Table 5 gives a summary of all of the spent nuclear fuel signatures we have discussed. Feel free to use this as a quick reference in subsequent chapters of this module.
Signature  Explanation  

Physical Signatures  

Irradiation during reactor operation can corrode and deform fuel assemblies.  
Gamma Radiation Signatures  
Total γray activity  The total amount of gamma radiation from fission products is proportional to the total number of fissions (i.e., burnup adjusted for the cooling time). High γray activity comes from irradiation in a nuclear reactor.  
Single fission product γray activity  Cs137 activity  Fission product γray activity is emitted only by spent fuel. The fission yield of Cs137 is equal for both U235 and Pu239. Cs137 has a small neutron cross section and long halflife relative to typical reactor irradiation cycle shutdowns. These factors make Cs137 concentration a function of burnup and cooling time after irradiation. 
Fission product γray activity ratios 
Cs134 / Cs137 
Activity ratios are more easily determined than absolute activity because underlying detection biases will cancel. Both ratios yield a mostly linear curve dependent on burnup when corrected for cooling time decay. Cs134/Cs137 also depends on shutdowns in irradiation history. 
Cerenkov Radiation Signatures  
Cerenkov radiation  Cerenkov radiation indicates an object has been irradiated. Cerenkov intensity (dependent on total γray emission) is proportional to burnup and decays with cooling time (precise measurement of Cerenkov intensity is required).  
Neutron Radiation Signatures  
Total neutron activity  Neutron emissions come from only spent nuclear fuel. Total neutron activity is proportional to the burnup. Neutronburnup proportionality is impacted by both initial U235 enrichment and the number of shutdowns. Fuels with lower initial enrichment require a greater neutron fluence to reach the same burnup as fuels with higher initial enrichment. The larger neutron fluence of lower enriched fuels results in a greater amount of neutron emitting actinides in the spent fuel. Spent fuel neutron activity is dominated by Cm. During shutdowns between irradiation cycles Pu241 which would otherwise absorb a neutron during reactor operation instead decays to Am241 which produces Cm242 through neutron absorption during subsequent irradiation. This produces greater neutron activity in spent fuel with more cycles or total shutdown time.  
Coincident neutron activity  Fission in spent fuel fissile materials emits multiple prompt neutrons simultaneously. Detection of 2 or more neutrons simultaneously at different locations around the assembly can be representative of a fission in fissile material. The coincident neutron count rate can be correlated to the fissile material content of the spent fuel. Active interrogation of the spent nuclear fuel with an external neutron source can enhance the coincident neutron signal.  
Combined Radiation Signatures  
Total γ activity / Total neutron activity  The total gamma / total neutron ratio is proportional to total burnup and insensitive to cooling time.  
Total neutron vs. fission product ratios  Burnup from the total neutron rate compared to the burnup from Cs134/Cs137 ratio is used to distinguish mixed oxide fuel (MOX) from LEU. 
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