Nevertheless a reactor accident could become exceedingly dangerous. The reactor is charged with radioactive fission products and some other radioactive substances produced by neutron absorption. Any accident which will allow even a portion of these products to escape into the air will endanger people at a considerable distance in the downwind direction. One reason why reactors can be dangerous is that in protracted operation of the reactor, fission products which have longer lives accumulate. It is precisely these longer-lived products which are more dangerous because they have a better chance to find their way into the human body. Reactors are now planned which will produce 300,000 kilowatts of electricity. If such a reactor operates for half a year and then explodes and releases its radioactive content into the atmosphere, its radioactivity will be comparable to that of a hydrogen bomb. In one important respect such an accident would be worse than a hydrogen explosion. The nuclear explosive lifts most of its radioactive products to a high altitude and the poisonous activity gets dispersed and diluted before it descends. The activity from a reactor on the other hand will remain close to the ground and might endanger the lives of the people in an area of hundreds of square miles. It will contaminate an even greater territory. In the extensive operation of many reactors in the United States no one has yet been killed by the radioactivity. This has been due to extremely careful operation and also to good luck. We must be prepared that sooner or later accidents will occur. On the other hand we must try to take sufficient precautions to avoid the kind of catastrophic accident which we have mentioned above. With great care such accidents can indeed be avoided. In thinking of all kinds of man-made machines we find some which move fast and seem dangerous like, for instance, airplanes; others which are stationary and apparently harmless, like the bath tub. Yet more accidents happen in bath tubs than in air travel. The most dangerous element in all operations is the human element. We ourselves constitute the greatest safety hazard. This is a situation no different in nuclear technology than in any other kind of technology. What is new in nuclear technology is that a reactor is usually very safe but may become extremely dangerous when something unexpected happens to it. Also we dare not use the method of trial and error. An error in the reactor business could exact a far heavier toll of lives than an error in the testing of H-bombs. We cannot wait to learn by experience; we must forestall accidents. An especially difficult safety problem is connected with the use of reactors in small countries. A serious accident could endanger the lives of people in adjacent countries. Thus modern technology may force cooperation across national boundaries.