Many possibilities are available for the choice of the geometrical location of the control rods. In the block design the control rods can be located in some of the hexagonal blocks (see Fig. 1.2).<4> The choice of the control-rod location is strongly related to the design of the number of penetrations in the upper part of the pressure vessel (which in modern HTRs is of pre-stressed concrete). The control rods can either be located in the same penetrations used for refuelling, or in separate penetrations.

In the case of pebble-bed reactors the control rods are directly inserted among the fuel elements. This requires higher forces and allows a smaller insertion velocity than in other reactors. In order to ease this problem it is possible to use also control rods located in the reflector (e. g. THTR). These rods alone are not sufficient to control the reactor, but may be used either for the fine regulation where frequent rod movements are needed, or for a fast insertion in case of an emergency shut-down.

1.2. Limitations to the performance of the HTR core

Whereas in other reactor types the melting point of some reactor components gives well-defined limits to the core performance, the limitations of the fully ceramic HTR core are much more difficult to define. Safe and reliable reactor operation requires mechanical stability of the core components and a primary circuit activity kept below limits established by maintenance and safety requirements. Damage to the core material is induced by the combined effect of temperature and irradiation. Collisions with neutrons above a certain energy (0.1 MeV) displace atoms from the original crystal structure, thus altering the properties of the reactor materials with resulting changes in dimensions, mechanical and heat transfer properties. The effect of neutron irradiation is temperature dependent. The thermal movement of the atoms influences the further consequences of the displacements due to neutron collision and can also cause the return of the displaced atoms to their original positions.

Gradients in temperature and fast neutron dose cause stresses, particularly in large fuel or reflector blocks.

Furthermore, fissions can cause damage to the coated particles because of fission fragment recoil, and cause build-up of pressure due to gaseous fission products. The latter effect is also influenced by temperature.

If coated particles operate under a significant temperature gradient an internal corrosive attack and transport of carbon takes place (amoeba effect). By proper choice of particle design and operating conditions the amount of attack on the inner PyC layer can be held within safe limits.

Sectional View of THTR Pressure Vessel

Another effect of temperature is to determine the rate of diffusion and the rate of reaction with graphite of H20 and C02 impurities present in the coolant. The temperature at the surface of the coolant channels is particularly important for this effect, whereas the central fuel temperature is responsible for coated particle damage.

The primary circuit coolant activity is determined by the balance between the emission of gaseous fission products from the fuel elements, and their elimination by the coolant clean-up system (this is a purification plant to which a part of the coolant flow is continuously by-passed).

Virtually all non-gaseous fission products emitted by the coated particles are either retained in the graphite structure or are evaporated in the coolant and subsequently deposited on the cooler parts of the circuit (e. g. heat exchangers, circulator blades, etc.).

The release of fission products from the fuel elements is due to migration through the coating of the particle, the possible presence of broken particles, and the presence of U impurities outside the coated particles in the fresh fuel. Before reaching the coolant the fission products have also to migrate through the carbon matrix and the outer graphite layers of the fuel elements. The migration of fission products through pyrocarbon coating and through graphite is strongly influenced by temperature. An improvement of the retention of metallic fission products at high temperature is obtained with a SiC layer in the coating of the particles.

From the above mentioned facts it results that temperature and dose limitations cannot be easily expressed by fixed data in HTRs.’791

Higher temperature transients usually give rise to only a temporary increase in activity which can be often accepted. This means that some temperature rises can be easily tolerated during accidents.

Also during normal operation the need of power and temperature flattening is less stringent than in other reactors: the same release rate can be theoretically obtained with all particles at a given temperature or with a few at a higher temperature and the rest at a lower level. This fact is present to an even greater degree in pebble-bed reactors where, because of the continuous movement, it is not always the same element which experiences the highest temperatures.’10’ Also the rupture of the coating can be tolerated if it is limited to a very small fuel fraction.

Besides higher temperatures usually occur in fresh fuel where, because of the low burn-up and dose, they are better tolerated.

References

1. G. E. Lockett and R. A. U. Huddle, Development of the design of the high temperature gas cooled reactor experiment. Dragon Project Report 1, Jan. 1960.

2. R. A. U. Huddle et al., Coated particle fuel for the Dragon reactor experiment. Dragon Project Report 116, Oct. 1962.

3. A. L. Bickerdike, H. C. Ranson, C. Vivante and G. Hughes. Studies on coated particle fuel involving coating, consolidation and evaluation. Dragon Project Report 139, Jan. 1963.

4. D. A. Nehrig, A. J. Neylan and E. O. Winkler, Design features of the core and support structures for the Fort St. Vrain Nuclear Generating Station. Conference on Graphite Structure for Nuclear Reactors, London, 7-9 March 1972.

5. M. V. Quick, D. G. Richardson and W. R. Hough, High temperature reactor core design and behaviour. Conference on Graphite Structure for Nuclear Reactors, London, 7-9 March 1972.

6. E. Cramer, G. Hagstotz and A. Eiermann: Components of the THTR300 heat transfer system. Conference on Component Design in High Temperature Reactors Using Helium as a Coolant, London, 3-4 May 1972.

7. L. W. Graham and H. Hick, Performance limits of coated particle fuel. Dragon Project Report 850, Sept. 1973.

8. H. Nickel, E. Balthesen, L. W. Graham and H. Hick, HTR fuel development for advanced applications. BNES Conference on the High Temperature Reactor and Process Heat Applications, London, Nov. 1974.

9. M. R. Everett, D. F. Leushacke and W. Delle, Graphite and matrix materials for very high temperature reactors. BNES Conference on the High Temperature Reactor and Process Heat Applica­tions, London, Nov. 1974.

10. L. Massimo and U. A. Schmid, Statistical temperature distribution calculation in pebble bed reactors. Nucl. Engng and Design, 10, 367-372 (1969).