The W81 went through several design iterations, starting with an enhanced radiation model, then a pure fission model, and finally was simply canceled in 1986 without ever being tested or produced. Characteristics are not known in detail, but the B61 it is derived from has a physics package (bomb core) of about 12 inch diameter with length of 32 inches, weighing around 300 pounds (see the W80, another B61 derived design). Available LASL images show a much shorter weapon, perhaps 12 by 16 to 18 inches, probably the final fission-only W81 concept, corresponding with the size of the B61 fission primary alone.