REACTIVITY COEFFICIENTSDOE-HDBK-1019/2-93 Reactor Theory (Nuclear Parameters)ModeratorTemperatureCoefficientThe change in reactivity per degree change in temperature is called the temperature coefficientof reactivity. Because different materials in the reactor have different reactivity changes withtemperature and the various materials are at different temperatures during reactor operation,several different temperature coefficients are used. Usually, the two dominant temperaturecoefficients are the moderator temperature coefficient and the fuel temperature coefficient.The change in reactivity per degree change in moderator temperature is called the moderatortemperature coefficientof reactivity. The magnitude and sign (+ or -) of the moderatortemperature coefficient is primarily a function of the moderator-to-fuel ratio. If a reactor isunder moderated, it will have a negative moderator temperature coefficient. If a reactor is overmoderated, it will have a positive moderator temperature coefficient. A negative moderatortemperature coefficient is desirable because of its self-regulating effect. For example, anincrease in reactivity causes the reactor to produce more power. This raises the temperature ofthe core and adds negative reactivity, which slows down, or turns, the power rise.FuelTemperatureCoefficientAnother temperature coefficient of reactivity, the fuel temperature coefficient, has a greater effectthan the moderator temperature coefficient for some reactors. The fuel temperature coefficientis the change in reactivity per degree change in fuel temperature. This coefficient is also calledthe "prompt" temperature coefficient because an increase in reactor power causes an immediatechange in fuel temperature. A negative fuel temperature coefficient is generally considered tobe even more important than a negative moderator temperature coefficient because fueltemperature immediately increases following an increase in reactor power. The time for heat tobe transferred to the moderator is measured in seconds. In the event of a large positive reactivityinsertion, the moderator temperature cannot turn the power rise for several seconds, whereas thefuel temperature coefficient starts adding negative reactivity immediately.Another name applied to the fuel temperature coefficient of reactivity is the fuel dopplerreactivity coefficient. This name is applied because in typical low enrichment, light water-moderated, thermal reactors the fuel temperature coefficient of reactivity is negative and is theresult of the doppler effect, also called doppler broadening. The phenomenon of the dopplereffect is caused by an apparent broadening of the resonances due to thermal motion of nuclei asillustrated in Figure 3. Stationary nuclei absorb only neutrons of energy Eo. If the nucleus ismoving away from the neutron, the velocity (and energy) of the neutron must be greater than Eoto undergo resonance absorption. Likewise, if the nucleus is moving toward the neutron, theneutron needs less energy than Eo to be absorbed. Raising the temperature causes the nuclei tovibrate more rapidly within their lattice structures, effectively broadening the energy range ofneutrons that may be resonantly absorbed in the fuel. Two nuclides present in large amounts inthe fuel of some reactors with large resonant peaks that dominate the doppler effect areuranium-238 and plutonium-240.NP-03Rev. 0Page 26
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