Patent Number: 039714446
Section: abstract

A reaction vessel which holds a charge of spherical reaction elements, for example a pebble bed nuclear reactor having a charge of spherical fuel elements, has the charge formed of at least two groups of spherical elements with all of the elements in each group being of equal diameter, but each group of elements having a different diameter from the other. The elements of both groups are uniformly mixed together and the different diameters are chosen in such a way that when the elements are continuously circulated through the reaction vessel during operation, the circulation does not cause the two groups of elements to segregate during the intended number of circulation cycles. Further, when control rods are used in a nuclear reactor which are advanced into and retracted from direct contact with the spherical elements for providing control and shutdown of the nuclear reaction, the use of spherical elements selected from at least two groups, each of which has a different diameter and where the difference in diameters is in a selected range so that the random arrangement of elements is maintained during continuous circulation of the core, prevents the elements from becoming tightly packed together so that insertion of the control rods into the core is facilitated.