
117Engineering Issues
level, with reactors fueled by
235
U being the easiest to control. If a reactor becomes
“prompt-critical” at any point in its operation (k
prompt
≥ 1), the delayed neutrons no
longer provide a means of control.
Another
issue affecting the control of a reactor is the production of long-lived
neutron poisons during its operation. The most important of these is the 9-h ssion
product
135
Xe, which is not only produced directly, but also arises from the β decay of
the 7-h
135
I ssion product.
135
Xe has an enormous cross section for capture reactions
with thermal neutrons:
135 136
3 106Xe n Xe b
th
( , ) ,γ σ = × .
A
reactor operati ...