April 18, 2008

Cause of high yield - Nuclear Bombs

The yield of 15 megatons was two and a half times what was expected. The cause of the high yield was a laboratory error made by designers of the device at Los Alamos National Laboratory.

It was expected that lithium-6 isotope would absorb a neutron from the fissioning plutonium, emit an alpha particle and tritium in the process, of which the latter would then fuse with deuterium (which was already present in the LiD) and increase the yield in a predicted manner.

The designers missed the fact that when the lithium-7 isotope (which was considered basically inert) is bombarded with high-energy neutrons, it absorbs a neutron then decomposes to form an alpha particle, another neutron, and a tritium nucleus. This means that much more tritium was produced than expected, and the extra tritium in fusion with deuterium (as well as the extra neutron from lithium-7 decomposition) produced many more neutrons than expected and induced more fission of the uranium tamper, increasing yield.

This resultant extra fuel (both lithium-6 and lithium-7) contributed greatly to the fusion reactions and neutron production, and in this manner greatly increased the device’s yield. The test used lithium with a high percentage of lithium-7 only because lithium-6 was (at the time) scarce and expensive; the later Castle Union test used almost pure lithium-6. Had more lithium-6 been available, the usability of the common lithium-7 might not have been discovered.