Yes that's what I should have said as a clarification.  The HE material would explode but the fissile materials would simply scatter. The same effect is possible with any HE encased with radioactive material, even nuclear waste or isotopes from a laboratory.  One idea is to obtain radioactive iodine from chemotherapeutic treatment facilities and use that.  Even the excreta from a person undergoing active chemotherapy with I-131 could be used, putting a new twist on the expression "shit bomb".