After reactor shutdown, the neutron flux drops to zero and there remains no channel of removal of samarium. On the other hand, there is significant amount of accumulated promethium that decays to samarium, therefore the concentration of samarium increases to a new level. This buildup is dependent upon the neutron flux level before the shutdown(i.e. upon the promethium equilibrium).
The new equilibrium is reached in approximately 20 days, and since samarium 149 is stable, the concentration remains essentially constant during reactor outage. Samarium 149 does not peak as xenon-135 does, but increases slowly to a maximum value as shown in figure. As was written samarium can be removed from the core by refueling. During refueling usually one third or one quarter of the core is removed to spent fuel pool and replaced by fresh fuel assemblies (without samarium), while the remainder is rearranged to a location in the core better suited to its remaining level of enrichment. Therefore refueling naturally leads to a decrease in the overall samarium content in the core.
After reactor startup samarium concentration usually decreases as a result of samarium burnup and very slow decay of promethium. Similarly, transient changes in samarium on power level changes are very small compared to xenon, and change very slowly over a week or so. In each case, the samarium level returns to the same equilibrium value.
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.
It was written, the samarium equilibrium is independent of the neutron flux level, therefore stationary samarium worth is independent as well. Note that, the promethium equilibrium, which determines especially the samarium peak concentration after shutdown, is flux dependent! The way of reactor shutdown must be taken into account in estimated critical conditions calculations and it can influence MTC at the beginning of the next cycle.
Moreover, especially high flux research reactors, must not be shut down suddenly. Sudden shutdown of a reactor with thermal flux of about 1019 neutrons/cm-2.s-1 can cause more than thirty times higher samarium poisoning as in PWRs. This can cause permanent poisoning and preclude any reactor startup. These reactors must lower the reactor power gradually in order to burn off as much samarium 149 as possible before shutdown.
Special reference: Paul Reuss, Neutron Physics. EDP Sciences, 2008. ISBN: 978-2759800414.