HEU reactors vs. LEU reactors

[Marasmusine]

Summary:: How do high-enriched uranium reactors compare with low-enriched uranium reactors?

Hello, I'm doing some research for a book, and there's some information about fission reactors I just can't find.

I am comparing HEU fission reactors with LEU fission reactors ... even ballpark figures will be helpful.

So for example, the S8G HEU reactor used on the Ohio-class submarines weighed 2750 tons (including shielding, coolant and turbines, I think, correct me if I'm wrong). It produced 60,000 hp for the driveshaft. The fuel endurance was 9 years.

What would be the performance of a LEU reactor of a similar mass? From what I've read, the fuel would need replacing 2 to 3 times more often. But I think LEU would be cheaper to manufacture (7 SWU instead of 230 SWU).

Performace per mass of the whole reactor is of most interest to me.

Thanks for reading, apologies if it is an inappropriate question, this is my first post.

 

[etudiant]

Difficult question, not only because there are no commercial reactors that are comparable in terms of size afaik.

Most LEU reactors are much bigger than the S8G and I do not think that performance per mass was ever a target parameter for LEUs, whereas it well should be for a vessel that needs to minimize mass to maximize boat performance.
So the design goal are entirely different, which further distorts the answer, perhaps totally.

 

[Astronuc]

HEU reactors are smaller, hence the need for HEU. Commercial power reactors are much larger and produce greater power levels. One mentions 60,000 shp marine power plant, or 45 MW, so let's use that number for comparison to commercial power reactors.

Large commercial power reactors produce ~900 MWe - 1200 MWe, so about 20 to 27 times larger. One has to look at individual reactors that produce more power. A 900 MWe reactor has a fuel inventory of ~72-73 tU (81.7 - 82.8 tUO₂), and larger 1100-1200 MWe reactor would have ~82-88 MTU (92 - 100 tUO₂), depending on fuel pellet diameter, pellet density, and whether or not the low enrichment blankets have annular or solid pellets. Many commercial power reactors in the US have been uprated to generate more power from the original design license.

Fuel management and fuel cycles have changed over the decades. LWR fuel was originally intended for 3 or 4 years on annual cycles (with third core or quarter core reload batches) and subsequently reprocessed/recycled. When reprocessing disappeared as an option in the US, fuel cycles were extended to 18 to 24 months, and fuel batch sizes were increased to nearly half-core in some cases.

There is no such thing as a typical power reactor, even those in the same design class, since basically every reactor is custom designed.

 

[anorlunda]

Performace per mass of the whole reactor is of most interest to me.

That's a very strange interest. Why?

 

[Marasmusine]

It's nothing important or academic I'm afraid!

It's for an role-playing game supplement I'm writing, to allow the design of vehicles. To simply the design process, vehicles are broken down into 20 components of equal mass. So that Ohio-class submarine will have 3 components of "HEU Fission Reactor Plant" providing X amount of power to 1 "Shrouded Screw Propeller" component.

This system has given me fairly accurate vehicle speeds for things like gas-turbine powered helicopters and sailing ships.

I can look to civilian nuclear ships such as the NS Savannah but I can't always find the numbers I need for the mass of the fission plant.

Astronuc's reply has given me more confidence in just making these numbers up :)

 

[Astronuc]

It's nothing important or academic I'm afraid!

It's for an role-playing game supplement I'm writing, to allow the design of vehicles. To simply the design process, vehicles are broken down into 20 components of equal mass. So that Ohio-class submarine will have 3 components of "HEU Fission Reactor Plant" providing X amount of power to 1 "Shrouded Screw Propeller" component.

This system has given me fairly accurate vehicle speeds for things like gas-turbine powered helicopters and sailing ships.

I can look to civilian nuclear ships such as the NS Savannah but I can't always find the numbers I need for the mass of the fission plant.

Astronuc's reply has given me more confidence in just making these numbers up :)

The numbers for the NS Savannah reactor can be found in a Wikipedia article on the ship,
https://en.wikipedia.org/wiki/NS_Savannah#Reactor

Note, the ship's power plant was designed for 20,000 horsepower (15 MW), as compared to a vehicle like a HMMWV with a power level about 190 hp (140 kW). One would need a small reactor, and assume a metal fuel, for compactness. Enrichment would not significantly affect the mass because U-235 is only 1% less mass than U-238. One has to decide on a power conversion cycle, e.g., steam (Rankine) or gas (Brayton). One might wish to look at nuclear rocket cores, e.g., Kiwi, Nerva or Phoebus.
See - https://en.wikipedia.org/wiki/Project_Rover

Power conversion systems and shielding add large masses.