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Thursday, April 03, 2014

Nuclear Research Reactors - The MK 4 Experiments *UPDATED*

I was a reactor operator at General Atomics in the late 1980's.  General Atomics then operated two swimming-pool type TRIGA reactors.  The first was a MK IV reactor that was the bread and butter of the reactor division of the company.  It had two long-running experiments, one of which piggy-backed on the primary one.

Below, a TRIGA reactor similar to the one I operated.  Yes, it glows just like that!



The primary experiment that the MK IV was used for was in-core thermionic cell testing.  The goal of testing the thermionic cells was to determine the longevity of these unique devices. 

What the heck is a thermionic cell?  In order to explain, we need to understand how a basic vacuum tube works.

Below is a diagram of a vacuum tube called a diode.  It has two elements inside the glass tube (the circle).  These elements are called the cathode and the anode.  I prefer to call them an emitter and a collector.  In this diagram, the bottom emitter is heated by an electrical current, labeled "Filament Supply".  So - there is a short electrical loop completely dedicated to heating the spiral-wound filament, which is made of tungsten.  Tungsten is chosen because it ejects electrons when heated.  This process is called "Thermionic Emission"

Next, voltage is applied at the right side of the diagram, to put a positive charge on the Anode (collector), and a negative charge on the cathode (emitter).  This causes electrical current to flow from the emitter at the bottom, to the collector at the top, just like it were a piece of copper wire.

Current will not flow in the other direction if the voltage on the right is reversed though, because the anode is not a heated piece of tungsten, ready to fling electrons out.  This is the simplest vacuum tube around.  It is called a diode, and it only lets current pass in one direction, from cathode (emitter) to anode (collector).


So with this understanding of a basic vacuum tube, let's see if we can figure out a thermionic cell.

Suppose in our drawing above, we crank up the power provided by "Filament Supply", so the tungsten emitter is really blasting electrons out.  Next, we put the emitter in the center of the vacuum tube, and the collector encircles it, so that no matter what direction the electrons go, the anode (collector) will capture them.  This process is called thermionic conversion.  You heat the cathode hot enough and it kicks out electrons, and now you have DC current from cathode (emitter) to anode (collector).  On the right side of our drawing above, you could power a light bulb with this current.

The clever part of thermionic conversion is you get DC power without the need for a boiler, steam turbine, generator, condenser, pumps, etc, etc.  It's a pretty simple and elegant arrangement.

In a nuclear thermionic cell, the emitter is a small thimble of uranium that is coated with tungsten.  Around the emitter is a slightly larger collector.  The assembly is placed in a reactor.  Fission begins in the uranium, which in turn heats the tungsten coating.  The tungsten ejects electrons, and current flows in the thermionic cell.  Even a tiny thermionic cell can generate 1KW of low-voltage but high amperage power.

If you had enough of these devices in a single small reactor core, you could easily get a Megawatt of power.  This would be perfect to launch into orbit as a power supply for your space laserplasma weapon, or coil gun to shoot down incoming nuclear missiles - and in fact the project was funded by the Department of Defense through the Strategic Defense Initiative (aka the "Star Wars" Program).

The MK IV reactor was run round the clock in order to put reactor hours on the thermionic devices, to see how they would hold up, year after year. 

We had some other people who wanted to use the MK IV reactor's spare neutrons, and run their "experiment" alongside the thermionic experiment.  These guys were jewelers.  That's right, they made jewelry.  I don't think the jewelers had the income stream to keep the reactor running 24-7, so it was good that they could use the opportunity provided by the government's experiment.

This is where I learned about gemstone color enhancement.  Improving the appearance of gemstones goes back to ancient times, when emeralds would be soaked in oil to hide their fractures.  Nowadays though, a generic stone like topaz can be made beautiful by irradiation. 

The stones were packed into canisters and placed in dry Cadmium-lined tubes (to provide fast neutron flux only).  Even with fast neutron flux, due to trace impurities in the stones, they become radioactive during the process.  The jewelers would come in once a week and rotate the stones using remote tools, so that the stones would be irradiated from all sides.  After the stones were cooked, they would be removed from the reactor, but left on a shelf well below the water level, because they were quite radioactive, fresh out of the core.

It would take quite a while for the radioactivity to die off enough for the stones to be released to the public.  The jewelers probably had 5-10 million dollar's worth of inventory stacked out in cheap sheet-metal lockers at a given time, waiting for the activity to fade. 

 Here is a raw uncut, uncolored topaz, as you would find it in nature.  Not impressive.


Below is a cut natural topaz.  Nice clarity and cut, but I am not a fan of pee-yellow gemstones, and most others aren't either.

 Below is a Topaz that has been irradiated with fast neutrons.  This color is called "London Blue", or "Swiss Blue", and is actually about the color of Windex glass cleaner.

Below is a Topaz that has been irradiated by fast neutrons, and afterwards been irradiated by an electron beam provided by a linear accelerator.  I like this color better, and the jewelers started producing these after a while.  I helped install a used linear accelerator in the underground bunker.


 Below is the "Sky Blue" color you get when you irradiate a topaz with pure gamma rays from a Cobalt-60 source.  Pretty.  Also no neutrons, no radioactivity.  You can wear it the next day.

As always, fascinating stuff!

****UPDATE****

An interested anonymous commenter has graciously done a lot of homework and helped out with content on the Gemstone Color Enhancement process.  Thank you for that!

Here is a great link for a layman explaining how the various color shifts are achieved

Here is a cool research paper with experiments showing gemstone color changes with dose

Some early (1981) irradiated topaz stones were released before the radioactivity had died down

A nice long-form explanation of topaz color enhancement

Lastly, the link where all the above links (and many more) Can be found:

All very cool information!  Thanks again Anon commenter :)

6 comments:

Anonymous said...

what are typical conditions to receive the "London Blue" color?

Flux? Neutron spectrum? Temperatures?

Mark said...

Hi and thanks for visiting, anon. That's an excellent question for a reactor physicist!

Alas, I am not a physicist, just a reactor operator. I'm also not an expert on irradiating gemstones.

However.... I did find something on the web that says 10^17 fast neutron per square cm will yield the London blue color :) Here's the link.

http://www.academia.edu/5840301/Research_on_radiation-induced_color_change_of_white_topaz

Anonymous said...

Thank you very much for your fast reply and the link to the document.

The Chinese authors could achieve only "Sky Blue" and "Swiss Blue", not the dark "London Blue".

But maybe it is only a matter of dose, time and size of samples (penetration depth).

Mark said...

You are correct sir! I was not able to locate any information regarding the flux required for the London Blue color.

It would require fast or intermediate speed, certainly. What follow-up treatment would be needed? Heat? electron beam? Surely the information is known and out there, but I'm not finding it :(

Anonymous said...

I was searching the internet and found a list of literature (articles & patents) about topaz in general and also irradiation of topaz, which might be of interest to other readers of your blog, as well:

http://www.meelis.de/topas_literatur

Mark said...

Thanks very much, anon. Sorry it took a little longer to get back to you this time. Sometimes real life interferes with Blogging ;) I will update the original post with the new information!