1、Chapter 3. Basic Instrumentation for Nuclear TechnologyAccelerators DetectorsReactorsOutline of experiment: get particles (e.g. protons, ) accelerate them throw them against each other observe and record what happens analyse and interpret the dataHistory-WhyParticle SourcesAcceleration stageSpace ch

2、argeDiagnosticsApplication1.AcceleratorsGas-Filled Radiation DetectorsScintillation DetectorsSemiconductor DetectorsPersonal DosimetersOthersParticle identificationMeasurement theoryDetection Equipmentionization chambersproportional countersGeiger-Muller countersE-E, TOFphotographic films photograph

3、ic emulsion platesCloud and Bubble ChambersPhotomultiplier tube2. Detectors3. ReactorsReactions Involving NeutronsThermal-Neutron Properties of FuelsGeneral featuresThe Neutron Life Cycle in a Thermal ReactorHomogeneous and Heterogeneous CoresReflectorsReactor KineticsReactivity Effects (feedback)Su

4、stained, moderation“The energy produced by the breaking down of the atom is a very poor kind of thing. Anyone who expects a source of power from the transformations of these atoms is talking moonshine.” Lord Ernest Rutherford, 1933.n + 235U X + Y+ n + + E 200 MeV Self-sustaining Chain reactionDec. 2

5、, 1942, Fermi achieved sustained chain reaction, and the first fission reactor provided data for future design of nuclear reactors.The Vision“It is not too much to expect that our children will enjoy in their homes nuclear generated electrical energy too cheap to meter.” Lewis Strauss, Chairman of t

6、he U.S.Atomic Energy Commission (1954)The total and fission cross section for 235U based on NJOY-processed ENDF/B (version V) data.Fission neutron energy spectrumWhy neutron moderation is needed?The average energy of prompt fission neutrons is about 2 MeVReactions Involving NeutronsNeutron Scatterin

7、gThe elastic scattering is the main mechanism in moderating neutrons in thermal nuclear reactors.The corresponding neutron energy lossAverage Logarithmic Energy Losson a logarithmic energy scale a neutron loses the same amount of logarithmic energy perelastic scatter, regardless of its initial energ

8、yaverage number of scatters required to bring a neutron of initial energy E1to a lower energy E2Slowing of neutron (moderation) by various materials.Here n is the number of elastic scatters to slow, on the average, a neutron from 2 MeV to 0.025 eVIs H2O a good moderator (慢化劑)Nuclear Fission15Thermal

9、 Neutrons Cross SectionsThermal neutron capture cross sections (c)Thermal neutron cross section for fission (f)1H2H 12C 14N 16O113Cd c /b0.330.00052 0.0034 1.82 0.000219,820Moderators: H2O vs. D2O vs. CFermis used Cd for emergencyNeutron Capture ReactionsNeutron leakageSafety considerationSpontaneou

10、s fission:The fission produced in these cases is insignificant for energy production However the phenomenon is important since represents an uncontrollable source of neutrons in a reactor and it is, furthermore, possible to make use of it in the start-up stage. An example of the use of this fission

11、is the neutron source of 252 californium.Induced fission: Certain heavy nuclei can be induced to fission, as result of one neutron capture. Consequently, several high-energy neutrons are produced, which permit to maintain the chain reaction process. The nuclei 235U,233U, 239Pu and 241Pu experience f

12、ission with low-energy thermal neutrons and they are called fissile materials. The nuclei 238U and 232Th fission with fast neutrons. The radiative capture of neutrons by 238U and 232Th leads to the formation of the fissionable materials 239Pu and 233U, so they are called fertile materials. 3. Reacto

13、rsReactions Involving NeutronsThermal-Neutron Properties of FuelsGeneral featuresThe Neutron Life Cycle in a Thermal ReactorHomogeneous and Heterogeneous CoresReflectorsReactor KineticsReactivity EffectsThermal-Neutron Properties of Fuelsf and are the cross-sections for fission and capturev is the a

14、verage number of emitted neutrons per nuclear fissionThe number of neutrons emitted when one neutron is absorbed in the nucleus expressed as 1. 233U has the largest value of , the number of fission neutrons produced per thermal neutron absorbed, and hence is the best prospect for a thermal breeder r

15、eactor (增值反應堆）. A breeder reactor needs an of at least two since one neutron is needed to sustain the chain reaction and one neutron must be absorbed in the fertile material （增值材料） to breed a new fissile fuel atom. Fertile materials are those such as 232Th and 238U that, upon thermal neutron absorpt

16、ion, may yield fissile materials2. Although the plutonium isotopes produce almost 3 fission neutrons per thermal fission, their relatively high radiative capture (n,) cross sections result in low values of . E 100 keV, 239Pu and 241Pu, 3. Thus fast reactors using plutonium as fuel are attractive as

17、breeder reactors.3. The fertile isotopes 232Th and 238U have absorption cross sections of about 1% or less than those of their conversion fissile isotopes4. The fertile isotope 240Pu has a large capture cross section for the production of the fissile isotope 241Pu.-values for important fissile nucli

18、des3. ReactorsReactions Involving NeutronsThermal-Neutron Properties of FuelsGeneral featuresThe Neutron Life Cycle in a Thermal ReactorHomogeneous and Heterogeneous CoresReflectorsReactor KineticsReactivity EffectsSustained, moderationgeneral featuresActive core: (1) fissile fuel which through its

19、fissioning is the main source of neutrons, (2) moderator material if the fission neutrons are to slow down, (3) coolant if the heat generated by the fissions is to be removed from the core, and (4) structural material which maintains the physical integrity of the core.Reflector: scatter neutrons bac

20、k towards the coreBlanket region: captures neutrons leaking from the core to produce useful isotopes such as 60CoShieldControl: allow the chain reaction to be started up, maintained at some desired level, and safely shutdownReactors are broadly classified according to the energy of the neutrons :fas

21、t reactor, the fast fission neutrons do not slowdown very much before they are absorbed by the fuel and cause the production of a new generation of fission neutrons. thermal reactor, almost all fissions are caused by neutrons that have slowed down and are moving with speeds comparable to those of th

22、e atoms of the core material, i.e., the neutrons are in thermal equilibrium with the surrounding material.The fast fission cross section for three fissionable uranium isotopes based on NJOY processed ENDF/B (version V) data3. ReactorsReactions Involving NeutronsThermal-Neutron Properties of FuelsGen

23、eral featuresThe Neutron Life Cycle in a Thermal ReactorHomogeneous and Heterogeneous CoresReflectorsReactor KineticsReactivity FeedbackThe neutron life cycle in a thermal reactor showing the major mechanisms for the loss and gain of neutrons. The n fast neutrons beginning the cycle produce n second

24、-generation fast neutrons which, in turn, begin their life cycleThe Neutron Life Cycle in a Thermal Reactorneutrons generated by nuclear fission +neutrons increased by (n,2n), etc. = absorbed neutrons + leaked neutronscritical statesupercritical statesubcritical stateright side left sideQuantificati

25、on of the Neutron Cycle1. fast fission factor （快中子增殖因數）: the ratio of the total number of fast neutrons produced by both thermal and fast fission to the number produced by thermal fission alone. - 1 238U2. resonance escape probability p （逃脫共振俘獲概率）: the probability that a fast fission neutron slows t

26、o thermal energies without being absorbed p - 1 235UThis is the measure of how many neutrons can go through resonances without being absorbed:3. thermal utilization f （熱中子利用系數）: probability that, when a thermal neutron is absorbed, it is absorbed by the fuel (F) and not by the nonfuel (NF)for a homo

27、geneous(均勻的） core=0-1 is called the macroscopic cross sectionExample 10.1 what is the thermal utilization factor in a mixture of graphite and natural uranium with a carbon-to-uranium atom-ratio / of 450?4. thermal fission factor （熱裂變中子數）: number of fast fission neutrons produced per thermal neutron

28、absorbed by the fuel.Equivalently, is the average number of neutrons per thermal fission (v) times the probability a fission occurs when a thermal neutron is absorbed by the fuel,is a property of the fuel material alone and is unaffected by the type and amount of nonfuel material in the core. 1, sus

29、taining chain reaction5. thermal non-leakage probability （熱中子在擴散過程中不泄漏概率） :probability a thermal neutron does not leak from the core before it is absorbed.D is the thermal diffusion coefficientfor a homogeneous mixture of fuel (F) and moderator (M)R: spherical core of radius there is no leakage臨界屈曲M

30、oderator properties for thermal (0.00253 eV) neutrons. L is the thermal diffusion length.the probability a fast neutron does not leak from the core as it slows to thermal energies.（快中子在慢化過程中不泄漏概率） ：one-sixth the mean squared distance between the point at which a fast fission neutron is born and begi

31、ns to slow down and the point at which it reaches thermal energies. is the Fermi age from fission to thermal energiesthermal utilizationfastabsorp.Effective Multiplication Factor （有效增殖因數）four-factor formulaFor an infinite medium, there is no neutron leakage.is a property of only the core material an

32、d is independent of the size and shape of the core.Variation of Keff and its factors with the fuel-to-moderatorratio. This example is for a homogeneous mixture of water and 2%-enriched uranium. Here NF /NM = atom density of uranium to molecular density of water.subcriticalsupercriticalself-sustainin

33、gcriticalWhat is of a homogeneous mixture of 235U and graphitewith an atomic uranium to carbon ratio of 1 to 40,000?For such a dilute mixture of fully enriched uranium and carbon,so thatWhat is the radius R of a critical bare sphere composed of a homogeneous mixture of 235U and graphite with a urani

34、um to carbon atom ratio of 1 to 40,000?For criticality,R = 125 cmfastabsorp.3. ReactorsGeneral featuresReactions Involving NeutronsThermal-Neutron Properties of FuelsThe Neutron Life Cycle in a Thermal ReactorHomogeneous and Heterogeneous CoresReflectorsReactor KineticsReactivity EffectsFission Prod

35、uct PoisonsHomogeneous and Heterogeneous CoresThe least expensive fuel to use in a reactor assembly is natural uranium (0.72 atom-% 235U). Buta fast fission neutron would lose so little energy in each scatter from a uranium nucleus that over 2000 scatters would be required to slow the neutron to thermal energies238U has large absorption