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1、Nuclear®ssionreactorsCharlesE.TillArgonneNationalLaboratory,Argonne,Illinois60439[S0034-6861(99)01902-9]Thestoryof®ssionisastoryofdramaandemotion,therecouldbesuf®cientneutroncaptureinuraniumtoanditisastorythatdemonstratespossiblybetterthanmakeaself-sust
2、aining®ssionreactionpossible.anyothertheoverwhelmingimportanceofmodernTheprinciplesunderlyingthe®rstsuccessfulreactorphysicstotheeventsofthispastcentury.Itsdiscovery,demonstrationwerequitesimple.Theprincipalobstacleormoreproperly,itsidenti®cation,wasdel
3、ayedforatoovercomewasthefactthatthefuelhadtobeuraniumsurprisinglylongtimeafterthephenomenonhadinfactthatcontainedonlythenaturallyoccurringtraceconcen-beeninducedinseveralofthe®rst-ranklaboratoriesintration(0.7%)oftheactual®ssileelementU-235.Notheworld.O
4、verfouryearspassedwithoutitsrecogni-enrichmentfacilitiestoincreasetheconcentrationofU-tion,butwhenrecognitiondidcome,itcameatthemost235hadyetbeencontemplated.Absorptionofneutronsdramaticpossibletime.Itwasjustbeforethewarthatinthefuelitselfthereforeneede
5、dtobemaximized,use-engulfedtheworld,andknowledgeablephysicistsinev-lessneutronabsorptioninnonfuelmaterialsminimized,erycountrywereinapositiontoguessatitspossiblethesurfacewhereneutronscouldleavethesystem,alsomilitarysigni®cance.uselessly,minimizedwithre
6、specttothevolumewhereThedelayintheidenti®cationofthephenomenonofusefulabsorptioncouldtakeplace,and®nally,absorp-®ssionwascausedbymisdirection,inthattheexperi-tionofneutronsinthefuelhadtotakeplaceatthe(low)mentersexpectedanotherresult,basedontheunder-ene
7、rgieswheretheprobabilityof®ssionwashigh.Onlystandingoftheday.Whenthiswascombinedwithmea-thencouldcriticalitybeachievedandachainreactionsurementimprecisioninherentinthetechniquesusedinbegintoproceedatsteadypower.Theprobabilityoftheexperiments,properinter
8、pretationoftheresultswas®ssionishighestatthermalenergies.Anef®cientmod-obscured.Forthediscoveryof®ssionrestedondelicateeratingmaterialwasneededtomoderatetheenergyofradiochemicalseparationsofunknownelementscreatedneutronsreleasedi
