I read Physics amongst other subjects at Cambridge University and have a Masters degree and Senate membership of that body . By incorporating the gamma radiation from a fission reaction along with the fission neutron in a PROCESS of PHOTODISINTEGRATION , a chain reaction may be sustained without a CRITICAL MASS of fissile material . Amongst other things Plutonium 239 may be produced for 300 GBP a kilogram .
The schematic below shows the radiation path within the cell which costs about 250 GBP to construct . This is how North Korea afforded its own Nuclear Program .