堆工原理 UNIT 5

1、1 PRINCIPLE OF REACTOR ENGINEERING核核 反反 应应 堆堆 工工 程程 原原 理理PU SHENGDI PU SHENGDI 2007.082 CHAPER 5 REACTOR CONTROL 3REACTOR CONTROLvPurpose of Reactor ControlvRequirementvControl methodsvControl MaterialsvControl loops4REACTOR CONTROL New Words and Phrasesvbuilt-in (excess) reactivity （或剩余）反应性（或剩余）反应性

2、vinitial cold condition 初始冷态初始冷态vaddition or removal of . 添加或减少添加或减少vneutron absorber 中子吸收剂中子吸收剂vinsertion or withdrawal 插入或抽出插入或抽出vbe regarded as competitors of . 看成是看成是.的竞争者的竞争者vthe larger the proportion.,the smaller the fraction. 份额越多，份额越多，.份额越少份额越少vvice versa 反之亦然反之亦然vcontrol element 控制元件控制元件vde

3、sired operating power level of the reactor 预定的反应堆运行功率水平预定的反应堆运行功率水平vremain steady 保持恒定保持恒定vcontrol loop 控制环路控制环路vcontrol mode: manual and automatic 控制模式控制模式：手动和自动手动和自动vsafety action 安全安全（保护保护）动作动作voperator action 操纵员动作操纵员动作（操作操作）vdirection flow of information, signal, or effect 信息、信号或作用传播的方向信息、信号或作用

4、传播的方向vexert appropriate action 做适当的操作做适当的操作vcontrol block 控制块控制块vpredetermined value 预定值预定值vservomechanism 伺服机构伺服机构vsilver-indium-cadmium 银银铟铟镉镉5 UNIT 5 REACTOR CONTROL Purpose of Reactor Control l1. In the normal operation of a reactor, the functions of the control system may be divided into three

5、phases, i.e. startup, power operation, and shutdown. The basic purpose of the reactor control system is to provide a means for starting the reactor, i.e., bringing the power output up to the desired level, for maintaining it at that level, and for shutting it down in the course of routine operations

6、. l2. If the potentially unsafe conditions should arise, a protection system would automatically shut down the reactor. 6UNIT 5 REACTOR CONTROL requirementv3. An essential requirement of the control system is that it must be capable of introducing enough negative reactivity to compensate for the bui

7、lt-in (excess) reactivity at initial startup of the reactor.In most power reactors, the fuel supply cannot be continuously replaced as it is consumed*. (burn-up) *CANDU type reactor s are designed so that fuel replacement is possible during normal operation . Consequently, at the beginning of each o

8、perational period, the reactor core must contain all the fuel (fissile material) that will be required to produce a predetermined quantity of energy. 7UNIT 5 REACTOR CONTROL requirementFurthermore, additional fuel is necessary to allow for the decrease in neutron multiplication arising from fission-

9、product poisons and from the high operating temperature, i.e., the negative temperature coefficient. lHence, when a thermal reactor core is assembled prior to commencing operation, it includes a considerable amount of excess fuel * - some 25 to 30 percent in water-moderated reactors-above that requi

10、red for criticality in the initial cold condition. *The additional fissile material in the core is said to represent built-in (or excess) reactivity. 8UNIT 5 REACTOR CONTROL fission ratevfission rate= f fissions/m3.s Where is the neutron flux (in units of neutrons/m2s ) f is the macroscopic cross se

11、ction for the fission (m-1) f N f where N is the number of target nuclei per cubic meter . f is the microscopic cross section for the fission , which applies to a single nucleus 9UNIT 5 REACTOR CONTROL reactor power and neutron fluxvthe thermal power (or heat generation rate) is given by V f P (ther

12、mal) = - watts (W) 3.1x1010 where v is active volume f is the average macroscopic cross section for the fission is the average neutron flux vthe power level at any instant can be considered proportional to the neutron flux. v In most situations a reactor is controlled by varying the neutron flux.10U

13、NIT 5 REACTOR CONTROLcontrol methodsv4. Four general methods are possible for changing the neutron flux in a reactor; they involve temporary addition or removal of (1) fuel (e.g., molten-salt reactor) (2) moderator, (3) reflector, or (4) a neutron absorber .v 6. The procedure most commonly employed,

14、 especially in power reactors, is the insertion or withdraw of a material, such as boron or cadmium, having a large cross section for the absorption of neutrons.The absorber and the fissile material may be regarded as competitors for neutrons; the larger the proportion absorbed by the control materi

15、al, the smaller the fraction available for fission, and vice versa. 11UNIT 5 REACTOR CONTROL control procedurev7. When a reactor core is being assembled, the neutron absorbing control rods are fully inserted. The neutron density (or flux) is thus maintained at such a low level that the effective mul

16、tiplication factor is significantly less unity, i.e., the system has a large negative reactivity and is subcritical. v8. During startup, the control rods are withdrawn slowly, thereby permitting a gradual increase in the reactivity until the reactor becomes critical and then slightly supercritical.1

17、2UNIT 5 REACTOR CONTROL control procedurev9. The neutron flux is thus allowed to increase at a safe, controlled rate until its magnitude corresponds to the desired operating power level of the reactor. v10. The rods are then inserted to the extent required to keep the system exactly critical, so tha

18、t the neutron flux and power level remain steady. v11.To shut the reactor down, the control rods would be reinserted into the core, thereby decreasing the reactivity, neutron flux, and power output.13UNIT 5 REACTOR CONTROL vTABLE 6.1 PROPERTIES OF CONTROL MATERLALS Major Resonances 0.0253-eV 0.0253-

19、eV Abundance a a Energy a Material (percent) (barns) (102m-1) (eV) (barns)-vBoron 759 107 v Boron-10 19.8 3838 NonevSilver 63 3.64 v Silver-107 51.8 37 16.6 630v Silver-109 48.2 92 5.1 12500vCadmium 2450 113 v Cadmium-113 12.3 19800 0.18 7200vIndium 195 7.3 v Indium-113 4.3 58 v Indium-115 95.7 202

20、1.46 30000vGadolinium 46000 1400v Gadolinium-155 14.6 61000 2.6 1400v Gadolinium-157 15.7 255000 17 1000vHafnium 113 5.06v Hafnium-177 18.5 390 2.36 6000v Hafnium-178 27.2 90 7.8 10000v Hafnium-179 13.8 84 5.69 1100v Hafnium-180 35.1 14 74 130-14UNIT 5 REACTOR CONTROL hafniumvAs far as its neutronic

21、s, mechanical, and physical properties are concerned, hafnium is an excellent control material for water-moderated reactors. vThe absorption cross section is fairly large for thermal neutrons and there are several resonances at somewhat higher energies. vMoreover, the metal is resistant to corrosion

22、 by high-temperature water. It can be readily fabricated, and has adequate mechanical strength. vHafnium was used as the control material in the Shippingport pressurized water reactor or some nuclear submarine.vbut it has been found to be too expensive for general use in nuclear power reactors.15UNI

23、T 5 REACTOR CONTROL PWR control system - control rods and boric acid dissolved vOperational reactivity control in a PWR is provide by boric acid dissolved in the coolant water, supplemented by control rods.lThe boric acid concentration is varied to control long-term reactivity changes, such as those

24、 resulting from fuel depletion and fission-product buildup, and to some extent coolant temperature changes.lThe control rods are used for startup, to follow load changes, and to control small transient changes in reactivity. In addition , the rods serve to control the power distribution.16UNIT 5 REA

25、CTOR CONTROL PWR control system - burnable poison assemblyvSince the initial fuel load contains no fission products, the core has more excess reactivity than in subsequent cycles.vHence, tubes containing boron (borosilicate borosilicate glass) as a burnable poison are placed in some of the control r

26、od guide tubes.vThis reduces the amount of chemical shim that might otherwise be required.vThe concentration of boron (boric acid) in the water can thus be kept below the level that would result on a positive moderator temperature coefficient.17UNIT 5 REACTOR CONTROL PWR control materialsvAlthough c

27、admium might be a suitable control material for such reactors from the neutronics standpoint, its low melting point of about 321, which is below the outlet water temperature (about 330), makes impractical. vAs seen in Table 6.1, both silver and indium have significant resonance in the epithermal neu

28、tron regions. Although indium melts at an even lower temperature than cadmium, the melting point of the silver-indium-cadmium alloy is substantially higher than that of cadmium.v5. The control material commonly used in pressurized-water reactor is an alloy of 80 (weight) percent silver, 15 percent i

29、ndium, and 5 percent cadmium. It is enclosed in a stainless steel tube to protect the neutron absorbing material from corrosion by the high-temperature water. 18UNIT 5 REACTOR CONTROL CANDU control systemvThe core reactivity inventory requiring compensation by the control system is small.Since there

30、 is only slight absorption in the coolant and the moderator is kept cool, there is little decrease in reactivity from shutdown to power in contrast to that for LWRs.Continuous refueling also avoids reactivity burnup change.vThe coolant temperature and void coefficients are slightly positive; neverth

31、eless, the power coefficient is negative, although small, as a result of the overriding effect of the negative Doppler fuel coefficient. Thus the system is inherently stable.19UNIT 5 REACTOR CONTROL CANDU control systemvSeveral types of control elements are provided in the CANDU reactor.lPrimary shu

32、tdown capability is provided by vertical absorber (shutdown) rods with secondary shutdown, if necessary, by high-pressure injection of gadolinium nitrate solution into the moderator.lFlux flattening is provided by absorber rods called “adjusters”.lIn addition, there are “zone control” absorbers for

33、bulk reactivity control and local suppression of flux oscillations. These absorbers consist of vertical chambers which can be filled with ordinary water to any desired level.lVertical (solid) absorber rods supplement the zone control absorbers. 20UNIT 5 REACTOR CONTROL control loopsl As a general ru

34、le, both an “operator loop” and an “automatic loop” form part of the reactor control system, as shown in Fig 6.1l A third loop is the “load loop”, lArrowheads indicate the direction of flow of information, signal, or effect21UNIT 5 REACTOR CONTROL control loops - operator loopvIn the operator loop,

35、the operator exerts appropriate action which, with the aid of an external power source, affects the control blocks, i.e., causes the control rods to move in or out of the core. vThe resulting change in the reactor conditions, e.g., the neutron flux, is recorded by instruments which pass the informat

36、ion on to the operator. vThe operator can then take such further action on the control block as may be desirable. vThe instrument block may include a process computer which provides information concerning the condition of the reactor that cannot be obtained from instruments alone. 22UNIT 5 REACTOR C

37、ONTROL control loops - operator loopv In the automatic loop, the information received from the reactor by the instrument block is fed directly to the control block, the operator being bypassed. vIn this loop, the instrument signal is compared with a predetermined value and difference (or error) serves to actuate the control block by means of a servo