22.08.2015   Comprehensive nuclear materials

Graphite Temperatures

Graphite component temperature depends on radia­tion and convection (and conduction in the case of light-water gas-cooled reactors) heat transfer from the fuel and heat generated in the graphite by neu­tron and g-heating, that is, energy deposition as dis­cussed above. Therefore, a detailed knowledge of the coolant flow is important.

Thermohydraulic codes such as Panther (http:// www. sercoassurance. com/answers/) are used to cal­culate heat generated in graphite blocks. These codes estimate the following:

1. The heat generated in the fuel.

2. The coolant flow.

3. The heat transfer to the graphite.

4. The heat ‘energy deposition’ in the graphite.

The calculations take account of graphite weight loss and change in thermal conductivity of the graph­ite due to fast neuron damage and radiolytic oxida­tion. The largest uncertainty is probably associated with the size of flow bypass paths and flow resistance.

In an AGR, the temperature at the outside of the brick is lower than the temperature at the inside because of the interstitial flow, whereas in an Reaktor Bolshoy Moshchnosti Kanalniy (RBMK) the temper­ature is hotter at the brick outside.

Using the brick ‘boundary conditions’ including energy deposition temperatures calculated by the thermohydraulic code, a standard finite element code such as ABAQUS can easily be used to calculate the spatial distribution of temperature with the graphite component. Thermal transient tempera­tures can also be calculated using a standard finite element code. Often, the temperature distribution is calculated for a central brick, and the temperatures in the bricks in the rest of the core are calcu­lated using interpolation/extrapolation, that is, form factors as described in Section 4.11.9. The calculated

 

temperatures are compared with the few brick ther­mocouples that are installed in the moderator. The codes are also fine-tuned to these.

In conclusion, the calculation of graphite tem­peratures is complex and involves the calculation of heat transfer flow to the fuel and flow calculations. Graphite temperature predictions should be com­pared to measurements taken from thermocouples located in most graphite cores.

 

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