Work began Monday on a refueling outage at Byron unit 1, bringing in a surge of temporary workers and beginning one of the most extensive maintenance efforts at the Illinois reactor in recent years.Roughly 2,200 temporary workers will join the 850 permanent workers employed at both units, according to plant owner Exelon. In addition to replacing 1/3 of unit 1's fuel, crews will complete about 14,000 inspections and maintenance tasks at the reactor. Exelon did not provide an estimate for when the unit will come back online.“This is our biggest outage in more than a decade and we’re glad so many people will be able to come to the site and work,” Byron Station Vice President Tim Tulon said in an Exelon release. “We need the extra assistance; and many of these professionals currently need work. It’s a wonderful opportunity for all to benefit.”Unit 2 will remain online. According to the Nuclear Regulatory Commission, both units are Westinghouse pressurized water reactors generating roughly 1,150 megawatts each and first licensed in the mid-1980s.