Reactor Theory (Nuclear Parameters)
SAMARIUM AND OTHER FISSION PRODUCT POISONS
The fission yield of samarium-149, however, is nearly zero; therefore, the equation becomes the
lPm NPm NSm s
Solving this equation for the equilibrium concentration of samarium-149 and substituting
f / lPm for NPm(eq) yields the following.
g Pm S
This expression for equilibrium samarium-149 concentration during reactor operation illustrates
that equilibrium samarium-149 concentration is independent of neutron flux and power level. The
samarium concentration will undergo a transient following a power level change, but it will return
to its original value.
Samarium-149 Response to Reactor Shutdown
Since the neutron flux drops to essentially zero after reactor shutdown, the rate of samarium-149
production becomes the following.
Because samarium-149 is not radioactive and is not removed by decay, it presents problems
somewhat different from those encountered with xenon-135, as illustrated in Figure 7. The
equilibrium concentration and the poisoning effect build to an equilibrium value during reactor
operation. This equilibrium is reached in approximately 20 days (500 hours), and since
samarium-149 is stable, the concentration remains essentially constant during reactor operation.
When the reactor is shutdown, the samarium-149 concentration builds up as a result of the decay
of the accumulated promethium-149. The buildup of samarium-149 after shutdown depends
upon the power level before shutdown. Samarium-149 does not peak as xenon-135 does, but
increases slowly to a maximum value as shown in Figure 7. After shutdown, if the reactor is
then operated at power, samarium-149 is burned up and its concentration returns to the
equilibrium value. Samarium poisoning is minor when compared to xenon poisoning. Although
samarium-149 has a constant poisoning effect during long-term sustained operation, its behavior
during initial startup and during post-shutdown and restart periods requires special considerations
in reactor design.