In this case the time between successive generations of the reaction, " T ", is only limited by the fission rate from the prompt neutrons, and the increase in the reaction will be extremely rapid, causing a rapid release of energy within a few milliseconds.
12.
Apart from traditional uranium charges, combinations of uranium with americium and thorium have been tried, as well as a mixed plutonium-neptunium charge, but they were less successful in terms of yield and was attributed to stronger losses of heavy isotopes due to enhanced fission rates in heavy-element charges.
13.
The low uranium-235 density in natural uranium ( 0.7 % 235 U ) compared with enriched uranium ( 3-5 % 235 U ) implies that less fuel can be consumed before the fission rate drops too low to sustain PWR reactors ( 50 GW . d / t ).
14.
In contrast, a critical assembly is said to be prompt-critical if it is critical ( " k = 1 " ) without any contribution from delayed neutrons and prompt-supercritical if it is supercritical ( the fission rate growing exponentially, " k > 1 " ) without any contribution from delayed neutrons.
15.
Unfortunately, experiments by Emilio G . Segr?and his P-5 Group at Los Alamos on reactor-produced plutonium showed that it contained impurities in the form of the isotope plutonium-240, which has a far higher spontaneous fission rate than plutonium-239, making it unsuitable for use in the Thin Man gun-type nuclear weapon design.
16.
However, without addition of a neutron poison or active neutron-absorber, " decreases " in fission rate are limited in speed, because even if the reactor is taken deeply subcritical to stop prompt fission neutron production, delayed neutrons are produced after ordinary beta decay of fission products already in place, and this decay-production of neutrons cannot be changed.
17.
Studies of these by Emilio G . Segr?and his P-5 Group at Los Alamos revealed that it contained impurities in the form of the isotope plutonium-240, which has a far higher spontaneous fission rate than plutonium-239 . This meant that it would be highly likely that a plutonium gun-type nuclear weapon would predetonate and blow itself apart during the initial formation of a critical mass.
18.
Because reactor-grade plutonium contains isotopes of plutonium with high spontaneous fission rates, and the ratios of these troublesome isotopes-from a weapons manufacturing point of view, only increases as the fuel is burnt up for longer and longer, it is considerably more difficult to produce fission nuclear weapons which will achieve a substantial yield from higher-burnup spent fuel than from conventional, moderately burnt up, LWR spent fuel.
