Researcher Peter Vogel has web-published a very detailed, book-length footnote to this historical incident entitled The Last Wave From Port Chicago. He claims that in addition to the Mark I uranium metal bomb with 20 kiloton yield under development at Los Alamos and first used in August 1945 on Hiroshima, there was also a Mark II uranium hydride bomb under development with a yield of “only” 2 kilotons and intended to destroy enemy defenses on invasion beaches of Japan. Because of its greater simplicity and lower uranium requirements, the Mark II was supposedly developed a year before the Mark I. Vogel claims the Mark II bomb was accidentally detonated, or perhaps even deliberately tested, at Port Chicago. To maintain strict secrecy for another year of Manhattan Project development of the Mark I, the fiction of a conventional munitions accident as a plausible cover story was implemented and continues to this day.
It sounds like sci-fi, but could it be true? Did the home-port detonation of a revolutionary experimental weapon and a follow-on cover-up really occur? Sounds horrific today, but the US was in a life-or-death war at that time, only a month after the Saving-Private-Ryan D-Day invasions and with planning underway for similar invasions of Japanese beaches that would have estimated American casualties approaching a million men. Vogel makes a surprisingly convincing argument that the Port Chicago explosion was in fact low-yield nuclear, worth reading for his insights into the Manhattan Project Mark II program alone.