The Kraftwerk Union concept of a small nuclear district-heating plant

Kraftwerk Union (KWU) is currently assessing the potential of nuclear district-heating plants ranging in size from 200 to 500 MW(th). An acceptably low capital cost rather than technical feasibility is seen as the major problem. To meet this challenge, experience with components and systems technology for large LWRs has been drawn upon considerably. The design, however, is tailored to the specific requirements dictated by the intended application. Compared with power reactors these have, in essence, a much smaller unit size, lower temperatures and pressures, and less demanding requirements for load-following capability. Core, control-rod drives and intermediate heat exchangers are integrated within the reactor pressure vessel. The vessel in turn is housed in a containment that is basically formed by closing the biological shield cavity with a top closure. For overall compactness and simplicity the primary system operates in natural circulation. Pressurizing is achieved by production of net steam in the core which substantially enhances natural circulation. Dictated by supplying heat in the tertiary circuit at up to 150 deg. C the primary circuit pressure was set to 15 bar. The core is formed by BWR-type fuel elements located in the lower part of the reactor pressure vessel (RPV). Riser ducts and gravity-actuated shutdown rods are placed above the core. The heat exchanger bundles are arranged between the pertinent structure and the RPV wall. Because of the low system pressure, heat is transferred almost entirely in the liquid phase, leading to a very docile dynamic behaviour of the system. The close-fitting containment limits any loss-of-coolant accident. This feature, together with the fact that the core always remains covered with water, and that there is always sufficient heat transfer available for decay heat removal, results in a very simple safety concept. (author)