  In this letter we demonstrate that if oxygen can be properly supplied to
(Nb-Zr)-Sn wires, ZrO2 precipitates will form during the heat treatment,
refining the Nb3Sn grain size markedly. Here, a Nb3Sn subelement was fabricated
in which Nb-1Zr alloy was used, and oxygen was supplied via SnO2 powder. The
results showed that such a design could supply sufficient oxygen to internally
oxidize the Zr in the Nb-1Zr alloy, and that the sample reacted at 650 {\deg}C
had grain sizes of ~45 nm, less than half the size of the grains in present
Nb3Sn conductors. Magnetic measurements showed that the peak of the pinning
force vs. field (Fp-B) curve was shifted to ~0.3Birr (the irreversibility
field).
