How Did Hanford's 100-B Reactor Overcome Xenon Poisoning During WWII?

[mheslep]

I'm just finishing the Rhodes history, "The Making of the ..." , and am curious about the resolution of the Xenon poisoning problem after it was first discovered in the first production reactor at Hanford, the 100-B pile.

According to Rhodes, shortly after starting the reactor for the first time they discovered the reactor dropping sub-critical every few hours due to Xenon poisoning, and the power up again as the Xenon decayed. The problem was resolved by adding another ~500 Uranium slugs, which overcame the neutron losses.

My question is, with the additional U slugs, I assume power levels would still cycle up and down with Xenon production while staying critical, so how was control accomplished at that time with the B-pile? Apparently Fermi used Cadium rods in the Chicago experimental pile. Were these also used with the B-pile, and would they then need to be constantly pushed and pulled in and out of the pile, manually, to maintain constant power?

 

[SteamKing]

This paper discusses the Xe-135 poisoning of the Hanford B reactors, including details of discovery of the problem and the fixes put in place to overcome it:

http://ww.c-n-t-a.com/srs50_files/049roggenkamp.pdf

http://en.wikipedia.org/wiki/Neutron_poison

A discussion of early reactor features is here:

http://www.osti.gov/manhattan-project-history/Events/1942-1944_pu/reactor_design.htm

Cadmium control rods are more than a means of control: they are an important safety feature.

The Chicago Pile CP-1 was a small reactor intended to demonstrate that a chain reaction could be created and sustained. It was an extremely low power reactor (about half a watt) which did not have any radiation shielding or cooling system. The Hanford production piles were much larger reactors than CP-1, which is probably why the xenon poisoning problem first surfaced during their operation.

 

[Astronuc]

The Hanford reactors apparently had horizontal shim rods for reactivity control.

"HCRs - Horizontal Control Rods - Elements of control consisting of long cylindrical or rectangular aluminum shells containing boron or cadmium, as well as cooling passages for carrying water into and out of the rods. Each Hanford reactor was designed with a particular number that could be inserted into and/or withdrawn from the reactor to control reactivity on an everyday basis. See also: Shim Rods."

"Shim Rods - A Hanford term for the hydraulically driven HCRs of the oldest production reactors. Some of the rods were electrically driven, but this term referred only to those that were hydraulically driven. They were used in startup and to offset long-term reactivity changes. See also: HCRs."

Ref: http://www.b-reactor.org/histglos.htm

Xe-135 has a high absorption cross-section in the thermal energy range, i.e., neutron energies < 0.1 eV. By shifting the neutron spectrum to higher (epithermal) energies, the Xe-135 poisoning effect can be diminished. The uranium slugs could have done that, but they also increase the amount of uranium. If they operated at low power, then the build-up of Xe would be low, and the U-slugs become a 'neutron reflector'. Increasing the enrichment of U-235 in the uranium, would also have an effect.

"When the B-Reactor was started with a 1500-tube cylindrical geometry, the effect of xenon poisoning caused the reactor to shut down. The correction was made by adding more nuclear fuel in the 500 peripheral tubes. The added neutron source overcame the xenon poisoning."
Ref: http://files.asme.org/ASMEORG/Communities/History/Landmarks/5564.pdf

Alternatively, removing control elements would also serve to overcome the Xe-135 effect, but then one would likely see some variation in axial and radial power distributions.

See also - http://wcpeace.org/history/Hanford/HAER_WA-164_B-Reactor.pdf - sections 3.2.2 - 3.2.4.

 

[mheslep]

...
Increasing the enrichment of U-235 in the uranium, would also have an effect.

Right, this was the original *natural* Uranium graphite moderated reactor (production version after Chicago). There was no enrichment to speak of on the planet when they lit it up. I'm curious about how they managed control under the conditions of the day. If the Cadium shims/rods were the only control method as you say, I assume the answer is they inserted/removed them in tandem with the production of Xe.

...Alternatively, removing control elements would also serve to overcome the Xe-135 effect, but then one would likely see some variation in axial and radial power distributions.

Not in that original reactor with the original set of U slugs, prior to the additional U slugs/tubes. They pulled out everything and it still went subcritical until the Xe decayed away.

 

[Morbius]

My question is, with the additional U slugs, I assume power levels would still cycle up and down with Xenon production while staying critical, so how was control accomplished at that time with the B-pile? Apparently Fermi used Cadium rods in the Chicago experimental pile. Were these also used with the B-pile, and would they then need to be constantly pushed and pulled in and out of the pile, manually, to maintain constant power?

mheslep,

The other thing you can do is to wait a day before restarting the reactor.

Xe-135 has a half-life of 9 hours. So after 18 hours; the Xe-135 amount will be down 75%

Greg

 

[mheslep]

mheslep,

The other thing you can do is to wait a day before restarting the reactor.

Xe-135 has a half-life of 9 hours. So after 18 hours; the Xe-135 amount will be down 75%

Greg

Which is what they discovered when first starting up the Hanford pile. But with the war ongoing running a day on, day off was not appealing, thus the addition of the other ~500 uranium slugs to maintain criticality in the presence of Xe-135.

I'm curious to know what they actually did at that time to regulate the pile given the Xe production. I assume that Xe production would reach an equilibrium and that therefore they could find an equilibrium point with the insertion of the cadmium fixtures.

 

[Morbius]

Which is what they discovered when first starting up the Hanford pile. But with the war ongoing running a day on, day off was not appealing, thus the addition of the other ~500 uranium slugs to maintain criticality in the presence of Xe-135.

I'm curious to know what they actually did at that time to regulate the pile given the Xe production. I assume that Xe production would reach an equilibrium and that therefore they could find an equilibrium point with the insertion of the cadmium fixtures.

mheslep,

Yes - there is an equilibrium level of Xe concentration in the operating reactor. However, it is power-level dependent. The source of Xe-135 is its I-135 parent, and the source of that is fission, hence the rate of I-135 production, and the equilibrium I-135 level is power level dependent.

There are two sinks for Xe-135; one is power level independent; it's radioactive so it decays. The other is that it is "burned up" via neutron capture, and that is power-level dependent.

Greg