There are various basic power reactor concepts under consideration in the United States today. There are the light water moderated reactors, the high-temperature gas-cooled reactors, and the sodium-cooled reactors, to name just three. Of these, only the water reactors have reached the

stage where many units are being built and operated. For this reason, I shall confine my examples to this concept, although many of my com­ments will apply equally well to the other reactor concepts.

Within the light water reactor concept, there are two major design approaches — pressurized water reactors (pwr) and boiling water reac­tors (bwr). Since most of the considerations relevant to this volume are, in general, applicable to both design approaches and since I have spent the last ten years associated with bwr’s (although the four years before that were with pwr’s), I shall speak about the bwr; most of my remarks will be equally applicable to the pwr design and, for that matter, to all the principal reactor types.