical water reactors...

 reactor design produced by Westinghouse in the 1970s. The design was developed for 4 USA utilities, and plants were built at Callaway

Callaway Nuclear Generating Station

The Callaway Plant is a nuclear power plant located on a 5,228-acre site in Callaway County, Missouri, near Fulton, Missouri. It began operating on December 19, 1984. The plant, which is the state's only commercial nuclear unit, has one 1,190-megawatt Westinghouse four-loop pressurized water...

 and Wolf Creek. The UK plant at Sizewell B was also based on SNUPPS but with significant modifications, such as a passive Emergency Boration System.