What is the practical size of a neutron research reactor?

[Josiah]

TL;DR Summary

Is there such things as a fission micro reactor

What is the size of the smallest reactor ever made?

 

[fresh_42]

The smallest reactor is 5 m long, 1 m wide and a few centimeters thick. This smallest reactor is located very close to the earth's surface and is therefore exposed to severe weathering. The actual reactor core consists of 5 to 20 cm thick layers of uraninite embedded in clay.

https://en.wikipedia.org/wiki/Oklo_Mine

I don't think you can get much smaller.

 

[scottdave]

This KRUSTY reactor could be a candidate.

https://www.nasa.gov/news-release/d...n-system-can-provide-space-exploration-power/

 

[Astronuc]

TL;DR Summary: Is there such things as a fission micro reactor

What is the size of the smallest reactor ever made?

Practical in what sense? Thermal output? Electrical generation? Propulsion?

One needs to define a set of functional requirements, e.g., thermal/electrical power output of the system, duration/life-cycle, refueling schedule (unless one-time), operating conditions, then apply technical constraints.

 

[Rive]

There are some fission micro reactors designed for satellites. SNAP-10A, BES-5, Kilopower, and their never-launched experimental sister-designs...

I think they might be the closest to 'smallest practical'.

 

[OmCheeto]

10 watts is the lowest listed at wiki: List of nuclear research reactors. Though there are numerous listed as being '0.00 kw', which I infer as meaning <10 watts.

Not sure what good 10 watts is outside of research. Maybe you could keep a guppy warm in a 1/2 gallon fish bowl.

 

[OmCheeto]

From the list above I found this operating manual for a 5 watt reactor model used at Texas A&M and the University of New Mexico: https://www.nrc.gov/docs/ML2019/ML20195E222.pdf

Wow! They are/were loaded with over a lb. (620 grams) of U-235. Those reactors must last forever at 5 watts.

 

[russ_watters]

Does an RTG count?
https://en.wikipedia.org/wiki/Radioisotope_thermoelectric_generator

 

[Astronuc]

From the list above I found this operating manual for a 5 watt reactor model used at Texas A&M and the University of New Mexico: https://www.nrc.gov/docs/ML2019/ML20195E222.pdf

Wow! They are/were loaded with over a lb. (620 grams) of U-235. Those reactors must last forever at 5 watts.

Texas A&M has an AGN-201 5W reactor.

https://engineering.tamu.edu/nuclear/research/facilities/agn-201m-nuclear-reactor-laboratory.html

The AGN-201M Nuclear Reactor Laboratory operates a 5 W AGN-201M nuclear reactor which teaches fundamentals of nuclear reactor operations and interactions of neutrons with matter and lets students conduct experiments on basic reactor physics parameters. The AGN is used primarily to support education programs rather than research.

Texas A&M University purchased the AGN reactor in 1957 to be used for the newly forming Department of Nuclear Engineering. Originally located in Thompson Hall, the reactor was moved to the Zachry Engineering Center in 1972, and was moved again in 2016 to the Nuclear Science Center. The reactor is currently being refurbished in its new location to facilitate undergraduate and graduate learning for the next generation of students.

The AGN reactor has a thermal power rating of 5W. The reactor utilizes a homogeneously mixed polyethylene and UO2 plate type fuel. The fuel is surrounded by graphite and is contained within a pressure tight vessel fabricated from aluminum. Natural convection maintains the core temperature relatively stable by removing heat that is generated being lost to the water surrounding the core. The reactor is controlled by four fueled control rods that are inserted into the core to maintain control of the nuclear reaction.

Students used to startups of the reactor and occasionally activation analysis. It was low temperature - with UO2 dispersed in polyethylene. With fuel control rods, they would be inserted from below and drop (out of the core) with gravity. It's been ~40 years since I've been involved with that reactor.

The manufacturer was Aerojet General Nucleonics (AGN), a division of Aerojet General.

AGN-201 for Catholic University
https://www.nrc.gov/docs/ML2003/ML20038B261.pdf

AGN-201 at Idaho State
https://www.isu.edu/cose/research/nel/

AGN ML-1 3.3 MW moblie reactor - total plant weight, < 40t
https://www.osti.gov/servlets/purl/4880928

Aeroject General was part of General Tire and Rubber Co.
https://en.wikipedia.org/wiki/Aerojet
https://en.wikipedia.org/wiki/Aerojet_Rocketdyne

https://www.l3harris.com/aerojet-rocketdyne-history

 

[Astronuc]

Does an RTG count?
https://en.wikipedia.org/wiki/Radioisotope_thermoelectric_generator

While RTG are nuclear (decay) based and produce thermal energy, they are not considered reactors in the sense that they do not produce much in the way of a neutron flux and they do not generate heat primarily by fission. RTGs are probably mostly Pu-238 which decays by alpha emission, but there is a small amount of spontaneous fission.

Plutonium 238 has a spontaneous fission half life of 4.8 x 10 to the 10th power years. Neutrons associated with this spontaneous fission are emitted at a rate of 28,000 neutrons per second per gram.

https://ntrs.nasa.gov/citations/19720010066

 

[berkeman]

Texas A&M has an AGN-201 5W reactor.

https://engineering.tamu.edu/nuclear/research/facilities/agn-201m-nuclear-reactor-laboratory.html

Holy smokes! That big thing is a 5 Watt reactor?

 

[OmCheeto]

Holy smokes! That big thing is a 5 Watt reactor?

View attachment 338433

As I mentioned, they must last forever at 5 watts.
If you release all the energy from ≈1 kg of U-235 fissioning all at once, the internet says that's the equivalent of 18,000 tons of TNT.

 

[Astronuc]

Holy smokes! That big thing is a 5 Watt reactor?

It's not that big. The vessel is made of an aluminum alloy, and it can be moved.

It was relocated from the Zachry Engineering building on the main campus to the Nuclear Science Center, which is located out near the airport.

https://nesc.tamu.edu/
https://engineering.tamu.edu/nuclear/research/facilities/nuclear-science-center.html

The core is very small, however, it is a source of neutrons when operating, so the vessel provide shielding.

 

[scottdave]

If the purpose of the reactor is to produce radio isotopes for medical or industrial purposes, then it is practical.

https://world-nuclear.org/informati...rld's nuclear,many are on university campuses.

 

[Astronuc]

ORNL's HFIR and similar reactors would be a small, practical reactor, if one need's only neutrons (with a high flux). It is used for neutron activation, neutron diffraction/scattering experiments (including materials testing) and neutron radiography. It has small core, and is about the size of a washing machine, excluding the pool in which it sits.

https://neutrons.ornl.gov/hfir

TRIGA reactors also sit in a pool of water, and the cores are much smaller than the pool. The pool provides shielding from radiation (gammas and betas). The reactor core is suspended from a moveable bridge, which sits over the pool. The reactor may be brought to the edge of the pool if a neutron flux is needed in one of the irradiation cells. Otherwise, experiments (specimens) can be inserted into special locations in the core, or shot into the core in a special pneumatic system.

Such research reactors are practical as neutron sources for research and isotope production, their intended functions, but they are not practical for power generation.