Penn State research reactor.

[Higgs]

Who knows how the Penn State test reactor works? I saw a video of them "pulsing" it and was curious as to the mechanism that causes and then stops the pulse. I am assuming that some sort of rapid insertion of positive reactivity occurs to cause the pulse but then Doppler broadening stops the reaction.

Thanks for the help.

Justin

[Higgs]

Hmm if you watch closely at 9-10 seconds, you can see what appears to be their control rod going back in, which is clearly after the reaction was already mostly over. I am liking my hypothesis.

[matthew.b]

It quits in milliseconds because the fuel gets too hot and the reactor goes sub-critical.  Remember these things work in a cool pool.  They only have enough reactivity to reach criticality while cold.  Once they reach temperatures that would seem normal for power reactor guys they no longer a capable of reaching criticality.

One of the control rods has a compressed air cylinder on it.  They bring the reactor up to a few watts of power and then pop the rod out with compressed air.  It is one of the coolest things I've watched.

[Higgs]

It quits in milliseconds because the fuel gets too hot and the reactor goes sub-critical. 

That is what I said, Doppler broadening.

One of the control rods has a compressed air cylinder on it.  They bring the reactor up to a few watts of power and then pop the rod out with compressed air.  It is one of the coolest things I've watched.

Thank you! That explains the 2 distinct "cuh chunks" and what I saw at 9-10 seconds.

Justin

[dselil]

TRIGA Reactor information. 

http://www.ga-esi.com/triga/about/index.php

[Higgs]

TRIGA Reactor information. 

http://www.ga-esi.com/triga/about/index.php

Thank you for that! Very interesting. Do you have any more technical information regarding this;

"In meeting this challenge, the idea of the "warm neutron principle" was introduced as a first step towards the design of an inherently safe reactor. In a water-cooled reactor, the general result from suddenly removing the control rods is a catastrophic accident, leading to a melting of the fuel. This is because the neutrons from the fission reaction remain "cold" from interacting with the cold water around the fuel and maintain their ability to cause further fissioning of uranium atoms in the fuel. This in turn results in the temperature of the fuel continuing to increase rapidly until it finally melts. However TRIGA is no ordinary light water reactor because much of its "moderation" of neutrons is due to the hydrogen that is mixed in with the fuel itself. Therefore, as the fuel temperature increases when the control rods are suddenly removed, the neutrons inside the hydrogen-containing fuel rod become warmer than the neutrons outside in the cold water. These warmer neutrons inside the fuel cause less fissioning in the fuel and escape into the surrounding water. The end result is that the reactor automatically reduces power within a few thousandths of a second, faster than any engineered device can operate. In other words, the fuel rods themselves act as an automatic power regulator, shutting the reactor down without engineered devices."


Thanks!

[Higgs]

Nevermind, wiki was surprisingly informative about this "warm neutron principle." So my hypothesis of doppler broadening appears to be incorrect, at least as far as what the prime moderator is.

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

And similarly;

http://en.wikipedia.org/wiki/Hydrogen_Moderated_Self-regulating_Nuclear_Power_Module

[Pman52]

I for one find this incredibly interesting...

The test reactors like these can have such a huge impact on nuclear technology and learning more about what we can do to make the future plants safer in the near future.  Engineering a reactor that has the attributes of a safe and practical design with the appropriate redundancies are what we need in the very near future.  Defense in depth coupled with self-sustaining emergency backup systems like natural circulation cooling that can remove decay heat in the case of a SBO will make our future plants safer.  Nuclear power just intrigues the hell out of me.  I just wanted to say I enjoyed this post.  Thanks Justin.

[Higgs]

You're welcome and I agree, it is very intriguing. I love reading about new things coming down the pike. Although this little reactor is old, there is still much to learn from it outside of "mainstream" nuclear power. Until I started studying this reactor, I only realized 3 neutron energy levels (fast, epithermal, thermal)..., now I am reading about "ultra cold" neutrons, etc.

[Higgs]

Oh, and I've found something else that suggests it IS doppler broadening..., so now I am going straight to the source and emailing some SROs there.

[Pman52]

You're welcome and I agree, it is very intriguing. I love reading about new things coming down the pike. Although this little reactor is old, there is still much to learn from it outside of "mainstream" nuclear power. Until I started studying this reactor, I only realized 3 neutron energy levels (fast, epithermal, thermal)..., now I am reading about "ultra cold" neutrons, etc.

I just enjoy learning more about fission and how we control it and harness it's power.  Its just an amazing process and the amount of technicalities involved just makes it mind boggling how we've come so far with it from the days of chicago-pile 1 to now.  Light water reactors are very interesting in the least, but a while back I was reading more about HTGRs and how the idea was abandoned and the existing plants decommissioned, like Peach Bottom Unit 1.  I understand the reactors were very complex with many different systems that would require a tedious and expensive maintenance plan, but still have to wonder if they were operating today how reliable and efficient they would be and how much safer nuclear power would be perceived by the general public.

Not to derail the topic at hand though...

[Higgs]

More info on these Triga reactors. This is where it is crediting doppler.

http://www.rcp.ijs.si/ric/pulse_operation-s.html

[Higgs]

So these pulses are making the reactor go prompt critical, self moderates, then scams? Am I reading this correctly?

Whats the purpose of this reactor?

Make cool youtube videos.

[Higgs]

So I sent an email to the director of the University of Arizona Triga reactor to get some clarification on what is going on. I provide the link I posted earlier so he knew what I was looking at. Here is his response;

"Justin,

The information in the link you sent is reliable, though the model he
describes is known to me as the Fuchs-Nordheim model, not Fuchs-Hanson.

There is a small Doppler broadening effect in TRIGAs, the reduces the
resonance escape probability as in other thermal reactors, but that is not the main
contribution to the negative temparature coefficient. The main contribution
comes from a spectral shift of the thermal neutrons. Because most of the
moderator is homogeneously mixed in the fuel, its temperature is the same as
the fuel temperature. Therefore a rise in fuel temperature increases the
neutron temperature of the Maxwellian part of the spectrum. Because the
fission cross section in uranium is approximately inversely proportional to neutron
speed, the higher the neutron temperature, the less the effective
fission cross section. This reduces the thermal utilization, eta, and hence reduces keff.
There is no comparable effect in reactors with the moderator separate from the
fuel."


Justin

[Llama]

At first I was a little confused with the response you got Justin. The Maxwell-Boltzmann distribution deals primarily with particle speeds in gases and at first glance would seem to be in conflict with TRIGA reactors since they are a pool type reactor. After a little research however (amazing what research can do) I found that the neutron moderation is primarily taken place in the hydrogen which is mixed in with the fuel. It is this configuration that allows for the "warm neutron principle", i.e. Maxwell-Boltzmann, to provide the prompt negative temperature coefficient of reactivity. In other words as temperature increases the speed of the neutron increases and the fission cross-section of the fuel decreases allowing the neutrons to escape to the water surrounding the fuel versus causing fissioning.

[Higgs]

At first I was a little confused with the response you got Justin. The Maxwell-Boltzmann distribution deals primarily with particle speeds in gases and at first glance would seem to be in conflict with TRIGA reactors since they are a pool type reactor. After a little research however (amazing what research can do) I found that the neutron moderation is primarily taken place in the hydrogen which is mixed in with the fuel. It is this configuration that allows for the "warm neutron principle", i.e. Maxwell-Boltzmann, to provide the prompt negative temperature coefficient of reactivity. In other words as temperature increases the speed of the neutron increases and the fission cross-section of the fuel decreases allowing the neutrons to escape to the water surrounding the fuel versus causing fissioning.

Yep that was my confusion as well, which I explained to him in my email. So yes, without realizing what you mentioned, his response would be confusing because it is really a half answer. Like I said though, I asked half a question.

Justin

[Higgs]

More research is good I suppose.

"The pulsing feature of UZrH fueled reactors, first demonstrated in this prototype TRIGA at General Atomics, are standard among many TRIGA reactors, and special designs of pulsed TRIGA's in use today routinely achieve power levels of 22,000 MW to test the safety of fuels for nuclear power reactors."

Holy cow! 22,000 MW O.o

Do/Can these reactors "pulse" continuously? Seems like a good idea for a BWR type reactor.  Would it have a continuous electrical output in a system?  

http://www.ga-esi.com/triga/about/index.php

Yes, they can pulse, or run somewhate continuously. No, it isn't a good idea for a BWR type reactor.

And, US installations.

http://www.ga-esi.com/triga/about/install_usa.pdf


If you dig further, like I have, and look at some of what they are used for, you will find many things from testing how materials react to neutron bombardment to irradiating sources for economic gain.

[Higgs]

how would they produce steam then? seems like in a pwr youd have to have ridiculous pressures to keep it from flashing to steam at 22,000 MW O.o

You grossly misunderstand the operation of this reactor, and need to study it more. Not trying to be a douche. I simply don't have the time or desire to spill everything I've learned the last few days.

This is a pool reactor, not a bwr and not a pwr. It has very little in common with commercial or navy reactors.

[matthew.b]

Its purpose is a neutron source.  There are all sorts of chemical and biological experiments that can be aided by activation of samples.

[fourteener]

From my undergraduate days (Class of '83) at PSU (BS-NE), the pulsing of the reactor was part of the curriculum.  We were not allowed to pulse the reactor and not allowed to prevent the control rods from dropping (gravity) back in.  Chain pulsing (allowing the reactor to shut down due to negative temperature coefficient and pulse again once the fuel cooled) was not allowed.  I'm not sure if we had any choice - or at least the staff did not communicate any choices to us.  Bottom line, one pulse and we had to let the reactor cool (about 15 minutes ).  In one experiment, we would pulse the reactor, measure the delta t (time) between the pulse and the first visible signs of boiling (departure from nucleat boiling - DNB) on the surface of the fuel cladding (viewed with binoculars).  From this information, we calcualted the gap between the fuel and the cladding.  The calcualtions were pretty close too, as I remember it.  I videotaped the pulsing with 8mm film.  The posted video is an exact replica of what I remember.