Samarium Production
Fission does not produce Sm 149 directly, and the fission yield of samarium 149 is nearly zero. Sm-149 is in the decay chain of the fission product neodymium 149, which decays to promethium 149. Because of the short half-life of Nd-149 (compared to the half-life of Pm-149), promethium may be considered as if it were formed directly from fission. The decay sequence is depicted in the figure.
Samarium Removal
An important difference between Xe-135 and Sm-149 is that samarium 149 is a stable isotope and remains in the core after shutdown. There are only two ways of samarium removal, and one of these processes is the samarium burn up when the reactor is at power operation. The samarium burnup rate depends upon the neutron flux and the samarium 149 concentration (i.e., the reaction rate).
The second process is associated with refueling. During refueling, one-third or one-quarter of the core is usually removed from the spent fuel pool and replaced by fresh fuel assemblies. At the same time, 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.
Samarium 149 – Differential Equations
Assuming that fission does not produce Sm 149 directly and neglects the small amount of promethium burnup, the situation is described as follows. The production and removal of samarium can be characterized by the following differential equations.
