The role of delayed neutrons in controlling a nuclear reactor

It's been a while since I did a nerdy nuke post.  I've had a subject in mind, and been thinking a bit about how to describe some of that nerdy stuff in an understandable way...  I guess it's time to give that a shot.

Today's post is about delayed neutrons, and how they allow a reactor to be operated without power taking off exponentially within microseconds.  I brushed over delayed neutrons in another post, but this will be just a little more in depth discussion of the how and why.

K-431 (Project 675)

K-431 was an Echo II class submarine.  Construction was started in January 1964, and she was commissioned in September of 1965, in a very rapid construction process.  The Echo II class was a twin reactor ship which carried cruise missiles for attacking surface ships.  Not very pretty.

TRIGA - an amazingly safe nuclear research reactor

So far we have talked about natural reactors, fast neutron reactors, plutonium production reactors, and thermal power plant reactors.

There is one cool reactor that I would be remiss in discussing, and that is the TRIGA reactor.  I worked at a facility where two of these were located, and was licensed to operate them both.  One was the very first TRIGA reactor ever built, rated at 250 KW (thermal), and the other was a MK IV model, rated at 1.5 MW (thermal).  These reactors are swimming-pool reactors, and so they don't generate steam or electrical power using steam turbines.

In fact TRIGA stands for (T)raining, (R)esearch, (I)sotope production, (GA) General Atomic, the manufacturer.

In the late 1950's there was a desire to promote "Atoms for Peace".  This was Eisenhower's attempt to invoke the power of the atom for peaceful purposes.  The world was understandably horrified by the images of Hiroshima and Nagasaki, as well as the possibility that the budding cold war might turn into a hot war at some point.

The invention of TRIGA reactors went a long way toward fulfilling that vision.  Unlike other reactors, TRIGA reactors have a solid moderator that is cast into the fuel itself.  Therefore there is a homogenous blend of solid moderator and fuel.  The moderator is Zirconium Hydride, and as you would expect, the hydrogen atoms do the moderating. 

Because about 60% of the moderator is solid and homogenous with the fuel, this reactor has what is known as a "prompt negative temperature coefficient of reactivity".  In other words, the very instant a runaway nuclear reaction begins and starts causing fuel temperature to increase, the solid moderator temperature also instantly increases, which in turn reduces the available thermal neutrons.  This provides a VERY rapid damping of the runaway nuclear reaction. 

If you recall in an earlier post, it is important (in all reactors except TRIGA) to never allow the reactor to be critical on prompt neutrons alone, because each generation of neutrons only last 10^-14 seconds.  There is no way to control a reaction that proceeds so quickly, so the small fraction of delayed neutrons are what allow us to control reactors.  Chernobyl, SL-1, and the Borax Experiment were each prompt criticality events that ended badly.

TRIGA reactors though, can easily tolerate a prompt critical event.  Doing this is called "Pulsing" the reactor. Any reactor can be pulsed, but only a TRIGA can do it more than once ;)  In fact, the record reactivity insertion into any reactor was  TRIGA, at 5.22 times the value needed to be prompt critical.  Because the moderator heats up as rapidly as the fuel, it shuts the reactor down just as soon as heat is generated, in a few thousandths of a second, without operator intervention.

TRIGA reactors ended up being sold around the world.   Being low-power, they weren't practical for making weapons, and the solid UZrH moderator was incredibly difficult to extract from the fissionable fuel, so using the fuel for making weapons was not possible.  Even so, currently manufactured TRIGA fuel has been reduced from 20% U-235 down to 7% to prevent proliferation.

Here is a video of a TRIGA reactor being pulsed to 2.5 x prompt criticality.  Any other reactor would vaporize the fuel and create a steam explosion, blowing water upwards out of the tank!



Below is an image of a TRIGA reactor at the bottom of the pool, while not in operation.  As you can see, it's quite simple.  The fuel elements rest on a bottom grid plate, and are kept vertical by the upper grid plate.  The fuel can be grabbed by a long-handled pole with a ball-type coupler at the end, similar to how modern hydraulic couplings work.

The rods sticking down into the core are just aluminum shafts that connect the drive motors to the control rods (which are partially out of the core).

The inner ring around the core is a lazy susan.  Samples can be dropped into a number of holders in this dry ring.  When the reactor is in operation, the ring rotates to ensure each sample is exposed to equal amounts of neutron flux.  This is useful for performing neutron activation analysis on several samples at once.

The outer ring is a graphite reflector/moderator, which reduces the amount of fuel needed.  The cans outside the reflector are neutron detectors, for determining what power level the reactor is at.  The little lanyard at the bottom is attached to a neutron source (usually Americium/Beryllium).  This makes sure there are enough neutrons available to start the reactor up.  Also its a daily test to pull it and stick it next to each neutron detector and make sure they work OK before you start the reactor up.

Nuclear Reactors and Nuclear Weapons - There is a difference!

I know in a lot of people's minds, there is a notion that a nuclear power plant could detonate like a nuclear weapon.  It's not possible, and I will use the previous few posts to explain why that is.

Recall that neutrons from fission have a very high energy level, about 2 MeV, which equates to a speed of 45 million miles per hour.  Neutrons traveling this fast do not interact with Uranium atoms as often as they do after they have been moderated (slowed down) to thermal speed, about 4900 miles per hour.

Nuclear weapons require an absolutely uncontrolled fission reaction. The more fissions that occur before the core is vaporized, the better.  This requires that the entire thing take place in microseconds, which also means the neutrons must be fast neutrons.  It also requires a VERY dense concentration of fissile material, because fast neutrons don't interact well with atoms.

There are some other techniques used to improve nuclear weapon performance.  A neutron reflector is one.  It bounces fission neutrons back into the reaction, and makes more fissions possible before the whole thing vaporizes.  Another trick is using a tamper - a thick casing to hold the exploding device together for a couple more microseconds, allowing a couple more generations of neutrons to build up.

With a weapon, if the neutrons were thermalized, it would slow down the chain reaction to the point that the uranium or plutonium device would melt or vaporize, rather than detonate.  AKA, a "fizzle".

On the other hand, nearly all nuclear reactors are "thermal" reactors, meaning the neutrons are slowed down.  This provides fine control of the nuclear reaction, but more importantly allows us to use lower enrichment of Uranium than would be possible in a Fast Neutron Reactor.  Fast neutron reactors require enrichment that would be appropriate for weapons, which is why nobody really likes them. 

Typical Thermal Neutron Reactors, such as those in power plants, use just 3-5% enriched Uranium, which is too low for a nuclear explosion of the weapons type.

What can occur in a thermal reactor though is a runaway chain reaction that creates enough heat to vaporize metal in the core and create a steam explosion.  All nuclear reactors rely on "delayed neutrons" to achieve control.  Most fission neutrons are released within 10^-14 seconds following a fission.  These neutrons are called "prompt neutrons".  That reaction speed is far too fast for humans or electronics to prevent a runaway chain reaction. 

Fortunately for us, a small fraction of neutrons are produced by the neutron-rich fission fragments of the split atoms.  So... an atom fissions (splits) and releases 2-3 prompt neutrons.  Some of the split nuclei decay later and release more neutrons.  These are the "delayed neutrons", which can be generated up to 55 seconds after the initial fission event.

The key to controlling a nuclear reactor is to never allow the reactor to be critical on Prompt Neutrons alone.  If a reactor does become critical on Prompt Neutrons, the condition is called "Prompt Critical".  This is a dangerous uncontrolled reaction, which led to the violent explosions at both Chernobyl and SL-1.  It was also done intentionally for the SPERT experiment.

Each of these reactors experienced a prompt criticality event, just like a nuclear weapon.  However, because the neutrons were thermalized, which greatly slows down the increase of neutrons, none of these reactors detonated with a nuclear weapon type of blast.

Here is some interesting footage of the Prompt Critical SPERT reactor experiment:


Below are two accounts of the account of the prompt critical SL-1 accident, where the first operator fatalities took place. The first is a YouTuber overview, and the second is an old newsreel that goes into greater depth.

And lastly, Chernobyl, nicely explained by Scott Manley.  I like to flatter myself that maybe he read this post before making that video :)  Chernobyl was the worlds biggest Dirty Bomb.