In the beginning of the 1990’s, in the course of the events which were rapidly cha- ing the political con?guration of the East European countries, the crisis which - vested the vast research apparatus of the former Soviet Union was entailing con- quences whose dimension and depth were immediately realized by the international scienti?c community. In the same years, however, the most important branch of nuclear energy - searchanddevelopment,inparticularthatconcerning?ssionreactor,wasworldwide undergoing a substantial reduction due to a variety of decisional situations. Yet, paradoxically, it was a very good fortune that a number of concerns on the future of nuclear research were shared by East- and West-European scientists, especially those who were working in advanced ?elds. In fact, the only hope for coping with an uncertain future was to erect bridges between similar institutions and employ safeguarding tactics linked to a long term collaboration strategy. A decade later, this proved to be a winning decision, since the revival of nuclear energy is presently starting from a basis of common intentions and a network of established cooperation, whose seeds are to be searched in those initial, individual e?orts.

The present volume Uranium C5 covers the physical properties of U0 - the production 2 and preparation of U0 were already treated in Uranium C4, whereas the chemical proper 2 ties will be the subject of the forthcoming part C6. U0 is the most important chemical compound in all aspects of nuclear technology. 2 It is and will be for the foreseeable future the fuel for all light and heavy water reactors as well as (in the mixed crystal with Pu0 ) for the fast breeder reactors. Therefore, the 2 nuclear engineer has to understand the behavior of U0 under all conditions existing during 2 operational (and possibly failure) states of a nuclear reactor, e. g. , not only in the solid state but also to some extent in the liquid and gaseous states. Besides high scientific interest in the sometimes unique or unusual properties, e. g. , at low temperatures, a lot of data and physical properties which are critical for its use as a nuclear fuel have been determined more or less accurately. Creep, swelling, irradiation densification, and fission gas behavior in the fuel are properties which have been evaluated up to the high temperatu res (near the melting point) which may exist in U0 fuel due to its low thermal conductivity. 2 Besides these more technical data there have been accumulated a lot of important physical data, e. g.