Pressure tube type reactor and its operation method

Purpose: To make the reactor core properties stable and increase the controllability in a pressure tube type reactor. Constitution: Gases such as heavy water steams or helium gases are used as moderators at the outside of pressure tubes, the periphery of the reactor core is surrounded with reflector layers and the fuel density near the boundary of the reflectors are made lower than other portions. In the course of the operation period, the gaseous moderators are replaced with liquid moderators having greater moderating performance thereby attaining the spectral shift operation. The neutron energy spectrum becomes hard by using gases as the moderators and, in addition, fertile materials in the fuel are effectively converted into nuclear fission materials by utilizing the reflection due to the reflectors. Then, by replacing them with liquid moderators in the course of the operation, since the neutron energy spectrum changes from hard to soft spectrum, it is possible to effectively burn the nuclear fission materials, facilitate the fabrication and improve the neutron economy. (Kamimura, M.)