Neutron continued fluence

The neutron fluence in massive stars is such that iron can capture neutrons up to Zr, but not much beyond that. This is the so-called weak component of the s process. In contrast, the region between the hydrogen and helium burning shells in an evolved low-mass star experiences a complex sequence of events that allows iron to capture all the way to Pb and Bi. Because the isotopes of Po are all unstable, however, s-process flow cannot continue to the actinides. This burning in low-mass stars provides what is known as the main component of the s process. 

The high-density iimer pyrolytic carbon (IPyC) layer protects the kernel and buffer from chemical attack by chlorine compounds, which are generated as byproducts during deposition of the silicon carbide (SiC) layer. The IPyC layer also provides a surface for deposition of the SiC layer and delays transport of radionuclides to the SiC layer. The IPyC layer shrinks with the accumulation of fast neutron fluence, which helps to maintain the SiC layer in compression, provided the bond between the IPyC and SiC layers remains strong and continuous during irradiation. 

Total fluence requirements are determined by the U concentration present in the mineral to be examined. Apatites typically have concentrations from 5 to 100 ppm, requiring a thermal neutron fluence of about 10 n cm. Zircons, with 50 to 500 ppm, require around 2 to 4 x 10 n cm. As a practical example, this translates to about 30 hours in the thermal column of a 1 MW reactor. However, the irradiations do not have to be continuous. 

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