Thermal  Utilization  Factor,  (f) - h1019v2_28

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NEUTRON LIFE CYCLE DOE-HDBK-1019/2-93 Reactor Theory (Nuclear Parameters) The  value  of  the  resonance  escape  probability  is  determined  largely  by  the  fuel-moderator arrangement  and  the  amount  of  enrichment  of  uranium-235  (if  any  is  used).    To  undergo resonance absorption, a neutron must pass close enough to a uranium-238 nucleus to be absorbed while slowing down.  In a homogeneous reactor the neutron does its slowing down in the region of  the  fuel  nuclei,  and  this  condition  is  easily  met.    This  means  that  a  neutron  has  a  high probability  of  being  absorbed  by  uranium-238  while  slowing  down;  therefore,  its  escape probability  is  lower.    In  a  heterogeneous  reactor,  however,  the  neutron  slows  down  in  the moderator where there are no atoms of uranium-238 present.  Therefore, it has a low probability of undergoing resonance absorption, and its escape probability is higher. The value of the resonance escape probability is not significantly affected by pressure or poison concentration. In   water   moderated,   low   uranium-235   enrichment   reactors,   raising   the temperature of the fuel will raise the resonance absorption in uranium-238 due to the doppler effect (an apparent broadening of the normally narrow resonance peaks due to thermal motion of nuclei).   The increase in resonance absorption lowers the resonance escape probability, and the fuel temperature coefficient for resonance escape is negative (explained in detail later).  The temperature  coefficient  of  resonance  escape  probability  for  the  moderator  temperature  is  also negative.  As water temperature increases, water density decreases.  The decrease in water density allows  more  resonance  energy  neutrons  to  enter  the  fuel  and  be  absorbed.   The  value  of  the resonance escape probability is always slightly less than one (normally 0.95 to 0.99). The product of the fast fission factor and the resonance escape probability (   p) is the ratio of the number of fast neutrons that survive slowing down (thermalization) compared to the number of fast neutrons originally starting the generation. Thermal  Utilization  Factor,  (f) Once  thermalized,  the  neutrons  continue  to  diffuse  throughout  the  reactor  and  are  subject  to absorption by other materials in the reactor as well as the fuel.   The thermal utilization factor describes  how  effectively  thermal  neutrons  are  absorbed  by  the  fuel,  or  how  well  they  are utilized  within  the  reactor.    The  thermal  utilization  factor  (f)  is  defined  as  the  ratio  of  the number of thermal neutrons absorbed in the fuel to the number of thermal neutrons absorbed in any reactor material.   This ratio is shown below. f   number  of  thermal  neutrons  absorbed  in  the  fuel number  of  thermal  neutrons  absorbed  in  all  reactor  materials The thermal utilization factor will always be less than one because some of the thermal neutrons absorbed within the reactor will be absorbed by atoms of non-fuel materials. NP-03 Rev. 0 Page 4


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