  We investigate the mixed-valent regime of a two-configuration Anderson
impurity model for uranium ions, with separate quadrupolar and magnetic
doublets. With a new Monte Carlo approach and the non-crossing approximation we
find: (i) A non-Fermi-liquid fixed point with two-channel Kondo model critical
behavior; (ii) Distinct energy scales for screening the low-lying and excited
doublets; (iii) A semi-quantitative explanation of magnetic-susceptibility data
for U$_{1-x}$Th$_x$Be$_{13}$ assuming 60-70% quadrupolar doublet ground-state
weight, supporting the quadrupolar-Kondo interpretation.
