def calc_thermal_fission_functions_with_Np239(t0, ts, read_data)
      read_data.theENp239 * read_data.theR * ((read_data.lamda1 / (read_data.lamda1 - read_data.lamda2)) *
      (1 - Math.exp(-read_data.lamda2 * t0)) * Math.exp(-read_data.lamda2 * ts) -
      (read_data.lamda2 / (read_data.lamda1 - read_data.lamda2)) * (1 - Math.exp(-read_data.lamda1 * t0)) * Math.exp(-read_data.lamda1 * ts))
    end
def calc_thermal_fission_functions_with_Np239(t0, ts, read_data)
      read_data.theENp239 * read_data.theR * ((read_data.lamda1 / (read_data.lamda1 - read_data.lamda2)) *
      (1 - Math.exp(-read_data.lamda2 * t0)) * Math.exp(-read_data.lamda2 * ts) -
      (read_data.lamda2 / (read_data.lamda1 - read_data.lamda2)) * (1 - Math.exp(-read_data.lamda1 * t0)) * Math.exp(-read_data.lamda1 * ts))
    end
Calculate Np239 fission product decay heat power from ANS-5.1-1979 Eq.15
 ts: Time after remove (sec)
 t0: Cumulative reactor operating time (sec)

 calc_thermal_fission_functions_with_Np239(t0, ts, read_data)

 return fNp239