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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
