As has been discussed before, a chain reaction may occur in a block of fissile material, big enough to assure that not too many fission neutrons manage to reach the surface from the inside and escape. This is the so-called “critical mass”.
But if a chain reaction occurred in a block of pure or nearly-pure fissile material, the consequences might be catastrophic (well, sometimes this is the goal it how nuclear weapon works). In this text, however, we want to talk only about peaceful applications of nuclear energy. A chain reaction may release much energy that can be converted to electricity – but only if there are means for controlling the process. There is no way of controlling the process in a bulk piece of pure fissile isotopes. The “avalanche” develops very fast, the chain reaction simply causes the material to explode. For making the chain reaction “controllable”, it must be slowed down. But how to achieve such a goal?
Let’s begin with a discussion of what means are at our disposal. The first and the most important question is: what kind of“nuclear fuel” can we use? Now, there are several different types available. But let’s talk about the situation at the very beginning of the Atomic Age: then, the pioneers were left by Mother Nature with just one choice: natural uranium.has already left us with little choice: natural uranium. Its is a mixture of two isotopes: it contains 99.3% Uranium-238, which is not fissile, and only 0.7% of Uranium-235, which is. Only one atom out of 140 not-fissile ones.
But it’s not the only problem: U-238 is not merely a “neutral” component, it is fissionable, meaning that it gladly absorbs fission neutrons from U-235, undergoes a fission, contributes 200 MeV of energy but releases no neutrons. No neutrons needed for maintaining the chain reaction! In other words, U-238 is a “kidnapper” of fission neutrons. There is no way that a chain reaction, controlled or uncontrolled, can ever occur in a piece of natural uranium
Nevertheless, Enrico Fermi, Leo Szilard, and a few other pioneers man- aged to successfully start a self-sustained and controlled chain reaction on Dec. 2, 1942, in a device they called “the pile”, which used nothing else but natural uranium as a fuel. After that success, a number of other reac- tors were built, all using natural uranium which remained the only available nuclear fuel for several more years. Then, many new types of fuel emerged (e.g., “enriched” Uranium with increased U-235 content, or uranium mixed with the “man-made” fissile element Plutonium). Most currently existing reactors in civilian nuclear power plants worldwide use those new types of fuel. But for explaining how a nuclear reactor works it still make sense to begin with an analysis of the design and the physical processes occurring in a simple reactor prototypes using natural uranium fuel4. And the basic principles of operation of modern reactors remain essentially the same as in those old prototypes
4In fact, the progress in developing new more sophisticated fuel types has not made natural uranium fuel completely obsolete: there is a CANDU family of Canadian-designed power reactors running on natural uranium – 29 operational in several different countries, and a few more under construction