15.46. The inherent characteristics of the HTGR concept provide the safety basis for the MHTGR system. First of all, graphite is stable at high temperatures and has a high heat capacity, which assures that core tem­perature transients will be slow and readily controllable. Helium is inert both chemically and neutronically. Therefore, coolant interactions with materials during the course of an accident are not possible and coolant

changes do not affect reactivity. Thorium loading in the fuel enhances the negative temperature coefficient of reactivity. These characteristics cause the reactor inherently to shut down.

15.47. In the MHTGR, should both the primary and secondary shut­down cooling systems become inoperative, decay heat would be removed by the passive reactor cavity cooling system (RCCS). The decay heat path is from the core to the surrounding reflector, then to the walls of the uninsulated reactor vessel. Heat would then be transferred by radiation and convection to RCCS cooling panels placed around the vessel. Outside air moved by natural circulation then cools the panels. The annular core geometry limits local heat generation so that this passive system prevents the fuel from reaching a temperature that would damage the refractory coatings on the microspheres.

15.48. The response of the MHTGR to a wide variety of accident scen­arios has been studied extensively, but space does not permit even summary coverage here. However, a common question concerns the possibility of chemical reaction of graphite with water and air. The water-graphite re­action is endothermic and normally would have minor impact with no fission product release likely. A worst-case scenario involving a combi­nation of failures results in about a 5-Ci iodine release to the environment, which would result in an acceptable offsite dosage. Air ingress would result in only a small amount of oxidation, primarily because of the large resist­ance to flow that the cooling tubes provide.

15.49. Another scenario worthy of mention is a depressurization acci­dent, with failure of all forced cooling systems. The reactor would trip, but decay heat would be removed only by the RCCS. A peak core tem­perature of about 1600°C would occur after about 80 hours. A release to the environment of only about 1 Ci of iodine is predicted.