Mechanisms of Positive Temperature Reactivity Coefficients of Dilute Plutonium Solutions

Mechanisms of a positive temperature reactivity coefficient that occurs in a dilute plutonium solution are investigated based on the perturbation theory and the four-factor formula. The temperature coefficient of a solution fuel is positive if the adjoint flux increases with neutron energy between 0.05 and 0.2 eV. As compared to ²³⁹Pu, ²⁴¹Pu has a tendency to make the temperature coefficient of a plutonium solution positive because of the energy dependence of the capture cross section of ²⁴¹Pu. As ²⁴¹Pu in a plutonium solution decays into ²⁴¹Am with time, the temperature coefficient of the solution becomes more positive. Since the capture cross sections of most neutron absorbers such as boron and gadolinium decrease with increasing neutron energy between 0.05 and 0.2 eV, soluble absorbers in a plutonium solution make the temperature coefficient positive for higher-concentration plutonium solutions. Cadmium and samarium dissolved in a dilute plutonium solution can exceptionally keep the temperature coefficient negative because of the energy dependence of the capture cross sections. A fixed neutron absorber generally makes the temperature coefficient of a plutonium solution negative regardless of the property of absorber materials.