Graphite oxidation modeling in the oxygen environment at temperatures 400-800 °C :: SPE RESST

Graphite oxidation modeling in the oxygen environment at temperatures 400-800 °C

Now operations on creation of nuclear power systems generation IV are actively carried that caused need of carrying out researches of core components, and, in particular, a research of properties and behavior of nuclear-pure graphites in oxidizing environments at high temperatures. A large amount of experimental work was carried out to study the regularities of the carbon materials oxidation process, which made it possible to comprehend the main phases of the oxidation process, as well as to carry out the corresponding physical and mathematical description. However, not enough attention is paid to researches on simulation of such processes: influence of particle size distribution and porosity of graphite is not considered. Accounting of these factors will allow simulate carbonic components oxidation under operating conditions of nuclear power systems generation IV.

Simulation model

The simulation model is based on splitting the sample into a set of cubic elementary cells (Fig. 1). Elementary cell is characterized by mass and linear dimensions. A graphite sample is described using a three-dimensional matrix V, each element of which has mass i, j, k-th unit cell.


a)		b)

Figure 1 - Scheme of the sample transformation into a three-dimensional matrix V:
a) sample; b) the unit cell i, j, k

The graphite sample was proposed to be developed step by step, taking into account the particle size distribution.

Formation of granules was carried out, using statistical approach that allows setting the necessary form of granules depending on grade and features of a graphite microstructure – the size, shape, percentage of filler granules in a sample.

The granules package in graphite sample model volume was carried out with using the developed original algorithm of granules placement.

The results of modeling a GMZ grade graphite sample are shown in Fig. 2, 3. Red color indicates the material that is on the boundary of the sample, blue - inside.


a)		b)		c)		d)

Figure 2 - Modeling of particle size distribution of GMZ grade nuclear-pure graphite sample: a) granules measuring 1.5 ± 0.5 mm; b) addition of granules 0.5 ± 0.3 mm; c) addition of granules 0.3 ± 0.1 mm. d) filler and binder

Figure 3 - Computer model of graphite: filler and binder (increased)

The simulation model is based on the assumption that the internal pores of the sample contain an oxidizer and oxidize the inner cells at the same rate as the external oxidizer.

A detailed mathematical description of the oxidation modeling of a graphite sample in the developed simulation model is presented in [21(2014)].

Approbation of the simulation model

Approbation of the simulation model was carried out on the oxidation example of GMZ grade nuclear-pure graphite sample.

The linear dimensions of the sample are 0.002 m in width, 0.003 m in length, 0.02 m in height, and the step of dividing the sample into cubic unit cells with a specified rib length 0.0001 m.

The parameters of the oxidation process: temperature 560 °C, oxidizer concentration 10% O₂, duration of the process - 30 days.

Analysis of simulation results (Figure 4) shows the presence of the sample oxidation dynamics, with a significant structural change observed after 10 days, after 30 days the sample loses the properties of the supporting structure, and after 30 days, complete destruction of the sample is observed.


a)		b)		c)		d)

Figure 4 - Type of GMZ grade graphite sample under oxidation in a environment with an oxidant concentration 10% O₂ at a temperature 560 °C: a) initial state; b) after 10 days; c) in 20 days; d) after 30 days (the scale of the change in the mass of the unit cell is shown on the right)

Simulation taking into account sample porosity allowed considering nonlinearity and stochasticity of graphite oxidation process.

The oxidation simulation error of a nuclear-pure graphite sample makes 7%.