The United States also had something similar -- the Mark 18[1], designed as a very large fission bomb in case the first thermonuclear bomb failed, had four critical masses of uranium-235 in its core. Even one of the implosion lens detonating could have led to a kiloton-range explosion. To prevent such an accident, a chain of aluminum and boron (the latter being an especially good neutron absorber) was inserted in the core until it was ready to be used.