Since the beginning of the last century, nuclear energy has gradually entered public view as a kind of clean energy。Compared with coal，A gram of nuclear fuel can release 1 million times more energy than a gram of coal, It is the most impressive feature of nuclear energy: the high Fuel calorific value。For more than 70 years, from the first man-made nuclear reactor Chicago Pile 1 to the very recent high temperature gas-cooled reactors, graphite has been used as a neutron moderator^[1]。Compare to other neutron moderator, H2O as a moderator in high temperature reactors requires a bulky pressurized water machine and beryllium is toxic and brittle^[2]，there is no more equipment requirements for graphite, It not only has sufficient mechanical strength, but also nontoxic。Graphite has a high moderating ratio, but due to its weak slowing-down power, the volume of moderator needs to be greatly increased to achieve the same moderating effect^[2]，But because graphite is cheaper than other materials, and the problem of volume is not the main problem in nuclear power plant，Therefore, the choice of graphite as the neutron moderator is more economy.