I don’t know anything about the plutonium fuel; are you sure it is fuel and not the fissioned spent fuel? My experience has been with PWRS, which are may be fueled differently.
I did find the following per Wikipedia:
Size
A modern BWR fuel assembly comprises 74 to 100 fuel rods, and there are up to approximately 800 assemblies in a reactor core, holding up to approximately 140 tons[vague] of uranium {see my note below]. The number of fuel assemblies in a specific reactor is based on considerations of desired reactor power output, reactor core size and reactor power density.
Safety systems Main article: Boiling water reactor safety systems
A modern reactor has many safety systems that are designed with a defence in depth philosophy, which is a design philosophy that is integrated throughout construction and commissioning.
A BWR is similar to a pressurized water reactor (PWR) in that the reactor will continue to produce heat even after the fission reactions have stopped, which could make a core damage incident possible. This heat is produced by the radioactive decay of fission products and materials that have been activated by neutron absorption. BWRs contain multiple safety systems for cooling the core after emergency shut down.
Refueling systems
The reactor fuel rods are occasionally replaced by removing them from the top of the containment vessel.
Because they are hot both radioactively and thermally, this is done via cranes and under water. For this reason the spent fuel storage pools are above the reactor in typical installations. They are shielded by borated water several times their height, and stored in rigid arrays in which their geometry is controlled to avoid criticality. In the Fukushima reactor incident this became problematic because water was lost from one or more spent fuel pools and the earthquake could have altered the geometry. The fact that the fuel rods’ cladding is a zirconium alloy was also problematic since this element can react with water at extreme temperatures to produce hydrogen and oxygen, and can ignite in air. Normally the fuel rods are kept sufficiently cool in the reactor and spent fuel ponds that this is not a concern, and the cladding remains intact for the life of the rod.
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Note: 140 tons of uranium can be a deceptive amount. It is a very heavy element, has very high density being approximately 70% denser than lead, and what appears to be a small amount can be surprisingly heavy in weight!
I think it is weapons grade material that has been processed to make it useful as a fuel, mainly, I think, as a way of turning it into something that terrorists can't use. I think the material in Japan came from France.