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Sun Dec 1, 2019, 01:23 AM

Experimental Determination of the Bare Sphere Critical Mass of Neptunium-237.

The paper I'll discuss in this post is this one: Criticality of a 237Np Sphere (Rene Sanchez et al., Nuclear Science and Engineering, Nuclear Science and Engineering, 158:1, 1-14 (2008)).

Neptunium is the only actinide element that is easy to obtain in an isotopically pure form simply by chemically isolating it. This is because all of the isotopes except Np-237, which has a half-life of 2,144,000 years, that are known and which form readily in thermal spectrum nuclear reactors - which represent almost all of the world's commercial nuclear reactors - are short lived. The half-life of Np-238, the parent of plutonium-238 is 2.117 days, and the half-life of Np-239, the parent of plutonium-239 is 2.356 days. Thus even in a continuous on line isolation system from a critical nuclear fluid of the types now under discussion, chiefly molten salt type reactors, any isolated neptunium would decay, with a few weeks time to essentially pure Np-237.

Neptunium is routinely formed in the operation of commercial nuclear reactors. In thermal reactors, neptunium has a high neutron capture cross section and its fission is rare. Chiefly it is transmuted into plutonium-238, the accumulation of which has the happy result, in high enough concentrations (albeit not necessarily routinely formed concentrations), to make reactor grade plutonium that is essentially unusable in nuclear weapons. (As a practical matter, it is much easier to make nuclear weapons from natural uranium by separating the U-235 than it is to make it from reactor grade plutonium, and since it is impossible for humanity to consume all of the natural uranium on the planet, it will never be possible to make nuclear war impossible.)

In a fast neutron nuclear spectrum, neptunium can form a critical mass, and thus can be utilized as a nuclear fuel (or in theory, a nuclear weapon).

I personally favor fast spectrum nuclear reactors, since they represent the potential to ban all energy related mining, dangerous natural gas wells, fracked and "normal," dangerous petroleum wells, fracked and "normal," all the world's coal mines, and in fact, all of the world's uranium mines for many centuries to come, utilizing the uranium already mined and the thorium already dumped by the lanthanide industry.

The so called "minor actinides," generally including neptunium, americium, curium and sometimes berkelium and californium, all have useful properties; there has been a lot of discussion in the scientific literature of using neptunium and americium as constituents of nuclear fuels, to eliminate the often discussed, but entirely unnecessary waste dumps for the components of used nuclear fuel.

From the introduction of the paper:

For the past 5 yr, scientists at Los Alamos National Laboratory LANL have mounted an unprecedented effort to obtain a better estimate of the critical mass of 237Np. To accomplish this task, a 6-kg neptunium sphere was recently cast1 at the Chemical and Metallurgy Research Facility, which is part of LANL. The neptunium sphere was clad with tungsten and nickel to reduce the dose rates from the 310-keV gamma rays originating from the first daughter of the a-decay of neptunium, namely,233Pa.

Neptunium-237 is a byproduct of power production in nuclear reactors. It is primarily produced by successive neutron captures in 235U or through the n, 2n reaction in 238U. These nuclear reactions lead to the production of 237U, which decays by beta emission into 237Np (Equation 1):



It is estimated that a typical 1000-MW electric reactor produces on the order of 12 to 13 kg/yr of neptunium.2 Some of this neptunium in irradiated fuel elements has been separated and is presently stored in containers in a liquid form. This method of storage is quite adequate because the fission cross section for 237Np at thermal energies is quite low, and any moderation of the neutron population by diluting the configurations with water would increase the critical mass to infinity. However, for long-term storage, the neptunium liquid solutions must be converted into oxides and metals because these forms are less movable and less likely to leak out of containers.

As noted in Ref. 3, metals and oxides made out of neptunium have finite critical masses, but there is a great uncertainty about these values because of the lack of experimental criticality data. Knowing precisely the critical mass of neptunium not only will help to validate mass storage limits and optimize storage configurations for safe disposition of these materials but will also save thousands of dollars in transportation and disposition costs.

The experimental results presented in this paper establish the critical masses of neptunium surrounded with highly enriched uranium (HEU) and reflected by various reflectors. The primary purpose of these experiments is to provide criticality data that will be used to validate models in support of decommissioning activities at the Savannah River plant and establish welldefined subcritical-mass limits that can be used in the transportation of these materials to other U.S. Department of Energy facilities. Finally, a critical experiment using an a-phase plutonium sphere surrounded with similar HEU shells and using the same setup used for the neptunium experiments was performed to validate plutonium and uranium cross-section data.


A brief excerpt of the materials utilized in these experiments:

The fissionable and fissile materials available consisted of a neptunium sphere, HEU shells, and an a-phase plutonium sphere. The neptunium sphere was ;8.29 cm in diameter and weighed 6070.4 g. Based on its weight and volume, the calculated density for the neptunium sphere was 20.29 g0cm3. A chemical analysis was performed on the neptunium sphere sprue…

…The analysis showed that the sphere was 98.8 wt% neptunium, 0.035 wt% uranium, and 0.0355 wt% plutonium. There were also traces of americium in the sphere. Table I shows the elements found in the chemical analysis of the sprue. Approximately 1% of the mass of the sphere was missing because the sprue sample did not dissolve completely.

To reduce the gamma-radiation exposure to workers, which comes mostly from the 310-keV gamma ray from the first daughter of 237Np, 233Pa, the neptunium sphere was clad with a 0.261-cm-thick layer of tungsten and two 0.191-cm-thick layers of nickel. The gamma radiation at contact with the bare sphere was reduced from 2 R/h to 300mR/h for the shielded sphere. Table II shows the dimensions, weights, and calculated densities of the neptunium sphere and different cladding materials. The total weight of the sphere, including cladding materials, was 8026.9 g. Figure 2 illustrates how the neptunium sphere was encapsulated. Except for the tungsten layer, both of the nickel-clad materials were electronbeam welded. In addition, a leak test was conducted for the nickel-clad layers to ensure that the neptunium metal and possibly some neptunium oxide produced in the event of a leak were contained within these materials and not released into the room or the environment.


Table 1:



This is a highly technical paper, and it is probably not of any value here to excerpt all that much of it. Nevertheless, there is a great deal of public mysticism about nuclear technology, mysticism that is killing the world, since nuclear energy is the only technology that might work to ameliorate, stop, or even reverse climate change. There is so much mysticism and misinformation that completely scientifically illiterate morons like say, Harvey Wasserman, can find people ignorant enough to believe he is, in fact, an "expert" on nuclear issues. (He's not. He is an abysmally ignorant fool, whose ignorance is killing people right now.)

With this in mind, I thought it might be useful to show some diagrams and photographs of the work that was performed here and that is found in the original paper:















A student of nuclear history will recognize that these experiments are very much like the experiments with the "demon core" that killed the nuclear weapons scientists Harry Daghlian and Louis Slotin in separate experiments in 1946. The remote equipment here is obviously designed to prevent that sort of accident from recurring.

The authors explored a number of different systems and reflectors, including both polyethylene and steel. In the process of conducting these studies, they refined some nuclear data on uranium isotopes, a valuable outcome.

From their conclusion:

Several experiments were performed at the Los Alamos Critical Experiments Facility to measure the critical mass of neptunium surrounded with HEU shells and reflected with various reflectors. For some experiments, Rossi-α measurements were performed to determine an eigenvalue that could be calculated by transport computer codes. These experiments were modeled with MCNP. For neptunium/HEU experiments, ENDF0B-VI data underestimated the keff of the experiment by ;1%. ENDF0B-V data and an evaluation provided by the T-16 group at LANL were in better agreement, although these cross sections continue to underestimate the keff by only 0.3% on average. After adjusting the neutron cross section for 237Np and 235U so that the MCNP simulations reproduce the experiments, we have estimated that the bare critical mass of 237Np is 57 +/- 4 kg.


Currently the main use for Np-237 is as a precursor for Pu-238 for use in deep space missions. Production of this important isotope has resumed at Oak Ridge National Laboratory, albeit on a small scale.

If we are interested in saving the world - there isn't much evidence that we are - neptunium can play a larger role in doing so, and thus this historical work is of considerable value.

A related minor actinide, which is also a potential source of Pu-238, although this plutonium will always be contaminated with Pu-242 owing to the branching ratio of the intermediate Curium-242, is americium-241.

It was estimated, in 2007, that the world inventory of these valuable elements was, as of 2005, was about 70 tons of Np-237, and 110 tons of Americium. It is desirable, critical actually (excuse the pun) that these materials be put to use.

I wish you a pleasant Sunday.

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Response to NNadir (Original post)

Sun Dec 1, 2019, 04:22 AM

1. I wonder, is this newly declassified?

Edit to add: Interesting reading and I can almost understand it.

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Response to 5X (Reply #1)

Sun Dec 1, 2019, 09:18 AM

4. I very much doubt that any of this was ever considered classified.

The paper was published in 2008 in a scientific journal to which anyone can subscribe or track down in a scientific library that subscribes to it. This sort of thing might have been considered for classification in the 1950's but not now.

Many similar papers have been publicly available for decades.

Although people often discuss the minor actinides in connection with nuclear weapons, as a practical matter, the conversation about neptunium, americium and californium "suitcase" bombs is somewhat absurd. It's easy to think about how it might be done, but the practical issues make it unlikely. At the end of the day, it would be far easier to make a nuclear weapon with uranium than with any higher transuranium actinide. This was true in 1945 and is true today. We will never be able to make nuclear war impossible, since uranium exists, but we can do a lot to make it more improbable. It turns out that neptunium is a key material for doing this.

I meant in the OP to include a reference to support the statement about inventories of neptunium and americium. The reference would have been to one of Kessler's papers suggesting how to make plutonium that is impossible to use in nuclear weapons.

People who don't know very much about nuclear technology like to pretend that its simple to make nuclear weapons and often write scare story along those lines to inspire fear. However all of the major terrorist and war events, except for the petroleum driven Second World War, involved petroleum. There's a reason for that. If we're concerned about weapons of mass destruction in a practical as opposed to theoretical weapons of mass destruction, we might restrict petroleum, which is possible using nuclear technologies.

If one spends enough time thinking about nuclear weapons, and in understanding the technology, one can understand how silly some of these scare stories are.

My strongly held opinion is that nuclear technology is the last, best hope for the human race to address it's dire environmental issues, only one of which is climate change. That is why I read and sometimes comment on papers like this one, all part of the public record.

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Response to NNadir (Original post)

Sun Dec 1, 2019, 08:06 AM

2. My first thought on seeing the title was, "oh my word, they're twisting the dragon's tail again" ...

... as it was known in the Manhattan Project. Glad to see common sense and the lack of wartime pressure led to a highly automated manipulation in an isolated chamber. No more "Demon cores".


JOOC, do you know anything about a company called Flibe Energy ? I've been trying to gather info on them, and while they have a nice-looking Web site, it seems the actual physical research is going on at ORNL and PNNL. The company gives its home as nearby Huntsville, AL where the founder and President (formerly?) worked for two companies where my father worked. In fact, he's described on several Web sites as still being chief nuclear technologist at Teledyne Brown Engineering, a position I didn't even know existed (my dad worked in the Apollo program, I don't remember TBE having *any* interest in nuclear). I'm still trying to find where they have actual labs, as opposed to paper offices.

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Response to eppur_se_muova (Reply #2)

Sun Dec 1, 2019, 09:04 AM

3. Yes, I know Flibe Energy. It's Kirk Sorensen's company. Years ago, before the company...

...was founded I corresponded with him indirectly and perhaps directly, on his website where I used to write from time to time.

Back in the 1980's or 1990's he collected all of the documents at Oak Ridge around the MSRE, and was quite driven to bring the technology back to life. He seems to have come a long way to doing so.

FLIBE is the FluorideLIthiumBEryllium eutectic salt developed by Alan Weinberg's group at Oak Ridge in the 1950's and 1960's.

Although I learned a lot in those years thinking about FLIBE and related salts like FLINAK, I kind of moved away from those concepts, in particular, FLIBE since I am not fond of the beryllium handling requirements for FLIBE.

My feeling was that Kirk regarded the Molten Salt Reactor as almost "off the shelf." I disagree.

FLIBE will also generate alot of Be-10 and tritium. The latter would be useful for fueling fusion reactors if they become practical, but Be-10 is problematic.

Nevertheless, the things I learned while writing on Kirk's website, and from what his other correspondents wrote, generated many ideas. There are, in fact, an infinite number of molten salts, organic, inorganic, and hybrid, and their use in nuclear technology has much to recommend it.

I knew Kirk before he founded FLIBE Energy - a company for which there are now many competitors - and I don't know where he set up offices. I never met him personally, although I have met some of the people who used to write on his site, albeit briefly. My feeling was that Kirk was a very nice guy, very smart, and very committed to nuclear energy.

I'm very much a fast reactor kind of guy these days, and I kind of lost contact with many of the MSR guys I used to know. I'm also into the U/Pu cycle since thorium is exhaustible and uranium is not, on geochemical grounds.

I once suggested on Kirk's website that the U-233 at Oak Ridge - I saw the building where it is stored during my tour of the lab this summer - should be mixed down with depleted uranium, which produced howls of horror, and frankly, my suggestion was a very bad idea, but just popped off the end of my keyboard.

I suspect if my name came up in a conversation with him; he'd remember me.

Because they can achieve high breeding ratios and produce lots of useful neutrons while providing very high burn up, I am a metal fuel kind of guy, which is why I loved this paper. However there are many applications for molten salts, both in separations, and in heat transfer. I'm glad I went down that pathway for a time.

In any case, I'm totally an arm chair nuclear person; nuclear issues have no connection with my professional life.

If I were to live over again, I would train as a nuclear engineer and not as an organic chemist, but perhaps to a limited extent, I have been involved in saving lives in ways other than working for the expansion of nuclear energy. All this said, organic chemistry is a beautiful science, with some interface with nuclear technology, and as such, it's made my life very good as opposed to perfect. There is no reason to object to "very good" because it's not "perfect." At the end of the day, to my total surprise, my professional life ended up in analytical chemistry, also a fascinating subject.

My son is studying materials science; and I very much encourage him to think about nuclear energy, where materials science will play a critical role, about which he sometimes does think. He did intern at Oak Ridge this summer, and that helped.

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Response to NNadir (Reply #3)

Sun Dec 1, 2019, 02:00 PM

5. Thanks for all the info. Interesting to see the personal connection.

What I have noticed in trying to get more info about Flibe Energy is that their objective at the moment appears to be to set up a molten-salt thorium reactor to produce Tc-99m for medical purposes. I was looking up info on Tc at the time and came across a group which was monitoring the availability of Tc-99m -- who was producing it, and how, and where, and plans for future output. UAH and Flibe Energy were mentioned.

A few years ago I had an undergrad TA at UAH who mentioned that he had a research job working with Tc, but I didn't ask for details. Wish I had; he's moved on and I don't know how to contact him.

Hey, I just found his advisor, apparently ! Didn't find her in earlier Web searches; just maybe UAH is slow at updating Dept. Web sites (yeah, they are, trust me). I'm off to get more info.

https://www.researchgate.net/profile/Hayley_Dent

Maybe they'll set up a small, specialty medical-isotope reactor, as opposed to power generation, if they can find something to do with Be-10. IIRC the ability to extract Tc continuously from the liquid, without interrupting reactor operation, was the advantage they were hoping to exploit.

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Response to eppur_se_muova (Reply #5)

Sun Dec 1, 2019, 02:31 PM

6. Tc-99m is generally "milked" from Mo-99.

Last edited Sun Dec 1, 2019, 07:50 PM - Edit history (1)

In theory, I suppose that it is possible to isolate Mo-99 from a molten salt reactor relatively quickly and continuously.

A fluid phased reactor's true advantage is precisely that. It allows for fast on line separations.

The distribution of mass numbers in nuclear fission looks rather like a camel's two humps. The low mass hump moves with the mass number of the original nucleus. (At high mass numbers, spontaneous fission, out around einsteinium and fermium, actually becomes gaussian.

The fission of U-233 seems to have a maximum close to A = 99:



When I knew Kirk, the goal was to make energy. If it is now to make Mo-99 for medical use, that represents the way funding start ups sometimes goes, you need to downsize your goals. It's unfortunate but true. Many people with money lack vision.

But I haven't kept up with his company. I was asked to consult for one of these MSR companies (for free), not FLIBE Energy, but when I got a look at the reactor, I wasn't fond of the chemistry, which was designed to eliminate the lithium/tritium problem, and declined to participate.

Actually one can engage in better on line processing where spontaneous phase separations take place or where on line extraction is possible using immiscible phases. (Distillation is also a very real potential, as the disasters at Chernobyl and Fukushima taught us.)

Isolating molybdenum from FLIBE seems more problematic, but I haven't looked into it in any detail.

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Response to NNadir (Reply #3)

Thu Dec 5, 2019, 04:59 AM

9. Energy from Thorium

Kirk's blog has gone off the boil in the last few years, but I learned a lot from it before then. I think some of the more erudite contributors hived off to different MSR start-ups, and probably have proprietary info they won't discuss. Do you mind telling us your nom de plume for the site ? It's well indexed by subject and by participant.

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Response to John ONeill (Reply #9)

Thu Dec 5, 2019, 09:22 AM

10. If I recall correctly - it was some years ago - my...

...my "handle" was the same as I use here.

I recognize that everyone thought they would get rich exploiting permutations of the technology.

Several different groups contacted me. I met one such group in person.

I myself contemplated an idea that I thought might be patentable, only to learn that a close idea had been patented independently by David LeBlanc.

I know that at least one chemical idea I posted there, about the separation of plutonium from uranium by exploiting cesium or rubidium complexes of the perfluoro anions was wrong, which I learned by finding some very obscure and old literature. That idea lingered with me a long time before I realized it won't work, which is why I remember it.

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Response to NNadir (Original post)

Mon Dec 2, 2019, 12:56 PM

7. If anyone asks, I want a tungsten nickel clad neptunium-237 sphere for Christmas.

I'll display it alongside the plutonium Doc stole for his DeLorean.

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Response to hunter (Reply #7)

Mon Dec 2, 2019, 10:42 PM

8. You may wish to consider utilizing different cladding for powering the DeLorean.

It's pretty straight forward to consider the energy requirements for the DeLorean by recognizing that a lightening bolt sent Marty back to 1985. According to this site at the University of Arizona

A single lightning flash is formed by a series of lightning strokes. Usually there are about four strokes per flash. An average duration of time for a stroke of lightning is about 30 microseconds. The average peak power of a stroke of lightning is about 10^12 watts.


The four strokes each lasting for 30 microseconds means that the overall duration is 0.12 seconds at a trillion watts giving a total energy of 120 billion joules. Now, it's been many years since I saw the movie, but I'd estimate the time for Marty to disappear entirely was about 5 seconds. This means that the power requirement, if it is continuous, averages 14 billion watts, 14,000 MW.

Now Doc had a "flux capacitor" which means he was storing some energy for later release. The "flux" designation may refer to an ion flux, so his name in 1955 may be equivalent to our modern supercapacitors, which also rely on ion "fluxes" to separate charges.

Still, it's a lot of energy.

A working figure for nuclear fission is around 200 MeV/fission, suggesting that we'd need to fission about 1.5 grams of neptunium in 5 seconds. At this power, given the low melting point of neptunium, it would surely melt into a liquid, and it would not be surprising if the melting point were suppressed further by the formation of a nickel/neptunium eutectic, since iron and cobalt plutonium eutectics are well known.

Pure tungsten and pure tantalum are known to resist attack by liquid plutonium. They may also work well with liquid neptunium. Certain modern ceramics might work well too. I'd stick with pure tungsten out of the box, perhaps alloyed with a little technetium to make it more machinable.

I have no idea about how Doc did heat transfer, but being a fictional character, he seems to have mastered the point.

Anyway, you'd have a pretty high neutron flux, and plenty of Pu-238 generated along with the fission products.

To protect yourself from neutrons, I'd go with a beryllium or carbon reflector.

If you travel to the future, say 2100, do come back and let us know if a working fusion reactor is the same 20 years off that it was in 1955 and how Greenpeace is doing with their 100% renewable energy scheme.

Here is the interesting distribution of fission products by the way, much flatter than uranium or plutonium fission:



Be careful with them.

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