  We investigate a two-channel Anderson impurity model with a $5f^1$ magnetic
and a $5f^2$ quadrupolar ground doublet, and a $5f^2$ excited triplet. Using
the numerical renormalization group method, we find a crossover to a non-Fermi
liquid state below a temperature $T^*$ varying as the $5f^2$ triplet-doublet
splitting to the 7/2 power. To within numerical accuracy, the non-linear
magnetic susceptibility and the $5f^1$ contribution to the linear
susceptibility are given by universal one-parameter scaling functions. These
results may explain UBe$_{13}$ as mixed valent with a small crossover scale
$T^*$.
