Samarium form factor

The perception that lanthanide metals were rare and therefore inaccessible or expensive was a contributing factor to their long-lasting neglect and slow develo nent as useful synthetic tools, hi fact, rare earths in general are relatively plentiful in terms of their abundance in the earth s crust. Samarium and ytterbium occur in proportions nearly equal to those of boron and tin, for example. Modem separation methods have made virtually all of the lanthanides readily available in pure form at reasonable cost. 

Poison addition to the reactor is the final general factor affecting neutron multiplication. Poisons can be in the form of boron in control rods xenon and samarium fission products or any absorbing nucleus that is introduced into the reactor. Poisons increase the denominator in the thermal utilization factor making the overall value of f decrease. In the case of resonance absorbers, resonance escape probability also decreases. 

As xenon-135 and samarium-149 are formed in a reactor, they reduce the multiplication factor by decreasing the thermal utilization factor, f, Since the formation of fission product poisons is a direct function of the fission rate, as power level changes the amount of poison present in the reactor also changes. Control system reactivity insertions such as rod motion and chemical shim must be made to compensate for fission product reactivity. 

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