Nuclear graphite thermal conductivity

Irradiated nuclear graphite thermal conductivity data are usually presented as the reciprocal, which represents the thermal resistivity. The thermal resistivity is usually plotted as a function of fluence and irradiation temperature similar to other irradiated material properties. Thermal resistivity data for Gilsocarbon plotted as fractional change are given in Fig. 14.12. Thermal resistivity initially increases rapidly to a slightly rising plateau before a secondary increase at high fluence. 

Data have been published for CFCs whos thermal conductivities are similar to nuclear graphites, and show degradation similar to that expected from the graphite literature. For example, Burchell [24] has shown that the saturation thermal conductivity for a 3-directional composite (FMI-222, = 200 W/m-K) is 40%... 

Because of their low thermal conductivity, high temperature capability, low cost, and neutron tolerance, carbon materials make ideal thermal insulators in nuclear reactor environments. For example, the HTTR currently under construction in Japan, uses a baked carbon material (Sigri, Germany grade ASR-ORB) as a thermal insulator layer at the base of the core, between the lower plenum graphite blocks and the bottom floor graphite blocks [47]. 

Sintered beryllia. This material exhibits an extraordinarily high thermal conductivity, only surpassed by graphite and metals — hence the high resistance to thermal shock which together with high chemical inertness is its main practically utilized property. In the application of sintered BeO in nuclear reactors, use is made of its low absorption cross section and of high scattering cross section of neutrons (moderators, reflectors). 

For physics reasons, uranium in the form of metal rods was extensively employed as fuel for the first generation of nuclear reactors. The requirement for metallic fuel for a natural uranium graphite-moderated reactor is based on the need for a high fuel density and a fuel rod of sufficiently large diameter to reduce the resonance capture to a level where criticality may be achieved. Only uranium in metalhc form has sufficiently high thermal conductivity to permit adequate heat removal from rods of the required diameter. 

As expected, the behavior of the thermal conductivity of graphite foam with temperature is similar to that of graphite that is, thermal conductivity decreases as temperature inereases. With increasing temperature, the dominant phonon interaction becomes phonon-phonon scattering (Umklapp processes) and, thus, the observed reduetion of thermal conductivity with increasing temperature. The temperature dependence of thermal conductivity for nuclear-grade graphite H 451 is shown, for reference, in Fig. 4.34. 

Fig. 4.34. Temperature dependence of thermal conductivity for nuclear-grade graphite H 451.

HoUenbach, D.F., Ott, L.J., 2010. Improving the thermal conductivity of UO2 fuel with the addition of graphite fibers. In Transactions of the American Nuclear Society and Embedded Topical Meeting TSluclear Fuels and Structural Materials for the Next Generation Nuclear Reactors , June 13—17, San Diego, CA, USA, pp. 485—487. 

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