1.1. Basic concepts

The temperature limitations of conventional reactors impose a low thermal effi­ciency, use of special turbines and consequently cause higher fuel consumption and thermal pollution of the environment. Furthermore, the low outlet temperature limits the possibilities of using nuclear power as an industrial source of heat.

High-temperature reactors have been developed in order to avoid these limitations using a core composed exclusively of ceramic materials and employing an inert coolant.10 This limits the choice of the moderator to graphite or beryllium.

Beryllium is a very good moderator, but its use is limited by technical and economic difficulties, so that nowadays graphite appears to be the only possible moderator for HTRs.

The coolant must be chemically inert, not undergo any phase change, have good heat-exchange properties and not be activated by neutrons. Helium is the only coolant able to satisfy these conditions and that is naturally available in sufficient quantity. The coolant outlet temperature of the HTRs now in operation ranges between 750° and 950°C with an inlet temperature around 300°C. As the high temperature excludes the use of metallic fuel sleeves, the fission product retention takes place initially at a microscopical level through the adoption of coated fuel particles.<2,3> These particles consist of kernels of oxides or carbides (of U, Th or Pu), whose diameters range between 200 and 800 /x m, coated with various layers of pyrolytic carbon (PyC). Layers of silicon carbide (SiC) are sometimes used in the coating in order to improve the retention of metallic fission products.® The coating thickness is of the order of 150-200 fi m.

If no SiC is present the coating consists of an inner low density buffer layer and an outer high density layer (BISO or duplex particles). If SiC is used the coating consists of an inner low density buffer layer, an inner high density pyrolytic carbon layer, a silicon carbide layer and an outer high density pyrolytic carbon layer (TRISO or triplex particles, Fig. 1.1).

These particles are dispersed in a graphite matrix.

In order to avoid corrosion due to impurities of the coolant and to further improve the fission product retention the part of the fuel element which contains the coated particles is separated from the coolant by means of graphite.

This basic concept leaves the possibility of various types of fuel geometries. In the first experimental reactors the fuel element had the form of hexagonal prismatic

(Dragon) or cylindrical (Peach Bottom) rods, with the coolant flowing between these rods.

Since then a more robust geometry has been devised for power reactors. The fuel consists of big hexagonal blocks (40 to 60 cm across flats) containing a regular pattern of fuel and coolant holes. The coolant holes can either contain the fuel in the form of pins or the fuel can be located in geometrically separated holes. In the second case (General Atomic version) the heat has to flow across the bulk moderator before reaching the coolant (Fig. 1.2).<4) In the first case (pin-block design) the fuel pins consist of a carbon matrix containing the coated particles, isolated from the coolant by a layer of unfuelled carbon (Fig. 1.3).<5) These pins can have different geometries, some of which are represented in Fig. 1.4.

The tubular design is shown on the left: the coolant flows inside and outside the pin. The dotted area represents the fuel containing matrix. On the right side is shown the teledial geometry, so called because of its resemblance to a telephone dial (Fig. 1.4).<5)

The core arrangement of a reactor with multi-hole block fuel is shown in Figs. 1.5 and

1.6.<4)

In parallel to the HTR with prismatic fuel a version with spherical fuel elements has been developed in Germany (pebble-bed reactor). The fuel elements have the form of graphite spheres of 6 cm diameter with a fuelled inner zone (see Fig. 1.7). It is possible to vary the fuel loading by mixing dummy graphite elements with the fuel spheres. These spheres which are contained in a reflector of graphite blocks are loaded pneumatically by means of tubes from the upper reflector and extracted from one or more tubes located in the bottom reflector (see Fig. 1.8).<6)

Apart from the described geometrical differences the HTR fuel elements differ also according to fabrication methods which influence the material structure and therefore the behaviour under heat flow and irradiation. Carbon can be in the form of machined graphite, or of a compacted matrix. The first pebble-bed fuel elements, for example,

Graphite

sleeves

consisted of a machined graphite shell which contained in the inner part a fuel compact of coated particles dispersed in a carbon matrix. The present THTR elements have a similar geometry, but the outer shell consists also of a pressed carbon matrix. The multi-hole blocks used by General Atomic for the Fort St. Vrain reactor are made of machined graphite, but by analogy with the THTR fuel, integral blocks made of a pressed carbon matrix are being developed in Germany (monolithic blocks). An ultimate development of this integral block technique may eventually consist in dispersing the fuel throughout the block instead of concentrating it in the fuel holes, only leaving small unfuelled layers around the coolant channels.

The principal differences between the various fabrication methods concern the behaviour under irradiation and the heat conduction properties, while for the reactor physicist the fuel is sufficiently characterized by geometry and atomic concentration of each region.