[en] The potential energy in Joules and the potential radioactive releases following accidents with FBR type reactors and thermonuclear reactors are used in a discussion of the accident potentials of such systems. The problems of routine releases during normal operation are also discussed. The release of plutonium from reprocessing plants is considered to be the main hazard with FBR based power production, and it is pointed out that virtually all Pu 239 released in a 10,000 year period will accumulate. This will require release rates as low as 10^-5-10^-6 %. Thermal pollution will also be a problem and an energy production of 10¹⁴W will require 15% of the total runoff from the land surface for cooling water. Finally the damage to structural materials by fast neutrons will present problems. The author considers that the problems considered will probably be less by several orders of magnitude for thermonuclear reactors than for FBR systems. (JIW)