How exactly would a fission reactor power a 'mech?

[Liam's Ghost]

  I'm having trouble wrapping my head around how a mech could get useable power out of a fission reactor.

  I admit I'm not scientist, but I at least understand more or less the basics of how the other types of propulsion (fusion, fuel cell, ICE) are supposed to work, but from what I understand the standard means of deriving electrical power from fission is using it to heat water to run a turbine.

  But somehow that just seems like way too much to actually fit in a mech's torso.

  Is there something I'm missing here? Are fission powered mechs also technically steam powered? Is there another mechanism for extracting useful power from a fission reactor?

[Gladius-XC]

Unless there's a spiffy explanation somewhere, I don't think you're missing anything.  You are correct that fission power plants create heat, which boils water in a steam generator to well, generate steam.  That steam drives a multi-stage turbine, which drives the main generator to create electricity.  The reactor and its support systems on the primary side, the steam generators, and the secondary side systems leading to the turbine generator are huge.  And the efficiency is only around 33%.  A big part of the limitation is the use of water for transporting energy.  There's a term called enthalpy, which is a measure of the energy in a material.  Water reaches its peak enthalpy at around 900°F.  In order to bump the efficiency, to get more power out, ideally, you'd want to heat your working fluid more, but with water, that doesn't actually get you anything because of the enthalpy.  So, to get more power, you need to make the whole thing bigger.  That works for something like a ship, but not so well for a battlemech.  Now, 1000 years of engineering and some handwaving can go a long way to making it more feasible.  Using an exotic working fluid instead of water, that can be heated well beyond 900°F to it's maximum enthalpy, and you're most of the way there.

Don't get me started on shielding the mechwarrior from the radiation...

[Frabby]

Don't get me started on shielding the mechwarrior from the radiation...

Hey, if submarines can do it...

A US Navy officer once told me that Soviet submarines seemed to be more capable ton-per-ton compared to western designs, but that was because they felt reactor shielding wasn't that terribly important. Not sure how accurate that is, but that's what he said. (He served one tour on a 688, but was a medical officer iirc.)

[AlphaMirage]

You could have a gas cooled reactor that does the same but with a noble gas driving the turbine instead of steam. There are a few more Gen IV-V fission designs that might be interesting.

[worktroll]

A US Navy officer once told me that Soviet submarines seemed to be more capable ton-per-ton compared to western designs, but that was because they felt reactor shielding wasn't that terribly important. Not sure how accurate that is, but that's what he said. (He served one tour on a 688, but was a medical officer iirc.)

Very true about Soviet nuc plants. They designed them to be supercritical - eg. quick to power up, but also annoyingly difficult to slow down. This increased energy density, reducing size. They also took a more casual view of shielding. "How do you know if they served in Northern Fleet? They glow in the dark!"

US/UK reactors are designed trickier to keep ticking over, and failure modes aim to reduce reactivity. Which requires more engineering, hence more mass & volume.)

Now to the point at hand ... fusion reactors in the real world could generate power by tapping the magnetic field of the fusing plasma to generate electricity directly, without needing cooling loops. Fission plants could not do that. They could in theory use advanced thermocouples - which generate electricity based on temperature differentials. But you'd need one end more or less inside the reactor core for maximum efficiency. But that would mass less than primary/secondary loops.

[klarg1]

Fusion has exactly the same issue.

Fission, fusion, oil, gas, coal all use the same principle of generating lots of heat and using that to generate electricity. In the real world that is generally via steam turbine.

How does that fit in a ‘mech? Very efficiently. *shrug*

[Giovanni Blasini]

That would basically make it essentially an overpowered RTG, wouldn’t it?  They use thermocouples to draw power from the heat of radioactive decay of their fuel.

[klarg1]

Now to the point at hand ... fusion reactors in the real world could generate power by tapping the magnetic field of the fusing plasma to generate electricity directly, without needing cooling loops. Fission plants could not do that. They could in theory use advanced thermocouples - which generate electricity based on temperature differentials. But you'd need one end more or less inside the reactor core for maximum efficiency. But that would mass less than primary/secondary loops.

Interesting

[Empyrus]

Fission fragment system is a possibility. Alpha and beta particles are charge, and can be used to generate electricity.

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

[mechasaurus]

Fusion has exactly the same issue.

Fission, fusion, oil, gas, coal all use the same principle of generating lots of heat and using that to generate electricity. In the real world that is generally via steam turbine.

How does that fit in a ‘mech? Very efficiently. *shrug*

Yep.  Before you get to fission, it's worth asking, "How do fusion reactors power a 'mech?"  Direct conversion is still mostly conceptual, but presumably, if you can imagine a fusion direct drive, you could imagine a fission direct drive.  The first commercial fusion reactors are mostly planned to have a steam cycle, just like a fission plant (or coal plant, for that matter).  That is, heat from the reaction is due to energetic particles being absorbed in a blanket and being used to create steam which turns a turbine (electricity generation for sale is well understood). Small, portable fusion reactors are currently even more sci-fi than large commercial reactors!

[Charistoph]

Now to the point at hand ... fusion reactors in the real world could generate power by tapping the magnetic field of the fusing plasma to generate electricity directly, without needing cooling loops. Fission plants could not do that. They could in theory use advanced thermocouples - which generate electricity based on temperature differentials. But you'd need one end more or less inside the reactor core for maximum efficiency. But that would mass less than primary/secondary loops.

Modern-day fusion reactors also require deuterium fuel to stay powering (albeit quite inefficiently), while fission reactors keep powering until their fuel decays after too many half-lives.

Still, they are a millenia ahead of us in both fusion and fission tech, who knows what other principles they could have learned in the process, not to mention materials found on new worlds.

[BATTLEMASTER]

I like the idea of Battletech military fission reactors being just really powerful RTGs.

Electric power plants in general are pretty neat.  Combined cycle plants are a little over 70% efficient thanks to exhaust heat recovery powering a steam turbine :)  I've been to a plant that used a Cat Solar turbine powered with natural gas to generate 40 MW.  It produced enough waste heat to provide steam for a 10 or 15 MW turbine.  The heat recovery stack was a giant box probably about 1200 sqft in footprint and about 5 stories tall

[glitterboy2098]

per Techmanual Page 35, Fusion engines produce power using turbines as well. while their main power production is via Magnetohydrodynamic processes (stripping electrons directly out of the fusion plasma), it also uses "regenerative cooling" as a suppliment. that is, using the waste heat from the reactor to run a turbine.

Hey, if submarines can do it...

A US Navy officer once told me that Soviet submarines seemed to be more capable ton-per-ton compared to western designs, but that was because they felt reactor shielding wasn't that terribly important. Not sure how accurate that is, but that's what he said. (He served one tour on a 688, but was a medical officer iirc.)

modern nuclear subs benefit from operating in and under a vast expanse of nuclear shielding material. usually the reactor area has thick walls between it and the crewed areas, but fairly thin ones along the hull. with just enough to protect people on a dock when the sub is in port with its power output reduced.