Question

1a Describe how thermal energy is obtained by fission, specifically when a U=235 atom is hit by a slow-motion neutron (less than 0.9 MeV). When one states that 200 MeV of energy are released by a fission event, what is the form of this energy?

1b. The energy released by fission is by the change in mass and Einstein’s equation (E=Δmc²). If the mass of one uranium 235 atom is 235.042924 amu and the mass of fission products is 234.92754 amu, determine the energy in Joules and MeV released by one fission. (Note: 1 amu = 1.66 x 10^-27kg and 1 ev = 1.6 \x 10^-19J).

1c. For an atom, the atomic number is the number of protons, while the atomic weight (i.e. molecular weight) can be 235 or 238 depending on the isotope. Determine the number of atomic particles (electrons, neutrons, and protons) for U-235, U-238, Kr-90. Note: The atomic number of Krypton is 36.

Answer 1

Solution :

Part 1 a : THERMAL ENERGY BY FISSION REACTION

Nuclear fission is a process of radioactive decay of a heavy nucleus generating two or more lighter daughter nuclei with the release of energy. This is an exoergic reaction.

When Uranium-235 is struck using thermal neutrons with energy less than 0.9 Mev, there is a greater probability of absorbing a neutron before splitting into daughter nuclei with the release of energy. There various components contributing to the total energy release in fission are :

1. Kinetic energy of fission fragments,
2. Kinetic energy of prompt neutrons,
3. Energy of prompt gamma- rays,
4. Energy of particles emitted by fission fragments,
5. Energy of anti neutroinos emitted by the fission fragments, and
6. Energy of gamma- rays emitted by fission fragments

The energy released during nuclear fission can be measured by bombarding a piece of uranium with thermal neutrons, which is found to be heated due to absorption of fission fragments and other residual products. The heat thus generated can be measured by calorimetric method. The enormous quantity of energy release in nuclear fission is nest explained by binding fraction curve. As U-235 is a heavy nucleus with binding energy fraction close to 7.6 MeV per nucleon, the fragments produces in its fission have mass numbers somewhere in the center of the periodic table.

PART 1(B) :

Consider the following fission decay in a typical case :

PART 1(C) :

For U-235,
Atomic Number, A = 92
Mass number of the isotope, Z= 235
Number of protons = Atomic number=92
Number of neutrons= Z-A = 235-92= 143
Number of electron = Number of protons = 92

For U-238,
Atomic Number, A = 92
Mass number of the isotope, Z= 238
Number of protons = Atomic number=92
Number of neutrons= Z-A = 238-92= 146
Number of electron = Number of protons = 92

For Kr-90,
Atomic Number, A = 36
Mass number of the isotope, Z= 90
Number of protons = Atomic number=36
Number of neutrons= Z-A = 90-36= 54
Number of electron = Number of protons = 36