It is possible, but not practical, to use l0B-hned chambers for fixed in-core neutron-flux monitoring in power reactors They are not practical because the thermal — neutron cross section for 10 В is more than six times that for 23SU and results in too-rapid burn out For example, the neutron sensitivity of a fission chamber is reduced to 50% of its initial value after nine months of operation in a
thermal neutron flux of 4 X 1013 neutrons cm"2 sec1 (typical for most water reactors at full power) In contrast, a 1 0 B-lined ion chamber under the same conditions is down to 50% of its initial sensitivity in l’/2 months At the end of nine months of operation, the 10B lined ion chamber would have less than 2% of its initial sensitivity Figure 3 9 compares sensitivity vs time for the fission chamber[10] and the 10 В-lined ion chamber in a flux of 4 X 101 3 neutrons

-2 -i

cm sec

Boron-lined ion chambers can be used satisfactorily as neutron sensors on traveling in-core probes since the total time of exposure to the neutron flux is only a small fraction of the total operating time of the reactor The time required to run a complete core traverse seldom exceeds 3 mm, and the frequency of traversing is seldom more than once a month, thus many years of satisfactory operation can be obtained from a boron lined ion chamber used on a traveling in-core probe The characteristics of an in core ion chamber detector for traveling in-core probe service are described in Table 3 2

*The useful life of fission chambers can be extended by using a mixture of uranium isotopes eg 10% 23SU and 90% 2 3 4 U In such a mixture the 2 3 4 U is transmuted to 2 3 5 U and the useful life is thus extended beyond that achieved with a 2 3 5 U lined chamber