HTR: Safety features and performance analysis

Full text: High Temperature Reactors are among the candidates of the possible next generation nuclear plant. HTR's are expected to offer inherent safety characteristics, low cost of electricity generation, and short construction period. Especially, the synergy of HTR's with hydrogen generation is another advantage. There are two mainstream designs; prismatic and pebble bed. Different fuel options can be considered such as uranium dioxide, uranium carboxide (UCO), and the (U,Pu) mixed oxide. There are possibility of using such reactors to eliminate weapon plutonium stockpiles and nuclear waste transmutation. Thorium can also be used in such reactors as fuel. Presence of only ceramic materials in the core, large graphite inventory as a heat sink, and inherent safety characteristics of such reactors make them very attractive. The basic building block of HTR fuel elements is TRISO coated particles. A TRISO particle is made of a fuel kernel. It is surrounded by a low density porous pyrolytic carbon, high density inner pyrolytic carbon, SiC, and high density outer pyrolytic carbon layers. The SiC layer acts as the primary pressure boundary. Normally, SiC is a very strong material and withstand temperature up to 1800 degrees. Beyond that temperature, thermal decomposition may be observed at extended over extended tine periods. Buffer layer is used to accommodate volume expansion by swelling and fission products. Pyrolytic carbon layers protect SiC layer, keep SiC under compression, and act as barrier against gaseous fission products. In this study, main features provided by TRISO particles, common failure modes, and basic elements for the performance analysis will be discussed. An important failure mode corresponds to over-pressurization and the mechanical failure of pressure vessel. Examples will be given in this context to compare mechanical response of the fuel with simple analytical techniques as well as finite element calculations