Just mass and geometry will do it. Critical mass for a sphere is about 10kg. This photo contains much more than 10kg tho. It doesn't need to be a sphere to go critical, it just needs the right geometry of the right mass. It won't be an atomic explosion like a bomb going off, but it will generate a burst of neutron radiation, gamma, and heat that will blow the geometry apart. The plutonium rods will be fine but anyone nearby will be as far from fine as it is possible to be