Fission Product Poisons

Question 1

Define fission product poison.

Answer 1

A fission product that has a substantial neutron absorption cross section and does not fission.

Question 2

State the characteristics of Xenon-135 as a fission product poison.

Answer 2

To be significant in the neutron balance, a fission product must be relatively abundant (that is have a high fission yield) and have a high absorption cross section.

Xenon-135 is near the top of the fission yield curve, and it has a high absorption cross section at 2.6 × 10⁶ barns.

The absorption cross section for U-235 is 684 barns and for water, about 0.3 barns.

Question 3

Describe the production of Xenon-135

Answer 3

The production of a fission product is due to the direct production from fission and the decay of other fission products into that isotope.

• About 0.3% of all fissions produce Xe-135 directly.
• Xe-135 is also the product of a radioactive decay chain.

Either tellurium-135 (Te-135) or iodine-135 (I-135) is produced in 5.6% of all fissions.

Question 4

Describe the removal of Xenon-135

Answer 4

Removal of Xe-135 from the core is by burnout and decay.

Burnout

• Xe-135 absorbs a thermal neutron and is transformed into Xe-136. The microscopic cross section for absorption of Xe-136 is negligible, and Xe-136 is stable.

Decay

• Xe-135 decays by beta emission, with a 9.1 hour half-life, to cesium-135 (Cs-135). Cs-135 has a very long half-life (>10⁶ years) and small absorption cross section.

Question 5

What is the formula for total production of Xenon-135?

Answer 5

Question 6

What is the formula for Xenon-135 removal?

Answer 6

Question 7

Describe Equilibrium Xenon and its effect on reactor operations.

Answer 7

• When the production and removal rates of Xe-135 equal each other, equilibrium is established. Equilibrium Xe-135 concentration is a function of the flux and, therefore, a function of reactor power level.
• Equilibrium Xe-135 concentration is reached faster at a higher power level. At 100% power for a typical PWR, the equilibrium Xe-135 concentration is reached in about 40 hours.

Question 8

Describe Xenon behaviour following power changes.

Answer 8

Equilibrium Xe-135 is reached in about 40 hours when starting up to full power, in about 44 hours when at 50% power, and about 48 hours at even lower power levels.

The equilibrium level is reached sooner at the higher power level due to the faster production rate.

To maintain constant power during the buildup of Xe-135, the operator must add positive reactivity by reducing boron concentrations or withdrawing additional control rods.

Question 9

Describe Xenon behaviour following reactor trip and its effect on operations.

Answer 9

From equilibrium Xenon it takes approximately 6 to 10 hours post shutdown to reach peak Xenon.

Xenon then decays to a xenon free condition at a rate that eventually approaches the 9.1 hour half-life of Xe-135.

Rule of Thumb: Time to peak Xenon (hrs) equals the square root of % power prior to the trip or downpower.

Question 10

Plot the curve for the reactivity insertion by Xenon-135 versus time.

Answer 10

Question 11

Explain the reason for the reactivity insertion by Xenon-135 versus time plot for initial reactor startup (xenon free).

Answer 11

On start-up from a xenon-free condition results in the immediate production of Xe-135 directly from fission and a large amount of I-135.

Additional Xe-135 is produced as a decay product of the iodine. As the Xe-135 starts building in, burnout and decay remove some of it.

Equilibrium Xe-135 is reached in

• 40 hours starting up to full power
• 44 hours going to 50% power
• 48 hours at even lower power levels.

At 100% power, the reactivity associated with equilibrium Xe-135 is about -2.7% delta k/k.

Question 12

Explain the reason for the reactivity insertion by Xenon-135 versus time plot for reactor startup with Xenon-135 already present in the core.

Answer 12

Once the reactor begins producing significant power (5-10%), the Xe-135 inventory starts to decrease faster than just the decay rate.

This is due to two factors. The first is that the burnout rate is again present and is even increased, due to the fact that a high concentration of Xe-135 already exists.

The second is that although direct production of I-135 and Xe-135 from fission is again occurring, it takes some time for the equilibrium conditions to be re-established.

If the reactor is taken critical but power is held low (1-2% or below), then the burnout mechanism is not sufficient to alter the xenon removal rate, and xenon follows its normal decay curve.

Question 13

Explain the reason for the reactivity insertion by Xenon-135 versus time plot for down power from steady state operations.

Answer 13

Assume xenon concentration is at equilibrium with reactor power at 100%. Reactor power is then decreased to 50%.

Xenon production directly from fission decreases by one half, but production from I-135 initially remains at the 100% level.

Xenon removal by decay remains constant, and removal by burnout decreases by one half.

Xenon production exceeds removal. Xenon concentration increases at a decreasing rate as I-135 decays. Peak xenon is reached 8 to 10 hours after the power decrease.

To maintain power constant during this time, the operator must add positive reactivity by withdrawing additional control rods or decreasing boron concentration.

At 8-10 hours, xenon production exactly equals xenon removal, but I-135 concentration is still decreasing, and xenon removal begins to exceed production. Xenon concentration decreases and reaches equilibrium for the 50% power level between 40 to 50 hours after the power decrease.

A greater power change requires a longer time for xenon to reach equilibrium.

Question 14

Explain the reason for the reactivity insertion by Xenon-135 versus time plot for up power changes from steady state operations.

Answer 14

If reactor power increased, an immediate increase in the burnout rate occurs due to the increased neutron flux level. An immediate increase in the direct Xe-135 production term also occurs.

Since direct Xe-135 only accounts for about 5% of the Xe-135 produced, the burnout term is predominate and causes the concentration of Xe-135 to decrease.

As the concentration of the I-135 increases, more Xe-135 is produced, and in about five hours, the concentration of Xe-135 stops decreasing.

Eventually a higher equilibrium level is established for the higher power level. The total time for the transient is 20 to 30 hours. The greater the power level changes the greater the time required to re-establish the equilibrium.

Question 15

Describe the effects that Xenon concentration has on neutron flux shape.

Answer 15

Since the neutron flux is not uniform over the core volume, the production and removal rates, and the isotopic concentrations of Xenon-135 and Iodine-135 are also not uniform over the core volume.

Thus, Xenon-135 has a local reactivity effect which tends to change the thermal neutron flux shape, especially during transient conditions where production and removal rates differ.

Question 16

What is a Xenon Oscillation?

Answer 16

A xenon oscillation occurs when the xenon concentration changes at different rates at different core locations, usually the top and bottom halves of the core.

During a reactor startup and power ascension, neutron flux will initially move from the bottom of the core to the top of the core as rods are withdrawn. During the continued ascension to 100% power, the reactivity effect of moderator temperature causes the equilibrium neutron flux to shift towards the bottom of the core.

Equilibrium iodine and xenon concentrations are directly proportional to the local neutron flux level, so they would be greater in the bottom half of the core as well.

Question 17

State the characteristics of Samarium 149 as a fission product poison.

Answer 17

Sm-149 has a half-life of 1016 years and is considered stable.

Therefore, the removal of Sm-149 is by neutron capture, converting it to Sm-150.

The microscopic cross section for absorption is 4 × 10⁴ barns. Sm-150 is stable and has a low absorption cross section (103 barns).

Question 18

Describe the production of Samarium-149

Answer 18

Negligible amounts of Sm-149 are produced directly from fission. However, 1.1% of all fissions result in the production of either neodymium (Nd-149) or promethium (Pm-149).

Sm-149 is the end product of the decay chain containing Nd and Pm: Since the half-life of Nd-149 is relatively short (1.7 hrs) when compared to the half-life of promethium (53 hours), it is a fairly accurate approximation to say that 1.1% of all fissions yield Pm-149 directly.

The production of Samarium-149 is governed by the equation multiplying the probability of a Pm-149 fission product times the macroscopic cross section for fission of the fuel times the nuetron flux. The Pm-149 then decays to Samarium-149 based taking the decay constanct for Pm-149 times its nuclear densiy.

Question 19

Describe the removal of Samarium-149.

Answer 19

The removal of Sm-149 is by neutron capture, converting it to Sm-150. The formula is the atomic density of Sm-149 time the microscopic cross section of absorption for Sm-149 time the neutron flux.

Question 20

Define equilibrium samarium.

Answer 20

The removal term for Pm-149 is the production term for Sm-149. At equilibrium, the production of Pm-149 equals the removal of Sm-149

In the end, the equilibrium density of Sm-149 does not depend on neutron flux. It is the probability of a Pm-149 fission fragement times the macroscopic cross section for fission of the fuel divided by the microscopic cross section of absortion for Sm-149.

The at-power reactivity worth of quilibrium samarium is about -1.0%k/k, regardless of power.

Question 21

Plot the curve for reactivity insertion vs. time for Samarium-149 during reactor startup.

Answer 21

Question 22

Explain the reason for reactivity insertion by Samarium-149 vs. time for initial reactor startup and ascension to power.

Answer 22

When the reactor is restarted, neutron flux begins burnout of Sm-149 immediately, but due to the long half life of Pm-149, Sm-149 formation is delayed. Sm-149 decreases below its equilibrium level in about one day, at full power.

The burnout continues, and the Sm-149 concentration continues to dip below the equilibrium concentration.

The concentration of Sm-149 remains below the equilibrium level for about one week, until the Pm-149 decay returns the Sm-149 concentration to equilibrium levels.

Question 23

Plot the curve for reactivity insertion vs. time for Samarium-149 during reactor shutdown.

Answer 23

Question 24

Explain the reason for reactivity insertion by Samarium-149 vs. time for reactor shutdown.

Answer 24

Sm-149 peaks after a reactor shutdown. The shutdown causes the Sm-149 burnout to stop, but the decay of the Pm-149 continues.

Essentially all the Pm-149 decays to Sm-149 after about 12.5 days, and the Sm-149, concentration stops increasing. The concentration of the Sm-149 remains constant until reactor power operations begin again.

Even though the operating value of equilibrium samarium is always -1.0% k/k, the peak value after shutdown does vary, based on power prior to shutdown.

Samarium peaks higher after a shutdown from higher power, due to a greater amount of Pm-149.

Question 25

Describe the effects of power changes on Samarium-149

Answer 25

The equilibrium value for the samarium concentration does not depend upon the flux, and therefore, does not depend upon power level.

The at-power reactivity worth of equilibrium samarium is about -1.0%k/k, regardless of power.

Question 26

Compare the effects of Samarium-149 and Xenon-135 on reactor operations.

Answer 26

Sm-149 is a much smaller operational problem than Xe-135. The negative reactivity value of samarium is much smaller, and the changes occur over periods of days rather than hours.

The negative reactivity value of Xe-135 at equilibrium is nearly three times that of Sm-149 at 100% power.

The negative reactivity value of Xe-135 at its peak is nearly as much as the positive reactivity added to the core to operate the reactor for a year (~5% k/k).

Samarium reaches a peak after reactor shutdown and remains at that peak, since it does not decay. Xenon reaches a peak after reactor shutdown and slowly decays after reaching the peak. The result is that samarium is not seen as the operational problem that xenon is.

Question 27

What factors in the six factor formula do fission product poisons impact?

Answer 27

Fission product poison buildup reduces the thermal utilization factor.