Radiation effect on the graphite of HTGR

For predicting the operability of graphite structures for modern nuclear installations, it is necessary to know the rules of radiation-induced changes in such properties of graphite as stability of dimensions, strength, creep, modulus of elasticity, coefficient of thermal expansion, heat conductivity and resistance to oxidation in a wide temperature range and at fast neutron fluence more than 1022/cm2. The dimensional change of graphite under irradiation is the most important parameter which determines the operability of HTGR graphite structures. The effect of irradiation on the thermophysical and mechanical properties of graphite is reported. The change of microstructure of graphite was also studied. One of the effective methods enabling to avoid cracking in nuclear graphite under irradiation is the production of the nuclear graphite with the difference of crystal grain size as small as possible. At low irradiation temperature, the change of crystal grain size occurs to a certain limit due to the change of the crystal lattice caused by radiation defects. At high irradiation temperature, the lattice parameters change less, and above 500 deg C, the change is negligible. Nuclear polycrystalline graphite changes complicatedly its dimensions due to porosity. One of the most important properties of stressed and strained graphite is the radiation creep. (Kako, I.)