Decay Heat generation in Fission Reactors

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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