# Executive Summary
MARVEL stands for Microreactor Applications Research Validation and Evaluation. MARVEL
is an 'open-source' microreactor design that is coming out of the Idaho National
Laboratory. This 'open-source' idea is a response to commerical reactors such as eVinci
from Westinghouse being closed-door endeavors. This isn't great, because it doesn't
allow a place for everyone to communicate about designs. This is especially relevant
for microreactors, where the low power model and economy-of-production model really 
benefits from common knowledge being out there. 

MARVEL is a unique design that I haven't really seen before. First of all, coolant
is by natural convection. This reactor system uses two coolant loops: the primary loop
is sodium-potassium salt, while the secondary coolant is lead. These two fluids
drive 'Stirling Engines', which are how the system generates power.


>[!info] # What are Stirling Engines?
>- [x] What are Stirling Engines? #MARVEL ✅ 2025-03-21
>
> Stirling engines are heat engines that directly convert heat energy into mechanical energy. 
> How they work is they have a piston that travels back and forth from a hot side to a cold side. 
> When the cylinder travels over the hot side the volume of air on that side heats up from the hot 
> walls and expands, forcing the cylinder in the direction of the cold fluid. At the same time,
> that fluid is getting colder, decreasing in pressure, and creating a vacuum. Then, once the piston
> moves to the cold side, the chamber compresses, removing the cooling, and heats up pushing 
> the piston back towards the hots side. Here's a graphic:
> ![[Pasted image 20250321163711.png]]
> 

Anyways, MARVEL also uses an interesting drum based system to do power control.
They use one big central 'insurance' rod, but also have four drums around the cylindrical 
core. These drums have a poison side, whose angle of incidence on the reactor plays a large part in how much neutron fission is allowed to happen. These drums have a fail-close state, that if something
were to happen to the controller, they would passively return to a shut position. To keep the poison
faces from killing with the reactor, the controller must actively apply force to the drums to prevent 
them rotating. 

These drum based control systems have a significant advantage that they don't create a 
longitudinal imbalance in reaction rate, compared to a typical control rod. Since they face / shy from
the entire core equally along its length, they can probably be more efficiient, and take up way less 
space. The insurance rod is a typical rod however, as it would not be possible to put a rotating
control drum in the middle of a cylinder. 🤷‍♂️