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1、Chapter8DECAYHEATGENERATIONINFISSIONREACTORS?M.Ragheb6/17/20121.INTRODUCTIONAfterareactorcoreisshutdown,throughtheinsertionofitscontrolrods,heatcontinuestobegeneratedbythedecayofthefissionproducts,eventhoughthefissionpowerwouldstoptobegenerated.Thefissionproductsheat
2、generation,alsocalledafterheat,afterglowordecayheat,wouldhavetobeextractedfromthesystem;otherwiseitwouldleadtofueldamage,steam-claddinginteractionleadingtohydrogengeneration,meltingorevenvaporizationofthecore.ItdependsonthedesignofthenuclearpowerplantasshowninFig.1,p
3、articularlyonitspowerdensity.LightWaterReactors,LWRsasPressurizedWaterReactors,PWRSandBoilingWaterReactors,BWRsofferthesametypeofresponse.Figure1.Powerinthermalkilowattsperunitmassofreactorfuelinmetrictonnesfordifferentreactordesigns.LWR:LightWaterReactor,AGR:Advance
4、dGas-cooledReactor,Magnox:Magnesiumoxidealloycladdingreactor.Thedecayheatpowergenerationdecreasesrapidlyasafunctionoftime,butcoolingprovisionsfordissipatingitimmediatelyaftershutdownmustbeincorporatedintothereactor’sdesign.Thepowerreleasefromsuchaprocessmustbeestimat
5、edandaccountedforinthesafetydesignofnuclearpowerplants.2.SOURCETERMSTwosourcetermscontributetothedecayheatrates.Onesourcetermresultsfromtherateofnegativebetaparticlesemissionbythefissionproductsandisgivenby:??612??particlesRT()=3.810×T(1)β????sec.fissionTheseconddeca
6、yheatsourceresultsfromtherateofgammaphotonsemissionsbythefissionproducts,andisgivenby:??612??photonsRT()1.910=×T(2)γ????sec.fissionwhere:Tisthetimeafterthefissioneventindays.Itcanbenoticedthattherateofbetaemissionsistwicetherateofgammasemissions.3.BETAANDGAMMAENERGYR
7、ELEASESWeconsiderthemeanenergiesofthebetaandgammaparticlesas:E=0.4MeV,βE=0.7MeV.γwheretheaverageenergyofthegammaphotonsisslightlylessthantwicethatofthebetaparticles.Thissuggeststhattheenergyreleasefromthebetaemissionsslightlyexceedstheenergyreleasefromthegammaemissio
8、nsfromthefissionproducts.Thecombinedrateofemissionofbetaandgammaraysenergycanbewrittenas:ET?()=RTERTE()+()ββγγ?6??612=××+××[3.810(0
