Operator Generic Fundamentals Reactor Theory – Fission Product Poisons

Operator Generic Fundamentals Reactor Theory – Fission Product Poisons
Ensure students have calculators that are permitted for use on the Generic Fundamentals Examination. Operator Generic Fundamentals Reactor Theory – Fission Product Poisons

Fission Product Poisons Introduction
Xe-135 Most significant fission product poison ≈ pcm at 100% power equilibrium Creates numerous operational issues for operators Production and removal half-lives different Samarium-149 Second most significant fission product poison ≈ pcm at equilibrium Minor operational issues associated with Sm Equilibrium concentration NOT dependent on power INTRO

Terminal Learning Objectives
At the completion of this training session, the trainee will demonstrate mastery of this topic by passing a written exam with a grade of ≥ 80% score on the following TLOs: Describe the behavior of Xe-135 in a nuclear reactor and its effects on reactor operation. Describe the production, removal, and effects of Sm-149 on the operation of a nuclear reactor. TLOs

Xe-135 Fission Product Poison
TLO 1 – Describe the behavior of Xe-135 in an operating nuclear reactor and its effects on reactor operation. 1.1 Describe fission product poisons and how fission product poisons affect the neutron life cycle. 1.2 List the most important fission product poisons to the operation of a nuclear reactor. 1.3 Explain how Xe-135 is produced and removed in the core of a nuclear reactor. 1.4 Explain the following terms: equilibrium iodine, equilibrium xenon , transient xenon, peak xenon, xenon free, and xenon precluded startup. Understanding how it is produced and removed and its response during various reactor operation transients is important to operators for predicting reactor response TLO 1

Enabling Learning Objectives for TLO 1
1.5 Explain how Xe-135 concentration reacts during the following nuclear reactor operations: xenon free initial reactor startup, reactor shutdown, decrease in reactor power, increase in reactor power, and reactor startup with xenon present in core. 1.6 Describe the causes and effects of a xenon oscillation. 1.7 Explain the effects of xenon concentration on a nuclear reactor core’s thermal flux profile for control rod motion and core life. ELOs

Fission Product Poison Effect on Neutron Life Cycle
ELO 1.1 – Describe fission product poisons and how fission product poisons affect the neutron life cycle. Fission fragments resulting from fission events decay to produce a variety of fission products Fission product poisons are of major concern because they absorb neutrons, removing them from the neutron life cycle There are dozens of long-lived and stable fission product poisons Small to large neutron absorption cross-sections Build up to equilibrium values over core life Add negative reactivity by decreasing thermal utilization factor (f) Related KA K1.01 Define fission product poison ELO 1.1

Most Abundant Fission Product Poisons
Fission yield curve shows probability of nuclei yield for U fission Xe-135 and Sm-149 fall on right side of curve Both have HIGH sa Other fission product poisons of high concentration Just not as high a sa on the left side The slide shows the fission yield curve for uranium-235, other fuel isotopes have a similar, but not exact, fission yield. Although several fission products have significant neutron absorption cross-sections, Xe-135 and Sm-149 have most substantial impact on reactor operation Figure: Fission Yield Curve for Uranium-235 ELO 1.1

Xe-135 and Sm-149 both have high absorption cross-sections: 2.6 x 106 barns for Xe-135 4.0 x 104 barns for Sm-149 Because they compete with the fuel, they impact the thermal utilization factor (ƒ), keff and reactivity These values contained in “Various GFE Values. pdf” file. ELO 1.1

Equation for ƒ shows that an increase in macroscopic cross-section for absorption by any neutron poison will result in a decrease in ƒ 𝑓= 𝑎 𝑓𝑢𝑒𝑙 𝑎 𝐹𝑢𝑒𝑙 + 𝑎 𝑚𝑜𝑑 + 𝑎 𝑜𝑡ℎ𝑒𝑟 + 𝑎 𝑝𝑜𝑖𝑠𝑜𝑛 ELO 1.1

Fission product poisons present in core at any given time depends: Poison’s production and removal rate Fission product poisons may be produced: Directly from fission From decay (or decay chain) of certain fission products Removal of fission product poisons occurs by: Radioactive decay Neutron absorption Removal generally results in an isotope with a much lower neutron absorption cross-section Term often associated with fission product poisons is equilibrium At equilibrium, production rate of poison equals removal rate of poison, therefore concentration of poison is constant Depending on poison, equilibrium levels are power dependent and time to reach equilibrium is related to both power and decay rates ELO 1.1

Other Fission Product Poisons
Other fission products besides xenon and samarium build in Macroscopic cross section not large enough to specify Still impacts temperature As a result, boron concentration diluted over core life to compensate for: Fuel depletion Buildup of these “other” fission product poisons ELO 1.1

Knowledge Check Fission product poisons contribute ___________ reactivity to a nuclear reactor as they buildup in the core and ____________ reactivity to a nuclear reactor as they burnout in the core. negative; positive negative; negative positive; negative positive; positive Correct answer is A. Correct answer is A. Once again, “less negative is more positive”! ELO 1.1

Xenon and Samarium ELO 1.2 – List the most important fission product poisons to the operation of a nuclear reactor. At equilibrium levels Xe-135 and Sm-149 add considerable negative reactivity: Xenon -2700 to -3,000 pcm (power dependent) Samarium -700 to - 1,000 pcm (not power dependent) Related KA - K1.02 State the characteristics of Xe-135 as a fission product poison ; K1.15 State the characteristics of Samarium-149 as a fission product poison. 1.9* 1.9* ELO 1.2

Xenon and Samarium On a reactor trip
Xenon will peak with a negative reactivity of almost 5,000 pcm and decay in 3 days back to 0 pcm Samarium (Sm-149), once it reaches equilibrium, does not decrease in its negative reactivity worth Will add ≈ -400 pcm on a reactor trip Due to promethium in the core (Pm-149) Pu-239 buildup from Np-239 in the core Will add ≈ +200 pcm on a reactor trip The reason Sm-149 does not decrease to 0 is because it has a very long half-life. This will be discussed in upcoming slides. ELO 1.2

Xenon and Samarium Knowledge Check
Which one of the following has the greatest microscopic cross section for absorption of a thermal neutron? Uranium-235 Boron-10 Samarium-149 Xenon-135 Correct answer is D. Correct answer is D. NRC Bank Question – P2458 Analysis: Isotope Absorption Cross-Section for a Thermal Neutron (Barns) U (includes sf and sa) B SM x 104 Xe x 106 ELO 1.2

Xenon and Samarium Knowledge Check – NRC Check
Fission product poisons can be differentiated from other fission products in that fission product poisons... have a longer half-life. are stronger absorbers of thermal neutrons. are produced in a larger percentage of fissions. have a higher fission cross-section for thermal neutrons. Correct answer is B. Correct answer is B. NRC Question P858 Analysis: A. WRONG. A fission product’s half-life has nothing to do with its ability to absorb a neutron. There may be several other fission products with longer half-lives that Xe-135 or Sm-149. B. CORRECT. The “differentiation” between fission product poisons and fission products is the “poisons” part – HIGH Microscopic Cross-Section for absorption of thermal neutrons. C. WRONG. Isotopes other than the fission product poisons are also prominent on the fission yield curve. D. WRONG. Fission product poisons have a high microscopic cross section for thermal neutron absorption, not fission. ELO 1.2

Production and Removal of Xenon
ELO 1.3 – Explain how Xe-135 is produced and removed in the core of a nuclear reactor. Xe-135 production terms: Direct term Xe % fission yield Indirect term I-135/Te-135 5.6% fission yield Xe-135 removal terms: Absorption Neutron capture reaction which creates Xe-136 (burnout) Decay Beta-Minus decay to Cesium-135 Related KA K1.02 State the characteristics of Xe-135 as a fission product poison 3.0, 1.1; K1.03 Describe the production of Xe ; K1.04 Describe the removal of Xe ELO 1.3

Xe-135 Production Xe-135 production decay chain
𝑆𝑏 𝛽 𝑇𝑒 𝛽 𝐼 𝛽 𝑋𝑒 𝛽 𝐶𝑠 𝛽 𝐵𝑎 (𝑠𝑡𝑎𝑏𝑙𝑒) 1.7 sec 𝑠𝑒𝑐 ℎ𝑟 ℎ𝑟 × 10 6 𝑦𝑒𝑎𝑟𝑠 Approximately 0.3% of all fissions yield Xe-135 as a fission fragment Approximately 5.6 % of all fissions yield above isotopes, with beta- minus to I-135 Because of short half-life of Sb-135 and Te-135 and longer half- life of I-135, Sb-135 and Te-135 are ignored and I-135 is assumed the source from fission I-135 is not a strong neutron absorber so it all decays to Xe-135 ELO 1.3

Xe-135 Production Of the total Xe-135 production: 5% is direct
95% is indirect (I-135) Based on % fission yield Direct: 0.3/( ) = 5% Indirect: 5.6/( ) = 95% Operation concern: Xe-135 decay half life (removal term) is 9.1 hrs I-135 decay half-life (production term) is 6.6 hrs Xenon concentration must be considered when changing power With 95% of the xenon coming from I-135 and I-135 having a half-life of 6.6 hours, a significant delay in production of Xe-135 occurs Important when considering changing power levels, since production of Xe-135 via I-135 is dependent on power level (number of fissions occurring) ELO 1.3

Xe-135 Removal Terms Burnout Absorption of thermal neutron
Results in Xe-136 (small sa) Decay Decays to Cs-135 Half life of 9.1 hrs Full Power Removal Percentages (approximate) 75% - Burnout 25% - Decay Low Power Removal Decay predominant removal term (≈ 10% power) There are some NRC bank questions asking which removal term is predominant. When asked at “full power” – burnout. When asked at “low power” – decay There isn’t a magical power level knowledge requirement where decay is the predominant term. Just remember, when the flux is HIGH – burnout, when flux is LOW – decay. ELO 1.3

Xe-135 Production = Removal
When production = removal Xenon is said to be in equilibrium Rate of change = 0 𝑥𝑒𝑛𝑜𝑛 _ 135 𝑦𝑖𝑒𝑙𝑑 𝑓𝑟𝑜𝑚 𝑓𝑖𝑠𝑠𝑖𝑜𝑛 + 𝑖𝑜𝑑𝑖𝑛𝑒 _ 135 𝑑𝑒𝑐𝑎𝑦 = 𝑥𝑒𝑛𝑜𝑛 _ 135 𝑑𝑒𝑐𝑎𝑦 + 𝑥𝑒𝑛𝑜𝑛 _ 135 𝑏𝑢𝑟𝑛𝑢𝑝 Where: NXe = Xe-135 concentration NI = iodine-135 concentration xe = fission yield of Xe-135 λI = decay constant for iodine-135 𝑓 𝑓𝑢𝑒𝑙 = macroscopic cross- section in fuel λXe = decay constant for Xe-135 𝜎 𝑋𝑒 𝑎 = microscopic absorption cross-section for Xe-135 Φ = thermal neutron flux ELO 1.3

Xe-135 Equilibrium – Solved for Concentration
This formula: Solved for NXe: ELO 1.3

Xe-135 Equilibrium Concentration
Formula shows: Xe-135 is power dependent Flux term (Φ)in formula NXe decreases over core life Fuel depletes, boron dilutes, flux increases NOTE: The flux increases by about 10% from BOL to EOL. Even though U-235 is depleting, Pu-239 and pu-241 are being produced. (convertor reactor) ELO 1.3

Xe-135 Production and Removal
Knowledge Check Xenon-135 is produced in a reactor by two primary methods. One is directly from fission; the other is from the decay of... cesium-135 iodine-135 xenon-136 iodine-136 Correct answer is B. Correct answer is B. NRC Bank Question – P358 Analysis: A. WRONG. Cesium-135 is what Xe-135 decays to. B. CORRECT. Indirect term: Te-135 and I-135 produced, but Te-135 half-life so short ALL considered I-135. C. WRONG. Xenon-136 is the resultant isotope from Xe-135 absorbing a thermal neutron. D. WRONG. Iodine-136 is just another fission product (but with a low microscopic absorption cross-section.. ELO 1.3

Xenon Transient Terms ELO 1.4 – Explain the following terms: equilibrium iodine, equilibrium xenon, transient xenon, peak xenon, xenon free, and xenon precluded startup. An understanding of various terms related to xenon transients will provide a deeper understanding of xenon characteristics and limitations during reactor operations Related KA - ELO 1.4

Equilibrium Iodine (NI eq)
Equilibrium iodine eventually occurs after a power change Directly proportional to reactor power Takes 20 to 25 hours after power change to reach equilibrium I-135 has to reach equilibrium before Xe-135 can reach equilibrium Equilibrium iodine is not tested by the NRC. ELO 1.4

Equilibrium Xenon (NXe eq)
When production and removal rates of Xe-135 are equal, equilibrium is established Equilibrium concentration of Xe-135 is designated NXe (eq) 𝑁 𝑋𝑒 𝑒𝑞 = Φ 𝑓 𝑓𝑢𝑒𝑙 ( 𝛾 𝑋𝑒 + 𝛾 𝐼 ) 𝜎 𝑋𝑒 𝑎 Φ + 𝜆 𝑋𝑒 Xe-135 increases as power increases, because numerator is proportional to fission reaction rate Thermal flux is also in the denominator for xenon removal by burnout, because of this it can be seen that equilibrium xenon concentration increases with higher reactor power, but is not directly proportional. ELO 1.4

Equilibrium xenon at 25% power ≈ equal to 50% of 100% power equilibrium Equilibrium xenon at 50% power is between 70 and 80% of 100% power equilibrium Not linearly proportional to power (function of burnout term) At low powers, decay is major removal term At high powers, burnout is major removal term At 100% power and 100% equilibrium, ≈ 75% of Xe-135 removal is burnout and ≈ 25% decay Graph on next slide ELO 1.4

Equilibrium iodine-135 and Xe-135 concentrations as a function of neutron flux are: The graph shows that as flux in increased, the “burnout” term has a bigger influence on xenon concentration that the “production” term. Therefore, xenon concentration is “dependent” on flux in the core, but not directly proportional to flux in the core (at least not at higher powers). You can also see that iodine concentration is linearly AND directly proportional to power. Figure: Equilibrium Iodine-135 and Xe-135 Concentrations Versus Neutron Flux ELO 1.4

Transient Xenon Power Increase
Xenon initially decreases due to increase in flux (burnout) Power decrease Xenon initially increases (less burnout and iodine in core) THUMBRULE - Time to Peak (on Rx trip) 7 to 9 hours Time to reach equilibrium 40 to 50 hours after power change Time to Peak thumbrule based on core size: 12 month core – Square root of power change (NRC bases bank questions on 1 year cores) 18 month core – Divide power change by 1.5, then take square root 24 month core – Divide power change by 2, then take square root Easiest way to remember the thumbrule is “the time to peak is slightly less than the square root of the power change”. ELO 1.4

Peak Xenon - Downpower Xenon peaks because
Decrease in burnout (less flux) Iodine already in the core Function of production and removal terms (times) 6.6 hours to produce (I- 135) 9.1 hours to remove (decay of Xe-135) Anytime the reactor undergoes a rapid down power, Xe-135 concentration will peak and then decrease to a new lower equilibrium value or go to zero (on a shutdown). The greater the flux level (power) prior to shutdown, the greater the concentration of iodine-135 at shutdown; therefore, the greater the peak after shutdown. Figure: Xe-135 Reactivity After Reactor Shutdown ELO 1.4

Peak Xenon - TRIP Trip from 100% power - peak xenon 7-9 hours
≈ 24 hours after trip [Xe] returns to pre-trip value From 24 to 80 hours after trip Xe-135 adds “net” positive reactivity post trip Reduces SDM Trip from 50% power - peak xenon 5.5 hours Initially an increase in SDM as [Xe] increases The longer the fuel cycle, the lower the flux in the core. The lower the flux in the core, the quicker the time to peak after a trip. The greater the flux level (power) prior to shutdown, the greater the concentration of iodine-135 at shutdown Therefore, greater peak after shutdown Xe starts adding positive reactivity after the peak (7-9 hours) but there is still “net” negative reactivity in the core from xenon until about 24 hours after the trip. Shutdown margin requirements should not be exceeded because of control rod negative reactivity insertion However, any additional cooldown adding positive reactivity could reduce shutdown margin further to point of exceeding its limits Adding boron would likely be performed ELO 1.4

Xenon Free Xenon-free 70-80 hours after a reactor trip
+2700 to pcm added to core Since last critical 100% condition Must be accounted for when calculating next startup ELO 1.4

Xenon Precluded Startup
NOTE: this concept is highly unlikely, but testable by NRC Following a reactor trip, xenon peaks Goes from ≈ pcm to pcm Kexcess is at its lowest at EOL Most “likely” precluded startup is: A trip from high power at EOL With startup time near xenon peak NRC tests this concept (P1358 and P3860) even though a “startup” should never be a problem. The concept of returning to 100% power might be thought of as a concern because of the “peak xenon” after the trip from 100% power (at EOL), but the return to power results in flux burning out the Xenon (peaking sooner and lower). Also at EOL the negative reactivity from “peak xenon” is easily offset by the positive reactivity of power defect (on the trip/shutdown). This concept is still presented, however, to understand the concept that: The higher the power tripped from The closer to the peak time The closer to EOL The more likely that xenon might preclude a return to power ELO 1.4

Xenon Transient Terms Knowledge Check
A reactor has been operating at 80 percent power for two months. A manual reactor trip is required for a test. The trip will be followed by a reactor startup with criticality scheduled to occur 24 hours after the trip. The greatest assurance that xenon-135 reactivity will permit criticality during the reactor startup will exist if the reactor is operated at __________ power for 48 hours prior to the trip; and if criticality is rescheduled for __________ hours after the trip. 60 percent; 18 60 percent; 30 100 percent; 18 100 percent; 30 Correct answer is B. Correct answer is B. NRC Bank Question – P3860 Analysis: The time to “peak” is slightly less than the square root of the power from the trip (~8 hours from 100% power; ~6 hours at 60% power). Therefore, the reactor that was operated at 60% power will provide the greatest assurance that fission product poison reactivity will permit criticality during the startup because Xenon worth will be less. Following the peak, the Xe-135 concentration will decrease at a rate controlled by the decay of Xe Approximately 24 hours from a trip from 100% power, Xe-135 concentration returns to the value prior to the trip. Note that the higher Xenon peak will occur in the reactor at 100% power due to the larger equilibrium Xenon value (-2700 pcm) compared to the reactor operated at 60% power (-2200 pcm). Therefore, the reactor that was operated at 60% power has a lower peak Xenon value which provides the greatest assurance that fission product poison reactivity will permit criticality during the startup. Also, waiting 30 hours to startup will result in a lower xenon concentration (more has decayed away) that if the reactor was started up after 18 hours. Therefore, choice “B” is correct. Keep in mind that the positive reactivity added by the Power Defect is more than adequate to overcome the negative reactivity from xenon after a trip so this condition is highly unlikely! ELO 1.4

Xenon Transient Terms Knowledge Check
Two identical reactors have been operating at a constant power level for one week. Reactor A is at 100 percent power and reactor B is at 50 percent power. If both reactors trip at the same time, xenon concentration will peak first in reactor _____; and the highest peak xenon-135 concentration will occur in reactor _____. B;B B;A A;B A;A Correct answer is B. Correct answer is B. NRC Bank Question – P1159 Analysis: Directly following a reactor trip, the neutron flux is reduced essentially to zero. Therefore, Xe-135 is no longer produced directly from fission, but is no longer removed by burnup. The only remaining production mechanism is the b- decay of the iodine-135; the only removal mechanism for xenon-135 is b- decay. Because the decay rate of iodine-135 is faster than the decay rate of xenon-135, the xenon concentration builds to a peak (about pcm if tripped from 100% power). The time to “peak” is slightly less than the square root of the power from the trip (Reactor A: 8 hours from 100% power; Reactor B: 5.5 hours at 50% power). Following the peak, the Xe-135 concentration will decrease at a rate controlled by the decay of Xe-135. Therefore, the correct answer is “B” because the peak will occur first in Reactor B because it was tripped from a lower power, and the peak will be higher in Reactor A because there was more Xenon and Iodine in the core to begin with, therefore a higher peak before turning. ELO 1.4

Xenon Response During Reactor Operations
ELO 1.5 – Explain how Xe-135 concentration reacts during the following nuclear reactor operations: initial reactor startup (xenon free), reactor shutdown, decrease in reactor power, increase in reactor power, and reactor startup with xenon present in core. Changes in Xe-135 concentration affect amount of reactivity present in the core Reactor operations can result in significant changes in concentration of Xe-135 and, therefore, reactivity In order to achieve or maintain a safe and desired reactor power level, reactor operators must be able to recognize and account for these effects Related KA – Plot the curve and explain the reasoning for the reactivity insertion by Xenon-124 versus time for the following: K Initial reactor startup and ascension to rated power K Reactor startup with Xe-135 already present in the core K Power changes from steady-state power to another K Reactor scram K Reactor shutdown ; K1.14 Explain the methods and reasons for the operator to compensate for the time dependent behavior of Xenon 135 concentration in the reactor ELO 1.5

Initial Reactor Startup (Xenon Free)
Xenon-free condition exists: at beginning of core life (BOL) prior to reactor operation OR Reactor shutdown more than 3 days During reactor startup: Xenon builds in based on half-life of production (activity curve) 40 – 50 hours to reach equilibrium Based on 7 – 8 half lives of Iodine-135 (6.6 hrs) Curve not exact due to the way that power is increased Note that the curve provided for xenon during a reactor startup is not how it really looks because as power is increased to some point, xenon will increase. If staying at that power for some time this curve is representative of how xenon builds in. However, we rarely go from 0% to 100% in one jump. Therefore this curve is not precise. For example if we went from 25% to 50%, xenon in the core would tend to initially decrease due to the increase in burnout term, THEN increase to new higher level. 50% to 75% the same, 75% to 100% the same. These curves (on next slide) assume going from 0 to 100% in one fail swoop then watching xenon build in. The reason this is stressed is because there are a few bank questions that ask how long it takes to reach xenon equilibrium. Regardless of the power change, the NRC is ALWAYS looking for “40 – 50 hours”! ELO 1.5

This figure shows the time required to reach equilibrium xenon concentration from a xenon free condition for three different power levels: 100%, 50% and 25% reactor power. Note the concentration of xenon at 25% power is approximately 50% of the xenon at 100% power. Figure: Time to Reach Equilibrium Xenon for Various Power Levels ELO 1.5

Immediate production of Xe-135 directly from fission Immediate production of iodine-135 (Te-135) directly from fission Delayed decay of I-135 to Xe-135 (half-life 6.6 hours) As power increased, production > removal (- reactivity added) rods or boron used to keep temperature on program These two mechanisms lead to an increase in concentration of Xe-135 in the core. 5% of xenon from direct fission and 95% of xenon from iodine decay As concentration is building, xenon removed by burnout and decay also increases If no operator action is taken during this time, xenon is building in (constant steam demand) RCS temperature will decrease to compensate for xenon negative reactivity Eventually equilibrium concentration of Xe-135 occurs Figure: Time to Reach Equilibrium Xenon for Various Power Levels ELO 1.5

Reactor Shutdown When reactor is shutdown from power:
Xenon concentration increases initially Production > removal Only removal is decay (no burnout) (9.1 hrs) No direct Xenon Indirect xenon from iodine in core produces xenon (6.6 hrs) Xenon peaks about 7-9 hours after trip/shutdown About to pcm (increase of about pcm) Time is “slightly less” than square root of power change Decay of Xe-135 now the only term Decays to xenon-free hours after trip/shutdown Thermal neutron flux drops to essentially zero Iodine-135 decaying to Xe-135 is adding to Xe-135 already in the core Xe-135 removal now only by decay Sudden drop in burnout and I-135 decay still temporarily producing xenon at a rate equal to pre-shutdown power level causes Xe-135 to peak. Unlike xenon, equilibrium iodine is directly and linearly proportional to power , therefore I-135 will continue for sometime to be producing Xe-135 at pre-shutdown levels Remember that 95% of xenon production at 100% power is from I-135 decay ELO 1.5

Decrease in Reactor Power
Same process as shutdown/trip, but smaller change in power Xenon concentration increases initially Production > removal Less burn out I-135 decays to Xe-135 Xenon peak (time) – slightly < square root of power change Removal now > production (less fissions at lower power) Xe-135 concentration decreases to new lower equilibrium level 40 – 50 hours to equilibrium If downpower was 100 to 50% (peak ≈ 5-6 hrs) New [Xe] > half of original concentration (not proportional to flux) Assume a reactor operating at 100% power with equilibrium xenon Decrease power to 50% Immediate decrease in xenon burnup Increase in Xe-135 concentration With a constant Xe-135 decay rate, reduced burnup rate, and iodine-135 concentration still at equilibrium level for 100% power – 95% of xenon is produced at 100% power rate Production > removal ELO 1.5

Decrease in Reactor Power
40-50 hours Recall: startup from Xenon free Power increase part of curve hidden by white SHAPES. Note that the new equilibrium concentration of Xe-135 on the down power from 100% to 50% is greater than half of the original concentration. Good values for [Xe] at BOL: 100%: pcm 50%: pcm 25%: pcm Therefore, going from 50% to 100% power [Xe] increases by “less than half of original”, etc. Several NRC questions test this terminology. Figure: Xe-135 Variations During Power Changes ELO 1.5

Increase in Reactor Power
Xenon concentration decreases initially Removal > production Flux Increase is immediate I-135 decaying to Xe-135 takes time Xenon dip (time) Slightly less than “peak” times Production now > removal (higher I-135 in core decaying to Xe-135) Xe-135 concentration increases to new higher equilibrium level 40 – 50 hours to equilibrium If increase in power was 50 to 100% New [Xe] < twice original concentration (not proportional to flux) The magnitude and the rate of change of xenon concentration during the initial 4 to 6 hours following the power change is dependent upon the initial power level and amount and rate of change in power level. Review dilution/boration required by the operator. The time to “bottom out” on an uppower is less than the “peak time”. Also, the time to equilibrium is actually a little less than hours, but ALWAYS answer hours on any “equilibrium” question! ELO 1.5

Increase in Reactor Power
Review this graphic with class – discuss the power increase. Notes: the “bottom dip” is slightly less than the square root of the power change The time to equilibrium is slightly less than hrs (but ALWAYS answer 40-50% on questions)! New 100% [Xe] is not double the 50% [Xe] This graph of [Xe] can be used as a “visual” of rod movement to keep temperature on program #4 Explanation: If power is raised from 50% to 100% in 2 hours you are still on the left hand side of the graph. For the next 3-5 hours as [Xe] decreases (temperature increases), you would have to insert rods. After the “dip”, you would have to withdraw rods as [Xe] increases (temperature decreases) to keep temperature on program. Figure: Xe-135 Variations During Power Changes ELO 1.5

Reactor Startup With Xenon Present
Startup shown 10 hours after trip from 100% Graph shows quick return to 100% power [Xe] decreases quickly due to burnout On a QUICK return to full power, a dip will occur On a SLOW return to full power, might just “decrease” to 100% equilibrium This figure shows a graph of xenon concentration versus time after startup with xenon present. The first portion of figure illustrates xenon concentration approaching peak xenon following a reactor shutdown. At time zero a reactor startup is commenced. As power increases, Xe-135 starts to decrease due to burnout with increasing flux levels. This decrease is much faster than normal xenon decay or xenon rate changes and must be monitored very carefully as very large reactivity addition rates can be created. This accelerated decrease in xenon concentration during startup is a result of two factors: Burnout rate very high and even above normal for an up power transient due to high concentration of Xe-135 Direct production of iodine-135 and Xe-135 from fission is again occurring, but takes several hours for equilibrium conditions to re-establish Lag in recovery of Xe-135 concentration during this kind of startup directly attributed to half-life of iodine-135 Although iodine-135 concentration starts to recover immediately, production of Xe-135 from iodine-135 lags due to 6.6 hour half-life Figure: Xenon Behavior During Reactor Startup With Xenon Present in the Core ELO 1.5

Knowledge Check – NRC Bank Reactor power is increased from 50% to 60% in 1 hour. The most significant contributor to the initial change in core xenon reactivity is the increase in xenon production of Xe-135 from fission production of Xe-135 from iodine-135 decay loss of Xe-135 due to absorption of neutrons loss of Xe-135 due to decay to cesium-135 Correct answer is C. Correct answer is C. NRC Question P460 ELO 1.5

Knowledge Check A reactor had been operating at 100 percent power for two weeks when power was reduced to 50 percent over a one-hour period. To maintain reactor power stable during the next 24 hours, which one of the following incremental control rod manipulations will be required? Withdraw rods slowly during the entire period. Withdraw rods slowly at first, and then insert rods slowly. Insert rods slowly during the entire period. Insert rods slowly at first, and then withdraw rods slowly. Correct answer is B. Correct answer is B. NRC Bank Question – P2061 Analysis: Draw [Xe] graph for downpower showing a)Time 1hrs, b)Peak Time 7hrs, c) Time 25hrs On a downpower from 100% to 10%, initially Xe-135 concentration rises due to the significant reduction in the burnout/neutron capture term. The first 8-10 hours of the transient results in an increase in Xenon concentration (negative reactivity). Therefore, to maintain stable Tave, rods must be withdrawn (positive reactivity). As Xenon concentration rises, the burnout term will increase. Also, after 8-10 hours, significant time has elapsed to see a reduction in Xenon production due to the b- decay of I-135. The next hours will be marked by a reduction in Xenon concentration (positive reactivity). Therefore, control rods must be inserted (negative reactivity). ELO 1.5

Xenon Oscillations ELO 1.6 – Describe the causes and effects of a xenon oscillation. Axial xenon oscillations fairly common in PWR operations Xenon oscillations can result in localized power peaking Power peaking can result in: unwanted power reductions uneven burnout high fuel temperature cladding failure Related KA K1.08 Describe the effects that Xenon concentration has on flux shape and control rod patterns This section is similar in nature to the next ELO and some of the graphs may be repeated ELO 1.6

Xenon Oscillations Usually caused by large downpower and/or large rod insertion affects axial flux distribution Xenon oscillations converge or diverge Converging oscillations dampen themselves Diverging oscillations continue to grow Usually occur near EOL Upper and lower halves of core “uncoupled” Procedurally controlled with rods/boron Example: When flux moving upwards in core, rods “bumped in” to dampen Large thermal reactors with little flux coupling between regions may experience spatial power oscillations because of non-uniform presence of Xe-135 Mechanism described in following four steps: An initial lack of symmetry in the core power distribution causes an imbalance in fission rates within the reactor core Affecting iodine-135 buildup and Xe-135 burnup Example, individual control rod movement or misalignment In the high-flux region, (higher burnout) Xe-135 burnout allows the flux to increase further In low-flux region, increase in Xe-135 (lower burnout) causes a further reduction in flux Iodine concentration increases where flux is high and decreases where flux is low As soon as iodine-135 levels build up sufficiently, I-135 beta minus decay to xenon reverses initial situation Flux decreases in higher power area causing power to decrease and former low-flux region increases in power Repetition of these patterns leads to xenon oscillations moving about the core with periods on the order of hours Operator actions would include specific instructions from reactor engineering involving control rod movement. ELO 1.6

Xenon Oscillations Oscillation times based on half lives of I-135 and Xe-135 (and power) [Xe] Peak/Flux Peak (top-bottom or bottom-top) Occur every hours Graphs shown in next section Xenon oscillations can cause AFD to exceed Tech Specs limits Reduce reactor power to < 50% In extreme cases, requires a reactor shutdown In Summary: Xenon oscillations worst (diverging) at high power at EOL ELO 1.6

Xenon Oscillations Knowledge Check
When a reactor experiences xenon-135 oscillations, the most significant shifts in power generation occur between the __________ of the core. top and bottom adjacent quadrants center and periphery opposite quadrants Correct answer is A. Correct answer is A. NRC Bank Question - P761 ELO 1.6

Xenon Effects on the Thermal Flux Profile
ELO 1.7 – Explain the effects of xenon concentration on a nuclear reactor core’s thermal flux profile for control rod motion and core life. When core neutron flux levels change from reactivity inputs, Xenon too is affected, in turn affecting flux distribution Xenon concentration and worth change from BOL to EOL This section also adds to the previously covered topic on xenon oscillations Related KA K1.08 Describe the effects that Xenon concentration has on flux shape and control rod patterns ELO 1.7

Xenon Effects to Thermal Flux Profile Control Rod Motion
Recall – [Xe] dependent on flux If rods inserted (constant power) Flux depressed at top of core Flux peaks lower in the core Xe-135 burns up faster in bottom of core Exaggerates bottom flux peak Xe-135 increases at top of core Less burnout and I-135 decay to Xe-135 Exaggerates flux depression even more Graphs start on next slide When control rods are inserted a small amount (while maintaining a constant reactor power), thermal flux in top half of core decreases, while thermal flux in bottom half of core increases Axial Flux Difference (AFD) will go negative Rate of Xe-135 burnup in lower portion of core increases immediately Exaggerates power peak in bottom of core over next several hours Xe-135 peak (less flux) in top of core will further suppress power in top at the same time AFD goes more negative ELO 1.7

Initial rod motion pushes flux pattern lower in core Power peak in bottom of core creates a larger I-135 concentration in the bottom Xenon concentration is dependent on, but not proportional to flux in the core. Therefore, based on the shape of this example “power” curve, the “Xenon” curve is similar in shape. When rods are inserted the flux gets pushed lower in the core. Since there is less flux burning out existing Xenon and the Iodine already in the core, Xenon concentration tends to increase in the top half of the core for approximately the next 6.5 hours. This tends to push the flux even lower in the core. Figure: Thermal Flux Versus Xenon Concentration After Control Rod Insertion ELO 1.7

After time 6.5 hours (xenon peaked in top, flux peaked in bottom) Xenon in top of core decreases (decay of existing Xenon) Xenon in bottom of core increases (iodine produced from fission) Xenon shifts to bottom, flux shifts to top in ≈ 13 hours About 6.5 hours after power peaks in bottom, Xe-135 from I-135 will produce so much Xe-135 that power will be suppressed in bottom of core and increase in top After about 19.5 hours, axial thermal flux and xenon concentration profiles shown in figure (D) will exist in core After the power increases in the top of the reactor, the I-135 concentration increases and eventually produces a Xe-135 peak in the top of the reactor Once again suppressing the power in the top to repeat the cycle. The period of the cycles (peak in top to peak in top, or peak in bottom to peak in bottom) is 24 to 28 hours. Figure: Thermal Flux Versus Xenon Concentration After Control Rod Insertion ELO 1.7

Cycle repeats itself from time 19.5 to 32.5 (13 hours half cycle) Fuel integrity concerns are: Bottom flux peak – kW/ft, fuel pellet melting Top flux peak – DNBR, cladding oxidation The two hot channel factors introduced in Control Rods and explained further in – Core Thermal Limits are: Heat Flux Hot Channel Factor – hot spot below core midplane, peak fuel centerline temperature of 4700 degrees, excessive kw/ft Enthalpy Rise Hot Channel Factor – departure from nucleate boiling, cladding temperature of 2200 degrees, caused by adding high kw/ft to already hot water at top of core. Figure: Thermal Flux Versus Xenon Concentration After Control Rod Insertion ELO 1.7

Xenon oscillations caused by rapid down power Causes flux to shift higher in core initially Smaller difference between Tcold and Thot Xenon oscillations caused by rod insertion Causes flux to shift lower in core initially Depression caused by rod insertion At BOL, reactivity effects due to MTC tend to dampen xenon oscillations At EOL xenon oscillations more prevalent because: Fuel mostly depleted in axial center of core Less neutronic coupling between upper and lower halves of core Recall that flux follows rods during the initial ascension to approximately 30-50% power (less flux depression as rods withdrawn), then shifts to just below core midplane after rods fully withdraws as the core Delta-T increases (bigger difference between bottom and top temperatures). If power is quickly reduced from 100% power to 50% power using boron only (rods in manual), flux will shift upwards due to the smaller Delta-T. If rods are inserted while at high power, flux will tend to shift downwards (be depressed by rods). These two types of “xenon oscillations” produce opposite effects. Westinghouse has created a fuel design that reduces the probability of creating diverging xenon oscillations Fuel pellets near top and bottom of fuel element have annular holes Reduces disparity in fuel concentration at EOL between edges and axial center, better neutron coupling Also creates more space for helium and fission product gasses when clad creep and fuel pellet swell closes the gap between fuel pellets and cladding ELO 1.7

Xenon Worth vs Concentration
BOL Xenon concentration higher Lower flux, less burnout Xenon worth is lower More competition from boron EOL Xenon concentration lower Higher flux, more burnout Xenon worth higher Less competition from boron There are a few NRC bank questions asking the differences between xenon concentration and xenon worth from BOL to EOL (and why). ELO 1.7

Xenon Oscillations Knowledge Check
Xenon-135 oscillations take about __________ hours to get from maximum xenon-135 negative reactivity to minimum xenon negative reactivity. 40 to 50 24 to 28 12 to 14 6 to 7 Correct answer is C. Correct answer is C. NRC Bank Question – P1160 Analysis: These oscillations tend to take about 12 to 14 hours to get from maximum Xe-135 negative reactivity to minimum Xe-135 negative reactivity. This can be attributed to the 6.6 hour half-life of I-135 and 9.1 hour half-life of Xe-135. Also the training materials show the time to peak Xe from top back to the top is 26 hours. This question is asking for one-half of that cycle (13 hours). ELO 1.7

Xenon Effects to Thermal Flux Profile
Knowledge Check Reactors A and B are operating at steady-state 100 percent power with equilibrium xenon-135. The reactors are identical except that reactor A is operating near the end of a fuel cycle (EOC) and reactor B is operating near the beginning of a fuel cycle (BOC). Which reactor has the greater concentration of xenon-135, and why? Reactor A (EOC), due to the smaller 100 percent power thermal neutron flux. Reactor A (EOC), due to the larger 100 percent power thermal neutron flux. Reactor B (BOC), due to the smaller 100 percent power thermal neutron flux. Reactor B (BOC), due to the larger 100 percent power thermal neutron flux. Correct answer is C. Correct answer is C. NRC Bank Question – P2558 Analysis: Xe-135 concentration decreases slightly over core life due to the slightly larger neutron flux. Therefore, at BOL the concentration will be larger due to less flux (burnout). ELO 1.7

Xenon Effects to Thermal Flux Profile
Knowledge Check – NRC Bank Xe-135 poisoning in a nuclear reactor core is most likely to prevent a reactor startup following a reactor shutdown from ____________ power at the ____________ of core life. high; beginning low; beginning high; end low; end Correct answer is C. Correct answer is C. NRC Question P1262 Analysis: Xe-135 poisoning is another way of saying “Xe-135 precluded startup”. The higher Xenon peak will occur in the reactor at HIGH power due to the larger equilibrium Xenon value (-2700 pcm) compared to the reactor operated at 50% power (approximately pcm). Therefore, a “Xenon precluded startup” is more likely in a reactor operated at high power. The worth of Xe-135 increases because there is less competition (boron concentration decreases over core life to compensate for fuel burning out) with Xe-135 to absorb neutrons. Therefore, a “Xenon precluded startup” is more likely in a reactor at end of life, that is tripped at high power (since the peak Xe-135 will be higher). Keep in mind that the positive reactivity added by the Power Defect is more than adequate to overcome the negative reactivity from xenon after a trip so this condition is highly unlikely! ELO 1.7

Samarium-149 TLO 2 – Describe the production, removal, and effects of Sm-149 on the operation of a nuclear reactor. 2.1 Explain how Sm-149 is produced and removed from the reactor core during reactor operation. 2.2 Describe equilibrium Sm-149 concentration. 2.3 Explain how equilibrium Sm-149 concentration varies with the following reactor operations: initial reactor startup, reactor shutdown, and reactor startup after shutdown. 2.4 Describe the effects of Sm-149 concentration on reactor operation over core life. 2.5 Compare the effects of Sm-149 to the effects of Xe-135 on reactor operation. K1.15 State the characteristics of Samarium-149 as a fission product poison. K1.16 Describe the production of Samarium-149. K1.17 Describe the removal of Samarium-149. K1.18 Define equilibrium samarium. K1.19 Plot the curve and explain the reasoning for reactivity insertion by Samarium-149 versus time for the following: Initial reactor startup and ascension to rated power K1.20 Plot the curve and explain the reasoning for reactivity insertion by Samarium-149 versus time for the following: Reactor shutdown K1.21 Describe the effects of power changes on samarium concentration. K1.22 Compare effects of Samarium-149 on reactor operation with those of Xenon-135. Even though there are currently NO NRC bank questions on these KA’s, understanding of Samarium is still important with respects to its effect on a reactor startup (included in ECC calculation discussed in – Reactor Operational Physics). TLO 2

Samarium Production and Removal
ELO 2.1 – Explain how Sm-149 is produced and removed from the reactor core during reactor operation. This section will explain the production and removal terms associated with Sm-149. Related KAs - KA K1.16 Describe the production of Samarium * 1.8*, K1.17 Describe the removal of Samarium * 1.8* ELO 2.1

Samarium-149 Production 149 60 𝑁𝑑 𝛽− 149 61 𝑃𝑚 𝛽− 149 62 𝑆𝑚 Direct
Negligible amounts of Sm-149 produced directly from fission Indirect 1.1% of all fissions result in production of either neodymium or promethium-149 𝑁𝑑 𝛽− 𝑃𝑚 𝛽− 𝑆𝑚 1.73 ℎ𝑜𝑢𝑟𝑠 ℎ𝑜𝑢𝑟𝑠 Nd-149 half-life – 1.7 hours Pm-149 half life – 53 hours Total indirect time – 55 hours Both neodymium and promethium decay by beta-minus; However, Pm-149 half-life is much longer than Nd-149 and all other 149 precursors (other fission products that decay to Nd-149). Consequently, the time dependence behavior of Sm-149 is accurately explained by assuming that the only Sm-149 precursor is Pm-149 with a yield of 1.1%. This is similar to Te-135 in the Xe-135 chain. 7 to 8 half lives of Pm-149 (55 hour half life) is what leads to the 2-3 weeks to reach Sm-149 equilibrium upon startup of a clean core. ELO 2.1

Samarium-149 Removal No Decay term
Sm-149 has half‑life of 1016 years and is considered stable Burnout Only removal mechanism for Sm-149 is by neutron capture sa for Sm-149 is 4.1 x 104 barns Results in conversion of Sm-149 to Sm-150 𝑆𝑚 𝑛 → 𝑆𝑚+𝛾 Sm-150 is stable with low sa One and Done ELO 2.1

Equilibrium Samarium ELO 2.2 – Describe equilibrium Sm-149 concentration and how it varies with reactor power changes. Equilibrium Sm-149 concentration is independent of power level (time to equilibrium is not) Since Sm-149 does not decay, core is NEVER “clean” again Unless entire core replaced Related KAs - KA K1.18 Define equilibrium samarium. 1.8* 1.8*; K1.21 Describe the effects of power changes on samarium concentration. 1.7* 1.8* ELO 2.2

Equilibrium Sm-149 Production = Removal
Production term – Indirect (from Pm-149) Removal term – burnout Solved for [Sm-149] 𝑁 𝑆𝑚 = 𝛾 𝑃𝑚 𝑓 𝑓𝑢𝑒𝑙 𝜎 𝑆𝑚 𝑎 Flux term factored out ([Sm-149] not power dependent) NOTE: There is a slight power dependence of equilibrium Sm-149 concentration. It is due to the fact that Pm-149 has about a 1400 b cross-section for neutron absorption. At higher powers Pm-149 absorbs some neutrons so there is less of it to decay to Sm-149. It is not a huge difference, only a small one. This would mean the 50% concentration of Sm-149 would be slightly higher than the 100% concentration. ELO 2.2

Samarium Concentration Transients
ELO 2.3 – Explain how equilibrium Sm-149 concentration varies with the following reactor operations: initial reactor startup, reactor shutdown, and reactor startup after shutdown. This session discusses Sm-149 and promethium-149 response for startup and shutdown scenarios Changes to Sm-149 have an impact on reactivity in the core Related KAs - KA K Initial reactor startup and ascension to rated power. 1.8* 1.9*; K Reactor shutdown. 1.7* 1.8* ELO 2.3

Samarium Concentration on Startup
Equilibrium Sm-149 concentration is reached in about 3 weeks Sm-149 worth: ≈ -700 pcm to pcm Although equilibrium Sm-149 is independent of power level, amount of time required to reach equilibrium samarium is related to power level When a new core is taken critical and power increased first time, production of promethium-149 begins with no Sm-149 present As promethium-149 builds, it decays to Sm-149 The time to reach equilibrium Sm-149 is longer at lower power levels. Since Sm-149 does not decay and is very stable, the concentration remains essentially constant during reactor operation and the reactor is never again samarium free. This means reused fuel on a refueling will contain Sm-149. Figure: Samarium-149 Buildup to Equilibrium ELO 2.3

Samarium Concentration on SD/Trip
Sm-149 will increase after shutdown/trip Sm-149 does not decay (no removal term) Decay of Pm-149 in core (production term) More Pm-149 in core, higher the peak Adds about – 400pcm of additional negative reactivity Usually takes about 2 weeks to reach peak value ELO 2.3

Takes about two weeks after shutdown for Sm-149 to peak and level off. Note this is a clean core WBN2 curve. Reload cores start off with Sm from once and twice burned fuel. Figure: Behavior of Samarium-149 in a Pressurized Water Reactor ELO 2.3

Samarium Concentration - Restart
After restart and increasing thermal flux levels: Sm-149 decreases due to burnout term Due to long half-life of promethium-149, significant formation of additional Sm-149 is delayed Sm-149 concentration essentially decreases to pre-trip value Approximately 3 weeks after restart Since Sm-149 changes by small amounts of reactivity Minimal impact operationally ELO 2.3

Whether there is a “dip” on the return to 100% power operation or a gradual decrease in [Sm-149] is a function of the rate of return to power, but not tested. Figure: Behavior of Samarium-149 in a Pressurized Water Reactor ELO 2.3

Samarium-149 Effects Over Core Life
ELO 2.4 – Describe the effects of Sm-149 concentration on reactor operation over core life. Greatest change in Sm-149 Initial startup of clean core However, Sm-149 and other unspecified poisons continue to increase in concentration throughout core life and remain in the core Presents a constant source of negative reactivity Related KA - KA K1.21 Describe the effects of power changes on samarium concentration. 1.7* 1.8* ELO 2.4

Sm-149 Effects Over Core Life
SM-149 worth may increase as core ages due to: Less competition from boron Depleting burnable poisons Increasing thermal neutron flux levels over core life Peak Sm-149 not likely to preclude Rx startup While Sm-149 reactivity should never prevent a restart, probability would be greatest at EOL Negative reactivity added by Samarium-149 combined with lack of positive reactivity available due to fuel burnout at EOL could prevent the reactor from immediately being taken critical ELO 2.4

Samarium-149 to Xe-135 Comparison
ELO 2.5 – Compare the effects of Sm-149 to the effects of Xe-135 on reactor operation. This section gives a brief overview of differences between xenon and samarium neutron poisoning Related KA - KA K1.22 Compare effects of Samarium-149 on reactor operation with those of Xe * 1.8* ELO 2.5

Highest worth at 100% power Xe-135 (about pcm) Highest worth when S/D 3 days Sm-149 (about pcm) Shortest time to equilibrium Xe-135 (40-50 hours) Power dependent Xe-135 Linearly proportional to power level Neither Operational concern (oscillations) – Xe-135 ELO 2.5

Effect Xe-135 Samarium-149 Microscopic Cross-Section for Absorption (σa) 2.6 x 106 Barns 4.1.x 104 Barns Time to Peak Concentration Square root of power prior to S/D or trip  Two weeks Time to Equilibrium Concentration 40 – 48 hours 25 – 35 days Reactivity Worth (these are approximate reactivity values — change over core life) -2.7% Δk/k at 100% power equilibrium -0.7% Δk/k at power equilibrium -4.7% Δk/k at peak -1.1% Δk/k at peak Removal by Decay Yes No Equilibrium Dependent on Power Distribution Problem (Oscillations) Table summary slide with a few other differences ELO 2.5

Knowledge Check A commercial pressurized water reactor experienced a reactor trip from 100% reactor power. Which of the following would contribute the greatest magnitude of negative reactivity to the reactor core, assuming the reactor remains shutdown for an extended period of time? Sm-149 concentration approximately 12.5 days after the trip Sm-149 concentration days after the trip Xe-135 concentration approximately 10 hours after the trip Xe-135 concentration approximately 70 hours after the trip Correct answer is C. Correct answer is C. Analysis: Sm-149 might add about -100 to -200 pcm 12.5 days after a trip Sm-149 might add -400 pcm 3 weeks after a trip Xe-135 adds about pcm 10 hours after a trip Xe-135 adds about pcm 70 hours after a trip (since it is almost decayed away) Therefore, Choice “C” is correct. ELO 2.5

KA to ELO Tie KA # KA Statement RO SRO ELO K1.01
Define fission product poison. 2.5 2.6 1.1 K1.02 State the characteristics of Xenon-135 as a fission product poison. 3.0 1.2 K1.03 Describe the production of Xenon-135. 2.7 2.8 1.3 K1.04 Describe the removal of Xenon-135. K1.05 Describe the following processes and state their effect on reactor operations: Equilibrium Xenon 3.1 1.4 K1.06 Describe the following processes and state their effect on reactor operations: Transient Xenon 3.2 3.4 K1.07 Describe the following processes and state their effect on reactor operations: Xenon following a SCRAM K1.08 Describe the effects that Xenon concentration has on flux shape and control rod patterns. 3.3 1.6, 1.7 K1.09 Plot the curve and explain the reasoning for the reactivity insertion by Xenon-124 versus time for the following: Initial reactor startup and ascension to rated power 1.5 K1.10 Plot the curve and explain the reasoning for the reactivity insertion by Xenon-124 versus time for the following: Reactor startup with Xenon-135 already present in the core K1.11 Plot the curve and explain the reasoning for the reactivity insertion by Xenon-124 versus time for the following: Power changes from steady-state power to another K1.12 Plot the curve and explain the reasoning for the reactivity insertion by Xenon-124 versus time for the following: Reactor scram K1.13 Plot the curve and explain the reasoning for the reactivity insertion by Xenon-124 versus time for the following: Reactor shutdown 2.9 K1.14 Explain the methods and reasons for the operator to compensate for the time dependent behavior of Xenon 135 concentration in the reactor. K1.15 State the characteristics of Samarium-149 as a fission product poison. 1.9 K1.16 Describe the production of Samarium-149. 1.8 2.1 K1.17 Describe the removal of Samarium-149. K1.18 Define equilibrium samarium. 2.2 K1.19 Plot the curve and explain the reasoning for reactivity insertion by Samarium-149 versus time for the following: Initial reactor startup and ascension to rated power 2.3 K1.20 Plot the curve and explain the reasoning for reactivity insertion by Samarium-149 versus time for the following: Reactor shutdown 1.7 K1.21 Describe the effects of power changes on samarium concentration. 2.4 K1.22 Compare effects of Samarium-149 on reactor operation with those of Xenon-135.

Operator Generic Fundamentals Reactor Theory – Fission Product Poisons

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Operator Generic Fundamentals Reactor Theory – Fission Product Poisons

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