MSRE prompt supercriticality event, looking for information

[mesa]

I am looking for information on a supposed prompt supercriticality event that occurred at ORNL while the MSRE reactor was running under Syd Ball. This incident was told to me by an engineer that is actively involved in reigniting interest in MSR based reactors.

According to his summary, "Syd Ball took the MSRE prompt supercritical. It was no big deal. It ran up, and came right back down. Strong negative temperature coefficient.".

A prompt supercritical event seems to me like it would occur much too rapidly to allow for even the highest feasible negative temperature coefficients to stand a chance of keeping the reactor together as the heat would still be localized as the prompt neutron generation would grow out of control. Perhaps the neutron multiplication was very very low, or perhaps this was simply an exaggeration; regardless more information could shed some light on this possible mishap.

If this did indeed occur, it would be fascinating to know more. I tried reaching out directly to Syd with no luck at this time. If anyone has any information, please share it.

 

[Astronuc]

"prompt supercritical" is a state that is typical unallowed in a nuclear reactor, and would usually connote an accident or unintentional excursion. Supercritical means the multiplication ratio, k, is greater than unity (1). This allows power to increase in a controlled manner, with the multiplication in the neutron population dependent on the fraction of delayed neutrons. A prompt (super)criticality event implies that the reactor is supercritical or critical on prompt neutrons, which is a forbidden or unallowable state for a commercial power reactor.

I've heard that MSRE had a criticality event, but I have not seen a written report on the matter.

 

[mesa]

"prompt supercritical" is a state that is typical unallowed in a nuclear reactor, and would usually connote an accident or unintentional excursion...

...A prompt (super)criticality event implies that the reactor is supercritical or critical on prompt neutrons, which is a forbidden or unallowable state for a commercial power reactor.

My thoughts as well. This is a significantly smaller research reactor compared to a commercial unit which would ease the repercussions for such an event, but it still seems problematic, although it may respond similarly to a TRIGA reactor in a pulse?

I've heard that MSRE had a criticality event, but I have not seen a written report on the matter.

I've looked for more information as well, but also to no avail.

 

[Astronuc]

but it still seems problematic, although it may respond similarly to a TRIGA reactor in a pulse?

It's difficult to say without knowing several design parameters.

Here are some examples of TRIGA pulses at different reactivity insertion levels, the highest value was $2.00, which is pretty substantial. A lot depends on the mass of fuel, enrichment, moderation, Doppler feedback, type of coolant and its thermophysical properties, . . .

 

[etudiant]

Fascinating. I'd no idea that the TRIGA could be made to pulse like that.
Just underscores how much more there is in even simple, safe and presumably well characterized nuclear designs.

 

[jim hardy]

You'd probably enjoy reading Freeman Dyson's "Disturbing the Universe" .
It's autobiographical and describes his work from Manhattan Project to General Atomic's HTGR . He advocates fuel with moderating hydrogen in the same molecule because the stabilizing temperature feedback is near instantaneous.
I loved the book. So far as i know he's the only remaining Manhattan project physicist. I sent him a birthday card last year.

from http://www.ga.com/triga-fuels

Low-enriched, long-lifetime uranium zirconium hydride (UZrH) fuel is the fundamental feature of the TRIGA® family of reactors that accounts for its widely recognized safety, rugged, dependable performance, economy of operation, and its acceptance worldwide. The large prompt negative temperature coefficient of reactivity characteristic of UZrH fuel results in safety margins far above those achieved by any other research reactor fuel. Large reactivity insertions are readily accommodated and are routine operation for some applications. Inadvertent reactivity insertions have been demonstrated to produce no fuel damage in TRIGA cores. Power coast-down from full power after loss of forced flow cooling (and resultant power scram) has been demonstrated to be a very benign event with the reactor immediately available to return to full power.

https://www.goodreads.com/book/show/134225.Disturbing_the_Universe
see also http://www.sns.ias.edu/dyson

old jim

 

[etudiant]

Thank you for this fascinating bit of information. I'd no idea that the TRIGA was designed with a special fuel.
Presumably the UZrH is too costly for more general use in power reactors, even though it appears to be a much more tolerant fuel.

Separately, very much agree that Freeman Dyson is one of our authentic 'Living National Treasures'.
He is of course first and foremost a world class scientist, but to me, he and Ted Taylor were by far the most credible and most inspiring voices of the nuclear community.

 

[Astronuc]

Presumably the UZrH is too costly for more general use in power reactors, even though it appears to be a much more tolerant fuel.

General Atomics summarizes numerous advantages of TRIGA fuel as compared to aluminum clad fuel for research reactors. http://www.ga.com/triga-advantages

GA states, "High-temperature strength and ductility of the stainless steel or Alloy 800 fuel cladding provides total clad integrity at temperatures as high as 950°C. The aluminum cladding on plate-type fuel melts and fails at about 650°C." So the fuel uses stainless steel or Alloy 800 (Incoloy 800), which would not be suitable for power reactors since that would require greater enrichments. The UZrH fuel is more expensive, partly because it is higher enrichment, and it is not a high volume commodity.

Also, TRIGA reactors generally operate at lower temperatures and lower pressures than commercial reactors. TRIGA reactors I've seen operate in a pool, without a pressure vessel, and were cooled by natural convection. I would expect fission gas release and fission product behavior would be different if UZrH was operated like power reactor fuel.
https://ansn.iaea.org/Common/docume...ety and Technology)/chapter3/mainsystem21.htmhttps://ansn.iaea.org/Common/docume...ty and Technology)/chapter3/mainsystem221.htmhttps://www-pub.iaea.org/MTCD/Publications/PDF/trs482Web-94435407.pdf
Keep in mind a large power reactor, like a Westinghouse 17x17 plant with 193 fuel assemblies (and 4 loops) was originally rated at ~3411-3415 MWt, as compared to a TRIGA core designed for 250 kW, or later larger units at 1-2 MW. So power reactor cores are very different from TRIGA research reactor cores.