Metalic Uranium as a Fuel: Reactor Types and Disadvantages

[The Prince]

Hey there ,

It's known that metalic U was uesed as a fuel in certain types of reactors. What I 'd like to know :

1-Which type of reactor use metalic Uranium ?
2-Is Uranium metal used nowdays as a fuel?
3-what are the disadvantages of using U as a metal beside reacting highly with water ?

 

[Morbius]

Hey there ,

It's known that metalic U was uesed as a fuel in certain types of reactors. What I 'd like to know :

1-Which type of reactor use metalic Uranium ?
2-Is Uranium metal used nowdays as a fuel?
3-what are the disadvantages of using U as a metal beside reacting highly with water ?

The Prince,

The Integral Fast Reactor [ IFR ] designed by Argonne National Labs in the '80s and '90s
used metallic Uranium as fuel. For more on the IFR see:

http://www.pbs.org/wgbh/pages/frontline/shows/reaction/interviews/till.html

http://www.nuc.berkeley.edu/designs/ifr/ifr1.html

The reason the IFR uses metallic uranium fuel is so that the reprocessing of the fuel
can be done on-site. If the fuel is metallic, metallurgical techniques; namely halide
slagging and electrorefining can be used to reprocess the spent fuel and recycle the
actinides back as fuel, as Dr. Till explains. Ceramic fuel, UO2; requires a more complex
chemical processing plant. The reason for having on-site reprocessing is to make the
IFR cycle proliferation resistant. In the IFR fuel cycle, there would be no shipments of
"weapons usable" material to / from the reactor. Any plutonium created by the IFR stays
in the high radiation area of the reactor plant. Additionally, as Dr. Till explains; the IFR
reprocessing technology doesn't create weapons usable material in the first place.

I don't know of any other reactor designs that use metal fuel. Metal fuel has the problem
of swelling under irradiation. Argonne was able to solve that problem, or at least
accommodate it in the IFR design.

Dr. Gregory Greenman
Physicist

 

[Astronuc]

There were issues with dimensional stability with metal fuel, and the lower melting point - fuel centerline melting is forbidden in commercial reactors - and the higher thermal conductivity means a lesser thermal time constant which would create a boiling and stability issue in certain transients.

No commercial plants use metal fuel. The IFR fuel was U-alloy, IIRC U-Zr.

Production reactors used slugs of U-238 for making Pu-239.

 

[Morbius]

There were issues with dimensional stability with metal fuel, and the lower melting point - fuel centerline melting is forbidden in commercial reactors - and the higher thermal conductivity means a lesser thermal time constant which would create a boiling and stability issue in certain transients.

No commercial plants use metal fuel. The IFR fuel was U-alloy, IIRC U-Zr.

Production reactors used slugs of U-238 for making Pu-239.

Astronuc,

Yes - the IFR fuel was an alloy of Uranium, Plutonium and Zirconium: See under "Safety":

http://www.nuc.berkeley.edu/designs/ifr/anlw.html

The higher thermal conductivity of the metal fuel was also part of the IFR's inherent
safety features:

http://www.nuc.berkeley.edu/designs/ifr/ifr2.html

The dimensional stability, i.e. "swelling" problem with metal fuel was accommodated
by giving the fuel some room to expand.

Commercial reactors can approach melting temperatures at the fuel centerline.
However, the UO2 ceramic fuel sinters at these high temperatures, and densifies.
The solid fuel pellet turns into a annulus with a cylindrical void in the middle. Once
that happens, one can increase the power because the limiting temperature will
be at the inside radius of the annular pellet, and not the centerline. Since the
innermost portion of the fuel is closer to the outer surface, where heat is removed;
than is the centerline; you can boost power and still not exceed temperature limits.

Dr. Gregory Greenman
Physicist

 

[Astronuc]

Commercial reactors can approach melting temperatures at the fuel centerline.

However, the UO2 ceramic fuel sinters at these high temperatures, and densifies.

The solid fuel pellet turns into a annulus with a cylindrical void in the middle. Once that happens, one can increase the power because the limiting temperature will be at the inside radius of the annular pellet, and not the centerline. Since the innermost portion of the fuel is closer to the outer surface, where heat is removed; than is the centerline; you can boost power and still not exceed temperature limits.

Commercial fuel typically does not reach temperatures that would allow a central void. Fission gas release would result in rod internal pressure issues. Fast reactor fuel on the other hand would achieve temperatures yielding centerline melt. The DOE was not contrained as are the utilities - See NUREG 0800, SRP4.2

 

[Morbius]

Commercial fuel typically does not reach temperatures that would allow a central void.

Astronuc,

Actually, central voids DO form in commercial reactor fuel. If you've ever
seen photos of "dissected" irradiated fuel - there is a central void with
cracks propagating outward.

Courtesy of the Nuclear Engineering Dept. of University of Californai - Berkeley:

http://www.nuc.berkeley.edu/thyd/ne161/mspeer/uo2fuel.html

under the heading of "Fuel restructuring":

"Pore migration to the center of the fuel occurs in this region and produces a
central void. One mechanism suggested for this migration is the vaporization
of fuel on the hotter side of existing pores and subsequent condensation on
the cooler side resulting in the migration of pores to the highest temperature
region by solid state difffusion."

The photos I've seen look very much like the following graphic from the above
UC-Berkeley article:

http://www.nuc.berkeley.edu/thyd/ne161/mspeer/cracksl.gif

Dr. Gregory Greenman
Physicist

 

[Astronuc]

Astronuc,

Actually, central voids DO form in commercial reactor fuel. If you've ever
seen photos of "dissected" irradiated fuel - there is a central void with
cracks propagating outward.

The photos I've seen look very much like the following graphic from the above
UC-Berkeley article:

http://www.nuc.berkeley.edu/thyd/ne161/mspeer/cracksl.gif

Dr. Gregory Greenman
Physicist

I know that picture from Don's book. That would certainly be accurate for FFTF or EBR-II fast reactor fuel, but not commercial fuel. I'll have to talk to Don about that picture. Some information published by universities is fairly old. Don and another professor are working on a more up-to-date textbook, which I had hoped would be finished by now.

Most of my work involves performance of commercial nuclear fuel, and I've also seen a lot of ceramographs from PIE. We don't allow central voids to form - NUREG 800 (SRP 4.2) - under normal operation. The NUREG prohibits centerline melt, 1% total strain or RIP exceeding cladding lift-off, whichever is limiting. I've done design reviews for utilities on most of the advanced nuclear fuel designs, which includes modeling under normal and abnormal conditions.

Just checking some of my calcs - a 10x10 BWR fuel rod at 13.4 kW/ft (~44 kW/m) would have a centerline temperature of about 2874°F (1578°C), and this would be less for fuel with burnups < 15 GWd/tU.

As for density, 95% is pretty much an old standard. PWR fuels run 95-96%TD and BWR fuels 96-97.5%TD these days.

 

[Morbius]

I know that picture from Don's book. That would certainly be accurate for FFTF or EBR-II fast reactor fuel, but not commercial fuel. I'll have to talk to Don about that picture. Some information published by universities is fairly old. Don and another professor are working on a more up-to-date textbook, which I had hoped would be finished by now.

Astronuc,

Perhaps my info is out of date. I'm remembering this from graduate school
in the mid to late '70s.

Nuclear materials engineering was not my forte.

Dr. Gregory Greenman
Physicist

 

[Astronuc]

Astronuc,

Perhaps my info is out of date. I'm remembering this from graduate school
in the mid to late '70s.

Nuclear materials engineering was not my forte.

Dr. Gregory Greenman
Physicist

We've gotten better over the years, but we are still surprised sometimes. These days we are pushing the limits on the technology.

I have seen central voids in failed fuel where the oxidation of the UO₂ decreases thermal conductivity by a factor of 2 or 3. In fact, I was just looking at a cross-section which has a central void. Fortunately, there aren't too many failed fuel rods out there.

I just read a paper on a Russian metal fuel with U and Mo. Looks interesting! Certain cermets may be the way to go, but the thermal time constant is an issue for core stability during transients.

 

[Morbius]

The Prince,

The Integral Fast Reactor [ IFR ] designed by Argonne National Labs in the '80s and '90s
used metallic Uranium as fuel. For more on the IFR see:

http://www.pbs.org/wgbh/pages/frontline/shows/reaction/interviews/till.html

http://www.nuc.berkeley.edu/designs/ifr/ifr1.html

I recently came across another article, actually a series, written by
Dr. Charles Till, formerly of Argonne National Laboratory:

http://www.sustainablenuclear.org/PADs/pad0509till.html

It gives some more information on the unfortunate demise of the IFR:

"The anti-IFR forces were led by John Kerry. He was the principal speaker
and the floor manager of the anti forces in the Senate debate. He spoke at
length, with visual aids; he had been well prepared. His arguments against
the merits of the IFR were not well informed—and many were clearly wrong.
But what his presentation lacked in accuracy it made up in emotion. He
attacked from many angles, but principally he argued proliferation dangers
from civilian nuclear power."

Proliferation concerns that Dr. Till in that article and in the aforementioned
Frontline article shows to be bogus.

Another nice article by Dr. George Stanford, formerly of Argonne National Lab
is at:

http://www.nationalcenter.org/NPA378.html

also addresses the non-issue of the IFR proliferation concern; as well as
addressing the reasons for metal fuel.

Dr. Gregory Greenman
Physicist

 

[theCandyman]

I recently came across another article, actually a series, written by
Dr. Charles Till, formerly of Argonne National Laboratory:

http://www.sustainablenuclear.org/PADs/pad0509till.html

From that same article in it's closing it says

The process was demonstrated successfully at small, laboratory scale. But it is a very big step to scale up to practical amounts. And this is precisely where the development was aborted; the large scale equipment was largely in place, as were the skilled personnel, but the work had not yet started.

Years later, two or three inconclusive tests were tried, but did little to settle questions of practicality.

Why were these tests performed? Curiosity or because the administration wanted to see if it would be successful (I take it that this was funded by those who cut it off)?

 

[Morbius]

Why were these tests performed? Curiosity or because the administration wanted to see if it would be successful (I take it that this was funded by those who cut it off)?

Candyman,

As Dr. Till states, the major expenses of equipment and facilities were
already built by funds provided by the previous administration.

The one or two tests could be funded by the discretionary funds that
DOE gives to the Laboratory Administration. As long as they don't spend
too much money, the Lab Director can spend funds without getting the
blessing from Washington. So the tests really weren't funded by those
that cut it off.

One doesn't discover whether something is practical or not by funding
one or two tests. There's probably a 0% chance that everything would
be "tuned up" properly on the first and second tests. No - to really
discover the practically requires a test, and an analysis of what went
wrong, then another test... and so forth.

It's not unprecendented to build a facility and not provide an operations
budget. That's what happened to MFTF-B "Mirror Fusion Test Facility B"
at LLNL. MFTF-B was a big "solenoid" whose ends where "plugged" by a
pair of "yin-yang" magnetic coils. The "yin-yang" magnet configuration
had been previously researched and found to be too "leaky" to support
fusion. However, if they were the end caps to a much larger confinement
system - the solenoid - their degree of leakage could be tolerated.

One of the yin-yang magnets is pictured at:

http://www.llnl.gov/pao/WYOP/Fusion_Energy.html

About $450 Million was spent constructing the facility. However, when it
came time to actually run and do experiments, there was no money for that.
So MFTF-B was dedicated and mothballed the same day.

The facility sat unused for a couple decades and only in the last few years
has it been dismantled and the space and materials reclaimed.

Dr. Gregory Greenman
Physicist