Page 35 - Inorganic Mass Spectrometry : Fundamentals and Applications

cross sections and other nuclear parameters [91-931. The half-life of 241Pu (14.35
years), about which there was much uncertainty, was measured by DeBievre et al.
to unprecedented accuracy [94]. Kelly described procedures to eliminate system-
atic bias due to isotopic fractionation and interference from 241Am in this measure-
ment [95]. Economical operation of power reactors is dependent upon changing of
fuel rods at optimum intervals. To do this, the amount of fuel consumed (called
burn-up) with time must be determined; an ASTM procedure prescribes how this
is achieved [96]. The information required is the isotopic compositions and
concentrations of uranium, plutonium, and neodymium, all of which are deter-
mined using thermal ionization mass spectrometry. Green et al. described deter-
mination of this important parameter [97]. De Laeter has published a review of the
role of mass spectrometry, including thermal ionization, in studies of nuclear
fission [3].
One of the most fascinating events in the Earth’s history was the naturally
occurring reactor at Oklo in Gabon. It was discovered in 1972 through the
insistence of a French mass spectrometrist that the measured 235U abundance of
0.7171% (0.5% low) was significantly different from the natural value and war-
ranted inves~gation. Subsequent measure~ent of the isotopic composition of the
rare earth elements (particularly neodymium) revealed the existence of a natural
reactor about 2 billion years ago when the abundance of 235U was much higher
than it is today [98,99]. Subsequent studies were performed to characterize
various reactor parameters; mobility and retentivity, properties of importance in
the isolation of nuclear waste, of many elements have also been studied. These
investigations revealed that criticality was reached in several zones in the ore
body; new ones are still being discovered. The uranium itself has been preserved
virtually intact in its original distribution, a remarkable fact after the passage of
so much time. Loss et al. were able to show that the fission process at Oklo lasted
for hundreds of thousands of years [ 1001; criticality in Zone 9, for example, lasted
approximately 2.2 x 105 years. Investigations of the Oklo phenomenon have been
summarized by De Laeter [3] and extensively described in two publications from
the Inte~ational Atomic Energy Agency [ 101,1021. Aside from the inherent
scientific interest in such an apparently unlikely phenomenon, Oklo studies have
direct bearing on various issues involving isolating nuclear waste, as has been
pointed out by Ruffenach et al. [ 1031.

Thermal emission has played a role in determination of atomic weights. Such
work is of great importance and requires scrupulous attention to detail. For the
many elements for which no certified isotopic standard is available, it is the
abundance measurements made during such experiments that serve as the refer-
ence values for calibration of mass spectrometers in other laboratories. Sample