Effects of Compressing Strength of Core Ball on Crushing Strength of Spherical Fuel Element

The spherical fuel element (pebble fuel) for high temperature gas-cooled reactor (HTGR) is the fourth-generation nuclear fuel element with completely independent intellectual property rights. The most notable feature of pebble fuel is its inherent safety, which is guaranteed by the four coating layers serving as its major security barrier and the external matrix graphite. The pebble fuel is composed of inner core and nonfuel zone, the former is composed of coated uranium dioxide particles and matrix graphite powder and molded by quasi-cold isostatic pressing (CIP) technique, while the nonfuel zone is matrix graphite shell wrapping on the core pebble by CIP again. After subsequent processes of low temperature carbonization, lathing and high temperature purification, spherical fuel elements complying specific performance requirements can be obtained. Currently, as the first fuel element production line supplying fuel for HTR power plant in the world, the HTR fuel element plant of China North Nuclear Fuel Co., Ltd. has completed its process technology development. The molding characteristics of spherical fuel elements is “composite quasi-cold isostatic pressing” of both fuel zone and nonfuel zone. Fuel particles and matrix graphite powder will be mixed up uniformly and premolded into core pebble using CIP, then by adding matrix graphite powder wrapping the core pebble to use CIP again for nonfuel zone creation. Through specific process, matrix graphite powder, the main raw material for spherical fuel elements, has certain composition of artificial graphite, natural graphite, and phenolic resin. In the production process of spherical fuel element, some destructive tests on graphite pebble without fuel particle but prepared by the same process for fuel pebble are required, among which the crushing strength is one of the most important testing items. For each batch of fuel pebbles, the crushing strength on directions both axial and perpendicular to the molding direction is tested. The results of the trial production of matrix graphite spheres indicate that compressing strength of the core pebble directly affects crushing strength of graphite pebble. Compressing strength of core ball depends both on the characteristics of matrix graphite powder including particle morphology, particle size distribution, content of phenolic resin, and the compressing pressure. In this paper, major study efforts are concentrated on the effect of matrix graphite powder characteristics on core ball compressing strength in order to discover the relations between powder characteristics and the crushing strength of substrate graphite sphere.