*Nuclear Thermal Propulsion DOI: http://dx.doi.org/10.5772/intechopen.103895*

material with absorption resonances exhibits Doppler broadening, the most prominent effect is usually stemming from 238U. Due to the much larger amount of 238U relative to HEU, Doppler broadening is much more important in LEU reactor concepts. Doppler broadening is always a negative feedback effect with increasing temperature and is effectively an immediate effect with respect to the temperature of the fuel.


Fast reactors have the smallest feedback coefficients. Burnup and hydrogen content do not have an appreciable effect, while temperature via expansion and Doppler and spectral shift have a comparatively small and equal effect. HALEU-fueled reactors react predominantly to temperature via the Doppler/ spectral shift. Burnup affects reactors with smaller loading of fissile isotopes more than reactors with higher fissile loading (e.g., GWDF typically has a smaller fuel loading than CERMET). The largest feedback effect for HEU GWDF is the hydrogen content of the core because Doppler broadening effects are small and the spectral shift is not as strong a feedback mechanism as that of hydrogen. Note that the sensitivity of the reactor to hydrogen content is used to introduce positive reactivity into the core by increasing the flow. The large positive reactivity coefficient does not make the HEU GWDF core dynamically unstable because an increase in reactor power leads to a reduction in hydrogen density, and thus, a negative feedback effect. Note that many observations here are based on feedback effects tabulated in Ref. [37].

Feedback is important for the controllability of the core. Large negative feedback coefficients as present in LEU cores with respect to fuel temperature require the control mechanism to have sufficient excess reactivity in reserve; therefore, thermal NTP reactors, especially if LEU fueled, must have control mechanisms with a much larger magnitude of reactivity relative to fast systems.
