NXe (eq)XeIfuelfXeXeaXeaReactor Theory (Nuclear Parameters)DOE-HDBK-1019/2-93XENONNP-03Page 37Rev. 0Figure 4 Equilibrium Iodine-135 and Xenon-135 Concentrations Versus Neutron FluxFor xenon-135 to be in equilibrium, iodine-135 must also be in equilibrium. Substituting theexpression for equilibrium iodine-135 concentration into the equation for equilibrium xenonresults in the following.From this equation it can be seen that the equilibrium value for xenon-135 increases as powerincreases, because the numerator is proportional to the fission reaction rate. Thermal flux is alsoin the denominator; therefore, as the thermal flux exceeds 10 neutrons/cm -sec, the term122begins to dominate, and at approximately 10 neutrons/cm -sec, the xenon-135 concentration152approaches a limiting value. The equilibrium iodine-135 and xenon-135 concentrations as afunction of neutron flux are illustrated in Figure 4.The higher the power level, or flux, the higher the equilibrium xenon-135 concentration, butequilibrium xenon-135 is not directly proportional to power level. For example, equilibriumxenon-135 at 25% power is more than half the value for equilibrium xenon-135 at 100% powerfor many reactors. Because the xenon-135 concentration directly affects the reactivity level inthe reactor core, the negative reactivity due to the xenon concentrations for different powerlevels or conditions are frequently plotted instead of the xenon concentration.
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