Abstract: The invention relates to a graphite side reflector in block form for a gas-cooled high-temperature nuclear reactor. The blocks of the reflector extend radially continuously through the entire reflector wall thickness and recesses are provided in the inner end faces of at least the blocks disposed in the upper region of the reactor core.

Abstract: A method of testing the circulation of a bulk charge of fuel elements in the core of a pebble bed type of nuclear reactor comprises increasing the electrical conductivity of a number of fuel elements and then injecting the marked elements into a bulk charge of other unmarked elements having a lower electrical conductivity at predetermined positions in the core of a nuclear reactor and a measured times. The marked fuel elements are subsequently detected electromagnetically as they are carried in the bulk charge of unmarked elements through an electromagnetic field generated by a detection and measurement station. The elements are marked by including a material having a greater electrical conductivity than the remainder of the element and this material may be coated on the surface of the elements or incorporated in the body of the elements.