Chapter 7 & 8 Control Rods Fission Product Poisons. Ryan Schow

Transcription

1 Chapter 7 & 8 Control Rods Fission Product Poisons Ryan Schow

2 Ch. 7 OBJECTIVES 1. Define rod shadow and describe its causes and effects. 2. Sketch typical differential and integral rod worth curves and explain the shapes. 3. Explain the significance of a point on the differential and integral rod worth curves. 3. Construct an integral rod worth curve from a differential curve or from differential data.

3 Ch. 8 OBJECTIVES 1. Explain the dependence of equilibrium xenon and samarium on core neutron flux. 2. Describe the equilibrium xenon and samarium balance in a reactor.

4 Differential control rod worth is the change in per change in rod position. a. reactivity, total b. reactor power, unit c. reactivity, unit d. reactor power, total CORRECT ANSWER c. reactivity, unit

5 AXIAL FLUX CORE MID PLANE TOP AXIAL POSITION BOTTOM

6 FLUX SHIFT AS RODS ARE INSERTED TOP CONTROL ROD BANK ALMOST FULLY WITHDRAWN TOP CONTROL ROD BANK INSERTED AS SHOWN CORE HEIGHT CORE HEIGHT BOTTOM BOTTOM AXIAL AXIAL

7 CONTROL ROD BANK HEIGHT (INCHES) DIFFERENTIAL ROD WORTH (PCM/INCH) CONTROL ROD BANK HEIGHT (INCHES)

8 Which one of the following describes the change in magnitude (absolute value) of differential control rod worth during the complete withdrawal of a fullyinserted control rod? a. Increases, then decreases b. Decreases, then increases c. Increases continuously d. Decreases continuously CORRECT ANSWER a. Increases, then decreases

9 INTEGRAL ROD WORTH vs. ROD HEIGHT IRW (PCM) CONTROL ROD BANK HEIGHT (INCHES) (A) Positive reactivity inserted in the core as rod bank is withdrawn IRW (PCM) CONTROL ROD BANK HEIGHT (INCHES) (B) Negative reactivity removed from the core as rod bank is withdrawn

10 Integral rod worth is the: a. change in reactivity per unit change in rod position. b. reactivity inserted by moving a control rod from one reference point to another point. c. change in worth of a rod per unit change in reactor power. d. reactivity inserted by a rod on a power change. CORRECT ANSWER b. reactivity inserted by moving a control rod from one reference point to another point.

11 MODERATOR TEMPERATURE EFFECT ON ROD WORTH n n (T 1 ) n ABSORBER ROD n NEUTRON ABSORPTION BY A CONTOL ROD AT TEMPERATURE T 1 n n n n n n (T 2 ) n n ABSORBER ROD n n GREATER NEUTRON ABSORPTION BY A CONTROL ROD AT TEMPERATURE T 2 WHERE T 2 > T 1 DUE TO INCREASED "SPHERE OF INFLUENCE" n n

12 CONTROL ROD # 1 ROD SHADOWING EFFECT (r) WITHOUT CONTROL ROD NO. 1 INSERTED AVG ( r ) WITH CONTROL ROD NO.1 INSERTED (a) CONTROL ROD NO. 1

13 CONTROL ROD # 1 CONTROL ROD # 2 ROD SHADOWING EFFECT (r) WITHOUT CONTROL ROD NO. 1 INSERTED A C B AVG ( r ) WITH CONTROL ROD NO.1 INSERTED SHADOW ROD ASSUME TWO IDENTICAL CONTROL RODS (b) CONTROL ROD NO. 2 AT POSITION A

14 CONTROL ROD # 1 CONTROL ROD #2 ROD SHADOWING EFFECT (r) WITHOUT CONTROL ROD NO. 1 INSERTED A C B AVG ( r ) WITH CONTROL ROD NO.1 INSERTED (c) CONTROL ROD NO. 2 AT POSITION B

15 CONTROL ROD # 1 CONTROL ROD # 2 ROD SHADOWING EFFECT (r) WITHOUT CONTROL ROD NO. 1 INSERTED A C B AVG ( r ) WITH CONTROL ROD NO.1 INSERTED (d) CONTROL ROD NO. 2 AT POSITION C

16 FISSION YIELD (%) MASS NUMBER FISSION PRODUCT YIELD CURVE 10 A 95 A LIGHT FISSION FRAGMENTS HEAVY FISSION FRAGMENTS

17 THERMAL UTILIZATION FACTOR f fuel a mod a fuel a other a poison a

18 FISSION PRODUCT POISON RATE OF CHANGE Production Rate - Removal Rate = Rate of Change of Concentration

19 A fission product poison is defined as a fission product: a. that is loaded into the core during fabrication. b. that absorbs a neutron and fissions. c. that has a substantial neutron absorption cross section and does not fission. d. that emits a neutron sometime after the initial fission event. Answer - c

20 PRODUCTION OF XENON THROUGH DECAY Te sec 6.6 hrs I Xe

21 PRODUCTION OF XENON Fission: fuel Xe f I-135 Decay: I N I Total production: Xe fuel f I N I Where: xe fuel f = probability of Xe-135 formation directly from fission = neutron flux = macroscopic cross section for fission in the fuel I = decay constant for I-135 N I = atomic density of I-135

22 XENON REMOVAL Absorption Xe a N Xe Xe Decay Xe N Total removal rate: Xe a N Xe Xe N Xe Where: Xe a = microscopic cross section of absorption for xenon N Xe = atomic density of Xe 135 = neutron flux Xe = decay constant for Xe 135

23 Question: a. Describe the two methods of Xe-135 production. b. Describe the two methods of Xe-135 removal. Answer: a. Xe-135 has two methods of production: about 95% of the xenon is produced by the decay of iodine- 135 (a fission product, the concentration of which is proportional to power), and the remaining 5% of the xenon results directly from fission. The production of xenon by iodine decay is: 135 I hrs Xe

24 b. Xenon also has two removal methods: by radioactive decay, with a half-life of 9.1 hours, and by neutron absorption, also referred to as xenon burnout. The removal reactions are illustrated below. Xenon Decay Xe Cs hrs ( Stable) Xenon Burnout Xe n Xe ( a 0) At low power levels, decay is the major removal method. At high power levels, due to the high neutron flux, burnout becomes the prominent method of xenon removal.

25 Which of the decay chains describes the production of xenon-135? a. Ba Cs Xe b. n 54 Xe Xe c. Te I Xe d. 56Ba Xe Answer - c

26 Two characteristics of Xe-135 that result in it being a major reactor poison are its relatively production from fission and relatively absorption cross section. a. low; large b. low; small c. high; large d. high; small Answer - c

27 EQUILIBRIUM XENON Xe fuel f I N I Xe a N Xe Xe N Xe

28 EQUILIBRIUM IODINE I fuel f I N I

29 EQUILIBRIUM XENON Xe fuel f I fuel f Xe a N Xe Xe (5%) (95%) (80%) (20%) N Xe

30 EQUILIBRIUM XENON CONCENTRATION N Xe fuel f Xe a ( Xe Xe I )

31 XENON REACTIVITY (k/k) CLEAN CORE STARTUP MW EQUILIBRIUM Xe MW EQUILIBRIUM Xe TIME (HOURS)

32 Which one of the following is a characteristic of Xe-135? a. Thermal neutron flux level affects both the production and removal of Xe-135. b. Thermal neutrons interact with Xe-135 primarily through scattering reactions. c. Xe-135 is primarily a resonant absorber of epithermal neutrons. d. Xe-135 remains a significant poison after capturing a thermal neutron. Answer - a

33 Xe REACTIVITY (% k/k) XENON PEAK AFTER SHUTDOWN -3.1 EQUILIBRIUM XENON SHUTDOWN PEAK XENON AT 100% POWER MW ABOUT 18 HOURS AFTER REACTOR SHUTDOWN Hours TIME (HOURS)

34 XENON CONCENTRATION XENON CHANGES DURING POWER TRANSIENT 100% EQUILIBRIUM XENON POWER DECREASE TO 50% POWER INCREASE TO 100% POWER INCREASE TO 100% 50% EQUILIBRIUM XENON 8-10 HRS HRS 5 HRS 25 HRS HRS TIME (HOURS)

35 Question: a. Describe the Xe-135 behavior during a power reduction from 100% down to 50%. b. How does the equilibrium xenon concentration at 100% power compare to that at 50% power? Answer: a. Xe-135 levels will first increase as the burn-out is reduced with the production remaining the same. It will peak about 8-10 hours after the reduction. The Xe-135 level then decreases to a new lower steady state value, settling out about 50 hours after the decrease. b. Xe-135 level at 100% will be less than double the value at 50% power. The burn-out becomes a larger portion of the total depletion term making the equilibrium value closer to a constant.

36 Describe the Xe-135 concentration behavior in a startup from a xenon-free condition. Answer: Commencing power operations from a xenon free condition results in the immediate production of some xenon directly from fission and a large amount of I Xe-135 is produced directly from 0.3% of all fissions; 5.6% of all fissions result in the production of I-135. Additional xenon is produced as the iodine decays (half-life = 6.6 hours). As Xe-135 starts building, some of it is removed by decay and burnup. Equilibrium xenon will be reached in about 48 hours when starting up at full power.

37 A reactor has just started power operation and is ramped to full power in six hours. How long after the reactor reaches full power will it take to achieve an equilibrium xenon condition? a. 8 to 10 hours b. 20 to 30 hours c. 40 to 50 hours d. 70 to 80 hours Answer - c

38 Describe the xenon-135 concentration behavior during a power decrease. Answer: A reduction in power causes a slight reduction in xenon production (due to less xenon being produced directly from fission) and a very large reduction in xenon burnout (due to the decreased neutron flux). A relatively high inventory of iodine still exists, however, and xenon production remains relatively high. This effect causes N Xe to increase. This increase in N Xe causes more xenon decay to start occurring, and in about 5 hours, the iodine- 135 concentration will have decayed sufficiently low enough to cause N Xe to stop increasing. In the next 30 to 40 hours, N Xe will decrease until a lower equilibrium level is reached for the lower power level. The total time of the xenon transient is 40 to 50 hours; the greater power changes requiring the longer time to reach equilibrium.

39 Describe the xenon-135 concentration behavior during a power increase. Answer: If reactor power is increased, an immediate increase in xenon burnout occurs due to the increased neutron flux, and an immediate increase in the direct xenon production term also occurs. Since direct xenon accounts for only 5% of the production, the burnout term will have the predominant effect, and will cause the Xe-135 concentration (N Xe )to decrease. As the I-135 concentration (N I ) starts to increase, more xenon will be produced, and in about 5 hours, N Xe will stop decreasing. Additional production of xenon, from iodine, will cause N Xe to increase over the next 15 to 25 hours. Eventually a higher equilibrium value of N Xe will be attained for the higher power level. The total time of the xenon transient is 20 to 30 hours; the greater power changes requiring the longer time to reach equilibrium.

40 A reactor has been operating at 50% power for one week when power is ramped up in four hours to 100% power. Which statement best describes the new equilibrium xenon concentration? a. The new equilibrium xenon will be twice the 50% value. b. The new equilibrium xenon value will be less than twice the 50% value. c. The new equilibrium xenon value will be more than twice the 50% value. d. The new equilibrium xenon value will remain the same since it is independent of power. Answer - b

41 A reactor has been operating at 50% power for two weeks when power is quickly ramped to 100%. How would the xenon-135 production terms below change? Xenon direct from fission + Xenon from radioactive decay of iodine a. Fission term decreases, and iodine decay term decreases until iodine builds in. b. Fission term increases, and iodine decay term is constant. c. Fission term decreases, and iodine decay term is constant. d. Fission term increases, and iodine decay term increases as iodine builds in. Answer - d

42 Describe the behavior of the Xe-135 concentration following a reactor shutdown. Answer: Immediately following a reactor shutdown, or trip, 5% of the xenon production is lost, and all the xenon burnout ceases. The only production of xenon is by the decay of I-135 in the core at the time of the shutdown. The only removal is by Xe- 135 decay. N Xe will increase for the first few hours following the shutdown time since N I > N Xe and I-135 decays faster than Xe-135. The increase in N Xe ceases when the iodine-toxenon ratio reaches a value of (This is a ratio of the respective half-lives, 6.6 hours/9.1 hours.) At a N I /N Xe = 0.73, the production and removal rates are equal, and the peak Xe- 135 concentration has been reached.

43 Answer (cont): At high power levels, the length of time to reach the peak is relatively long since a high iodine-to-xenon ratio existed in the core at time of shutdown. Therefore, a longer period of time is required for the ratio of N I /N Xe to decrease to a value of In short, if the power level has been 100%, the peak will be reached in 7 hours. A xenon- free condition will be reached approximately 3 days (72 hours) after a reactor trip.

44 Following a reactor trip from sustained power operation, the xenon-135 removal process consists primarily of: a. beta decay b. gamma decay c. electron capture d. gamma capture Answer - c

45 Following a reactor scram from sustained high power operation, xenon-135 concentration in the reactor will: a. decrease because xenon is produced directly from fission. b. increase due to the decay of iodine already in the core. c. remain the same because the decay of iodine and xenon balance each other out. d. decrease immediately, then slowly increase due to the differences in the half-lives of iodine and xenon. Answer - b

46 A reactor that has been operating at 100 percent power for two weeks experiences a trip. How long would it take for the xenon concentration to peak following the trip? a. 4 to 6 hours b. 6 to 10 hours c. 40 to 50 hours d. 70 to 80 hours Answer - b

47 XENON CONCENTRATION SHUTDOWN STARTUP XENON CHANGE DURING RESTART FULL POWER EQUILIBRIUM TIME AFTER STARTUP (HOURS)

48 A reactor startup (S/U) to full power is begun five hours after a trip from full power equilibrium conditions. If a 2%/min ramp were used rather than a 0.5%/min ramp, the xenon peak would occur, and the magnitude of the peak would be. a. sooner, larger b. sooner, smaller c. later, larger d. later, smaller Answer - b

49 PRODUCTION OF SAMARIUM THROUGH DECAY Nd Pm hr 53 hr Sm

50 SAMARIUM REMOVAL BY NEUTRON ABSORPTION Sm 1 0 n Sm

51 PRODUCTION, DECAY, AND EQUILIBRIUM OF PROMETHIUM Production from Fission Pm fuel f Decay Pm N Pm Equilibrium Pm fuel f Pm N Pm

52 PRODUCTION & REMOVAL BY NEUTRON ABSORPTION Production by Decay of Pm-149 Burnout Pm N Pm N Sm Sm a Equilibrium Pm N Pm N Sm Sm a

53 PRODUCTION OF PROMETHIUM EQUALS REMOVAL OF SAMARIUM Pm fuel f N Sm Sm a

54 EQUILIBRIUM SAMARIUM CONCENTRATION N Sm fuel Pm f Sm a

55 Describe the production and removal methods for Sm-149. Answer: The only significant method of samarium production is the decay of promethium-149, a fission product with a fission yield of approximately 1% and a half-life of about 53 hours. The decay scheme of Pm-149 is: Pm Sm ( hr Stable) The only significant method of samarium removal is by neutron absorption (samarium burnout). The absorption process is: Sm 0n 62Sm ( a 0)

56 Sm REACTIVITY (% k/k) CLEAN CORE STARTUP 100 % POWER 50 % POWER DAYS OF REACTOR OPERATION

57 Sm REACTIVITY (% k/k) SAMARIUM PEAK AFTER SHUTDOWN EQUILIBRIUM SAMARIUM REACTOR SHUTDOWN 100 % PEAK SAMARIUM CLEAN-CORE STARTUP TIME AFTER SHUTDOWN (DAYS)

58 Sm-149 REACTIVITY (% k/k) SAMARIUM CHANGE DURING RESTART REACTOR RESTARTED -1.4 EQUILIBRIUM DAYS OF POWER OPERATION AFTER REACTOR RESTART 40

59 COMPARISION OF XENON AND SAMARIUM COMPARISON Xe-135 Sm-149 Barns Time to Peak Time to Equilibrium Reactivity Worth Decays? Power Dependent? Distribution Problem? Square root of power prior to shutdown or trip hours -2.7% k/k at 100% equilibrium -4.7% k/k at peak Yes Yes Yes 12.5 days 20 to 25 days -1% k/k at power equilibrium -1.4% k/k at peak No Small dependency No

60 A reactor has been operating at 100% power for 2 weeks. Power is then decreased over a 1-hour period to 10%. Assuming manual rod control, which one of the following operator actions is required to maintain a constant reactor coolant temperature at 10% power during the next 24 hours? a. Insert negative reactivity during the entire period. b. Insert positive reactivity during the entire period. c. Insert positive reactivity, then negative reactivity. d. Insert negative reactivity, then positive reactivity. Answer - c

61 Xenon poisoning in a reactor core is most likely to prevent a reactor startup following a reactor shutdown from power at the of core life. a. high; beginning b. low; beginning c. high; end d. low; end Answer - c