UNi4B exhibits unusual properties and, in particular, a unique antifer
romagnetic arrangement involving only 2/3 of the U sites. Based on the
low temperature behavior of this compound, we propose that the remain
ing 1/3 U sites are nonmagnetic due to the Kondo effect. We derive a m
odel in which the coexistence of magnetic and nonmagnetic U sites is t
he consequence of the competition between frustration of the crystallo
graphic structure and instability of the 5f moments.