Engineers are evaluating two candidate fuel cycles for a deep-space propulsion system
PHYSICS · Multi-part · AP · Physics 2: Algebra-based · Modern Physics · Modern Physics
Problem
Engineers are evaluating two candidate fuel cycles for a deep-space propulsion system. The first cycle employs deuterium–tritium fusion through the reaction
2H+3H→4He+n+17.6MeV.
The second cycle couples this fusion reaction with a neutron-driven fission of 235U using the fast neutron produced:
n+235U→92Kr+141Ba+3n+200MeV.
Assume each fission neutron is completely absorbed within the fuel assembly and does not trigger additional reactions beyond the single fission specified.
(1) Classify each individual reaction in the combined cycle as fusion, fission, or radioactive decay, stating for each whether it is spontaneous or induced.
(2) Using the energy values provided, derive an expression for the net energy released per initial deuterium–tritium fusion event when exactly one of the emitted fusion neutrons induces the 235U fission described. Show all reasoning in symbolic form before substituting numerical values.
(3) Compare the neutron economy of the combined cycle with that of an isolated deuterium–tritium fusion cycle in which none of the fusion neutrons is harvested. Discuss how neutron availability influences both energy output and radiation shielding requirements for a spacecraft.
(4) Given equal initial masses of deuterium–tritium fuel mixture and 235U, evaluate which fuel contributes more to the total energy output after one complete pass through the combined cycle described. Support your conclusion with quantitative or properly scaled qualitative arguments based on atomic mass numbers.