Reproducing (almost) Oklo reactor conditions in the lab

My stepdaughter was looking at projects for a school science fair. She's done "classic" nuclear science fair projects (cloud chamber, etc) but wanted to do something different... She wanted to build a reactor.

And before we go any further, no, she DIDN'T want to build a fusor or attempt a "nuclear boy scout"... Instead, she's fascinated by Oklo and had it that she might be able to make a demonstration version by putting chunks of uraninite in a graphite crucible (as is used for smelting), and seeing if there was any difference in temperature when filled with light versus heavy water (the former compensating for the higher u235 concentration at the time of Oklo); this would also be repeated/ compared with doing the same in a boron crucible.

I put the kibosh on it, mainly out of concerns for legality if it worked, and contamination concerns even if it didn't, but I told her I'd ask the experts: would it have worked?

Any explanation (preferably, why it WOULDN'T have!) would be appreciated, as would recommendations for alternative, safe(ish), projects--before she doubles down and tries to build a CANDU in the basement.

galvdoc said:

mainly out of concerns for legality if it worked

The legality concern comes way before that: how would you go about legally obtaining uraninite?

galvdoc said:

Any explanation (preferably, why it WOULDN'T have!)

How much uraninite would you need to have a critical mass of U-235? Remember that the Oklo deposit is a mine--a lot bigger than could fit in a crucible. Also, the concentration of U-235 in natural uranium 2 billion years ago was about 5 times higher than today.

galvdoc said:

before she doubles down and tries to build a CANDU in the basement.

If you can't stop this from happening, you have a bigger problem that we here at PF certainly can't help you solve.

galvdoc said:

as would recommendations for alternative, safe(ish), projects

She could make a fun demonstration project that doesn't actually cause any nuclear reactions, but illustrates some characteristic of fusion or fission.

For example, she could put together a setup with some number of mechanical piston actuators arranged in a sphere so that when you drop a ping-pong ball down into it, it triggers and squishes the ball spherically. This would be intended to illustrate Laser ICF. She would need to have some timed trigger/sensor mechanism that senses when the ball reaches the center, and triggers the solenoids or other actuators to fire all of the pistons inward simultaneously.

She would write up a paper or poster presentation on ICF and how it works, and included details of where the current projects like NIF are in their timelines...



https://chemconnections.org/crystals/icf.html

I don't think it's possible to build a reactor from natural uranium and graphite smaller than 10's of tons. The first British reactor was 100 tons I think. I think the smallest heavy water reactor is more than a ton. That is about a million dollars worth btw. As Peter said, aside from the massive amount of material in the ground, Oklo was made possible because that far back in time natural uranium was reactor grade.

Uranium minerals are legal in most of the world that I know, but putting them in water probably isn't a good idea lest they leech anything. You may also find that processing uranium ore is illegal, so I definitely wouldn't do that either.

If you can get an alpha spectrum of a uranium mineral one cool thing that tells you is how enriched it is. It's possible with a modified ionisation chamber and an MCA but I don't know if this is an achievable science fair project. If an ionisation chamber is built, then with uranium in it, a background hiss can be heard when it's connected to an amplifier (alpha background), with occasional pops - which are nuclei naturally fissioning. Maybe this forum has physics teachers that know this demonstration.

galvdoc said:

My stepdaughter wanted to make a mini Oklo reactor for a school science fair.

Just curious: just on what level of the education system did this pop up?
Especially, together with that hint about "classic nuclear science fair projects" already done.

Rive said:

Just curious: just what level of the education system could this pop up?

There are university programs in which one would do approach to criticality on very low power systems, e.g., AGN-201, or higher power systems, e.g., TRIGA reactor. Systems producing neutrons are highly regulated, and it is not something done at one's home or in a primary education institution. Such devices are protected because they use special nuclear material and/or produce neutrons.

There is a third type of system of natural uranium rod suspended in a pool of water where one can change the pitch of the rods and measure the 'neutron multiplication' of the system. It is a subcritical system, incapable of criticality. One typically looks at undermoderated (smaller pitch) and overmoderated (too much moderator/water, or too large a spacing). One measures the 'neutron multiplication' as a function of rod pitch, or rod-to-rod spacing.

This is a very ambitious science fair project, and it sounds like your stepdaughter is pretty creative.
I'll just answer one part of your question - why this wouldn't work.

First, the main reason that the Oklo reactor worked is that the uranium enrichment when Oklo was active than it is today. U235 decays faster than U238, so over time, the natural enrichment of U235 is going to drop.
When Oklo was active, the U235 enrichment was about 3%, and today it is only 0.7%.

Second, if you want to create a reactor today, you need to bring the uranium metal close together. This means that you need to convert the uranium ore (U3O8) into pure uranium metal. This is very dangerous and messy to do. They are still cleaning up early waste sites where this was done without proper protection during the Manhatten project. I would not recommend it.

Third, you will need to come up with an efficient neutron moderator. Normal water worked when the enrichment was 3%, but not that the enrichment is only 0.7%, you need a moderator such as pure graphite or heavy water. Natural graphite contains a lot of impurities, like boron, so it needs to be "reactor grade". Again, this is difficult and expensive. But if you have enough money, this is probably the easiest part.

Fourth, and final, you need a LOT of uranium and moderator to bring it to a "critical mass". Take a look at the size of Chicago Pile 1 to get an idea of how much uranium and graphite you would need to make it critical.

If your stepdaughter has already done a cloud chamber, it sounds like she already has a lot of knowledge about nuclear physics. If you live close to a university with a nuclear engineering program, you may be able to contact one of the professors there who might be willing to do a simple lab experiment for a high school science project. The university programs usually have some type of high school outreach program and are always looking for bright and ambitious students.