Need for Processing: Fuel is removed from the reactor before all of 
the fissionable material is consumed for several reasons: the neutron 
absorbing fission products build up; radiation damage to fuel elements in- 
ereases proportionally with time; fuel elements become corroded by reactor 
coolants; the heat generated causes distortion; and the quantity of fissionable 
material decreases to the point where a chain reaction cannot be supported. 
Unreacted fissionable material must be separated from the fission products. 
Similarly, it is necessary to recover fertile material and the new fissionable 
material produced by excess eeuenon capture from reactor systems where fertile 
material is present. Obviously, fissionable material produced in the fertile 
material is fissioned along with the fuel but, fortunately, not as fast as 
it is produced. However, after irradiation, the fertile material contains 
fission products which must be separated from the unused fertile material 
and from the new fissionable material. 
Cooling Time; After irradiation in a pile, the contaminated mixture 
of fuel and fertile material normally is allowed to stand for a period of 
time (cooling period) to allow the fission products to decay and to permit 
the buildup of new fissionable material from the predecessors, Np? and 
pan33 | The fuel elements at the time of discharge are exceptionally 
radioactive and are too "hot" to be processed conveniently. 
Decay of Fission Products: Fission products decay from the instant 
they are produced ina pile. They decay by emitting beta particles and 
gamma rays. After 100 days cooling approximately twenty fission products 
remain which are significant from a waste standpoint. Each one has a 
characteristic and unchangeable decay rate which is customarily expressed 
as the half life of the isotope. The decay constant, INS is related to 
half life T , by the following simple expression: 
1/2 
x by 0.693 
Ty /e where: 0.693 = In 2 
DECLASSIFIED 
