Experimental and theoretical work on the Doppler effect in thermal reactors is reviewed for uranium metal, UO2, thorium metal, and ThO2. The experimental values of a, the fractional increase in resonance capture per °C, have a spread many times the quoted errors. The use of different slowing-down spectra has contributed to the discrepancies. For uranium metal, approximate corrections are made to obtain the coefficient α0 appropriate to a spectrum. The spread in the corrected values α0 is smaller than that for α, but remains unsatisfactory. Other experimental difficulties arise in reactivity normalizations, in obtaining the statistical weight of samples and from spurious temperature effects. Theory and experiment agree on an increase of α0 with increasing surface-to-mass ratio and that this is caused by an increase in the contribution of lower-energy resonances to the Doppler effect. It is also in agreement with the theoretical interpretation of the radial dependence of the Doppler effect in a lump. However in the region of practical interest where the surface-to-mass ratio is small, α0 is almost constant. Experimental evidence on the temperature behaviour of α0 is unsatisfactory but indicates that α0, decreases with increasing temperature. Theory predicts that α0 will vary approximately as where T is the Kelvin temperature. In the case of non-uniform temperature distribution in a fuel element, both experimental and theoretical effort is needed.
Irradiation of rubber-like fluorinated copolymers yields rubbers with poor mechanical properties, but in the presence of fillers it is possible to obtain rubbers with useful properties. The surface activity of the filler greatly influences the physical properties of the resulting rubbers. The best are irradiated rubbers containing active channel carbon; these are more stable to thermal ageing, action of solvents and static deformation than are the usual peroxide rubbers. Unlike PTFE and polytrifluorochlorethylene, rubberlike Kel-F type fluorinated copolymers become cross-linked when irradiated. The degree of cross-linking depends on their composition, on the radiation dose and on the medium in which the polymer is irradiated. The presence of oxygen favours a greater degree of cross-linking.
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