emission in nuclear reactors arises from the beta decay of the neutron-rich
daughters produced in heavy element fissions. The average fission is followed
by the production of about six antineutrinos that emerge from the core
isotropically and without attenuation. Thus the high fission rate - 10**21/second
in a typical power reactor - implies a similarly high antineutrino rate.
The high antineutrino flux near reactors compensates to a degree for the
low interaction probablity of the antineutrino, making possible high statistics
measurements in detectors of tractable size. This fact is a necessary condition
for use of antineutrino detectors as safeguards and monitoring tools.
Aside from reasonable event rates
(a few thousand per day in a standard configuration) reactor antineutrinos
have a second feature that is particularly interesting from the standpoint
of safeguards. The antineutrino rate, when normalized to the reactor
power, is correlated with the plutonium and uranium content of the core.
This correlation, known as the "burnup effect", may be used
to track changes in the reactor's fissile inventory while the reactor
is operating. As a reactor core proceeds through its irradiation cycle,
the mass of each isotope varies in time. Initially, only uranium is consumed
by fission. As the cycle proceeds, plutonium gradually builds up and
begins to fission. Because the number of antineutrinos per fission differs
between isotopes, the variation in the fissile content of the core is
reflected in the antineutrino rate measured in a nearby detector. This
correlation is such that at constant power, the antineutrino rate will
vary by some 5 to 10 percent of its initial rate as the reactor proceeds
through its normal burnup cycle. The change in rate is directly correlated
with the ingrowth of a few hundred kilograms of plutonium. As demonstrated
by the Chooz experiment
in France, the absolute antineutrino rate can be measured to 3% precision
including all systematic errors. Measurements made relative to reactor
startup can be made even more precise, since many systematic errors are
absent from such measurements.