中子的减速教学教程电子教案.ppt

Click to edit Master title style,Click to edit Master text styles,Second level,Third level,Fourth level,Fifth level,HT2004:Rector Physics,Slowing down,*,SLOWING DOWN OF NEUTRONS,Elastic scattering of neutrons.,Lethargy.Average Energy Loss per Collision.,Neutron moderation without space distribution.Space dependent moderation.Age Theory.,Neutron Spectrum in a Core.,HT2004:Rector Physics,1,Slowing down,Chain Reaction,235,U,2,MeV,n,235,U,n,0.1eV,235,U,n,n,HT2004:Rector Physics,2,Slowing down,HT2004:Rector Physics,3,Slowing down,Principles of a Nuclear Reactor,n,n/fission,N,1,N,2,Leakage,Fast fission,Resonance abs.,Non-fuel abs.,Leakage,Non-fissile abs.,Fission,Slowing down,Energy,E,2,MeV,1,eV,200 MeV/fission,2.5,HT2004:Rector Physics,4,Slowing down,HT2004:Rector Physics,5,Slowing down,HT2004:Rector Physics,6,Slowing down,HT2004:Rector Physics,7,Slowing down,HT2004:Rector Physics,8,Slowing down,Differential Cross Sections,r,x,y,z,HT2004:Rector Physics,9,Slowing down,Hard Spheres Model,r,Total scatteringcross section,s,=2,r,2,n,r,HT2004:Rector Physics,10,Slowing down,r,b,(,),impact parameter,cross section,(,),d,I,(,),I,is the number of neutrons deflected by an angle greater than,HT2004Rector Physics,11,Slowing down,Unit sphere r=1,I,HT2004:Rector Physics,12,Slowing down,Detector,I,HT2004:Rector Physics,13,Slowing down,Neutron Slowing Down,u,0,U,0,U,u,v,c,v,q,HT2004:Rector Physics,14,Slowing down,=0,=180,E,v,E+dE,v+dv,HT2004:Rector Physics,15,Slowing down,E,E-dE,E,a,E,0,E,0,p(E),HT2004:Rector Physics,16,Slowing down,Quantum mechanics+detailed nuclear physics analysis conclude,Elastic scattering is isotropic in CM system for:,neutrons with energies,E,10,MeV,light nuclei with A 13,HT2004:Rector Physics,17,Slowing down,E,a,E,0,E,0,HT2004:Rector Physics,18,Slowing down,HT2004Rector Physics,19,Slowing down,Number of collision required for,thermalisation,:,For non-,homogenuous,medium:,Average cosine,value of the scattering angle in CM-system,HT2004:Rector Physics,20,Slowing down,Average cosine,value of the scattering angle in Lab-system,HT2004:Rector Physics,21,Slowing down,Material,A,m,0,1,H,1,0,0.667,2,D,2,0.111,0.333,4,He,4,0.360,0.167,6,Li,6,0.510,0.095,9,Be,9,0.640,0.074,10,B,10,0.669,0.061,12,C,12,0.716,0.056,238,U,238,0.938,0.003,H,2,O,*,*,0.037,D,2,O,*,*,0.033,HT2004:Rector Physics,22,Slowing down,Slowing-down features of some moderators,Moderator,N,s,s,/,aH,2,O,0.927,19.7,1.36,62,D,2,O,0.510,36,0.180,5860,Be,0.209,87,0.153,138,C,0.158,115,0.060,166,U,.0084,2170,.0040,0.011,N-number of collision to thermal energy,xS,s,-slowing down power,xS,s,/,S,a,-moderation ratio(quality factor),HT2004:Rector Physics,23,Slowing down,Neutron velocity distribution,Att,.!,k,B,=1.38110,-23,J/K=8.61710,-5,eV,/K,Velocity space:,v+dv,v,4,v,2,dv,Probability that energy level,E,=,mv,2,/2,is occupied:,HT2004:Rector Physics,24,Slowing down,The most probable velocity:,and corresponding energy:,Maxwell-Distribution for Neutron Density,HT2004:Rector Physics,25,Slowing down,Dont forget:,Maxwell-Distribution for Neutron Flux,HT2004:Rector Physics,26,Slowing down,HT2004:Rector Physics,27,Slowing down,Average energy for neutrons,Neutron flux distribution:,For thermal neutrons,HT2004:Rector Physics,28,Slowing down,Average cosine of scattering angle:,CM:,LAB-system:,The consequence of,0,0 in the laboratory-system is that the neutron scatters preferably forward,specially for A=1 i.e.hydrogen and practically isotropic scattering for A=238 i.e.Uranium,because,0,0 i.e.,Y=,90,o,in average.This corresponds to isotropic scattering.,l,tr,is defined,as effective mean free path for non-isotropic scattering.,HT2004:Rector Physics,29,Slowing down,Y,Y,Y,l,s,cos,Y,l,s,cos,Y,2,Transport Mean Free Path,l,s,l,tr,HT2004:Rector Physics,30,Slowing down,Slowing-down of Fast Neutrons,Infinite medium,Homogeneous mixture of absorbing and scattering matter,Continues slowing down,Uniformly distributed neutron source,Q(E),(,E,)=n/(cm,2,s,eV),(,E,),dE,=number of neutrons with energies in dE about E,HT2004:Rector Physics,31,Slowing down,Continues Slowing-Down,E,t,dE,dt,assumed slowing-down,real slowing-down,HT2004:Rector Physics,32,Slowing down,Slowing-down density,q(E),-,number of neutrons,which per cubic-centimeter and second pass energy E.If no absorption exists in medium,so:,q(E,)=Q;Q,-source yield(n,cm,-3,s,-1,),Assuming,no or weak absorption(without resonances),Energy,E,q(E,),HT2004:Rector Physics,33,Slowing down,Energy,Lethargy,E,max,0,Lethargy,E,u,q,(,E,),u+du,E-,dE,u-du,HT2004:Rector Physics,34,Slowing down,Neutron spectrum,E,F,(E),u,F,(u),E,0.025,eV,20,15,10,5,0,10,MeV,HT2004:Rector Physics,35,Slowing down,Slowing-down with resonance absorption,Probability for absorption per collision:,Number of collisions in,du,or,dE,:,Probability for absorption in,du,or,dE,:This corresponds en relative decrease in,q,:,HT2004:Rector Physics,36,Slowing down,F,(u),E,u,s,t,=,s,s,+s,c,s,c,s,F,0,(u),q,0,F,(u),q,HT2004:Rector Physics,37,Slowing down,Example.,How long time exists the neutron under slowing-down phase respectively as thermal?,Slowing-down in time-,t,s,:,Number of collisions in,du,:,Number of collisions in,dt,:,v,(1,eV,)=1.39 10,6,cm/s,v,(,0.1 MeV,)=,4.4,10,8,cm/s,Thermal life-length-,t,t,:,HT2004:Rector Physics,38,Slowing down,Material,t,fast,(,m,s),t,thermal,(,m,s),H,2,O,1,200,D,2,O,8,1.5,10,5,Be,10,4300,C,25,1.2,10,4,Neutrons slowing-down time,and thermal life-time,HT2004:Rector Physics,39,Slowing down,Under the neutron life-time,Fission neutrons-fast neutrons,(10 MeV-0.1,MeV,),Slowing-down neutrons-resonance neutrons,(0.1MeV-1,eV,),Thermal neutrons,(1eV-0.),HT2004:Rector Physics,40,Slowing down,