4. (20) In our thermal reactor utilizing a novel fuel of 15%U235, we want to track our neutron population during slowing down through fission as we need to slow our neutrons down to thermal energies. Of 100 neutrons born at fast energies in one generation, 1 is lost to fast fission, 15 leak while at fast energies, 15 are caught in the resonances, 4 leak while at thermal energies, and 80% of the remaining neutrons are absorbed in the fuel. Each productive absorption in the fuel results in the birth of 2 new neutrons. (a) Write the six factors of the keff formula. (b) Calculate keff and the associated reactivity. Discuss the results