In a conventional stationary NTR, the energy E for heating the propellant is almost from the fission fragments which encompass almost the 95 % of the total energy, and the faction of energy from prompt neutrons f _ \ text { n } is only around 5 %, and therefore, in comparison, is almost negligible.
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The neutrons that occur directly from fission are called " prompt neutrons, " and the ones that are a result of radioactive decay of fission fragments are called " delayed neutrons . " The fraction of neutrons that are delayed is called ?, and this fraction is typically less than 1 % of all the neutrons in the chain reaction.
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However, if the nuclear core is pulsed-as previously discussed, it is able to produce N times more energy than the stationary mode, and then the fraction of prompt neutrons or f _ \ text { n } N could be equal or larger than the total energy in the stationary mode; and because this neutron energy is directly transported from the fuel into the propellant as kinetic energy-unlike the energy from fission fragments which is transported as heat from the fuel into the propellant, then is not constrained by the second law of thermodynamics, meaning that there is no impediment to transport this energy from the fuel to the propellant even if the fuel is colder than the propellant, in other words, it is possible a " propellant hooter than the fuel " which is the very limit for specific impulse enhancement in classics NTRs.
