Times are tough for some nuclear power plants in Pennsylvania.  After five years of no profits at Three Mile Island near Harrisburg, the owner, Exelon Corporation, announced plans to shut down TMI this year.  The Beaver Valley nuclear plant near Pittsburgh faces a similar fate in 2021.  But Rep. Tom Mehaffie, a co-chairman of the state's Nuclear Energy Caucus,  has an idea that he hopes will save TMI and Beaver Valley.    If an existing law is updated by adding nuclear energy to a list of clean forms of energy, it may allow the industry to thrive.