核反应堆工程核反应堆工程 (8).ppt

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1、Nuclear Engineering ReactorRadiation Effects in MaterialsnIntense nuclear radiations of various types that occur in a typical reactor environment are capable of changing the microstructure of many materials.Often,these changes have an unfavorable effect on physical and mechanical properties.nSince t

2、he materials are in a sense“damaged”,the term radiation damage is often used.This deleterious change in properties is an important design consideration and is unique to nuclear engineering.nThe radiation in a nuclear reactor consist mainly of alpha and beta particles(from radioactive decay),gamma ra

3、ys,neutrons,and fission products.As far as nonfuel materials,which are mainly metals,are concerned,neutrons are the most important in producing radiation effects.Beta particles and gamma rays can cause ionization and electronic excitation,but these do not produce substantial permanent changes except

4、 in nonmetallic substances,such as water and organic compounds.In metals,the electronic excitation energy is rapidly dissipated as heat.Radiation in a nuclear reactor High-energy fission products and alpha particles from radioactive source are restricted to the fuel because of their short range.They

5、 have little if any effect on other materials.The effects of neutrons on materials arise largely from the transfer of kinetic energy to atomic nuclei in one way or another.Thus recoil nuclei may be produced as a result of(n,)and other neutron reactions or of elastic scattering.Radiation in a nuclear

6、 reactor nIf the energy of the recoil nucleus is sufficient to permit it to be displaced from its normal(or equilibrium)position in the space lattice of a solid,physical changes of an essentially permanent character maybe observed.nThe effect of nuclear radiation in disrupting(or damaging)the crysta

7、l lattice by the displacement of atoms is commonly referred to as“radiation damage”.（a）Arrangement of crystal atoms（b）Space lattice or crystal lattice（c）Unit cellCrystal structure of pure iron metalnThe interactions of radiations,especially thermal and fast neutrons,with solids are summarized and sh

8、own in the figure below.Interactions of thermal and fast neutrons with a solidnIf the kinetic energy of a recoil nucleus exceeds a certain minimum values,called the displacement energy,which is about 25 to 40 eV for most metals,the recoiling atom will be displaced from its equilibrium position in th

9、e crystal lattice.nThe primary knock-on atom acquires sufficient kinetic energy to cause displacement of another atom by collision,and the latter becomes secondary knock-on.The progress will until the displaced atom does not have sufficient energy to eject another atom from the crystal lattice.nThis

10、 effect is especially important when the knock-on atom(or nucleus)is produced as the result of an elastic collision with a fast neutron(or other energetic heavy particle).nDefects in real metal crystals Point defect Line defect Surface defectnSlow neutrons cannot produce atomic displacements directl

11、y but they can do so indirectly as the result of radiative capture(n,)reactions.n The maximum of a gamma ray accompanying an(n,)reaction is roughly 6 to 8 MeV,and so for an element of low atomic mass,e.g.,about 10,the recoil energy could be 2 or 3 MeV.It requires only about 40 eV to displace an atom

12、,the recoil energy should be capable of causing an appreciable number of atomic displacements.nIn a thermal reactor,(n,)reactions may produce a significant fraction of the total displacements.In fact,in some circumstances,the radiation damage caused by thermal neutrons may be greater than that from

13、fast neutrons.nIrradiation frequently decreases the density of a metal over a certain temperature range,so that a specimen exhibits an increase in volume,i.e.,it swells.nThe swelling can cause changes in dimensions of the coolant channels and also interfere with the free movement of control elements

14、.The swelling increases with the fluence.nThe swelling of stainless-steel structural components and fuel-rod cladding in fast reactors,as a result of fast neutron irradiation at the temperatures existing in such reactors,is a matter of great concern.nWhen an atom is ejected from a crystal lattice,it

15、 leaves a vacant normal site,known as a vacancy.Some of the knock-ons produced by neutrons(or other radiations)will eventually fill vacancies left by the removal of other atoms.But not all knock-on atoms can find nearby vacancies.VacancyFrenkel pairnAn atom occupying an interstitial position is call

16、ed an interstitial atom,or interstitial.For each interstitial produced by the action of radiation,there is a vacancy somewhere in the lattice.A lattice vacancy and an interstitial when considered as a unit are referred to as a Frenkel pair(or Frenkel defect).Solids commonly contain some Frenkel defe

17、cts,but the number increases markedly as a result of exposure to fast neutrons.nBoth vacancies and interstitials migrate at random through the solid at rates that increase with temperature.An interstitial migrates by moving to another nonequilibrium site,whereas a vacancy effectively migrates when a

18、 neighboring atom(from an equilibrium site)moves into the vacancy leaving another vacancy behind.nDuring the course of migration,an interstitial and a vacancy occasionally suffer mutual annihilation when an interstitial atom enters the vacancy and remains there.nProvided the temperature is high enou

19、gh to permit the interstitials and vacancies to be fairly mobile but not so high that they are moved by recombination and by migration to sinks,a relatively large(supersaturated)concentration of defects can be maintained under irradiation.nIn these circumstances,the interstitials tend to agglomerate

20、 or cluster to form roughly circular two-dimensional disks or platelets,commonly called interstice loops.These loops can then grow to the point where they transform to network dislocations.Interstice loopsnVacancies,on the other hand,can agglomerate to form either two-dimensional vacancy loops,which

21、 collapse into dislocation loops,or three-dimensional clusters called voids.nSubstantial void growth following a prolonged incubation results in swelling,primarily under the fast fluence and temperature conditions found in fast reactors.Also important in fast-reactor stainless steel is the effect of

22、 helium formed by(n,)reactions,which can contribute to swelling as well as result in a ductility loss.Vacancy loops and voids General Effects of Fast-Neutron Irradiation on MetalsnThe general physical and mechanical effects of the irradiation of metals by fast neutrons and other high-energy particle

23、s are summarized as follows:nFor fast neutrons,the changes are usually undetectable at fluences below 1022 neutrons/m2nWith increasing fluence,the magnitude of the effect then increases,possibly approaching a limit at very large fluencesnFor a given fluence,the effects of irradiation are generally l

24、ess at elevated than at lower temperatures.nIncrease of temperature facilitates recovery(annealing)of the Frenkel defects produced by irradiation in a metal.Migration of the interstitials and vacancies increases with temperature,and this may be expected to increase the removal rate.nBoth the yield s

25、trength and ultimate tensile strength of a metal are increased by irradiation,but the increase in yield strength is generally greater than the increase in the tensile strength.nIrradiation results in a decrease in ductility.The increase in the yield strength also results in a corresponding increase

26、in the NDT temperature characterizing the transition from ductile to brittle fracture.Qualitative representation of neutron irradiation effect on many metalsnThe increase in the NDT temperature is one of the most important effects of irradiation from the standpoint of nuclear power system design.nIt

27、 is not likely that the transition temperature would rise sufficiently to approach the temperature of the steel curing normal operation.nHowever,there is a possibility that accident sequences leading to the injection of emergency coolant water would result in combinations of vessel temperatures with thermal and pressure stresses that could lead to catastrophic vessel fracture.nAttention must also be given to the role of the transition temperature during heat-up and low-pressure testing.