."”( OAK RIDGE NATIONAL LABORATORY operated by UNION CARBIDE CORPORATION NUCLEAR DIVISION for the U.S. ATOMIC ENERGY COMMISSION ORNL- TM-2489 €5 COPY NO. - DATE - June 2, 1969 Instrumentation and Controls Division MSBR CONTROL STUDIES W. H. Sides, Jr. ABSTRACT A preliminary study was made of the dynamics and control of a 1000 Mw(e), single-fluid MSBR by an analog computer simulation. An abbreviated, lumped- parameter model was used. The control system included a steam temperature con- troller and a simplified version of the MSRE reactor temperature control system. The results of the study indicate a need for a variable speed, secondary=salt pump for close control of the steam temperature. During severe transients, considerable care must be taken in designing the control system if freezing or overheating of the salts is to be avoided. NOTICE This document contains information of a preliminary nature and was prepared primorily for internal use ot the Quk Ridge National L.aboratory. It is subject to revision or correction and therefore does not represent. a final report. FSTRIBUTION Qb LHts OQCUMENE # UNLIMITER, - - - - LEGAL NOTICE -~ - = =-eme This report was prepared as an account of Government sponsored work. Neither the United States, nor the Commission, nor any person acting on behalf of the Commission: A. Makes any warranty or representation, expressed or implied, with respect to the accuracy, completeness, or usefulness of the information contained in this report, or that the use of any information, apparotus, method, or process disclosed in this report may not infringe privately owned rights; or B. Assumes any liabilities with respect to the use of, or for damages resulting from the use of any information, apparatus, method, or process disclosed in this report. As used in the above, *'person acting on behalf of the Commission'” includes any employee or contractor of the Commission, or employee of such contractor, to the extent that such employee or contractor of the Commission, or employee of such contracter prepares, disseminates, or provides access to, any information pursuant to his employment or contract with the Commission, or his emplaoyment with such contractor. CONTENTS . | 1. Infroduction « « « ¢ ¢ « 4 o v o v o o v v 4 e 0 o s e e ... 5 lation of the Control System . s > FO""‘2U 10 |Invesflgclfion of Plant Conditions for Less than Full-Load 5 Oeraflon........................... 2.2 Trznsienf Behavior of Control Method with Constant . Secondary-Salt Flow Rate. . . . . . Ce e 2.3 Study of Steam Temperature Control with Variable . Secondary-Salt Flow Rate. . . . . . . . .. .. oo | ... 14 3. Resu|ts e 3.1 Decrease in Load Demand. . . . . . . .. . .. .. e oFReachit............:....... gg g’r,’:;n%isunges of Reué/tivi'ry with Controller Dis.connect:add. Coe gg 3.4 Ramp Changes of Reactivity with Controller Disconnected . . - 3.5 Step Loss of One Secondary-Salt Cool-an‘r loop. . v+ v .. s 3.6 Measurement of System Transfer Function. . . . . .. . .. - 4 4, Concluding Remarks . « . « « ¢« v o v v v v oo LEGAL NOTICE This report was prepared as an a States, nor the Commission, * A. Makes any warranty ccount of Government spansored work. Neither the United ROr any person acting on behalf of the Commission: or representation, B. Assumes any liabilities wi use of any information, apparatus, As used in the above, *¢ pleyee or contractor of the C ih respect to the use of, or for damages resulting from the method, or process disclosed in this report. person acting on behalf of the Commiasion?’ includes any em- ommission, or employee of such coniractor, to the extent that 1. INTRODUCTION By means of an analog computer simulation, a preliminary investigation was made of the dynamics and possibilities for control of the proposed 1000~-Mw(e) single- fluid Molten-Salt Breeder Reactor (MSBR). For the purposes of this analysis the MSBR plant consisted of a graphite-moderated, circulating-fuel (primary salt) reactor, a shell-and-tube heat exchanger for transferring the generated heat to a coolant (sec- ondary) salt, and a shell-and-tube supercritical steam generator. The analog simula~ tion of the plant consisted of a lumped-~parameter heat transfer model for the core, primary heat exchanger, and steam generator; a six-group model of the circulating- fuel nuclear kinetics with temperature reactivity feedbacks; and an external control system. This investigation was concerned with the formulation of this control system and the integrated plant response; it was not concerned with a safety analysis of the system, although some of the transients introduced would be of an abnormal nature (e.g., step changes in the load demand on the plant). It was an initial probe into the response of the system initiated by such perturbations as changes in load demand, reactivity changes, and sudden loss of a secondary-salt coolant loop. The simulation was carried out on the ORNL Reactor Controls Department an- alog computer. So that the model would have the maximum dynamic range, the system differential equations were not linearized, and, as aresult, the requisite quantity of nonlinear equipment required the model to be severely limited spatially to minimize the number of equations. In addition, the pressure in the water side of the steam gen- erator, as well as in the rest of the plant, and the physical properties of the salts and water were taken to be time invariant. The flow rate of the primary salt and the tem- perature of the feedwater to the steam generators were also held constant. In this report, the path taken to arrive at the conceptual control system is out- lined along with the equations and values of the system parameters used in the simula- tion. The results are given as summary curves and graphs of the variations encountered in the system variables (femperatures, flows, etc.). 2. FORMULATION OF THE CONTROL SYSTEM 2.1 investigation of Plant Conditions for Less than Full-Load Operation The primary objective of this study was to formulate a control system that would maintain the temperature of the steam delivered to the turbines at a design value of 1000°F during all steady-state conditions and to within a narrow band around this value during plant transients. To accomplish this objective, the first step was to investigate the plant conditions (temperature profile, flows, etc.) for less than full-load operation. (The full-load temperature profile is shown in Fig. 1. The steady-state heat transfer equations and the method used in calculating off-design conditions are given in the Appendix, Sect. 5.1.) Three basic methods of plant operation at less than full foad were investigated: 1. The average reactor temperature was held at its 100% power level value, and a secondary-salt bypass line was included in parallel with the steam generators. The salt flow in the bypass line was given by P Fa = on(] " ‘P‘“) (M) 0 where Foo is the secondary-salt flow rate at the 100% power level PO. ORNL DWG. 69-6633 1300 °F T2 Te yisoor Ty > ———————— —— 1000°F | : f2 ) (F:a 7 <) | | PRIMARY g CORE HEAT ngggiron EXCHANGER : | l | | - 1050 °F T 850°F Ty Ts 850°F Te 700°F Fig. 1. Model for Calculating Off-Design Steady-State System Temperature Profiles. Temperature Values are for 100% Power Level. 2. The average reactor temperature was held fixed ot its 100% power-level value, and there was no secondary-salt bypass. 3. The secondary-salt flow rate was held fixed at its 100% power-level value, and there was no secondary-salt bypass. With the first two methods of plant operation the temperature of the secondary salt approached its freezing point of 725°F at power levels = 50% of full power. With the third method, however, the secondary-salt temperatures remained well above the freezing point. With the second method, the AT from primary to secondary salt in the primary heat exchanger increased from 150 to 335°F at the primary-salt exit end of the exchanger at 50% power, and the AT increased at the steam outlet end of the steam generator as well. The increases in these AT's were reduced for the first method where a valved bypass line had been placed around the steam generator, but this also was more complex because a flow control valve and a variable speed pump were required. The simpler arrangement of the third method showed that the AT's from primary to sec- ondary salt and from secondary salt to steam decreased with decreasing power level except at the feedwater inlet end of the steam generator where it increased only 65°F at 30% power (Fig. 2). This AT increase occurred in the coolest part of the system, however. Therefore, the third method of plant control, in which the secondary-salt flow rate was held constant, appeared to be the most promising from the viewpoint of simplicity and thermal stresses on the heat exchangers. 2.2 Transient Behavior of Control Method with Constant Secondary=Salt Flow Rate A steam-temperature controller was devised to vary the plant temperature pro- file as shown in Fig. 2. The transient behavior of such a control scheme was investi- gated by use of the analog computer simulation model shown in Fig. 3. Each heat exchanger was divided into five lumps: two for each of the two fluids and one for the tube walls. The reactor heat transfer system was approximated by two lumps for the circulating primary salt and one for the graphite moderator. A two-group approxi~ mation of the circulating-fuel nuclear kinetics equations was used, and temperature reactivity feedbacks were included. (The system equations that describe the model are given in the Appendix, Sect. 5.2. The values of the physical constants and system parameters used as "given" information are shown in Table 1. The values for various system volumes, masses, etc., were calculated from these constants and are listed in Sect. 5.2.) The steady-state partial-load calculations showed that a reasonable system- temperature profile could be obtained for off-design conditions by maintaining a con- stant secondary=-salt flow rate and by allowing the reactor and secondary-salt temper- atures to vary. The transient behavior of such a system was investigated, using at first only that part of the simulation model that included the primary heat exchanger and the steam generator. The secondary=-salt flow rate was held constant, and the steam ORNL DWG. 69-6634 REACTOR OUTLET (T3) ————100% POWER LEVEL PROFILE 1300~ ———=-30% POWER LEVEL PROFILE 12004— AVERAGE PRIMARY /—SECONDARY SALT HOT LEG (T,) SALT TEMPERATURE _ 1100 o - x > - - & 1000 O : ————— et '—- 00— SECONDARY SALT \ COLD LEG (T3) N -} N N N 800 N\ \ FEEDWATER (Tg) N oo PRIMARY PRIMARY S ECONDARY STEAM STEAM -~ CORE SALT LOOP —»le——HEAT EXCHANGER SALT LOOP'—_"_ GENERATOR —-I—_AND_.I TUBE WALL TUBE WALL FEEDWAT ER Levels. Fig. 2. Steady-State System Temperature Profiles for 100 and 30% Power FLOW RATE AT ORNL DWG. 69-6635 REACTOR REACTOR OUTLET — POWER TEMP SETPOINT OOSGETOC&?:‘AER?I ;i‘fiL RROR SETPOINT (Toser) - FLOW RATE _ STEAM ROD (€] = (Prger) ERROR (F2) ERROR = — stavo - X f = - i At r— ©m = SeTeomT : + — 2 7 + | CONTROLLER 1| | Y (1000 °F) REACTOR POWER l | REACTOR INLET 7 - l T, e . TEMP 4 = | - [ o [ - Tq ' STEAM CONTROL Ts ROD | Tz | /FJ“" | | | L PRIMARY A PRIMARY SECONDARY SECONDARY STEAM SALT [ 120N SALT SALT Fa SALT Fy V] \ N\ T, T2 T \ T Tss I\ g LOAD l ‘ DEMAND T | TUBE TUBE GRATZH! N\ | TuBe Lt LY 12 \\ I < \ | PRIMARY PRIMARY \ SECONDARY SECONDARY STEAM SALT | SALT W sALT SALT T Tr | Ty TSP Ts - S n T T, Ts FEEDWATER PRIMARY (700°F) REACTOR CORE HEAT EXCHANGER STEAM GENERATOR Fig. 3. Lumped Model of MSBR for Plant Simulation. Table 1. Physical Constants Primary Secondary Parameter _Solt Salt Steam Hostelloy N Graphite Ce B b o™ 0.324 0.36 2.17 0.126 0.409 o, |b/fr3 207.8 ot 1175°F 117 at 1000°F 19.7, 7.01 548 17 k, Bty helop T _— _— _— 11.0 4] Primary Heat Steam Parameter Exchanger Generator Length, f 19 63.8 Triangulor tube pitch, in. 0.625 0.875 Tube OD, in. 0.375 0.5 Wall thickness, in. 0.035 0.077 Heat transfer coefficients, Btu hr_]ft_2°F—]: tube side fluid to tube wall 2786 4000 tube wall conductance 3770 1715 shell side fluid to tube wall 1624 3745 Reactor Core Octagon: 13 ft across flats. Height: 13 ft. Fuel: 233U. Primary-salt volume fraction: 0.16, Graphite to primary-salt heat tramfer coefficient: 1700 Btu hr—IH_2°F—] ] Temperature coefficient of reactivity primary salt 1,333 x 107>/°F. grephite 1.056 x 10_5/°F. Thermal neutron lifetime: 3.6 x 10-4sec. Deloyed neutron constants for 233U: ; Bi x 104 \, sec-] 1 2.3 0.0126 2 7.9 0.0337 3 6.7 0.139 4 7.3 0.325 5 1.3 1.13 é 0.9 2.50 10 temperature was controlled by altering the temperature of the primary salt entering the primary heat exchanger. The rate of change of this primary-salt temperature was pro- portional to the steam temperature error, or dT2 ar - GP} T 70} @) where a is the controller gain, and T7 is the design value of the steam temperature T7. 0 A brief parameter study of a showed that with a gain of about 1°F/sec change in To per 10°F error in T the steam temperature returned to its design value of 1000°F and remained stable. At higher gains the system became unstable. For a typical trans- ient initiated by a 10% step decrease in load demand from 100 to 90% of full load (Fig.4), 100 sec was required for the steam temperature to return to within 1°F of its design value. One method of reducing the time required for the steam temperature to return to its design value would be to vary the secondary-salt flow rate during a trans- ient. This would enable close control of the amount of heat delivered to the steam generator, resulting in more restrictive temperature control. ORNL DWG. 69-6636 150 o B~ 0 1050 L 1000 950~ 50 100 150 200 SECONDS Fig. 4. Transient for 10% Step Decrease in Load Demand with Constant Secondary-Salt Flow Model. 11 2.3 Study of Steam Temperature Control with Variable Secondary-Salt Flow Rate Restrictive temperature control was accomplished by the control system shown in Fig. 3. The steam-temperature error was allowed to control the rate of change of the secondary-salt flow rate by dF — -a(T7 ~ 170). (3) Then, after the steam temperature transient, the flow rate was slowly adjusted to its original full-power value by allowing the flow rate error to change the temperature of the primary salt entering the primary heat exchanger by a second controller having a gain of fB: dT F - - Bl - =), ) 20 where F20 is the initial full-power value of the secondary-salt flow rate Fy A brief parameter study of a and B indicated that a reasonable steam-temperature response was obtained when a 1°F steam-temperature error produced a secondary=-salt flow rate change of 10%/min and a 1% error in flow rate produced a primary-salt tem~- perature change of 1°F/min. With this control system, a 10% step decrease in power demand from 100 to 90% of full load produced the transient shown in Fig. 5. The steam temperature re- turned to within 1°F of the design value in about 30 sec, as compared with 100 sec described previously. The maximum change in the secondary-salt flow rate was about 20%, i.e., from 100 to 80% of the design value; the maximum rate of change was about 50%/min. The flow rate returned to within 3% of its design value in approx- imately 250 sec (4.2 min). With the addition of the reactor heat transfer and nuclear kinetics equations to the model, the temperature of the primary salt entering the primary heat exchanger (reactor outlet temperature) was controlled by inserting or withdrawing control rods to change the reactor power according to the error in the secondary=-salt flow rate. Steam temperature control by means of the secondary=-salt flow rate remained the same. The rate of change of the set point for the reactor outlet temperature Ty op WOS obtained from the error in the secondary=~salt flow rate, as follows: 2 set - Bl - =), (5) 12 which is similar to Eq (4). This controller slowly adjusted the reactor outlet temperature set point in the proper direction until the secondary-salt flow rate returned to its 100% power value. The values for the controller gains a and B were adjusted for the transient runs such that a 3°F error in steam temperature yielded a 10% per min rate of change of the secondary-salt flow rate and a 1% error in the secondary-salt flow rate yielded a 1.7°F per min rate of change of the reactor outlet temperature set point. These values were obtained after a brief parameter study in which a 10% step decrease was initiated in the plant load demand and the gains were adjusted to yield the minimum steam temper= ature deviation. The required set point for the reactor power level Pr sef is obtained from the reactor outlet temperature set point by = - 6 lDr set A 12 set T] ! ©) where A is the proportionality constant between reactor power and reactor AT. Thus, as the reactor outlet temperature set point is altered by the secondary-salt flow rate, the reactor power level set point will be altered as well. The error in the reactor power level is given by & =P -P (7) r r set’ This error signal is the input to a proportional servo rod controller which is described by the second-order transfer function 2 T6) = 28 ®) 2 s t 20ws tw where G is the controller gain, w is the bandwith, and ( is the damping factor. In this simulation w equaled 31.42 radians/sec (5 Hz) and equaled 0.5. These values are typical of the kind and size of servo which may be used in this control-rod-drive service. The gain of the controller was such that for €. 2 1% of full reactor power the control reactivity addition or withdrawl rate was doc —d'i_— = O.TA:/sec. (9) 13 For errors less than 1%, the rate of change of reactivity was proportional to the error. For errors greater than 1%, the reactivity rate was maintained constant at the 0.1%/sec valve. Integration of Eq. (9) yields the term p _ in the reactivity equa- tion (see Sect. 5.2). This method of con’rrollin% the reactor outlet temperature is similar to that used in the MSRE control system. To obtain more realistic transient results from the simulation, limits were im- posed on several of the system variables, as follows: 1. The secondary-salt flow rate was limited to a range from 40 to 110% of the full- power flow rate. 2. The maximum steam flow rate was limited to 110% of the full-power flow rate. 3. The reactor outlet temperature set point was constrained to a range from 1000 to 1400°F (100°F over and 300°F under its full-power value of 1300°F). 4, A 5-sec first-order lag was introduced between the steam flow rate demand F in Eq. (43) in Sect. 5.2 and the steam flow rate in the system F, in Egs. (363 - (40), in order to better simulate the response of a turbine throttle valve. ORNL DWG. 69-6637 150 0 30 100 150 200 250 300 SECONDS Fig. 5. Transient for 10% Step Decrease in Load Demand with Variable Secondary-Salt Flow Model. 14 3. RESULTS Calculations of the temperature profiles with the system under partial load at steady state were made using the steady-state form of the analog simulation equations in Sect. 5.2. The variation of salt temperatures with various steady-state power levels is shown in Fig. 6. Figure 6 also shows the variation of the same salt temper- atures obtained by use of another method of calculation described in Sect. 5.1. Divergence of the two sets of curves begins at about the 50% power level. Thus, if it is assumed that the calculation method of Sect. 5.1 is more reliable for predicting system temperature profiles at steady state, the present analog simulation model is valid only at power levels greater than approximately 50%. Several transient cases were run which included (1) decreases in load demand P from 100% by steps of 10, 30, 50, and 60%; (2) ramp changes of 30 and 70% each of 5 and 10%/min; (3) changes in reactivity of steps of +0.05, +0.1, and -0.5%; and (4), with the reactivity controller disconnected, (a) reactivity steps of £0.05, +0.01, and -0.1% and (b) ramp changes in reactivity of +0.05 and =0.1% at 0.1%/min. Also, the step loss of one secondary-salt coolant loop was simulated. The system transfer function Pe(s)/p(s) was also measured. ORNL DWG. 69-6638 *300p— / 12501— e LOG MEAN AT CALC { APPENDIX, SECT. 3. ————— STEADY-STATF FORM OF ANALOG SWULATIO}/ EQUATIONS (APPENDIX, SECT. 5.2) Z 12001— REACTOR QUTLET, T2 # 50— / / 1100+— // : e s SECONDARY SALT HOT LEG, T, s : 7 7 e / o 1050 — /- - S—e ¢ 2 1000 :‘,’7'_' - o REACTOR INLET, Ty a —— = i 3 7/ > —_ X // // 50— ~ - / . -— - > L7~ po Q00— \ ——— T, SECONDARY SALT COLD LEG, T2 e 850|— T T — 800 +— 750}— 200 | ] | ] 1 1 | ] J 0.1 0.2 0.3 0.4 05 6.6 0.7 0.8 0.9 1.0 P!/p!l Fig. 6. Variation of Steady-State Salt Temperatures with Power Level. 15 3.1 Decrease in Load Demand The action of the system during a typical load demand transient was as follows. When the load demand P, was decreased, the steam flow rate decreased, transferring less heat out of the steam generators and decreasing the heat transfer coefficient be- tween the steam generator tubes and the steam. This caused the steam femperature to begin to rise. The steam temperature controller sensed the steam temperature error and began to decrease the secondary-salt flow rate at a rate proportional to the tem- perature error in order to transfer less heat into the steam generator and to decrease the heat transfer coefficient between the secondary salt and the steam-generator tubes. The transfer of heat from the secondary salt at the reduced flow necessary for steam temperature control caused the temperature of the secondary salt leaving the steam generator to decrease. The reduction in flow rate also decreased the amount of heat transferred out of the primary heat exchanger as well as the heat transfer coefficient between the primary-heat-exchanger tubes and the secondary salt. The primary salt was, thus, returned to the reactor at a higher temperature, producing the reactor power error signal . As this error was sensed by the reactivity servo controller, the control rods were withdrawn (inserting negative reactivity) to reduce the reactor power commensurate with the decrease in load demand. The secondary=~salt flow rate con- troller sensed the decrease in the flow rate and began to decrease the reactor outlet temperature set point at a rate proportional to the secondary-salt flow rate error. A new steady-state operation was achieved when the steam temperature reached its design value of 1000°F and the secondary-salt flow rate its full-power value. The results of a 30% decrease in load demand from 100% as o step and af rates of 10 and 5%/min are shown in Figs. 7-9. The reduction of the temperature of the secondary-salt leaving the steam generator after a change in load demand is shown in these figures. For large, rapid changes in load demand, the temperafure of the sec- ondary-salt may approach its freezing point (725°F). This possibility exists also when the load demand on the plant is increased. Under such conditions the steam temper- ature initially tends to decrease, causing the steam temperature controller to accel- erate the flow of secondary salt, which will transfer more heat into the steam gen- erator. Since this flow may increase only to 110% of full flow, a sufficiently rapid load increase will cause the temperature of the secondary salt leaving the steam gen- erator to decrease and, perhaps, approach the freezing point. However, increases in plant load will usually occur in a more orderly and controlled fashion than decreases since, under accident conditions, decreases will be more probable. Increases in load must be accomplished in a carefully controlled manner. The data from Figs. 7-9 and the results of other runs made with changes in load demand are summarized in Table 2 which also lists the maximum steam temperature de- viations from 1000°F, the maximum required rates of change of the secondary=-salt flow rate and of the control reactivity, and the maximum magnitude of the control reactivity requited. The highest reactivity rate was well below the 0.1%/sec maximum allowed by the controller. 16 ORNL DWG. 69-6639 + 150>§Y,, ¢ o e -t Pt 1 & 0 - 150 e° 0 150 . 0 160 200 300 400 500 600 700 800 900 1000 o i 0 100 200 300 400 500 6§00 SECONDS 700 800 900 Fig. 7. Transient for 30% Step Decrease in Load Demand. 17 ORNL DWG. 69-6640 150 oo T ' + md e e e e e bt b b b b e e e s e P . , ! : ; oLl gt bagly i i i ; P ; | | 1 7T 1 LOAD DEMAND,Pg * ° | I e oot rereem s et TR ey Lo b * Poboce b e e s? L 4 ‘ v I oo 1 doopoep o deg d g b S P e ~ P ! ! ! = = _ -4 B i t T * i } ¥ : , C 4 ? 4 -4 . ’ : : |r 4 —— i : o i"{'i ! Pyl o P i ! b P i o i P i t ' ! : 3 . | ; : . P b e 0 bRy vl i be e i 4 Ledo w3000 008 4 d i 150 t A To 150 Do 150 o 0 100 200 300 400 500 600 700 800 900 1050 & 1000 950 . 950 | - 850 4 0o 100 200 300 400 500 600 700 800 900 SECONDS Fig. 8. Transient for 30% Ramp Decrease in Load Demand at 10%/min. 18 ORNL DWG. 69-6641 150 = Prorrr s et - 3 : : L P i T e SR LT - 5 0 150 & 0 150 53 0 150 & 0 1050 L 1000 Yl %;,fl;; 950 . B 950 <~ e - o '; STEAM GENERATOR SALT OUTLET TEWP, T, & B850 —— . o - - | o 150 ‘ 0 100 200 300 400 500 600 700 800 900 1000 Fig. 9. Transient for 30% Ramp Decrease in Load Demand at 5%/min. 19 Table 2. Results of Load Demand Perturbations A. For Step Losses of Load Demand (from 100%) Magnitude of Step (%) 10 30 50 Final steady-state temperatures, °F T] — 1009 976 T2 — 1183 1098 T — 867 875 Ty — 1077 1025 Max steam temperature error, °F 13 44 147 Max rate of change of secondary- salt flow rate, %/sec -0.74 -2.3 -4.3 Max rate of change of reactivity, %/sec 3.5x10 21.06x 107 -3.0x 107 Max value of o, % -0.014 -0.045 -0.075 B. For 30% Ramp Loss of Load Demand from 100 to 70% Ramp Rate (%/min) 10 5 Max steam temperature error, °F 9 5 Max rate of change of secondary- salt flow rate, %/sec -0.41 ~0.26 Max rate of change of reactivity, %/sec 1.75% 107 1.5x 107 Max value of control reactivity required, % -0.045 -0.045 20 A plot was made of the maximum steam temperature variation from 1000°F as a function of change in load demand (Fig. 10). The plot shows, for example, that a step change of 30% in load demand from 100 to 70% of full power produced a max- imum steam temperature deviation of about +42°F at some point in the transient. The break upwards in the three curves for load changes greater than 30% was caused by the 40% lower limit imposed on the secondary=-salt flow rate. Changes in load of more than 30% required a change of greater than 60% in the secondary-salt flow rate to maintain control of the steam temperature. When this lower [imit was reached, control of the steam temperature was considerably reduced and higher deviations allowed to occur. 3.2 Changes of Reactivity The results of reactivity steps of 0.1 and -0.5% are shown in Figs. 11 and 12. A +0.1% step yielded a peak reactor power of about 155% as a puise with a fwhm (full-width, half maximum) of about 0.75 sec. This is an excess energy input of approximately 930 Mw~sec. The reactor outlet temperature peak deviation from 1300°F was about 25°F. The reactor inlet temperature and steam temperatures varied only a few degrees. The control reactivity changed at its maximum allowable speed in a direction te counter the reactivity step. A negative reactivity insertion of -0.5% decreased the reactor power sharply to about 18% before the control reactivity re- turned it to its 100% level ofter a 35% overshoot (Fig. 12). The reactor outlet tem- perature peak change was about =100°F, and the peak steam temperafure variation was about -15°F. ORNL DWG. 69-6642 250 STEP(WITH 40 % LOWER LIMIT ON SECONDARY SALT FLOW) 50 STEP (WITHOUT 409, LOWER LIMIT ON SECONDARY (- SALT FLOW) MAX STEAM TEMP CRROR, ATy (°F) 30— WITH 40 % LOWER LIMIT ON SECONDARY RAMP AT 1090 / MIN SALT FLOW RAMP AT 5% /MIN B_~WITHOUT 40% LOWER LIMIT ON SECONDARY SALT FLOW ‘ 1 | _ ¢ 10 20 30 40 50 60 70 80 90 100 CHANGE IN LOAD DEMAND (%) Fig. 10. Maximum Steam Temperature Error for Changes in Load Demand from 100%. 21 ORNL DWG. 69-6643 Ty T T OF rot = 150~ e 0125 1275 10 t5 0 5 10" 15 20 O 5 10 150 5 10 15 20 SECONDS Fig. 11. Transient for Steps in Reactivity of £0.1% with Reactivity Controller Active. 22 CRNLDWG 69-6644 ! lpi - 1 0 R, P T }L - : L ] | :_ .) ._-; -4 - . E 4. 4 ) ._E,