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Nuclear Safeguards Education Portal

Isotopic Characteristics

The initial fissile content (enrichment) of the fuel and the unique fuel assembly operational characteristics, irradiation history (burnup) and cooling time, can lead to unique spent fuel assembly isotopic characteristics.  Fission caused by fuel irradiation during reactor operation depletes fissile materials and produces fission products.  Irradiation also causes neutron capture which produces other transuranic isotopes and structural activation products in the non-fuel assembly materials.    Figure 2 shows concentration as a function of burnup for U-238, U-235, and Pu-239 (on a secondary axis) for fuel assembly radiation Case 1.  As U-235 is depleted through fission in Figure 2, Pu-239 accumulates from neutron capture in U-238 until fission of Pu-239 counteracts the buildup.   The concentrations of these isotopes is primarily a function of burnup.  The concentrations of Figure 2. were simulated with the TransLat lattice physics software package (Ref. 4).

 Isotopic -char -fig1

Figure 2.  Major actinide concentration in atoms/barn-cm  of nuclear fuel with initial enrichment of approximately 3 wt% U-235 during reactor operation.  Uranium concentration is plotted on a log scale.

Figure 3 illustrates the buildup of two fission products isotopes: (a) Nd-144 and (b) Sm-147.  Nd-144 production in Figure 3 (a) is clearly a linear function of burnup.  However, Sm-147 concentration in Figure 3 (b) is dependent on the complete irradiation history; both specific power and reactor shutdown time (Ref. 3).  Sm-147 is the stable daughter isotope of the radioactive fission product Pm-147.  Pm-147 has a large neutron absorption cross-section.  Neutron capture by Pm-147 is proportional to the neutron flux and specific power during reactor operation.  When the reactor is shut down, specific power is equal to zero and the Sm-147 concentration spikes from the unabated decay of Pm-147.  The concentration of Sm-147 may be further altered by additional radioactive decay of Pm-147 during post-irradiation cooling time.  The concentrations of Figure 3 were simulated by the TransLat lattice physics software package (Ref. 4).  The differences in core location, cycles, and shutdown lengths can result in fairly unique spent fuel isotopic composition even though final burnup may be the same.  

Isotopic -char -fig2 

Isotopic -char -fig 2b

Figure 3. Concentration in atoms/barn-cm of fission products (a) Nd-144 and (b) Sm-147 (from Ref. 5) for the 5 fuel assembly irradiation case histories of Figure 2.

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