Yangping Zhou

Effects of reaction temperature and inlet oxidizing gas flow rate on IG-110 graphite oxidation used in HTR-PM

ABSTRACT The oxidation behavior of a selected nuclear graphite (IG-110) used in Pebble-bed Module High Temperature gas-cooled Reactor was investigated under the condition of air ingress accident. The oblate rectangular specimen was oxidized by oxidant gas with oxygen mole fraction of 20% and flow rates of 125–500 ml/min at temperature of 400–1200 °C. Experiment results indicate that the oxidation behavior can also be classified into three regimes according to temperature. The regime I at 400–550 °C has lower apparent activation energies of 75.57–138.59 kJ/mol when the gas flow rate is 125–500 ml/min. In the regime II at 600–900 °C, the oxidation rate restricted by the oxygen supply to graphite is almost stable with the increase of temperature. In the regime III above 900 °C, the oxidation rate increases obviously with the increase of temperature. With the increase of inlet gas flow from 125 to 500 ml/min, the apparent activation energy in regime I is increased and the stableness of oxidation rate in regime II is reduced.

Investigation of oxidation behaviors of nuclear graphite being developed and IG-110 based on gas analysis

ABSTRACT The oxidation behaviors of the nuclear graphite being developed were investigated using gas chromatograph at 873–1373 K. The oxidation experiments were carried out with the gas flow rate of 0.2 L/min and the oxygen concentrations of 7, 10 and 20 mol%. The oxidation reaction began at 973 K and was accelerated with the increase of temperature. At 1173–1273 K, the oxidation was limited by oxygen supplied to graphite and the reaction rate held steady. From 1273 to 1373 K, the oxidation rate increased obviously due to the significant reaction between CO2 and graphite. At the low temperature regime (973–1073 K), the apparent activation energies with the oxygen mole fractions of 7%, 10% and 20% were 298, 324 and 321 kJ/mol, respectively. Scanning electron microscope was applied to reveal the pore development of the graphite oxidized at different temperatures. The effect of CO combustion at temperature below 1173 K was discussed based on the oxidation behaviors of the graphite being developed and IG-110. It was suggested that the ASTM D7542-15 standard should be adjusted to fit some popular graphite, such as graphite IG-110.

Simulation and Analysis of Helium Circulator Trip ATWS Test at Full Power on the HTR-10

Safety demonstration tests on the 10 MW High Temperature Gas-cooled Reactor-Test Module (HTR-10) were conducted to verify the inherent safety characteristics of modular High Temperature Gas-cooled Reactors (HTGRs) as well as to obtain the reactor core and primary cooling system transient data for validation of HTGR safety analysis models and codes. As one of these safety demonstration tests, a simulated anticipated transient without scram (ATWS) test called loss of forced cooling by tripping the helium circulator without reactor scram was carried out at 100% rated power level in July, 2005. This paper simulates the reactor transient behaviour during the test by using the THERMIX code system. The reactor power transition and a comparison with the test result are presented. Owing to the negative temperature coefficient of reactivity, the reactor undergoes a self-shutdown after the stop of the helium circulator and keeps subcritical till the end of the test. Due to the loss of forced cooling, the residual heat is slowly transferred from the core to the Reactor Cavity Cooling System (RCCS) by conduction, radiation and natural convection. The thermal response of this heat removal process is investigated. The calculated and test temperature transients of the measuring points in the reactor internals are given and the differences are preliminarily discussed. With respect to the safety features of the HTR-10, it is of most importance that the maximum fuel center temperature is always lower than 1230 °C which is the limited value at the first phase of the HTR-10 project. The simulation and test results show that the HTR-10 has the built-in passive safety features, and the THERMIX code system is applicable and reasonable for simulating and analyzing the helium circulator trip ATWS test.Copyright

Characterizing thermal-oxidation behaviors of nuclear graphite by combining O 2 supply and micro surface area of graphite

The effects of different parameters on oxidation rate are non-linear, interactive and diversified in which the change of adequacy of O2 supply is an important indicator. The influence of microstructure on oxidation rate became stronger worsening the fitting linearity to calculate the activation energy based on present method with the decreased adequacy of O2 supply due to the increase of temperature, the decrease of gas flow rate, etc. Here, we proposed a method to characterize thermal-oxidation behaviors of nuclear graphite by combining O2 supply and micro surface area of graphite. The proposed method improved the linearity and reduced the standard error of Arrhenius plots of oxidized graphite IG-110 (10 L/min reactant gas) and ET-10 (0.2 L/min reactant gas). The value of activation energy of graphite IG-110 oxidized under ASTM D7542 condition is calculated as 220 kJ/mol by this method echoing the results of previous studies with sufficient O2 supply. For the conditions with less O2 supply at low gas flow rate and/or high temperature, the change of microstructure of oxidized graphite should be obtained as an important factor influencing oxidation rate of graphite.

Thermal Mixing Performance of Mixing Structure at Reactor Outlet of HTR-PM under Complex Conditions

In order to investigate the effects of bypass flow and power change and deviation on radial temperature difference of the helium flow entering the steam generator, model experiments and simulation calculations are proposed to evaluate the Thermal Mixing Performance (TMP) of the thermal mixing structure at Pebble-bed Module High Temperature gas-cooled Reactor (HTR-PM) core outlet. The experiments on Test Facility - Hot Gas Mixing (TF-HGM) are carried out to observe the influences of bypass flow and reactor power deviation. Simulation calculations are conducted for the mixing structure of TF-HGM and HTR-PM. According to the results of experiments and CFD simulation, the maximum radial temperature difference at the outlet of the mixing structure fulfils the requirement by steam generator, under all considered conditions. In addition, the TMP obtained by the temperature difference between main flow and bypass flow is suitable for the experiment and the simulation results while the new definition of TMP integrating specific heat, flow rate, inlet position and thermal uniformity needs further improvement.

Thermal-Hydraulic Characteristics of the 250MW Pebble-Bed Modular High Temperature Gas-Cooled Reactor in Start-Up and Shutdown Processes

The start-up and shutdown processes are two important issues of the reactor control and operation for high temperature gas-cooled reactor plants. The analysis of thermal-hydraulic characteristics in these two processes can provide useful reference to the reactor design and manufacture, especially to the final establishment of the operation procedure. In this paper, according to the preliminary design of the 250MW Pebble-bed Modular High Temperature Gas-cooled Reactor (HTR-PM), the start-up and shutdown processes are studied by the help of two computer codes THERMIX and BLAST. The key parameters of these two processes, including the reactor power, the core inlet and outlet helium temperatures, the primary pressure and flow rate, as well as the secondary water flow rate and the outlet steam temperature, are analyzed in detail. Moreover, some uncertainties, such as the adjustment of power and secondary water temperature in the start-up process, the flow rate adjustment between the primary helium and the secondary water in the shutdown process, etc., are analyzed to evaluate the influences on the key parameters. The analysis results indicate that the pre-established start-up and shutdown rules are reasonable.Copyright