def calc_thermal_fission_functions_with_U239(t0, ts, read_data)
      read_data.theEU239 * read_data.theR * (1 - Math.exp(-read_data.lamda1 * t0)) * Math.exp(-read_data.lamda1 * ts)
    end
def calc_thermal_fission_functions_with_U239(t0, ts, read_data)
      read_data.theEU239 * read_data.theR * (1 - Math.exp(-read_data.lamda1 * t0)) * Math.exp(-read_data.lamda1 * ts)
    end
Calculate U239 fission product decay heat power from ANS-5.1-1979 Eq.14
 ts: Time after remove (sec)
 t0: Cumulative reactor operating time (sec)

 calc_thermal_fission_functions_with_U239(t0, ts, read_data)

 return fU239