Fission
1. The fission of 235U can be triggered by the absorption of a slow neutron by a nucleus. Can a slow
proton be used to trigger 235
92 U fission?
2. When a large nucleus splits during nuclear fission, the daughter nuclei of the fission fly apart with
enormous kinetic energy. Why does this happen?
3. What mass of 235
92 U has to undergo fission each day to provide 3000 MW of thermal power?
4. Calculate the energy released in the fission reaction Qfis
1
235
140
94
1
0 n + 92 U → 54 Xe+ 38 Sr + x 0 n
You can ignore the initial kinetic energy of the absorbed neutron. What is the values of x for this
reaction? The atomic masses are: mu= 235.043923u; mXe= 139.921636u; and mSr= 93.915360u.
Calculate the total fission energy of 1 kg of 235
92 U assuming all fissions proceed via this reaction.
5. A 186
76 Os nucleus at rest decays by the emission of a 2.76-MeV α particle. Calculate the atomic mass of
the daughter nuclide produced by this decay, assuming that it is produced in its ground state. The atomic
mass of 186
76 Os is 185.953838 u.
6. Assume that the average time τ between production and absorption of a neutron in a reactor
is 10−3 s. Calculate the number of free neutrons present at any time in the core when the reactor
is operating at a power level of 1GW.
7. A beam of neutrons of 0.1eV is incident on 1 cm3 of natural uranium. The beam flux is 1012
235
neutrons s−1cm2. The fission cross section of 235
92 U at that energy is 250 b. The amount of 92 U is
0.72%. The density of uranium is 19 g cm−3. Each fission produces 165MeV in the material.
What is the nuclear power produced?
8. Consider a nuclear plant producing an electric power of 900 MW with thermal neutrons and
enriched uranium at 3.32% in 235
92 U . The total yield of nuclear energy into electric energy is
R = 1/3 (including the thermal yield). The total uranium mass is 70 tons.
1. How many 235
92 U atoms are burnt per second?
2. What mass of 235
92 U is used per day?
3. Assuming the plant works at constant full power, how long can it run before changing the
fuel?
9. Using the Table in Appendix G, explain why 142
60 Nd would not be expected to be abundantly
produced in a nuclear reactor, unlike the other stable Nd isotopes.
10. Estimate the amount of uranium needed to create 100 Mw-yr of electrical energy assuming
a thermal-to-electricity efficiency of 0.3. This is the amount 328 6. Fission of uranium considered
in Fig. 6.15. In this figure, translate the thermal power to decay rate (in Bq) by assuming
5MeV per decay. Discuss the origin of the nuclides shown in the figure.
G. Table of Nuclei