Resonance broadening feedback

The surface deformation could be reduced even further with intermittent contact SFM. Tapping mode imaging in liquids has been described by several groups [185-191]. The main focus has been put on biological systems such as DNA, cells, chromosomes and proteins. However, it turned out to be rather tricky to perform the measurements. The resonant frequencies were usually 2-5 times lower than in air and the resonant peak gets strongly dampened and broadened [ 192,193]. Because of acoustic excitation of the cantilever holder and the body of fluid, the spectrum can be superimposed by other resonance s which not sensitive to the surface approach and cannot be used for the feedback control [185]. 

Autonomous reactor operation requires intrinsic self-regulation of reactor power. This behaviour can be obtained by negative reactivity feedback such as that provided promptly by Doppler broadening of neutron absorption resonances in resulting from increased fuel temperature, and quickly by efrects associated with coolant temperature increases and density decreases (i.e., negative void coefficient), and fuel thermal expansion. However, adequate safety consideration would also have to be given to possible reactivity insertion transients initiated by overcooling events. 

The isothermal temperature coefficient is a measure of the combined thermal reactivity feedback response to heating both the light water moderator and fuel uniformly. The isothermal temperature coefficient includes feedback mechanisms from (a) Doppler broadened resonances (prompt), (b) moderator thermal spectral shift to... 

The resonance absorbers provide negative feedback because of Doppler broadening of the resonances. As mentioned above, the resonance absorbers may be tungsten, dysprosium, iridium, europium, erbium, and other rare earth metals with resonances in the eV to tens of eV range. Specific neutronic analyses performed to obtain the temperature coefficient of reactivity and quantities needed in the fuel alloy are under way for several of the candidate resonance absorbers. 

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