Graphite Temperature Coefficient of Reactivity

The total teo ratiire coefficient cannot he easily stated since none of the separate teoqperatures (graphite fuel and coolant) are representative of a single reactor temperature (as vould he the case In a homogeneous reactor) The teiqperatures are all related to the power of the reactor however (assuming cozistant coolant flow and Inlet coolant temperature) and it Is sometimes useful to define a power coefficient of reactivity as follows  

The power coefficient is more convenient to use in reactor oparation since the power is measured directly whereas the individual temperatures are often not measured Usually the power coefficient is determined from operational esgper-ience in the reactor and once known can he correlated with the temperature coefficient if the teoperature-power relationships are known  

In general, the neutron temperature will vary over the lattice and its calculation is exceedingly complex. approximation used in the NDFEA and 

The coefficient is simplest to calculate since it involves at most only two regions of the lattice cell (the two fuel regions). The following eguations illustrate the.form of the -coefficients for the siaple case where is the only thermally-fissionable isotope present. 

MOFM calculations averaged over a graphite temperature range of 20 to 5 0 C and with equilibrium coolant teoperature and density (232 C and 0.02 gm/cm ) is -1.0 X 10 5/oc in the reactor with green fuel and -5. x 10 / C In the reactor with an average fuel exposure of TOO MHD/T. The coefficient is less negative for the exposed fuel due to the positive tenperature coefficient of the plutonium port of 

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