Why You Can’t Build a Bomb From Spent Fuel

February 20th, 2010
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To build a nuclear weapon you need weapons grade fissile material. This could be plutonium or uranium. (It could theoretically also be something like neptunium or americium, but nobody has ever bothered with that, as it would be far more difficult.) In the case of uranium, it must be highly enriched uranium and in the case of plutonium, it must be “weapons grade” plutonium.

The process of extracting plutonium from spent fuel for reprocessing, use in fast reactors or MOX fuel usage is similar to that used to extract plutonium for use in nuclear weapons. For this reason, many nuclear energy opponents will scream “AH HA!” and say that a nuclear power reactor is clearly a “proliferation hazard” regardless of what type it is. Furthermore, they’ll tell you that reprocessing is the ultimate danger and that if we dare recycling fuel, then others will recycle their fuel too (many already do, by the way) and if they do that then certainly they’ll be building weapons.

Of course, there are plenty of countries that reprocess fuel to one degree or another and don’t have nuclear weapons. Germany, South Korea, Japan, Belgium and Italy either reprocess fuel or have done so in the past but never had a nuclear weapon. Countries like Russia, France and the UK reprocess civilian reactor fuel but have never used this fuel to build a nuclear weapon.

There’s a good reason for this. As it turns out, the spent fuel from a modern power reactor, such as a BWR or PWR reactor is completely unusable for a nuclear weapon.

Here is why:

You’ve probably heard of the two earliest combat-ready nuclear bombs, developed by the Manhattan Project: Little Boy and Fat Man. Little Boy was the codename for the uranium bomb which used a “gun triggered” design, firing a subcritical uranium slug into a subcritical uranium target to produce a supercritical mass. Fat Man was the code name for a plutonium-based bomb that used a spherical core with a semantically explosives to compress the plutonium, resulting in an “implosion” that would bring the core to critical mass.

Well, here is “Thin man” the third bomb design that was developed during the Manhattan project: the one which never made it past the early test phase.

To be more accurate, those are not nuclear bombs, but rather just the empty casings. No “thin man” type bomb was ever built, because a combination of laboratory experiments and calculations proved that the design would either not work at all, or work so poorly that it wouldn’t produce an explosion any larger than a nominal size conventional munition. The problem is plutonium, or rather, an isotope of plutonium, plutonium-240.

Plutonium is produced by neutron bombarding uranium-238 in a reactor. When uranium-238 absorbs a neutron, it becomes uranium-239. Uranium-239 has a half-life of only 34 minutes and decays to neptunium-239, which has a half-life of 2.3 days and decays to plutonium-239. Therefore, when uranium-238 is irradiated with neutrons, a few days later, some of that uranium will have become plutonium-239, which can be separated chemically. Plutonium-239 is fissile and it’s exactly what you want if you’re looking to build a nuclear weapon.

However, there’s a problem that comes with the irradiation process: During the irradiation period, some of that plutonium-239 will also absorb a neutron. If that happens, it will usually fission, but up to a third of the time it won’t – instead it will become Pu-240. Likewise, some of that neptunium-239 will absorb a neutron before it gets a chance to decay to plutonium-239, thus resulting in neptunium-240, which quickly decays to plutonium-240. Because of these reactions, any reactor-generated plutonium will have some plutonium-240 in it.

Plutonium-240 is what you do not want when you’re building a nuclear weapon. For one thing, it’s not fissile and for another, it’s highly radioactive (four times more so than Pu-239), but what really makes Pu-240 so problematic for weapons use is that it has a high rate of decay by spontaneous fission, which produces neutrons. The rate of spontaneous fission of Pu-240 is so high that even with a tiny bit in a sample of plutonium, it will have significant effects on critical mass. If plutonium were used in a gun-triggered weapon, the spontaneous fission rate would mean that the bomb would begin to fission before the two portions came fully together. This is per-initiation, and it results in the weapon blowing itself apart before it actually gets a chance to achieve a full blown nuclear reaction. It’s commonly known as a fizzle.

To avoid this, a much more efficient design would be needed, one which could slam the fuel together very quickly and start the reaction before per-initiation could destroy the weapon in a very small explosion. This is why the implosion triggered system was designed. It proved to be capable producing a reaction from plutonium. Although the designers were not as confident about the design, leading to the need for a test before deployment – something which was not done with the gun-triggered uranium bomb.

Yet even using the implosion design, the presence of Pu-240 was still a problem. Having some Pu-240 present in the plutonium turned out to be inevitable, but it was still necessary to keep it to a minimum. If the level of Pu-240 were too high, even the implosion design would fail.

In order to do this, a special plutonium breeding cycle needed to be developed. The key to producing plutonium that is high in Pu-239 and low in Pu-240 is to make sure that it only spends a short period of time being irradiated. Uranium targets, in the form of small “slugs” would be irradiated in a reactor for a period of only one to two months. This would allow a small amount of neptunium and thus plutonium to build up, but only a small number of the atoms would absorb a second neutron. The slug would then be processed to remove the plutonium and the uranium would then be irradiated again, once again for a couple of months.

In practice, this meant that the target had to be completely dissolved and reprocessed, since that’s the only way to extract the plutonium. The uranium that was left could be reused, but it had to be completely re-fabricated into a new target. It also required a “cool down” period to assure the neptunium had all decayed. This period of time also eliminated most of the short-lived fission byproducts, which made the material difficult to handle and complicated the separation process. Typically, the cool down period would last several weeks. Each target would only produce a tiny amount of plutonium, so the process had to be repeated thousands of times to accumulate significant amounts of plutonium.

If the targets had been left in the reactor longer, then they’d produce more plutonium, but more of the plutonium would be Pu-240, the type which needs to be avoided. All and all, the process of producing weapons grade plutonium yields only about half a pound of plutonium for every ton of uranium irradiated and processed.

To accomplish this, the United States built a truly massive complex at Hanford Washington. It took three reactors working full time from 1944 on to produce enough plutonium for the weapons used in 1945. A total of nine plutonium production reactors were built at Hanford and produced plutonium for US nuclear weapons until 1987. Hanford was joined by the Savannah River Site in 1953. Six reactors would be built at the Savannah River site for plutonium production. All of the plutonium in US nuclear weapons came from the reactors at Hanford and Savannah River. Not one ounce of weapons plutonium ever came from a commercial nuclear power plant.

Grades of plutonium:

In current terminology, “weapons grade” plutonium is considered to have roughly 93% Pu-2239 or more, while “fuel grade” or “mid grade” plutonium has at least 81% and “reactor grade” contains less than 81% Pu-239. Weapons grade plutonium is considered to be the type well suited for use in nuclear weapons, but this does not mean that lower grades can’t theoretically be used, at least to a point.

As the concentration of Pu-240 increases, the difficulties it presents become greater and greater, causing the weapon to become less reliable, yield is reduced and a failure becomes more and more likely. Advanced weapons designs employ features like neutron reflectors, super high velocity, highly precise implosion lenses and multi-point detonation mechanisms, boosted cores and pulse neutron generators. These technologies, possessed by only a handful of countries could theoretically be used to create a more efficient weapon that could potentially utilize lower grade plutonium – but only to a point. A highly advanced, super-efficient weapon could make use of plutonium with a content of 91% Pu-239 or possibly even 88%. However, much beyond this would be difficult to impossible for even the most advanced weapons.

This kind of technology would not be available to a country just starting a weapons program anyway. The designs used by the United States and Russia are the result of hundreds of tests and decades of intense research. They also require exotic materials like tritium. However, no country with the technology to make such weapons would ever bother using the lower quality plutonium – doing so would reduce the weapons yield and make fabrication more difficult. Both the US and Russia already have a surplus of old weapons grade plutonium. Regardless of whether you could use sub-weapons grade plutonium in an effective weapon, it still would never work with reactor grade plutonium.

The reason for this is simple: commercial power plants don’t produce the kind of high grade, low Pu-240 material that is required for weapons. The most common type of nuclear reactor used around the world for power generation is the pressurized water reactor. In addition to this, there are many boiling water reactors in use. There are also some heavy water reactors and a few gas cooled reactors in use. With a couple of exceptions (more on that later) these reactors are NOT designed to produce weapons grade material.

Power reactors are designed to burn their fuel until there isn’t much left in it to burn, or at least, until the fuel no longer efficiently sustains critical mass in the core. Utility companies would rather be cranking out gigawatts than shuffling around fuel rods, and for this reason, most PWR’s and BWR’s are only refueled every year or two. Typically only a portion of the fuel is replaced and each fuel rod is used for more than one refueling interval. Most fuel rods spend at least three years in a reactor before being replaced and some spend even longer.

Some reactor designs allow for online refueling. These reactors are often refueled every few months, with only one or two fuel assemblies changed each time and fuel rods spending two or more years in the reactor, depending on burnup and enrichment. The CANDU reactor is refueled in this manner, but its spent fuel still spends far more time being irradiated than a weapons plutonium target ever would.

Because of this the plutonium found in spent fuel from power reactors has a very high concentration of Pu-240, making it unsuitable for use in nuclear weapons. The high levels of Pu-240 are not as important when the plutonium is to be reused in reactor fuel. Fast reactors will burn Pu-240 without issue and in thermal reactors, while plutonium-240 usually requires two neutrons to fission, more than Pu-239, this does not preclude its use. The pre-initiation issues of a weapon do not exist in reactors.

The plutonium produced by power reactors has yet another issue: it contains the isotope plutonium-241, an isotope which is only present in negligible quantities in weapons grade plutonium. Pu-241 decays into Am-241 with a half-life of only fourteen years. Since most reactor spent fuel sits for years in storage, by the time it is processed, a significant quantity has decayed to Am-241. The combination of Pu-241 and Am-241 makes the material highly radioactive, causing potential problems with material irradiation, the self-irradiation of a weapons core can cause embrittlement and heating of the plutonium pit and potentially compromise the weapons integrity.

In higher burnup reactors significant quantities of Pu-242 also begin ton build up. Plutonium-238, a source of intense heat also can be found in the spent fuel of high burnup reactors and can present problems for weapons us even when present at quantities of less than 1%. In a pressurized water reactor’s spent fuel, only about 53% of the plutonium present is plutonium-239, the type needed for a weapon! Creating a weapon out of plutonium with such extremely low levels of the critical isotope is absolutely impossible.

In face, 93% pu-239 is considered the low end of what is generally acceptable for weapons use and would work rather poorly in most weapons designs. Countries like the US, Russia, France and other advanced nuclear weapons states usually use even purer plutonium with concentrations of unwanted isotopes as low as 3% or less.

The possibility of using power reactors to produce weapons grade plutonium:

Simply reprocessing the fuel from a power reactor would yield plutonium that is utterly useless for weapons use, but could the reactor’s fuel cycle be modified to produce higher grade plutonium? Perhaps, but it wouldn’t be easy. PWR and BWR reactors are complex to refuel and normally are only refueled on relatively rare occasions, every year or so at the most. Refueling requires shutting down the reactor, allowing it to cool and depressurize, opening the lid of the reactor, replacing the fuel and finally replacing the lid. It takes more than a week to do this, during which time the reactor is shut down.

Producing weapons grade plutonium would mean irradiation cycles as short as a month. This would mean the reactor would be shut down almost as much as it was running, dramatically compromising its power producing capabilities. The cumbersome procedure would be made even worse because of the fact that the fuel assemblies use long, cladding rods, not the easily processed slugs that weapons reactors use. A power reactor of this type might be able to handle a fuel system more favorable to such frequent reprocessing and re-fabrication, but only with very extensive modification.

CANDU reactors can be refueled online, but the spent fuel they produce is very low in plutonium. A CANDU reactor could theoretically be used to produce weapons grade plutonium, but again, it would require extensive modification of the fuel cycle. Fuel would have to be ejected more frequently and doing so would reduce the power output of the reactor. Additionally, since the breed ratio of a CANDU under normal operation would not produce enough plutonium to make it a viable weapons reactor, there would need to be some modification of the fuel, likely using some level of enrichment combined with natural or depleted uranium target rods. It could be done, but like the PWR, it wouldn’t be especially easy and it would be pretty obvious to the world what you were doing.

There are two types of power reactors which are designed in a manner that allows them to produce weapons grade plutonium. The RBMK and Magnox reactors were both conceived as dual-purpose reactors and as such have the features necessary to produce weapons grade plutonium. The spent fuel from both of these reactors is useless for weapons production when they are run in a manner that maximizes their efficiency as power reactors, but the fuel cycle can be modified in order to produce nuclear weapons. However, both of these reactors are considered obsolete designs. The last Magnox reactor was built in 1971 and will be decommissioned this year. Most of the RBMK’s built have been decommissioned, but a handful are still in operation in Russia – a country which has no reason to produce more plutonium for weapons, given the enormous unused stockpile they already have.

A country wishing to produce plutonium-based nuclear weapons would be better off building a dedicated reactor or reactors for plutonium breeding. That is what every nuclear armed nation has done. The difficulties in making a power reactor produce weapons grade material are likely to be greater than simply building a purpose-built reactor, and in both cases the intentions would be fairly obvious. The fuel that comes out of a power reactor is useless for weapons production and therefore could never be spirited away to reprocessing to yield weapons grade material.

The great deception:

In 1977, President Jimmy Carter ended the reprocessing of nuclear fuel in the United States, thus making the US the only major nuclear nation with no reprocessing of spent fuel at all. In doing so, he created the nuclear “waste” problem, which prior to the end of reprocessing had been non-existent. At the time, he ordered some very selective information declassified to support this decision.

As anyone who understands nuclear weapons design would immediately realize that reprocessing of spent fuel absolutely does not yield weapons grade material, Carter asserted that, in fact, weapons grade material was not required to build a nuclear weapon. To support this claim he announced that the US had conducted a test in 1962 of a device containing “Reactor Grade Plutonium.” The problem was that Carter lied, if only by omission.

As is mentioned above, there’s not a single hard line at the 7% concentration. Plutonium containing 8% or 9% Pu-240 could certainly be used in a reasonably sophisticated weapon, although with reduced yield. When the 1962 test was conducted, there were only two terms for plutonium grades: “weapons grade” and “reactor grade.” In the contemporary terminology, “reactor grade” plutonium was anything with less than 93% plutonium-239, so a sample with 92.5% would qualify as reactor grade.

Additional information on the test was released in 1997. The biggest revelation was the source of the plutonium: it was not American but was imported from the UK. If the US wanted to test how lower grade plutonium would preform in a weapon, why not just use plutonium recovered from one of the power reactors in the US? After all, by 1962, the US had a number of PWR’s, including Shippingport, Yankee Rowe, Vallecitos Nuclear Center as well as a number of prototype naval reactors in operation at the Idaho National Laboratory.

The reason is simple: researchers were aware that the plutonium produced by these kind of reactors was so low in Pu-239 that it just plain would not work in a weapon. The test was likely intended to assess the effect of using lower grade plutonium in a weapon and confirm theoretical calculations, but to do this they needed plutonium that was lower than weapons grade, but not by too much, or the test would fail completely.

The DOE has never released the details of exactly what the composition of the plutonium was, nor have they released information about what the weapons yield was or whether the weapon incorporated advanced design features. The fact that it came from a Magnox reactor, however, is very telling.

Note in the graph seen above that of all the power reactor designs, the Magnox produces the least Pu-240 in its spent fuel and thus the most Pu-239. This is not by accident, as the Magnox reactor was originally designed as a weapons material reactor. Magnox reactors that continued to run in recent years produced plutonium with about 18% Pu-240, but this is only because they were operated at a high burnup level to produce power efficiently. When Magox reactors first came online, they were run at a much shorter fueling interval, more similar to the reactors at Savannah River and Hanford. Their primary purpose was to produce weapons grade plutonium. Electricity was also generated from the reactor’s heat, but this was seen as a byproduct, not the primary function of the reactor.

The first Magnox reactors became operational in 1956 at Calder Hall at the Stellafield nuclear site. The facility operated in a low-burnup mode intended for weapons production until 1964. After 1964 the reactors continued to produce weapons grade plutonium intermittently until 1995. The Chapelcross nuclear power station opened in 1959 and was considered the “sister plant” to the Calder Hall reactors. Chapelcross was also intended primarily to produce nuclear weapons material. In its early years, it ran on natural uranium at a low burnup that produced weapons grade plutonium. Berkely station opened in July of 1962. Berkeley was the first nuclear power plant in the UK that was owned by a civilian agency, the “Central Electricity Generating Board.”

Thus, there were only three nuclear reactors in the UK that potentially could have provided plutonium in 1962. Calder Hall can be ruled out because it was known to be operating at full capacity in weapons production mode at the time. Thus, the material it produced would be considered “weapons grade.” Reports indicate that in its early years, Berkeley was also used for producing weapons grade material. To this day the British government remains secretive about its historical plutonium breeding capabilities, and the level to which Berkely was used to produce nuclear weapons material is not entirely clear.

Had the material come from Chapplecross, it would have represented an abnormally long period of irradiation, since Chapplecross was primarily producing weapons material. Had it come from Berkeley, it would also have had to be low burnup, if only because of timing. Considering that it takes a good two to three months to cool, reprocess and fabricate fresh uranium rods into a weapons pit, that would have left only about three months for irradiation at Berkeley, even if the test took place in late December of 1962. Since the plant came online in July, there wouldn’t be enough time to heavily irradiate fuel rods. Also, at the time Magnox reactors ran exclusively on very low enrichment or unenriched fuel, limiting burnup, and because burnup directly effects fuel element integrity, early experiments with Magox reactors at higher burnup progressed conservatively, with usage extended in small intervals.

Therefore, the source of the plutonium can be determined to be either the very first round of spent fuel discharged from Berkeley, after having been irradiated for only a period of about three months max or possibly was from an extended irradiation period at Chapplecross. The fuel was not comparable to modern spent fuel, because, at the time, no British reactor was capable of producing such material. The fuel would have been lower than the US standard for weapons grade material – but only slightly.

The nature of the test can be inferred by a combination of its timing and the vague statement released in 1997: “This test was conducted to obtain nuclear design information concerning the feasibility of using reactor-grade plutonium as the nuclear explosive material. “

The test was probably done to establish a lower threshold for what would constitute acceptable plutonium for use in a weapon. This had already been established by calculations and sub-critical experiments, but this test would confirm how lower quality plutonium worked in a weapon. For example: Would the fuel cycle of the new Berkley nuclear plant produce material with weapons potential? Theory predicts that the use of plutonium of a quality slightly lower than the weapons grade standard would still produce a reasonable tactical yield in an advanced weapon design, but would diminish the total yield significantly. This is probably what happened in 1962.

One must remember that the material was only slightly lower grade than weapons grade plutonium – this is beyond question because of the source of the material being the low-burnup Magox reactors. Had it been from a modern power reactor or the power reactors that were being built outside the UK, it simply would not have worked at all.

And finally, on a “fizzle bomb”:

Some have said that this would be a deadly weapon as a “super dirty bomb” – the reality is that it would be a very big, bulky weapon that wouldn’t be much more powerful than its own weight in dynamite. It might spread around some plutonium, but the radio-toxicity of plutonium is not all that high and the spread of such material would be fairly local. Given that it’s primarily an alpha emitter, it would have to be inhaled or ingested to cause harm. If one went off, a couple blocks would need to be shut down while some guys in Tyvex suits washed everything down. We’ve actually experienced some weapons accidents that spread plutonium in a local area before. They weren’t the end of the world.

As a strategic weapon, it’s useless, as a tactical weapon, it’s pointless and as a weapon of terror, it’s only as terrifying as you let it be.



This entry was posted on Saturday, February 20th, 2010 at 6:58 pm and is filed under Bad Science, Enviornment, Good Science, History, Nuclear, Obfuscation, Politics. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.
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257 Responses to “Why You Can’t Build a Bomb From Spent Fuel”

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  1. 251
    Shafe Says:

            jeppen said:

    yada, yada, yada…

    That’s quite an ego you have there. Somehow the only part of the discussion that is neither tiresome nor fallacious is the part where you justify your role as the style police. Nice.

    Carrying on a discussion with folks like you is like watching a cat vomit. Nothing good is going to come out of it, but I have to see what you’ll come up with next.

    I’m going to take a page from DV82XL’s book and move on to greener pastures. You’re welcome to the last word.

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  2. 252
    jeppen Says:

            Shafe said:

    Somehow the only part of the discussion that is neither tiresome nor fallacious is the part where you justify your role as the style police. Nice.

    Right there, I didn’t justify, I simply asked you what you thought of DV’s conduct, and whether it warranted some meta-reply. You refused to answer for real, of course. My guess is that you were annoyed that a nobody came in and “lectured” a regular that you look up to, so you mounted an attack that didn’t really stick. That was frustrating to you, so now you’re doing an ungraceful exit, flinging some extra mud.

            Shafe said:

    Carrying on a discussion with folks like you is like watching a cat vomit.

    Hmm, folks like me? I’m not sure how I became a target worthy of contempt. But again, I guess that it boils down to tribalism. You would like outsiders to silently take whatever the insiders dishes out. If the outsiders do that, it proves that they are bad people, and if they don’t, that also proves they are bad people.

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  3. 253
    dyson vacuum cleaners Says:

    Hello, I ebjoy reading all of your article post.

    I wanted to write a little comment to support you.

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  4. 254
    Barbara Billig Says:

    Japan has the atomic bomb and has admitted it, even tho they were forbidden by the WWII treaty. How do you think they got it? How do you think North Korea got theirs? Do you think they purchased the material necessary to build the atomic bomb from another country? And if you are correct in your article (which you may be???) then why are so many people stating exactly the opposite from you? You have quite a bit of opposition out there – Why are the Iran negotiations even going on if you are correct?

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  5. 255
    Michael Karnerfors Says:

    @Barbara Billig: If you want to claim that Japan has a nuclear bomb (or more) you’d better cough up some evidence for it. Then you’d do best not to post it HERE but to go to the UN/IAEA to let them deal with it. Until then this is a case for Hitchen’s Razor.

    Second… North Korea doesn’t have nuclear power… it never did. So that part of the argument goes out the window right there.

    Third… Iran’s nuclear power reactors are already approved and good to go. The international community has no issue with them. It the enrichment facilities that are being debated… not the reactors.

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  6. 256
    DV82XL Says:

            Barbara Billig said:

    Japan has the atomic bomb and has admitted it, even tho they were forbidden by the WWII treaty. How do you think they got it? How do you think North Korea got theirs? Do you think they purchased the material necessary to build the atomic bomb from another country? And if you are correct in your article (which you may be???) then why are so many people stating exactly the opposite from you? You have quite a bit of opposition out there – Why are the Iran negotiations even going on if you are correct?

    While Japan’s nuclear energy infrastructure makes it eminently capable of constructing nuclear weapons at will, it has not, nor has it said it has. What Japan has done however is placed itself “a screwdriver’s turn” from making a weapon if it feels it needs to, To that end it has it has its own indigenous uranium enrichment plan and stockpiled approximately 1,200 to 1,400 kg of HEU. ostensibly for research reactors.

    Please understand that we know that any nuclear reactor can be used to breed plutonium if one sets out to do so and has a source of HEU (Highly Enriched Uranium), and a reprocessing facility – what this post is about is the use of spent fuel, which is what is left when LEU (Low Enriched Uranium) is burnt in a power reactor, and this is a very different argument.

    North Korea, Iran, India , Pakistan, South Africa and every other nuclear weapons state went the same route, and made HEU for their bombs, or bred Pu from it – nobody has used spent fuel.

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  7. 257
    NYT Reports Iran 'Breakthrough'—Obama: 'Iran Negotiating Seriously' - NewsFeeding.Net Says:

    […] power fuel rods are thought impossible to convert into fuel for nuclear weapons. According to Sanger, such a concession would remove a […]

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