Understanding Criticality Safety in Nuclear Operations
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Understanding Criticality Safety in Nuclear Operations

By Dr. Zoomie

Let’s not get critical!

Dear Dr. Z – every now and again I read or hear a mention of “criticality safety” but I’m not sure what that means. From some of your earlier postings here I know that nuclear reactors have the “good” criticality and nuclear weapons have “bad” criticality – but criticality safety doesn’t seem to apply to either of those. I’m stumped – can you help explain?

You know, it does get confusing, doesn’t it? And to add to the terminology, in Naval Nuclear Power School we learned about a few other flavors of criticality as well – prompt criticality, delayed criticality, supercriticality, and subcriticality. Although I know I’ve discussed some of this in other postings, let’s quickly go over what “criticality” means and what it takes to achieve criticality, then we can get into how we keep criticality from happening except when (and where) we want it.

The simplest definition of nuclear criticality is that, in a critical system, the number of neutrons is steady. If the number of neutrons is increasing then the system is supercritical, subcriticality occurs when the number of neutrons is dropping over time. This is sort of abstract – think of a bathtub with both faucet and drain open. If water is flowing in faster than it is draining then water level in the tub will rise; if water is flowing in more slowly than the drain can remove it then the level in the tub will drop. Only when the faucet is adding exactly as much was as is being removed by the drain will water level remain steady – just as in a critical nuclear chain reaction the number of neutrons causing fissions (the “water level”) will remain steady over time. Since the neutrons come from fission then a steady neutron population means the amount of fission is also steady and, in the case of a nuclear reactor, power output is steady as well.

In other posts I’ve discussed how criticality is controlled in nuclear reactors, how the geometry and mass of fissile material affects criticality, what can happen when a reactor gets out of control, and discussed some criticality accidents in the early days of America’s nuclear weapons research – in this piece we’ll take a look at how we try to prevent unwanted criticality.

To get a self-sustaining nuclear chain reaction what we need to do is to put together enough fissile material (a critical mass) in a configuration (a critical geometry) such that one neutron from every fissioned atom will cause another fission. In natural uranium there are too few of the fissile U-235 atoms for this to happen, even in pure uranium – as we enrich the amount of U-235 present then it becomes increasingly easy to sustain a chain reaction. So it’s easier to prevent criticality with low-enriched uranium than for higher grades of enrichment – and it’s not required at all for natural or for depleted uranium.

As I pointed out in an earlier post, criticality requires a critical mass of uranium or plutonium and a geometry that lets the highest possible number of neutrons from fission encounter one of those fissile atoms. So if we want to prevent a criticality from occurring all we need to do is to avoid assembling a critical mass of uranium in a critical geometry. Easy-peasy, right?

Well…most of the time it’s not too bad, especially when the uranium is only enriched to low levels. But in the handful of nations that enrich uranium to weapons-grade criticality safety becomes increasingly difficult, and increasingly important. In his memoir Surely You’re Joking Mr. Feynman the Nobel laureate recounts visiting the Oak Ridge uranium enrichment facility as part of his work on the Manhattan Project. Among other things, he realized that even something as simple as the way containers of enriched uranium were arranged was important – that stacking containers too close together could lead to criticality, just as an assembly of uranium fuel rods could sustain a critical chain reaction even if no individual fuel rod was capable of doing so. In other words, it’s not enough to keep a single container safe from criticality – criticality safety requires keeping the entire assemblage safe.

We can start by making sure that no individual container can accumulate a critical mass of uranium in a critical geometry. One way to do this is to limit container sizes – if it requires (for example) 5 gallons of a particular solution of enriched uranium to sustain fission, limiting all containers to less than 2 gallons will ensure that no single container can experience a criticality. The biggest problem with this approach is that it’s hard for Housekeeping to empty wastebaskets into 2-gallon waste containers – sometimes we have to have larger containers. This is where we break out the drills, making holes an inch or so above the bottom of the larger waste cans so that an enriched uranium solution will drain from the barrel before a critical mass can accumulate. On the one hand, this can make a mess (as well as radioactive spill) if too much uranium solution is dumped into the container by mistake. On the other hand, it’s better to have a spill than a criticality. Along these same lines, we can also limit the depth of drip trays (trays put beneath valves, pumps, and pipes to collect drips and small leaks) so that they’ll overflow before a critical mass can accumulate. If we can control the amount of material that can collect in any one place then we can prevent criticality.

This approach also helps to control the geometry of the material – uranium solution that drains from a perforated trash can will form a thin, flat sheet of uranium solution that’s unable to become critical.  Keeping highly enriched uranium in thin pipes has the same effect – more neutrons escape from the pipe than go on to cause fission. But then we get back to the problem that plagued Feynman at Oak Ridge – if you get enough 2-gallon buckets close enough together and in the right arrangement you can still have a criticality. The solution to that, unsurprisingly, is to calculate safe arrangements for different container sizes and levels of enrichment – containers holding low-enriched uranium can be larger and/or more closely spaced than containers of weapons-grade uranium. What this means in practice is that container storage areas will have markings on the floor to designate where containers can be placed, and they’ll even have lines painted on the floors along which containers must be moved to prevent inadvertently creating a momentary critical configuration when moving containers from place to place.

In 1958 a worker at the Los Alamos Scientific Laboratory was processing a plutonium solution; one step of the process called for turning on a rotating stirrer to mix the liquids after adding the plutonium to a tank. Due to the chemistry of plutonium the fissile material collected in a layer of organic solvents that rose to the top of the liquid in the container – this didn’t cause any problems. But when the worker turned on the stirrer it caused the plutonium-containing layer to thicken slightly as the surface in the center of the tank was pulled down by the suction of the stirrer – in this configuration the plutonium-bearing layer pooled in the “dimple” in the center and achieved a critical geometry, producing enough energy to move the tank a half inch and enough radiation to deliver a lethal dose to the operator and set off radiation alarms more than 50 meters away.

This brings us to yet another criticality safety practice – procedural controls; developing procedures to prevent accumulating a critical mass in a critical geometry as well as changing procedures that are shown to have been flawed. Of course, procedures have to be followed to be effective – a failure to follow procedures was responsible for the 1999 criticality accident that killed two operators in Tokaimura Japan (here, technicians were rushing to finish a task, skipping steps and triggering the criticality.

There have been several criticality accidents originating from the over-accumulation of liquids in tanks and sumps – one way to approach this is to add neutron-absorbing compounds into the tank. Boron, for example, is great at absorbing neutrons; it’s also a common additive to glass (borosilicate glass is used in laboratories because it can withstand large temperature changes without breaking). What uranium enrichment, spent fuel reprocessing, and similar facilities will do, then, is to put rings of borosilicate glass (called Raschig rings) into these tanks and sumps – using rings leaves plenty of room to hold liquids in the tanks and the boron in the glass absorb enough neutrons to prevent a criticality.

The key to criticality safety, though, doesn’t lie in finding the best practice for a particular situation; the key is using multiple techniques so that, if one method is ineffective, impaired, or simply overlooked or ignored, the other safeguards are still in place. Any system that relies on a single protective feature or that relies on personnel following procedures 100% of the time is more likely to fail – in this case, to experience an unwanted criticality – than one that relies on multiple overlapping features.