How do metal alloy fuel fast reactors respond to rapid reactivity insertion events?

Update: (Posted Feb 21, 2015 at 7:22) The title has been modified after initial discussion indicated it was incomplete. Other related updates are in blue font.

Fast neutron spectrum reactors offer one answer to the trump question that is often used to halt informative discussions about using more atomic energy to reduce our excessive dependence on burning hydrocarbon based fuels — “What do you do with the waste?”

In a fast neutron spectrum, very little of the long-lived radioactive material is unusable waste; instead, it becomes a valuable fuel resource. There are detailed explanations available on Atomic Insights and throughout the web that explain why that statement is true and why it is so important to mankind’s future prosperity.

The simple version is that we currently produce about 11-12 million barrels of oil per day equivalent in nuclear plants that use only 0.5% of the initial potential energy of mined uranium. The rest of the mined uranium is often misclassified as “nuclear waste” because we are not putting it to good use. It is being stored as either depleted uranium or as the major component of reusable nuclear fuel — a valuable energy reservoir that many publications still refer to as “spent nuclear fuel.”

Fast spectrum reactors offer the long term potential to use much closer to 90-99% of the potential energy, consuming the portion sometimes demonized as “waste.” In the process, they produce a stream of byproducts that might have commercial uses, but even if they are not used, they have half lives that are generally substantially less than 30 years. That means they decay away in less than a few hundred years rather than in tens of thousands of years.

Though there is plenty of refinement remaining before fast reactors can be widely used as commercial power plants, their technical potential is incredibly exciting for those of us who believe that human society needs as many sources of reliable, abundant, ultra-low pollution power as our creative minds can invent.

I like providing power to the people; my interest in nuclear energy stems from my belief that we should use “the best of the above” for a given energy application like propelling ships or generating industrial heat in preference to equal measures of “all of the above.”

The above paragraphs are background information for the remainder of this post.

On numerous occasions when Atomic Insights has mentioned using fast spectrum reactors as an additional tool in our quest for improving society’s power options, there have been comments that question their response in the event of rapid reactivity insertion accidents. They fear that fast reactors can suffer failures that carry the risk of harming the public.

Some of the commenters pose their reservations in a credible way, suggesting that they have done some serious research and been unable to reassure themselves that there are good answers. Because they have been unable to find answers to questions that are worth worrying about if unaswerable, they are adamantly opposed to fast reactors. Here is a concluding quote from a note submitted by Bill Hannahan.

Supporters of solid fueled fast neutron reactors have not published computer modeling of a high reactivity rate accident in a full sized core. Have they no curiosity? I suspect it is because they know or suspect that the results would terrify the public, and their beloved IFR would never be built.

I really enjoyed reviewing the Bethe calculation because he writes like a brilliant teacher who wants to share his knowledge with others. He lays out all of his assumptions and simplifications, AND, explains the underlying reasons each of them.

NUREG 1368 is the exact opposite. The initial conditions, the assumptions, the sequence of events are all hidden away, we must just take their word on everything.

I wish we could bring back Bethe in his prime and give him the knowledge and the tools we have today to investigate criticality accidents in a full size IFR. I think he would say ‘forget the IFR, mass produce simple MSR’s for this century and design breeder MSR’s for the long run’.


1… If a slow gravitationally induced, low reactivity rate accident in a 91 liter reactor yields 160 kg TNT equivalent, what would the yield be for a 6,000 liter core given the same reactivity profile?

2… What would the yield be for a 6,000 liter core given a high reactivity rate accident?

3… Why don’t proponents model a high reactivity rate accident?

4…Why don’t proponents model multiple criticality accidents or prove that they are impossible?

5… Do you agree that a higher reactivity rate of insertion will increase yield resulting in more prompt fatalities?

I’ve found answers to those questions for a specific metal alloy fuel design that I hope will reassure both the specific people who have expressed their concerns to me and anyone else who might have heard or read something about the issue from other people who have been unsuccessful in finding answers.

I’ve be aided in my quest by a number of experts who have done ground-breaking work in the field, including Yoon Chang, Dan Meneley, John Sackett, Neil Brown and Arthur Goldman. Though I gratefully acknowledge their assistance in providing answers and technical document references, this post is my own interpretation of the material, which is applicable to the alloy fuel used in the EBR-II as part of the Integral Fast Reactor project. If I get anything wrong, it’s my responsibility, not their’s.

The first part of the response is to remind Mr. Hannahan that Bethe’s 1956 vintage Hypothetical Core Disruptive Accidents calculation was based on an assumed fuel form, not a real one that had been refined based on years worth of fuel testing to be more resilient to various influences like overheating, excessive reactivity addition rates, and insufficient cooling.

The modern fuel form that was developed for the IFR included such innovations as sodium bonded metal alloy inside multiple tubes covered with a protective cladding. The sodium bonding provides good heat transfer paths in lightly used fuel, while being able to move to allow the fuel to expand inside the cladding as it grows under irradiation.

Source: Page 122 of Plentiful Energy

Source: Page 122 of Plentiful Energy

Each tube has a plenum above the fueled portion of the rods to allow space for fission product gases to accumulate and to provide a place for sodium to migrate as it is squeezed out by the expanding fuel alloy. In addition to the refined, resilient design of the fuel pins, the core is protected against seismic compaction by both a heavy structure lower grid plate and two sets of seismic load pads along the length of the hexagonal ducts that constrain radial tube movement during seismic events.

The combination of those features is called the core restraint system; it physically keeps fuel rod spaced apart to limit the maximum reactivity insertion under even beyond design basis events.

Of course, those design features cannot just be assumed to work if you want to actually build such devices in the United States. They had to be tested in ways that were adequate to convince both skeptical, questioning-attitude designers and regulators.

A series of tests were performed in a facility called TREAT (Transient Reactor Test Facility) between 1984 and 1987. These tests were carefully designed to determine the behavior of the proposed fast reactor fuel forms in the event of severe overpower conditions caused by rapid reactivity insertion. They demonstrated consistent behavior that would not be altered by different methods of inserting reactivity. (Ref: Behavior of Modern Metallic Fuel in TREAT Transient Overpower Tests – 4 MB PDF.)

The tests were performed with an exponential power increase at an 8 second period. (A reactor period is the time it takes for power to increase by a factor of e – roughly 2.72). Destructive analysis of the fuel rods was performed to determine their physical responses to the overpower condition and their mode of failure.

The tested rods withstood power that was about 4 times the operational maximum before they failed. Before reaching a point where the cladding failed, the rods expanded in the axial direction with fuel moving towards the plenum that held fission product gases. As the fuel began to melt inside the cladding tubes, fission product gases that had been dispersed in the fuel matrix pushed the fuel inside the cladding in an extrusion response that would be familiar to a cake decorator.

Fuel extrusion was greater than would have been caused by thermal expansion. It inserted a large amount of negative reactivity, limiting the peak power achievable. The cladding failures occurred at the hottest point in the fuel rods – which was at the top of the fuel column or in the plenum just above the fuel column. The pressure of the molten fuel tended to squeeze the fuel into the flowing sodium, resulting in even more dispersal and more negative reactivity insertion.

All of the fuel movements that resulted from the overpower experiments moved fuel outward from the reactor center, reducing reactivity and limiting peak power. None of them moved fuel closer together.

Here is a quote from Chuck Till and Yoon Chang’s Plentiful Energy, an excellent resource for people who are interested in a summary of the lessons learned during the 20-year-long IFR research and development effort. Plentiful Energy is also available in PDF format.

The TREAT tests demonstrated this further dispersal when fuel is molten. Both the pre-failure extrusion and the post-failure dispersal found in these tests provide intrinsic large negative reactivity feedbacks. They terminate overpower transients no matter what their cause.
(Emphasis added.)

So, going back to the questions that Bill Hannahan asked:

1… If a slow gravitationally induced, low reactivity rate accident in a 91 liter reactor yields 160 kg TNT equivalent, what would the yield be for a 6,000 liter core given the same reactivity profile?

2… What would the yield be for a 6,000 liter core given a high reactivity rate accident?

ANSWER: The reactor core design assumed for that hypothetical event bears no resemblance to a modern fast spectrum reactor. There is no yield, even in the event of a rapid reactivity insertion. It’s possible to experience core damage, but the fission products are retained by the coolant, the coolant container, and the containment.

3… Why don’t proponents model a high reactivity rate accident?

ANSWER: They do, and they base that model on physical test results from a series of carefully designed experiments.

4…Why don’t proponents model multiple criticality accidents or prove that they are impossible?

5… Do you agree that a higher reactivity rate of insertion will increase yield resulting in more prompt fatalities?

ANSWER: The test results show that overpower conditions are limited by strong negative reactivity feedback. They disperse fuel; they do not concentrate it as is done in an implosion device. There is no yield and there are no prompt fatalities. In fact, there are also no delayed fatalities; fission products are retained in the vessel.

The public can be adequately protected from radioactive material releases from fast spectrum reactors even in the event of a rapid reactivity insertion. Those results shouldn’t terrify anyone; they should result in celebratory rejoicing.

Physical testing at the IFR in April 1986 — mere weeks before Chernobyl — also demonstrated passive safety in the event of a loss of cooling flow without a reactor scram and in the event of a loss of all electrical power. It is demonstrably possible to design and build safe fast spectrum reactors.

I’m not in favor of a crash program to build them, but I strongly support implementing policies that enable their near term deployment, including favorable investment tax credits, accelerated depreciation appropriate for early generation machinery, and dedicated regulatory resources that can provide effective and efficient oversight of this specialized branch of nuclear reactor technology.

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