A Critical Problem


This photo gives me the creeps.

When you work with dangerous stuff, you have to condition yourself to respond at the level of emotion. You have to think things out too, but having a sense of what you can and can’t do that operates immediately is important. Children learn that hot stoves and automobile traffic are dangerous. We automatically stop at the street. It’s tourist season in Santa Fe, though, and I have observed that some lack that automatic response.

I’ve dealt with dangerous stuff since I was a child. I had a real chemistry set, not the baking-powder-and-vinegar wimp boxes that are the only things sold now. My parents walked with me a lot in the outdoors and made it clear that wild strawberries were good to eat and deadly nightshade was not.

I loved my lab courses in high school and college. Working at a national laboratory added more precautions I needed to learn. I worked with high-voltage lasers. I did a little bit of plutonium chemistry in a glovebox. And then I managed people who were developing cans for storing plutonium.

We expected the plutonium to be in the form of flat cylindrical ingots, like the enriched uranium in this photo. 400px-HEUraniumCIt had to be protected from the air, from unauthorized removal, and from criticality incidents. The criticality safety group played an important part in designing those cans. The cans were about a foot high. The ingots were about an inch thick. The extra space was for criticality safety.

Probably before I came to Los Alamos, I read about Louis Slotin and Harry Daghlian, who were killed by criticality incidents. They were doing experiments to determine criticality properties for nuclear weapons design. They died horribly. Don’t read about them unless you have a strong stomach.

The metal rods in the top photo are plutonium. Rods can roll. These rods could roll closer to each other and perhaps produce the kind of runaway neutron reaction that killed Slotin and Daghlian. Putting a hand in to separate them could make the reaction worse because the water in a human body reflects the neutrons.

I had formal safety training, informal discussions with more experienced people, and made it a point to internalize rules of thumb. Keep pieces of plutonium separate. Abide by glovebox limitations; every glovebox has a sign with the limits of plutonium allowed in it. For solutions, keep them dilute and in flat containers. Flat/thin is safer; the closer a shape is to spherical, the less material is needed to go critical. IIRC, there were racks to put rods in if you were working with that shape of metal, so that they didn’t accidentally roll together.

That photo is at the center of two articles from the Center for Public Integrity (NMPolitics.net, Washington Post). They are based on an investigation reported here. According to those articles, a technician ignored glovebox limits and arranged the plutonium to take that photo for management. A Los Alamos manager is also quoted in the articles as saying that the criticality safety group was an unnecessary expense. A number of the senior people in the criticality safety group were of my vintage and were expected to retire about when I did. According to the articles, management’s signal was heard loud and clear, and the rest left.

It took the criticality safety group two months to work out the criticality aspects of our can design. I found that frustrating, too. They had to consider the way the cans might be stacked, how they might fall together and how the ingots inside might move if a stack of cans fell. What if they were in a flooded area? If people decided to rescue them from that flooded area? Water, by itself and in human bodies, enhances the neutron reactions that lead to criticality.

The criticality safety group develops those criticality limits posted on gloveboxes. They take into account the kinds of operations in the glovebox, the equipment inside, and the effects of operators’ hands and bodies nearby.

I learned from people who recalled personally what happened to Slotin and Daghlian. Today’s managers at Los Alamos are rotated through, I’ve heard, every two years. There are not many other places where one learns criticality safety. If Los Alamos is to manufacture nuclear weapons pits, criticality safety evaluation is essential.

A criticality accident affects people who are close to it. It is not a nuclear explosion; the neutron reactions are the same but occur much more slowly. The closest people die horribly, but some tens of feet distance and walls between will shield others. Raemer Schrieber was present at the Slotin accident and lived into his eighties.

It is unconscionable not to educate workers to the dangers involved with handling plutonium. It is worse to encourage poor practice. Did the manager for whom that photo was made understand criticality safety? Or the manager who said to just keep working?

One of the reasons for adding industrial partners to the management of Los Alamos along with the University of California was to improve safety practices. That was done without considering the standard industrial management practice of rotating managers rapidly and the ever-present profit motive. Industry does things better, period. But perhaps not for a singular enterprise like designing and building nuclear weapons.

Coda: I am usually highly critical of articles on nuclear issues coming from the Center for Public Integrity. They often get the science wrong and display an excessive fear of radioactivity. They did a much better job with this investigation. There are a number of small errors and infelicities of word use in these articles, but nothing like the bloopers they have produced before.

Update: The National Nuclear Security Administration says that the issues of criticality safety have been cleared up.

This is plausible because the photo is said to have been made in 2011. That would allow time to reassert the importance of criticality safety and rebuild the group responsible for it. And, as I said above, the Center for Public Integrity has been sensationalistic in the past. But I’d like to hear more from NNSA. The fact that that incident occurred at all is disturbing.


Addendum (June 21, 2017)

How much plutonium is in that photo?

It’s pretty clearly a sharpie in the bottom of the photo. Sharpies are about 13.5 – 14 cm long. Scaling from there to the rods gives 12 cm long for the top four, and 5.7, 5.1, 4.5, and 3.8 for the bottom four. All have rounded caps of about 0.3 cm radius. I’m approximating, so we’ll neglect the rounded caps. The article says something about the yellow-and-black tape being raised, and the shadows agree, so the lengths of all the rods may be more than what I measured, depending on the angle of the photo. So the estimate is likely to be low, although probably less than tens of grams per rod.

Length (cm) Radius (cm) # of rods Mass in g (alpha phase, 19.86 g/cm3) Mass in g (delta phase, 15.92 g/cm3)
12 1.1 4 906 726
5.7 0.83 1 248 199
5.1 0.83 1 222 178
4.5 0.83 1 197 158
3.8 0.74 1 129 103

We can’t tell by looking what phase the plutonium is in, and the densities of the alpha and delta phases are quite different. So a high estimate, for the denser alpha phase, is a total of 4,420 grams in all eight rods; for the less dense delta plutonium, the total is 3,542.

Wikipedia gives the critical mass of plutonium-239 as about 11 kg, about 3 times what is in the photo. But there could be other isotopes. A critical mass depends on its surroundings, so depending what else is in the glovebox and the presence of a human body nearby, the critical mass could be less.

I have additional questions that pertain to the involvement of management and the safety procedures. How did the technician come to have that much plutonium? Plutonium is kept in the vault and must be signed out. Getting a photographer into PF-4 used to require a requisition for the services, and then there are procedural issues about taking a camera in that would have to be signed off by someone above technician level. Who signed off on the photo shoot?

Many thanks to Christopher Willis (@BeCurious) for checking my math, and Alex Wellerstein (@Wellerstein) for doing a parallel estimate.


Cross-posted to Balloon Juice.


  1. The Blog Fodder · June 19, 2017

    Good God!


  2. Peter Davis · June 20, 2017

    Excellent piece. My mother’s family and the Slotin family grew up together in Winnipeg.

    Liked by 1 person

  3. robgoldston · June 21, 2017

    Yup. Criticality safety is Job 0 when you are working with fissile material.

    Liked by 1 person

  4. John · June 21, 2017

    I know nothing of criticality safety, and have never been trained to handle anything more dangerous than a hot dog, but to my untrained eyes it would seem that the row of nipples along the length of the rods are there to reduce the chance of the rods rolling. Is this correct? If so, the technician has skillfully bypassed that safety feature as well by placing the rods closer than twice their diameter!


    • John · June 21, 2017

      Er, circumference. D’oh!


  5. Raymond Swenson · June 22, 2017

    Los Alamos created a chemical explosion four years ago in the plutonium disposal repository a half mile underground in a salt mine outside Carlsbad NM. The supervisor in charge of placing plutonium waste into 55 gsllon drums for disposal decided to repkace the clay absorbent, which captures stray liquids, with “biodegradable” organic kitty litter. The idiot did not understand that “biodegradable” stuff undergoes chemical changes, such as releasing hydrogen gas that can explode inside a drum. He shut down the disposal facility for three years, leaving plutonium spread around the cpuntry. Managers don’t have to be trained in all the cpurses ordinary workers take, and when they discourage their subordinates from speakng up and asking questions and telling the boss “No!” the stupidest are running things.


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  8. km · June 28, 2017

    Looks like it’s a 5 part series, or perhaps there are more to come. Curious of your opinion on the other articles.


    • Cheryl Rofer · July 5, 2017

      I just read the fifth article in the series and have some concerns about it, but won’t be able to write them up for a while. In general, it seems to me that the authors of the series have a narrative in mind that sometimes crowds out more complete reporting. That narrative is that the contractors are being paid too much and doing a poor job of safety. I don’t disagree too much with that, but I think the more interesting story is why they are doing such a poor job of safety. There are occasional indications of what could have been a fuller story, like the executive who thinks that criticality safety is a waste of money. How did he get a place at Los Alamos? What qualifications are required for executives who are rotated through the nuclear labs? How frequently are they rotated?


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