We started in Los Alamos. Mici and my son, Paul, arrived a few weeks later. We got a little house, or rath- rather, a little apartment in a house and were comfortable. And Wigner's prediction that we would be not facing any important points, any important problems, turned out to be very, very wrong. At first we thought we knew what to do. We had to bring together an amount of active material, the material to be produced in Chicago so that each half of it would be harmless, because more neutrons could dif- diffuse out of it than where produced in the same time by the fission process. But if the two pieces were brought together, then they would explode and the whole point was to bring them together fast enough so that they should not explode prematurely when the neutrons were barely able to multiply. Now that did not seem so difficult because to start the whole process needed a neutron and there were a few- few enough neutrons around naturally so that the two pieces could be brought together fast enough, if one piece was shot into the other by a gun. And that's what we were working on. And then came the discovery. Yes, if you used uranium 235 as the active multiplication material, it could be done but if you tried to use the material that came out of the reactors, plutonium, then it would be a very, very difficult problem, why? The material we tried to make was plutonium, one unit more heavy than the ample uranium, uranium 238, from which we started. We fed uranium into a reactor, the uranium would ab- would absorb a neutron, become uranium, of the weight 239; that made two disintegrations, turned into plutonium and that is a very good bomb material. Nothing wrong with that so far, except that when you leave the material for a long enough time in the reactor, you also made another kind of plutonium, plutonium 240 and that was a material which spontaneously emitted neutrons, spontaneously split into two much too frequently and following that it would emit a neutron. And so, our pieces of plutonium that we were to bring together were swamped with neutrons to a sufficient extent where- where at the time when they became just slightly critical, when the neutrons just began to multiply, they multiplied fast enough to throw the whole thing apart and get a small explosion instead of a big one. What to do about that?
