In 2015, tests of candidate carbon materials for high-temperature gas-cooled reactors (HTGR) of the new generation at temperatures of 1400 ... 1600 °С were carried out. This temperature range corresponds to hypothetical emergency situations in the core of the HTGR generation IV.

 The operating conditions of carbon materials in the core of the HTGR in the hypothetical emergency situations

 World studies of the operation parameters of the experimental HTGR reactors revealed the possibility new type emergency situations in new generation nuclear power plant, which associated with the depressurization of the first circuit or the appearance of a leak in the steam generator. The main parameters of such situations are determined by IAEA expert groups.

 The maximum operating temperatures of carbon materials in normal operation were determined by the maximum fuel temperatures of 1250 °C (at an average helium temperature of 750 °C to 1250 °C with allowance for overheating factors) and at 1400 °C in the equilibrium operating mode (at an average helium temperature of 950 °C).

 In case of emergency depressurization of the first circuit, the maximum fuel temperature can be 1600 °C. The project emergency conditions assume that at a temperature in the range from 1400 °C to 1600 °C for up to 100 hours there may be no more than 10% of the total number of fuel elements. In other emergency situations, the estimated maximum fuel temperature is 1500 °C.

 Another important factor in the design of simulation conditions is the concentration of corrosive oxidizing agents (air and steam) in their mixtures with the helium coolant. In emergency conditions, the air density in the air-helium mixture is assumed to be 0 ... 1 kg / m³ at a pressure of 0.1 MPa. The steam density in the vapor-helium mixture in the emergency mode lies within a wide range (0 ... 8 kg / m³) at a significantly higher pressure of 5 ... 6 MPa.

 The maximum velocity of helium flow of fuel elements does not exceed 15 m/s; for dense air-helium or vapor-helium mixtures, the flow velocity in emergency situations can be in a wide range from 0.1 m/s to 10.0 m/s.

 Thus, for assessment of carbonic elements oxidation rate of the in the core of the HTGR generation IV in the emergency event, it is necessary to conduct simulated tests under the following conditions:

 in the temperature range 600 ... 1600 °C with an air flow velocity of 5 ... 10 m/s;

 in the temperature range of 500 ... 1300 °C at a flow rate of 10 ... 15 m/s of the vapor-helium environment (10 ... 20% by weight of steam).

 Results of experiments

 Simulation tests at the temperatures of emergency situations were not conducted earlier at the NSC KIPT.

 Experimental equipment and research methods have been used.

 For carrying out of experimental researches: 120 samples from different grades of graphite (GSP, ARV, MPG) and CCCM with the size 40 × 2 × 3 mm and 60 ball elements Ø 45 mm and 60 spherical elements Ø 60 mm from the indicated grades of graphite are prepared.

 The main results of graphite oxidation studies at the emergency situations temperatures are given in Tables 1 and 2.

 It has been experimentally established that in the temperature range of 1000 ... 1300 °C in air and vapor-air medium flows, the oxidation rate of spherical elements from graphite ARV-2, MPG-7 and GSP under electron irradiation increases 4 ... 8 times, and at the same time the dependence of the decrease in the sample weight on the irradiation time is linear.

 Conclusions and recommendations

 An anomalous increase in the graphite oxidation rate at T = (1200 ... 1670) °C in air and vapor-air medium at a pressure of ~ 0.1 MPa was detected. Under these conditions, the maximum oxidation rate of the spherical elements from graphite APV-2, MPG-7 and GSP at T = (1300 ... 1400) °C is 30 ... 60 g/g⋅h.

 It was experimentally established that under electron irradiation graphite oxidation at T = (1200 ... 1300) °C and beam current 30 ... 300 μA in air and vapor-air environment goes many times more intensively, and the effect of oxidation acceleration threshold on a beam current, was for the first time watched. It obviously indicates the need to consider influence of irradiation on corrosion properties in case of a choice of candidates carbonic materials of HTGR generation IV.

 Spherical elements from graphite of ARV-2, MPG-7 and GSP grades have the same oxidation states at T = (1200 ... 1670) °C in air and vapor-air medium at a pressure of ~ 0.1 MPa and at T = (1000. ..1300) °C in air and vapor-air environment under irradiation with electrons with energy 2.5 MeV and beam current 10 ... 300 μA. At the same time for graphite grade MPG-7 samples under the specified test conditions, a "loose" surface layer with reduced strength characteristics was established. For this parameter, graphite of ARV and GSP grades are more corrosion resistant. However, for graphite ARV-2, an increased oxidation rate was observed at T = (1300 ... 1420) °C. Therefore, of the graphites studied, the best corrosion resistance in air and vapor-air environment at T = (1200 ... 1670) °C without irradiation and under electron irradiation at T = (1000 ... 1300) °C is graphite of GSP made using the technology of NSC KIPT with a density of 1.84 ... 1.90 g/cm³. This allows us to recommend this development for further consideration as a candidate material of the nuclear power unit generation IV.