Complex sentence: Thorium fueled reactors may pose a slightly higher proliferation risk than uranium based reactors because, while Pu-239 will fairly often fail to undergo fission after neutron capture and produce Pu-240, the corresponding process in the thorium cycle is relatively rare.
Split into simpler sentences: Thorium fueled reactors may pose a slightly higher proliferation risk than uranium based reactors. The reason for this is that while Pu-239 will fairly often fail to undergo fission on neutron capture, producing Pu-240, the corresponding process in the thorium cycle is relatively rare.