Dynamic Characteristics of CANDU Reactors

Transcription

1 Week 2 - Dynamic Characteritic of CANDU Reactor -0 Dynamic Characteritic of CANDU Reactor Prepared by Dr. Daniel A. Meneley, Senior Advior, Atomic Energy of Canada Ltd. and Adjunct Profeor, Department of Engineering Phyic McMater Univerity, Hamilton, Ontario, Canada Summary: Refinement of concept important to today CANDU power plant. Approximation ued for eparating thoe variable important for accident analyi, normal operation, fuel management, and tructural change. Effect of heterogeneity -- Lattice cell effect and neutron cro-ection averaging. Table of Content More about thi document Dynamic Characteritic of CANDU Reactor...2. Neutron Chain Reaction Operating Domain (afe operating envelope) Neutron migration area Reactor dynamic when the reactor i hut down Spatial Heterogeneity Long-term effect...9

2 Week 2 - Dynamic Characteritic of CANDU Reactor 2-0 Dynamic Characteritic of CANDU Reactor. Neutron Chain Reaction Figure depict the neutron cycle in a thermal reactor. (Fat reactor are omewhat different becaue fiion occur motly at high energie, and neutron either are aborbed or leak out before they low down to When Number of Slow Neutron i Leaked Neutron contant, the ytem i critical. thermal energy.) Fat Neutron low down in In normal operation, a ~ milliecond. thermal reactor i very near Delayed Neutron to a critical tate, wherein from Fiion Prompt Neutron Neutron the neutron population i Diffuing from Leaked Neutron Fiion independent of time (at leat CONTROL THIS TO "ASHES" CONTROL HEAT on the time cale of econd- (Fiion PRODUCTION Product) to-hour.) The population of U235 Captured Fiion Slow Neutron Neutron neutron determine the (Some neutron are HEAT captured in U238 and o fiion rate and, therefore produce ueful fuel - Pu239) the amount of power Figure The Neutron Cycle in a Thermal Reactor produced. To increae power, we temporarily reduce the number of neutron captured in control material the neutron population increae exponentially with time. When the deired power level i reached the control material capture rate i again increaed to keep the total neutron population contant. (Q approximately, what reactivity increae i required to change the power level at a rate of 0. percent per econd?). 2. Operating Domain (afe operating envelope) Automatic control ytem are deigned to control the reactor at contant power within a multi- Operating Trajectory parameter domain, a hown in Figure 2 for a typical CANDU power reactor. Aume that the reactor ha been deigned to Deign Center operate under Deign Center condition. During operation ome tate variable change and other are manipulated, cauing Operating Limit the reactor tate vector Trip Limit (operating trajectory) to wander Safety Limit like a worm in the n- dimenional pace a time progree. (The limitation of a two-dimenional heet of paper are obviou in thi Figure.) Delayed Neutron appear after Neutron Slowing ~ 0 econd. Down Operating Domain Operating Margin Safety Margin Figure 2 Safe Operating Domain of Control Sytem

3 Week 2 - Dynamic Characteritic of CANDU Reactor 3-0 The control ytem alway direct the tate vector toward Deign Center, a hown in Figure 2. Perturbation uch a fuelling, power change, temperature change, and o on tend to move the tate vector toward the Operating Limit. At thi boundary, the normal control ytem act immediately to return the tate vector toward Deign Center. If, for any reaon, (e.g. a large perturbation) the normal control ytem cannot do thi, the reactor tate vector may continue moving toward the Trip Limit at which pecial afety ytem repond automatically. Thee ytem act quickly, and olely, to reduce reactor power and/or increae heat removal from the core. They are cocked or poied at all time for thi purpoe. [Note: Several complex event and deciion led up to the diatrou Chernobyl Unit 4 reactor accident in April 986. However, in the end, the accident wa initiated by a combination of poor deign and poor operation. Poor deign of afety hutdown rod allowed the operator to reach an operating tate wherein the firt movement of afety rod (intended, of coure, to hut down the reactor) actually led to a reactivity increae and conequent power increae. Poor operation wa indicated by the fact that the operator deliberately put the reactor into an unafe tate prior to the accident.] Startup, power level tranient, teady-tate operation, and hutdown tranient tate are included implicitly in the n-dimenional tate pace repreented by the imple diagram in Figure 2. Add a few more tate variable and we can decribe ubcritical and maintenance tate. The concept i the ame - the outer boundary of the Operating Domain in the Figure can be compared with (for example) an aircraft tall warning. Protective action are taken if the aircraft tate reache the boundary of the operating domain (or operating envelope) o a to prevent equipment damage. There i no comparion between aircraft and a reactor automatic afety ytem a uch thee would be equivalent to a ytem that automatically landed the plane in cae of trouble. The tak equivalent to landing i relatively imple in the cae of a reactor hut off the chain reaction, cloe up the containment, and continue cooling the fuel. Aircraft do not have thi ultimate protective barrier. Another dimenion i added to the operating domain when conideration of fuel depletion are important. Referring to Figure, it i clear that action mut be taken to maintain a contant neutron population a the concentration of uranium-235 lowly decreae, and a the concentration of neutron-capturing fiion product increae. In the hort term the reactor control ytem act to keep the tate vector (Figure 2) inide the operating envelope. However, the control ytem eventually will reach the limit of it range and new fuel mut be added. In reactor uch a CANDU thi fuel changing i done daily at full power. Thi lead to an operating advantage for thi type of reactor becaue the reactor neutronic parameter reach an approximate equilibrium after a year or o of operation and remain contant after that time. Another advantage of thi cheme i that it become unneceary to change fuel with the reactor hut down when it i more difficult to meaure the degree to which the core tate i below a critical tate. In reactor uch a the preurized water reactor, fuel changing require reactor hutdown. A large fraction of the fuel i changed at one time and then the reactor i retarted. Thi major fuel change require that a large amount of neutron-aborbing material be added to control reactivity. A a reult core propertie are changed ignificantly by refuelling. The Beginning of Cycle tate

4 Week 2 - Dynamic Characteritic of CANDU Reactor 4-0 variable then change lowly over the cycle time of approximately one year, finally returning to End of Cycle condition jut prior to the next refueling. 3. Neutron migration area Reactor ize affect the patial tability of power generation in any fiion reactor. In general, large reactor are le table than are mall reactor. (We will ee the mathematical expreion of thi fact later in thi coure.) The characteritic ize that i mot relevant to comparion of different reactor type, at a given power output, i the ize expreed in unit of the mean travel ditance (migration length) between production and aborption in the core. We ee from Table that the HWR and the FBR are much maller in term of neutron diffuion that either the PWR or the BWR. A a reult, thee reactor are more table. In detail, other factor play a part in determining core tability, a will be examined later. Table : Characteritic Size of Power Reactor Reactor Type Reactor Core Diameter (cm., typ.) Neutron Migration Length (cm., typ.) Characteritic Size (migration length) PWR BWR HWR FBR The optimum power output of a CANDU reactor for a given number of channel i obtained when the reactivity of approximately half of the fuel channel, located in the center of the reactor, i deliberately lowered o that the flux (and therefore the power) i contant acro the o-called inner zone of the core. Thi flux flattening reult, of coure, in decreaed tability in thi portion of the reactor a mall addition of reactivity in a flat-flux zone produce a relatively large flux change. Thi condition uually i avoided by lightly reducing the reactivity of a few fuel channel near the center of the reactor, o a to retore ufficient tability condition at minor cot in term of power output. Large thermal reactor are ubject to another ort of intability, thi one induced by production, capture, and decay of the iotope Xenon-35. The bet deign trategy to eliminate reultant power fluctuation i to intall an automatic control ytem uch a that in the CANDU-HWR. Generally, it i impractical to control thee fluctuation manually. Reflector are ued in all power reactor to reduce neutron loe and to ait in flattening the power ditribution. Ecaping neutron are captured in a uranium radial blanket in the cae of the FBR. Production of plutonium for new fuel i greatly enhanced by thi deign feature. Reflector are relatively unimportant in the LWR becaue of their very mall migration length and the preence of a trongly aborbing teel core barrel urrounding the fuel.

5 Week 2 - Dynamic Characteritic of CANDU Reactor 5-0 The CANDU-HWR ue a heavy water radial reflector. Thi reflector alo ha an important effect on reactor dynamic becaue of it very long diffuion length. The mean flight path length of a neutron between colliion (the mean free path ) give an indication of the degree of inhomogeneity of any reactor aembly coniting of fuel element and coolant/moderator. If thi flight path i long relative to the thickne/diameter of a fuel element at all neutron energie, the reactor i aid to be quai-homogeneou. Mot fat reactor are nearly homogeneou in thi ene. The McMater Nuclear Reactor i typical of a quaihomogeneou thermal reactor. In mot thermal power reactor the flight path at ome neutron energie i hort relative to the dimenion of a fuel element, or to the moderator thickne between fuel element. Thee reactor mut be treated a heterogeneou, with conequent higher complexity in their analyi. The LWR and the HWR are typical heterogeneou reactor. Heterogeneity can be een at more than one level; for example, mechanical control aborber in a CANDU exhibit a econd level of heterogeneity that mut alo be conidered in analyi. Thee effect will be dicued in later chapter where pecific cae arie that are important to reactor dynamic behaviour. 4. Reactor dynamic when the reactor i hut down Many of the available theoretical treatment of reactor dynamic do not addre the important field of the hut down or ubcritical tate of the reactor. Thi obervation hould be conidered along with the hitorical obervation that the majority of reactor accident actually began from a hutdown tate. One could potulate everal reaon for thi common omiion, and the omiion itelf hould not be blamed for all uch accident. However, if the lack of knowledge of fiion reactor in their ubcritical tate might have been reponible even partly for one uch accident, we are fully jutified in tudying thi cae. A written, the imple neutron kinetic program referred to in the Week coure note doe not conider the ubcritical condition. In order to do o we mut add another term to the equation olved therein. The following equation et define the point-reactor kinetic behaviour in a reactor containing an external neutron ource. Thi equation will be derived later in thi coure however, it i expected that the tudent already i familiar with the general form: dn t S tc% k0 N t C t S t dt t k0 t œ % % l l ± dc% t C% dt l t % M M, 6 N t C t S G Z Dt * t F t M t t - Reactivity

6 Week 2 - Dynamic Characteritic of CANDU Reactor 6-0 C% G C Z Dt * F t t - Effective Delayed Neutron Fraction F(t) and M(t) are, repectively, the pace-and-time dependent fiion neutron ource and ink. D(t) i the weighted integral of the fiion neutron ource (inner product). œ C % C % l Dt * t G v Z t,6 - Delayed Neutron Fraction / - Prompt Neutron Lifetime % Dt * C t G C t l t - Effective Neutron Precuror Concentration S% t Dt l t G * S t - Effective Source Strength The non-phyical parameter k 0 i the invere of the larget eigenvalue of the ource-free reactor equation, for the known ubcritical teady tate. At teady-tate initial condition, the left hand ide of each of the even governing equation i zero and by definition, N(0) =.0, and ρ(0) = 0. Therefore, C% N C% 0,6 M l l S % k0 ž k žÿ 0 and Exitence of a teady-tate olution require that k 0 <.0; that i, the reactor mut be ubcritical. A the reactor approache a critical condition via any given tate trajectory (for example by increaing fuel concentration or by reducing the neutron leakage) k 0 tend toward unity and N(0) increae. But if the initial ource i identically zero, there will be no neutron preent and o N(0) = 0.0 even if k 0 >.0. Thi (hypothetical) ituation obviouly i very dangerou becaue addition of only one neutron to the ytem may lead to a very rapid energy releae. Such event in real ytem (for example the recent flah criticality accident in the fuel proceing facility at Tokai in Japan and the fuel loading incident in the McMater Nuclear Reactor) uually reult from mitaken phyical change that alter the reactor tate from k 0 le that unity to k 0 greater than unity. It i obviou that one neceary condition for afety in a ubcritical reactor i that an external neutron ource be preent at all time. It trength requirement i determined by the reultant

7 Week 2 - Dynamic Characteritic of CANDU Reactor 7-0 neutron flux level and the enitivity of neutron detector thee detector alo are eential o that any change in the tate of ubcriticality can be detected by the operating taff and o that automatic afety ytem will have time to repond. The econd part of thi afety requirement i, of coure, that udden phyical change that lead to an increae in k 0 be limited in rate or in abolute magnitude. The approach to critical (or otherwie, the method for afely tarting up a reactor) will be addreed later in thi coure. Referring back to Figure, it i obviou that the preence of an external ource of neutron mean that an operating reactor will never be preciely critical. It mut be kept in a tate lightly below critical o a to maintain contant power. However, thi apect of operation become important mainly under hutdown condition. Here again, the HWR i unique in that the gamma-neutron reaction in deuterium produce a ignificant neutron ource under all condition except the firt tartup with all-freh fuel. Spontaneou fiion of ome actinide element alo produce neutron, but in mall number. In all cae in which there i no other large ource of neutron, a pecial neutron-emitting ource mut be added to the reactor during tartup. Conideration of reactor dynamic under hutdown condition alo mut include the dynamic of high-aborption fiion product, epecially the Iodine-Xenon chain. Following hutdown from high power, the Xenon concentration increae o that a thermal reactor generally cannot be retarted. The eventual decay of the Xenon-35 iotope permit retart after hour delay. Retarting a thermal reactor containing ubtantial amount of Xenon-35 require careful manipulation of in-reactor control ytem, becaue the initial power increae burn up thi reidual Xenon and increae the reactivity of the core. Another important pecial ituation arie after reactor hutdown from high power. Fiion product produce about 7 percent of the total heat during operation; after the chain reaction i hut off, thi heat i till produced. The amount decay to about percent of initial power after 3 hour, and continue to decreae thereafter. Neverthele, removal of thi decay heat i an important conideration in fuel cooling. [Note: During the 979 accident at Three Mile Iland, the fuel wa well cooled for about 2 hour following reactor hutdown. Mitake in operation then led to removal of cooling from ome fuel after that time; the fuel that wa uncooled then melted (at ~2800 C) and poed a ignificant rik to the integrity of the preure veel. Fortunately, thi molten fuel wa re-olidified without extenive heat tranfer to the veel wall.] 5. Spatial Heterogeneity Figure to 4 of the general decription given in [Link-reference to Lecture 5 of 6-Lecture Coure ] how the geometry of the CANDU-6 reactor. Thoe Figure can be zoomed to view detail of the reactor core deign. The mot notable feature for our purpoe are (a) the horizontal (z-direction) orientation of fuel channel, the (b) vertical (y-direction) orientation of ome control device, and the (c) horizontal (x-direction) orientation of other control device. The fuel channel are filled with fuel plu heavy water; the control aemblie are either filled with neutron-aborbing material or heavy water. The aborbing and neutron producing propertie of the fuel change a the fuel i irradiated. Space between thee tructure are filled with pure heavy water moderator.

8 Week 2 - Dynamic Characteritic of CANDU Reactor 8-0 It i obviou that the modeling of neutron migration in thi complex lattice depend not only on the propertie of the iotope, but alo on their location in the lattice a well a on the proximity of other iotope in other part of the lattice. Thi extremely heterogeneou geometry i implified to ome extent by the fact that mot of the neutron preent are well thermalized. A typical neutron flux ditribution in a CANDU lattice i hown in Figure 3. A can be een, mot of the fat neutron produced in the fuel diffue to the moderator region where they are lowed down to thermal energie, and then diffue back to the fuel where they are aborbed. Relative Neutron Flux Propertie of the core material are Figure 3 Typical Flux Ditribution in CANDU Lattice Cell normally averaged over dicrete volume and over dicrete energy interval and collected in tabular form uitable for interpolation over pace and fuel irradiation o a to elect appropriate value to repreent the pecific tate of the core (fuel irradiation, temperature, near-neighbor effect) at any location. The time-dependence of thee propertie (due to change in denity, temperature, etc.) alo i pre-calculated and tored in interpolation table for ue in dynamic analyi. Mot reactor mut hut down for refueling. In thee ytem, extenive computer imulation are done to elect the optimum new-fuel loading (enrichment, burnable poion concentration, patial core layout, exce reactivity requirement, etc.) for the next cycle of about one year duration. Once the final calculation i made fuel i ordered and the deciion become very difficult to revere. The CANDU ytem i fuelled at full power, at a rate of 5-20 identical fuel bundle per day (2-3 fuel channel). Pre-calculation i neceary to elect the bet channel for refueling, to maintain the correct local power ditribution, core reactivity, etc. Neither enrichment nor burnable poion i required. The hift upervior make the final choice of channel to be fuelled and the fuelling ytem operator initiate the automatic fuelling ytem. Addition of new fuel in CANDU initiate a mall reactor tranient becaue the new-fuel reactivity i higher than that of old fuel. The zone controller compenate thi reactivity addition automatically, to maintain reactor power and correct local power level. Removal of any failed fuel element i eentially identical to refueling. New bundle are added to replace thoe containing failed element Fuel Bundle Thermal Flux Lattice Cell Fat Flux Equivalent Radiu (cm.)

9 Week 2 - Dynamic Characteritic of CANDU Reactor Long-term effect Power reactor mut be deigned to operate for 30 year or more. During uch long period, the high-intenity neutron and gamma irradiation lead to degradation of material. Example are: radiolyi of water, tranport of active corroion product to inhabited area, welling and embrittlement of teel, production and tranport of tritiated water, and creep of zirconium preure tube. Such procee are an intrinic part of the dynamic of any power reactor operation even though their characteritic time cale i quite long, becaue the operating life of a power reactor may itelf extend for 60 year or more.

10 Week 2 - Dynamic Characteritic of CANDU Reactor 0-0 About thi document Back to page Author and affiliation: Daniel A. Meneley, Adjunct Profeor, Department of Engineering Phyic, McMater Univerity, Hamilton, Ontarion, Canada; Senior Advior, Marketing and Sale, Atomic Energy of Canada ltd., Miiauga, Ontario, Canada Reviion Hitory: - Reviion 0, Febryary 2, 200, initial creation - Source document archive location: h:\violeta Word\web\Dynamic Characteritic of CANDU Reactor.doc - Contact peron: Violeta Sibana