It is a startup neutron source for nuclear reactors. It won't work as a neutron initiator in nuclear weapons (since you can't regulate its neutron flux), but it

It is a startup neutron source for nuclear reactors. It won't work as a neutron initiator in nuclear weapons (since you can't regulate its neutron flux), but it has the immense research value for criticality experiments if you are running your own nuclear program. P.S. you can't build a nuclear weapon out of californium, at all. To achieve the nuclear explosion, you need to assemble the supercritical configuration (minimum necessary k_eff about 1.53 for plutonium) without any stray neutrons leading to premature initiation, then, at the precise moment, inject neutrons in the system (even a single neutron is enough) to start the runaway chain reaction. Californium excels at spontaneous fission; there are so many neutrons in the system at any time, that premature initiation is guaranteed.