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MANY FAST CRITICAL ASSEMBLIES HAVE BEEN BUILT IN MANY COUNTRIES INTHE WORLD IN ORDER TO ACCUMULATE VALUABLE DATA ABOUT THE NUCLEARCHARACTERISTICS AND SAFETY OF THE FAST BREEDER REACTOR. THE EXPERI-MENTS ON THE DOPPLER REACTIVITY EFFECT ARE ALSO NOT THE EXCEPTIONALCASE AND HENCE ARE PERFORMED ENERGETICALLY AT EVERY RESEARCH LABORA-TORY. HOWEVER,ALMOST ALL THE MEASURING METHOD SO FAR EMPLOYED HAVEBEEN THE SAMPLE DOPPLER REACTIVITY METHODS,NAMELY,THOSE TO DETER-MINE THE REACTIVITY WORTH DIFFERENCE OF A HOT SAMPLE LOCATED AT THECENTER OF A CORE FROM THE SAME SAMPLE AT THE SAME LOCATION BUT INCOLD SITUATION.SEFOR(SOUTHWEST EXPERIMENTAL FAST OXIDE REACTOR)IS AN EXPERIMENTALFAST REACTOR,ONE OF THE MAIN PURPOSES OF WHICH IS THE STUDY OF THEWHOLE CORE DOPPLER REACTIVITY EFFECT WHICH HAS NOT BEEN ENOUGH STUDIEDUP TO NOW ON THE FAST CRITICAL ASSEMBLIES,AND A LOT OF DATA OBTAINEDFROM THE EXPERIMENTS PERFORMED ON SEFOR HAVE BEEN PUBLISHED.THE MAIN PURPOSE OF THE PRESENT WORK IS TO ESTABLISH THE MET
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