1、NUCLEAR ENERGYPROFESSIONAL(SPECIAL)ENGLISH COURSEChapter 10 Neutron Chain Reactions(中子链式反应中子链式反应)Chapter 14 Neutron Chain Reactions(中子链式反应中子链式反应)The possibility of a chain reaction involving neutrons in a mass of nuclear fuel such as uranium depends on:(a)nuclear properties(特性特性)such as cross sectio
2、ns(截面截面)and neutrons produced per absorption;(b)the size,shape,and arrangement of the materials.14.1 CRITICALITY AND MULTIPLI-CATION (临界性和增殖性临界性和增殖性)To achieve a self-sustaining chain reaction(自持链式反应自持链式反应),one needing no external neutron supply,a critical mass(临界质量临界质量)of uranium must be collected.
3、To appreciate this requirement we visualize the simplest nuclear reactor,consisting of a metal sphere of uranium-235.接上页(P199 第二段第四行中)Suppose that it consists of only one atom of U-235.If it absorbs a neutron and fissions,the resultant neutrons do nothing further,there being no more fuel.If instead
4、we have a small chunk(大块大块)of uranium,say a few grams,the introduction of a neutron might set off(导致导致)a chain of several reactions,producing more neutrons,but most of them would escape through the surface of the body,a process called leakage(泄漏泄漏).接上页（P200 第二行中）Such an amount of fuel is said to be“
5、subcritical”(次次临界的临界的).Now if we bring together about 50 kg of U-235 metal,the neutron production balances the leakage losses,and the system is self-sustaining or“critical(临界的临界的).”The size is the critical volume and the amount of fuel is the critical mass.Neutrons had to be introduced to start the
6、chain reaction,but the number is maintained without further additions.The term(术语术语)critical mass has become popular to describe any collection of entities large enough to operate independently.P200第二段第三行末If we add still more uranium to the 50 kg required for criticality(临界状态临界状态),more neutrons are
7、produced than are lost,the neutron population(数量数量)increases,and the reactor is“supercritical(超临界的超临界的).”Early nuclear weapons involved the use of such masses,in which the rapid growth of neutrons and resulting fission heat caused a violent explosion(爆炸爆炸).14.2 MULTIPLICATION FACTORS(增殖因子增殖因子)The be
8、havior of neutrons in a nuclear reactor can be understood through analogy(类推类推)with populations of living organisms(生物体生物体),for example,of human beings.There are two ways to look at changes in numbers of people:as individuals and as a group.A person is born and throughout life has various chances of
9、 fatal(致命致命的的)illness or accident.On average the life expectancy(期望值期望值)at birth might be 75 years,according to statistical data(统计数据统计数据).接上页(P200第三段第六行中)An individual may die without an heir(后嗣后嗣),with one,or with many.If on average there is exactly 1,the population is constant.From the other view
10、point,if the rates of birth and death are the same in a group of people,the population is again steady(稳定的稳定的).If there are more births than deaths by 1%per year,the population will grow accordingly.This approach(方法方法)emphasizes the competition of process rates.P200 倒数第二段The same ideas apply to neut
11、rons in a multiplying assembly.We can focus(聚焦聚焦,集中集中)attention on a typical neutron that was born in fission,and has various chances of dropping out of(不参与不参与)the cycle because of leakage and absorption in other materials besides fuel.we can compare the reaction rates for the processes of neutron a
12、bsorption,fission,and leakage to see if the number of neutrons is increasing,steady,or decreasing.Each of the methods has its merits(优点优点)for purposes of discussion,analysis,and calculation.P202 第二段The fractions of absorbed neutrons that form U-236 and that cause fission,respectively,are the ratios
13、of the cross section for capture c and fission f to that for absorption a.The average number of neutrons produced by fission is .Now let be the combination f/a,and note that it is the number of neutrons per absorption in uranium.Thus letting L be the fraction not escaping by leakage,k=L.P203 第一行The
14、system is critical if k=l,or L=l.Measurements show that L is around 2.2 for fast neutrons,thus L must be l/2.2=0.45,which says that as many as 45%of the neutrons must remain in the sphere,while no more than 55%escape through its boundary(边界边界).14.3 NEUTRON FLUX AND REACTOR POWER (中子通量和反应堆功率中子通量和反应堆功
15、率)The power developed by a reactor is a quantity of great interest for practical reasons.Power is related to the neutron population,and also to the mass of fissile material present.First,let us look at a typical cubic centimeter of the reactor,containing N fuel nuclei,each with cross section for fis
16、sion f at the typical neutron energy of the reactor,corresponding to neutron speed v.Suppose that there are n neutrons in the volume.接上页(P205第七行)The rate at which the fission reaction occurs is thus Rf=nvN f fissions per second.If each fission produces an energy w,then the power per unit volume is p
17、=wRf.For the whole reactor,of volume V,the rate of production of thermal energy is P=pV.If we have an average flux=nv and a total number of fuel atoms NT=NV,the total reactor power is seen to beP=NT f w.P205 第二段第五行The power developed by most familiar devices is closely related to fuel consumption(消耗
18、量消耗量).In a reactor,it is necessary to add fuel very infrequently because of the very large energy yield per pound,and the fuel content remains essentially constant.From the formula relating power,flux,and fuel,we see that the power can be readily raised or lowered by changing the flux.By manipulatio
19、n(操作操作)of control rods(控制棒控制棒),the neutron population is allowed to increase or decrease to the proper level.14.4 REACTOR TYPES(反应堆类型反应堆类型)Although the only requirement for a neutron chain reaction is a sufficient amount of a fissile element,many combinations of materials and arrangements can be use
20、d to construct an operable nuclear reactor.Several different types or concepts have been devised(设计设计)and tested over the period since 1942,when the first reactor started operation,(a)PurposeThe majority of reactors in operation or under construction have as purpose the generation of large blocks of
21、 commercial(商商业的业的)electric power.Others serve training or radiation research needs,and many provide propulsion power(推动力推动力)for submarines(潜水艇潜水艇).Available also are tested reactors for commercial surface ships and for spacecraft(宇宙飞船宇宙飞船).接上页At various stages of development of a new concept,such a
22、s the breeder reactor(增殖增殖堆堆),there will be constructed both a prototype reactor(原型堆原型堆),one which tests feasibility(可行性可行性),and a demonstration reactor(示范堆示范堆),one that evaluates(评定评定)commercial possibilities(可能性可能性).(b)Neutron EnergyA fast reactor(快堆快堆)is one in which most of the neutrons are in t
23、he energy range 0.l-l MeV,below but near the energy of neutrons released in fission.The neutrons remain at high energy because there is relatively little material present to cause them to slow down.In contrast,the thermal reactor(热堆热堆)contains a good neutron moderating(缓和的缓和的)material,and the bulk o
24、f (大多数大多数)the neutrons have energy in the vicinity(附近附近)of 0.l eV.(c)Moderator and Coolant (慢化剂与冷却剂慢化剂与冷却剂)In some reactors,one substance serves two functions-to assist(帮助帮助)in neutron slowing and to remove the fission heat.Others involve one material for moderator and another for coolant.The most f
25、requently used materials are listed below:Moderators Coolants light water(轻水轻水)light water heavy water(重水重水)carbon dioxide(CO2)graphite(石墨石墨)helium beryllium(铍铍)liquid sodium(液体钠液体钠)(d)FuelUranium with U-235 content varying from natural uranium(=0.7%)to slightly enriched(=3%)to highly enriched(=90%)
26、is employed in various reactors,with the enrichment(富集富集)depending upon what other absorbing materials are present.接上页The fissile isotopes Pu-239 and U-233 are produced and consumed in reactors containing significant amounts of U-238 or Th-232.Plutonium serves as fuel for fast breeder reactors and c
27、an be recycled as fuel for thermal reactors.The fuel may have various physical forms-a metal,or an alloy with a metal such as aluminum(铝铝),or a compound such as the oxide UO2 or carbide(碳化物碳化物)UC.(e)ArrangementIn most modern reactors,the fuel is isolated from the coolant in what is called a heteroge
28、neous(异类的异类的,非均匀的非均匀的)arrangement.The alternative(可供选择的方法可供选择的方法)is a homogeneous(同类的同类的,均匀的均匀的)mixture of fuel and moderator or fuel and moderator-coolant.(f)Structural MaterialsThe functions of support(支撑支撑),retention(保存保存)of fission products,and heat conduction(热传导热传导)are provided by various meta
29、ls.The main examples are aluminum,stainless steel,and zircaloy(锆锆合金合金),(an alloy of zirconium(锆锆).接上页By placing emphasis on one or more of the above features of reactors,reactor concepts are identified.Some of the more widely used or promising power reactor types are the following:PWR(pressurized wa
30、ter reactor)(压水堆压水堆),a thermal reactor with light water at high pressure(2200 psi(pounds per square inch)and temperature(600 oF)serving as moderator-coolant,and a heterogeneous arrangement of slightly enriched uranium fuel.接上页BWR(boiling water reactor)(沸水堆沸水堆),similar to the PWR except that the pres
31、sure and temperature are lower(1000 psi and 550 oF).HTGR(high temperature gas-cooled reactor)(高温气冷堆高温气冷堆),using graphite moderator,highly enriched uranium with thorium,and helium coolant(1430oF and 600 psi).接上页CANDU(Canadian deuterium uranium)(加拿加拿大重水铀反应堆大重水铀反应堆)using heavy water moderator and natur
32、al uranium fuel that can be loaded and discharged during operation.LMFBR(liquid metal fast breeder reactor)(液金液金属快增殖堆属快增殖堆),with no moderator,liquid sodium coolant,and plutonium fuel,surrounded by natural or depleted uranium(贫铀贫铀).P208倒数第二段To illustrate,we can identify the components and their funct
33、ions in a modern pressurized water reactor.Figure 14.5 gives some indication(迹迹象象)of the sizes of the various parts.P208 倒数第一段The fresh fuel installed in a typical PWR consists of cylindrical pellets(芯块芯块)of slightly enriched(3%U-235)uranium oxide(UO2)of diameter(直径直径)about 3/8 in.(l cm)and length a
34、bout 0.6 in.(l.5 cm).A zircaloy tube of wall thickness 0.025 in.(0.6 mm)is filled with the pellets to an“active length(工作长度工作长度)”of l2 ft(365 cm)and sealed(密封的密封的)to form a fuel rod(or pin).P209 第五行Control rods(控制棒控制棒),consisting of an alloy of cadmium(镉镉),silver(银银),and indium(铟铟),provide the abili
35、ty to change the amount of neutron absorption.The rods are inserted in some vacant(空的空的)fuel pin spaces and magnetically connected to drive mechanisms.14.6 THE NATURAL REACTOR(天然反应堆天然反应堆)Until the l970s,it had been assumed that the first nuclear reactor was put into operation by Enrico Fermi(恩里科恩里科.
36、费米费米)and his associates(同事同事)in l942.It appears,however,that a natural chain reaction involving neutrons and uranium took place in the African(非洲非洲)state of Gabon(加蓬加蓬),near Oklo,some 2 billion years ago.At that time,the isotope concentration of U-235 in natural uranium was higher than it is now bec
37、ause of the differences in half-lives:U-235,7.04 l08 years,U-238,4.47 l09 years.接上页(P214 第三行)The water content in a rich vein of ore(矿脉矿脉)was sufficient to moderate neutrons to thermal energy.It is believed that this“reactor”operated off and on(断断续续地断断续续地)for thousands of years at power levels of th
38、e order of kilowatts.The discovery of the Oklo phenomenon(奥克劳奥克劳现象现象)resulted from the observations(观测观测)of an unusually low U-235 abundance in the mined uranium.The effect was confirmed by the presence of fission products.SUMMARYA self-sustaining chain reaction involving neutrons and fission is pos
39、sible if a critical mass of fuel is accumulated(积聚积聚).The value of the multiplication factor k indicates whether a reactor is subcritical(l).接上页The reactor power,which is proportional to the product(乘积乘积)of flux and the number of fuel atoms,is readily adjustable(可调节可调节的的).A thermal reactor contains
40、a moderator and operates on slowed neutrons.Reactors are classified according to purpose,neutron energy,moderator and coolant,fuel,arrangement,and structural material.接上页Principal types are the pressurized water reactor,the boiling water reactor,the high-temperature gas-cooled reactor,and the liquid
41、 metal fast breeder reactor.Excess fuel is added to a reactor initially to take care of burning during the operating cycle,with adjustable control absorbers maintaining criticality.接上页Account must be taken of fission product absorbers such as Xe-135(氙氙)and of limitations related to thermal and radiation effects.About 2 billion years ago,deposits of uranium in Africa had a high enough concentration of U-235 to become natural chain reactors.
