Thermal Properties of ARV-U Graphite in the Temperature Interval of 293–1673 K

The demand for the production of various grades of reactor graphite is growing every year. This is due to their active use in nuclear power. To carry out engineering calculations of temperature fields in the core and simulate the operation of the reactor in the normal mode, as well as to predict the consequences of possible extreme situations, highly reliable data on the thermophysical properties of structural carbon materials in a wide temperature range are required. However, such data, which were obtained in limited temperature ranges and with an unknown error, are fragmentary in the literature. In this work, the thermal diffusivity a, isobaric heat capacity c_p, and linear thermal expansion coefficient (LTEC) β of ARV-U graphite at temperatures from room temperature to 1673 K are determined by the methods of laser flash, differential scanning calorimetry, and dilatometry. This graphite is considered as a cladding for fuel elements for neutron moderation. Thermal conductivity λ was calculated from the measurement results. Estimated error of the received data for a, λ, c_p, and β was 2–4, 3–5, 2–4, and 3%, respectively. By approximating the experimental results, expressions are constructed that describe the dependence of the studied properties on temperature and allow one to determine the thermophysical properties of ARV-U graphite with high accuracy, and they can be considered as reference values. Based on the obtained results, it was concluded that the properties of ARV-U graphite in the entire investigated temperature range change monotonically, without jumps or breaks, and are well reproduced in heating–cooling cycles. This indicates that the structure of the carbon composite remains unchanged. The comparison of the data of this work with the properties of graphite of other grades is carried out. It was confirmed that the heat capacity of graphite in the investigated temperature range is practically independent of its grade. A relationship has been established between the heat transfer coefficients (a, λ) of graphite and its macroscopic density (or porosity). The maximum difference in the density of the studied samples of 1.7% led to a difference of 13–17% in thermal diffusivity and 12–15% in thermal conductivity. A method has been developed for predicting the thermal conductivity of ARV-U graphite based on the density value at room temperature.