Which country first developed a neutron bomb?

Answer 1

us

Yes, the United States was the first to develop a Neutron bomb, it was originally part of the United States efforts following the fallout problems with the 1954 Castle Bravo test (15Mton) to develop a low fallout or "clean" Hydrogen bomb. In a standard Hydrogen bomb typically 90% of the yield and fallout is produced by fission of the depleted uranium tamper by the high energy fusion neutrons from the compressed hydrogen fuel assembly fusing. If one were to replace the depleted uranium tamper with one made from an element almost as dense as uranium, non-fissionable, and transparent to the high energy fusion neutrons (e.g. lead); then at some loss of yield one could eliminate almost all the fallout of a Hydrogen bomb! The fact that such a "clean" Hydrogen bomb would emit a very high local high energy neutron flux was not even considered at the time, just the reduction in fallout.

The first such "clean" Hydrogen bomb was tested in the 1956 Redwing Zuni test (3.5Mton) had a yield that was only 15% fission with a similar reduction in fallout. The nearly identical standard Hydrogen bomb the 1956 Redwing Tewa test (5Mton) had a yield that was 87% fission. A second, different, "clean" Hydrogen bomb was tested in the 1956 Redwing Navaho test (4.7MTon) had a yield that was only 5% fission! It was first realized in 1958 by Sam Cohen at LLNL that further changes (e.g. more neutron transparent tamper, high efficiency hollow core tritium gas boosted fission trigger) could produce a weapon generating more of its yield as neutron radiation than as blast (this was first tested underground in Nevada in 1963).

After long years of debate over the potential advantages/disadvantages of "clean" vs. standard vs. "dirty" (aka "salted") Hydrogen bombs, both "clean" and "dirty" designs were abandoned because of the lost yield for the same usage of expensive nuclear materials. The "clean" Hydrogen bomb returned (with some design changes, see above) in the middle 1970s, but now renamed the Neutron bomb or ERW (Enhanced Radiation Weapon). Carter delayed deployment, Reagan deployed it, Bush withdrew it.

Do not confuse a dirty radiological bomb with a "dirty" Hydrogen bomb. The radiological bomb is just an ordinary chemical explosive wrapped in highly radioactive isotopes, which it disperses when it explodes. Such a bomb will probably kill everyone involved in assembling it before it affects anyone at its target. A "dirty" Hydrogen bomb uses a tamper made of a material that captures neutrons and transmutes into highly radioactive isotopes, increasing the fallout.