  In this paper we present results of a long-term observation of the decay of
22Na in the presence of a nuclear fission reactor. The measurements were made
outside the containment wall of and underneath the Koeberg nuclear power plant
near Cape Town, South Africa. Antineutrino fluxes ranged from ~5*10^11 to
1.6*10^13 cm^-2 s^-1 during this period. We show that the coincidence summing
technique provides a sensitive tool to measure a change in the total decay
constant as well as the branching ratio between EC and beta+ decay of 22Na to
the first excited state in 22Ne. We observe a relative change in count rate
between reactor-ON and reactor-OFF equal to (-0.51+/-0.11)*10^-4. After
evaluating possible systematic uncertainties we conclude that the effect is
either due to a hidden instrumental cause or due to an interaction between
antineutrinos and the 22Na nucleus. An upper limit of ~0.03 barn has been
deduced for observing any change in the decay rate of 22Na due to antineutrino
interactions.
