QUIZ 2 - MORE TERMINOLOGY - EXPLANATION SHEET

last update August 28, 1996

  1. The fraction of fast fission neutrons that slow down to thermal energy and cause fission is equal to keff/ p (where p = the resonance escape probability). FALSE. It's keff/nu, where nu is the Greek letter representing the number of neutrons released per fission.

  2. Reactivity represents the fractional change in the fission neutron population in one second. FALSE. Reactivity is based on keff, which is derived in terms of generation time ... the fractional change in the neutron population in one generation time.

  3. At full power, the strongest neutron source in the core is the non-fission neutron source. FALSE. The strongest neutron source in the core at full power is the delayed neutrons, or their precursor atoms. See NUKEFACT #2

  4. The fraction of fission neutron production that is prompt neutrons is (1 - beta), where beta equals the precursor yield fraction. TRUE. Beta defines production fraction, or yield fraction, not fraction of the neutron population. See NUKEFACT #1

  5. The number of U-235 atoms fissioned per second to produce one watt of power is 31 billion. TRUE. The conversion factor as you know it, in scientific notation, is 3.1x10 10 fissions/sec/watt.

  6. A precursor atom is a fission fragment that undergoes radioactive decay to produce a prompt fission neutron. FALSE. Precursor decay produces a delayed neutron.

  7. Doubling time is always longer than the corresponding value of the reactor period. FALSE. The reactor period is the time required for power to change by a factor-of-e. Since e = 2.718 ... it obviously takes longer at a given rate to change by a factor-of-e than by a factor of 2. The mathematical relationship between doubling time and period is T = DT/0.443.

  8. If reactor power is increasing with time, the reactor is supercritical. FALSE: The direction of power change does not establish whether the reactor is subcritical or supercritical. Three factors in the denominator of the period equation contribute to the direction of power change. These are rho-dot (the reactivity rate), lambda x rho (the rate of delayed neutron source change), and lambda x (S-bar/P) (the rate of non-fission source change relative to the total neutron population).

  9. The number of prompt neutron life cycles occuring during each second of a typical chain reaction is 10,000. TRUE. The typical prompt neutron lifetime, from birth as a fast fission neutron until loss by fission, is 1x10-4 seconds. Since the prompt neutrons are the propagators of the chain reaction, the number of prompt neutron life cycles that must occur in one second is the inverse of the prompt neutron lifetime.

  10. A prompt neutron slows down in a shorter time interval than a delayed neutron because of its greater speed at birth. FALSE. The delayed neutron slows down in a shorter time, on average, because the birth energy of the delayed neutron, at about 0.25 Mev, is less than the mean birth energy of the prompt neutron at 1 Mev. The energy reduction, to thermal energy, for the delayed neutron is less than for the prompt neutron.

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