  At upcoming medium baseline reactor neutrino experiments the spallation 9Li
background will be somewhat larger than the inverse beta decay reactor neutrino
signal. We use new FLUKA simulations of spallation backgrounds to optimize a
class of veto strategies and find that surprisingly the optimal veto for the
mass hierarchy determination has a rejection efficiency below 90%. The
unrejected background has only a modest effect on the physics goals. For
example $\Delta\chi^2$ for the hierarchy determination falls by 1.4 to 3 points
depending on the muon tracking ability. The optimal veto strategy is
essentially insensitive to the tracking ability, consisting of 2 meter radius,
1.1 second cylindrical vetoes of well tracked muons with showering energies
above 3 to 4 GeV and 0.7 second full detector vetoes for poorly tracked muons
above 15 to 18 GeV. On the other hand, as the uncertainty in theta12 will be
dominated by the uncertainty in the reactor neutrino spectrum and not
statistical fluctuations, the optimal rejection efficiency for the measurement
of theta12 is 93% in the case of perfect tracking.
