Uranium fuel is a category of nuclear fuel made of uranium isotopes, specifically the fissile uranium-233 and uranium-235. They also often contain uranium-238 for stability and the byproduct of its neutron absorption, which produces the valuable plutonium-239. Uranium fuels can range from the stable MEU (medium enriched uranium), to the very powerful, weapons grade HEU-233 and 235 (highly enriched uranium). Its use is widespread among reactors.

Natural Uranium

Uranium Fuel
Heat/tick	50
Estimated lifespan	200,000 ticks
Start radiation	0.5 RAD/s
End radiation	150.0 RAD/s

Unenriched Uranium Fuel
Composition
Fissile material	U-235
Fertile material	U-238
Statistics
Yield	100,000,000
Self-igniting?	No
Splits with	Slow neutrons
Splits into	Fast neutrons
Flux function
Flux function type	Base 10 logarithm
Flux function danger	Medium
Depletion function	Raising slope
Xenon generation function
Xenon burn function
Heat at 100 flux	0.65°C
Diffusion	0.02 1/2
Melting point	2,865°C
Starting radiation	1.4 RAD/s

Natural Uranium Fuel
Heat/tick	30
Estimated lifespan	250,000 ticks
Start radiation	0.35 RAD/s
End radiation	172.5 RAD/s

Natural uranium (sometimes NU or UEU) is, as the name suggests, is simply natural uranium that was mined, without enrichment. Since natural uranium already contains some U-235, it can be used as a fuel, albeit a weak one and one that requires extra moderation.

Production

See: Uranium#Production

Natural uranium requires no additional processing beyond standard ore refinement needs.

ZIRNOX

It is quite a weak fuel, weaker than even thorium fuel. However, it lasts far longer and can be produced quickly and easily, meaning it may be the first nuclear fuel you use. While weak, it's quite stable and efficient. The recycling products are also quite valuable too, as it produces a good supply of plutonium isotopes, including pure plutonium-239, that can be used in other, more powerful fuels or nuclear weapons.

Assembly

Recycling

2 U-238 nuggets
4 reactor grade plutonium nuggets
2 Pu-239 nugget
4 tiny piles of nuclear waste

Per rod, divide by 2 to get a billet's worth of material.

RBMK

Similar to the ZIRNOX, it is quite weak, but now stronger than thorium, as it uses a base 10 logarithm function. So, while still fairly weak, it is better for self-sustaining criticality, as the function permits. More notably, it also produces plutonium-239 with low depletion levels. After 40% depletion, it becomes reactor grade plutonium, so it encourages removing the fuel quickly after usage to make use of the Pu-239. Additionally, as a result of the plutonium build-up, it features a "raising slope" depletion function, meaning it gradually becomes stronger as it depletes. So its use may depend on your needs, whether it be power or weapons grade material production.

Graph

Assembly

Recycling

Requires a SILEX with an infrared laser from the associated FEL.

(Xenon poison reduces short-lived waste by 1%)

Brand New
- 86.0% natural uranium
- 10.0% Pu-239
- 2% long-lived nuclear waste (U-235)
- 2% short-lived nuclear waste (U-235)
Barely Depleted
- 75.0% natural uranium
- 13.0% Pu-239
- 5.0% long-lived nuclear waste
- 7.0% short-lived nuclear waste
Moderately Depleted
- 64.0% natural uranium
- 16.0% reactor grade plutonium
- 8.0% long-lived nuclear waste
- 12.0% short-lived nuclear waste
Highly Depleted
- 53.0% natural uranium
- 19.0% RGPu
- 11.0% long-lived nuclear waste
- 17.0% short-lived nuclear waste
Fully Depleted
- 42.0% natural uranium
- 22.0% RGPu
- 14.0% long-lived nuclear waste
- 22.0% short-lived nuclear waste

Medium Enriched Uranium-235

MEU is one of the easiest fuel to produce in after natural uranium, as it can be made directly by centrifugal enrichment of uranium hexafluoride from natural uranium.

Production

It can also be produced from the pure isotopes:

6

ZIRNOX

It is quite stronger than natural uranium, but has a somewhat lower lifespan. Of note, it lacks the ability to produce plutonium-239, replaced with the raw plutonium isotope mixture. It also doesn't have any uranium to recycle at all, but it is one of the cheapest fuels that is able to produce technetium-99, a common fission isotope useful for technetium steel, a highly corrosion resistant alloy.

Assembly

Recycling

4 reactor grade plutonium nuggets
2 plutonium nuggets
2 Tc-99 nuggets
4 tiny piles of nuclear waste

Per rod, divide by 2 to get a billet's worth of material.

PWR

MEU Fuel
Composition

Fissile material	U-235
Fertile material	U-238
Statistics
Yield	1,000,000,000
Reaction function	$\log _{10}(x/2500+1)*600$
Function type	Logarithmic
Function danger	Medium
Heat per flux	5.0 TU
Hazards
Start radiation	0.5 RAD/s
End radiation	5.0 RAD/s

MEU fuel in the PWR is a very weak fuel, as are most of its cheap fuels. Its heat output is also below average.

Graph

Assembly

Recycling

3 U-238 nuggets
4 mixed plutonium nuggets
2 Tc-99 nuggets
3 tiny piles of nuclear waste

RBMK

Medium Enriched Uranium-235 Fuel
Composition

Fissile material	U-235
Fertile material	U-238
Statistics
Yield	100,000,000
Self-igniting?	No
Splits with	Slow neutrons
Splits into	Fast neutrons
Flux function	$\log _{10}(x+1)0.520$
Flux function type	Base 10 logarithm
Flux function danger	Medium
Depletion function	Raising slope
Xenon generation function	$x*0.5$
Xenon burn function	$x^{2}*50$
Heat at 100 flux	0.65°C
Diffusion	0.02 1/2
Melting point	2,865°C
Starting radiation	2.0 RAD/s

It is marginally stronger than natural uranium, but as expected, it loses some of the advantages in recycling. The plutonium-239 yield is much lower and the threshold before it becomes polluted with plutonium-240 is higher. Conversely, it is able to burn more of itself to yield more reactor grade plutonium and any byproducts in the produced nuclear waste.

Graph

Assembly

Recycling

Requires a SILEX with an infrared laser from the associated FEL.

(Xenon poison reduces MEU by 1%)

Brand New
- 84.0% MEU
- 6.0% Pu-239
- 4.0% long-lived nuclear waste (U-235)
- 6.0% short-lived nuclear waste (U-235)
Barely Depleted
- 68.0% MEU
- 10.0% reactor grade plutonium
- 9.0% long-lived nuclear waste
- 13.0% short-lived nuclear waste
Moderately Depleted
- 52.0% MEU
- 14.0% RGPu
- 14.0% long-lived nuclear waste
- 20.0% short-lived nuclear waste
Highly Depleted
- 36.0% MEU
- 18.0% RGPu
- 19.0% long-lived nuclear waste
- 27.0% short-lived nuclear waste
Fully Depleted
- 20.0% MEU
- 22.0% RGPu
- 24.0% long-lived nuclear waste
- 34.0% short-lived nuclear waste

Highly Enriched Uranium-235

HEU-235 is a highly potent fuel consisting of 100% U-235, which is weapons grade. It is very strong, so one must be careful when using it.

ZIRNOX

Uranium-235 Fuel

Heat/tick	85
Estimated lifespan	165,000 ticks
Start radiation	1.0 RAD/s
End radiation	165.0 RAD/s

Although it has a somewhat low lifespan, at 85 heat per tick, it is a very strong fuel nonetheless. It is highly unlikely that a ZIRNOX can be completely filled with HEU-235. One must take care when using this fuel in a setup. It produces more nuclear waste compared to less enriched fuels, but it can also produce useful neutron capture and fission products in higher quantities.

Assembly

Recycling

2 Pu-238 nuggets
2 Np-237 nuggets
2 Tc-99 nuggets
6 tiny piles of nuclear waste

Per rod, divide by 2 to get a billet's worth of material.

PWR

HEU-235 Fuel
Composition

Fissile material	U-235
Statistics
Yield	1,000,000,000
Reaction function	${\sqrt {x}}*22.5$
Function type	Square root
Function danger	Medium
Heat per flux	7.5 TU
Hazards
Start radiation	1.0 RAD/s
End radiation	10.0 RAD/s

HEU-235 is an average strength fuel for the PWR, with a strong square root function and 7.5 heat output.

Graph

Assembly

Recycling

3 Np-237 nuggets
3 Pu-238 nuggets
1 Tc-99 nugget
5 tiny piles of nuclear waste

RBMK

Highly Enriched Uranium-235 Fuel
Composition

Fissile material	U-235
Statistics
Yield	100,000,000
Self-igniting?	No
Splits with	Slow neutrons
Splits into	Fast neutrons
Flux function	${\sqrt {x}}*50/10$
Flux function type	Square root
Flux function danger	Medium
Depletion function	Gentle slope
Xenon generation function	$x*0.5$
Xenon burn function	$x^{2}*50$
Heat at 100 flux	1.0°C
Diffusion	0.02 1/2
Melting point	2,865°C
Starting radiation	4.0 RAD/s

While HEU-235 is much more powerful than MEU-235, it still uses a stable square root function, which makes it easier to use. Its stronger function is also paired with is much higher heat output, do take care with the cooling of the reactor.

As with all highly enriched fuels, especially in the RBMK, it can almost completely burn through during a fuel cycle to full depletion, resulting in a lot of nuclear waste and their byproducts.

Graph

Assembly

Recycling

Requires a SILEX with an infrared laser from the associated FEL.

(Xenon poison reduces U-235 by 1%)

Brand New
- 90.0% U-235
- 4.0% long-lived nuclear waste (U-235)
- 6.0% short-lived nuclear waste (U-235)
Barely Depleted
- 70.0% U-235
- 12.0% long-lived nuclear waste
- 18.0% short-lived nuclear waste
Moderately Depleted
- 50.0% U-235
- 20.0% long-lived nuclear waste
- 30.0% short-lived nuclear waste
Highly Depleted
- 30.0% U-235
- 28.0% long-lived nuclear waste
- 42.0% short-lived nuclear waste
Fully Depleted
- 10.0% U-235
- 36.0% long-lived nuclear waste
- 54.0% short-lived nuclear waste

Highly Enriched Uranium-233

Uranium-233 Fuel

Heat/tick	100
Estimated lifespan	150,000 ticks
Start radiation	5.0 RAD/s
End radiation	150.0 RAD/s

HEU-233 is an alternative to HEU-235, instead consisting of 100% U-233, also weapons grade. It is even stronger than U-235 due to its lower critical mass, meaning care must be taken to prevent meltdown.

ZIRNOX

HEU-233 is extremely strong, even higher than HEU-235. While it also has a low lifespan, one even slightly lower than HEU-235, its heat production of 100 is very potent, making it either very dangerous to use or a very valuable driver fuel. Similarly to HEU-235. it produces more nuclear waste, but also useful neutron capture and fission products. Of which is uranium-235, which can be reused in a reactor.

Assembly

Recycling

2 U-235 nuggets
2 Np-237 nuggets
2 Tc-99 nuggets
6 tiny piles of nuclear waste

Per rod, divide by 2 to get a billet's worth of material.

PWR

HEU-233 Fuel
Composition

Fissile material	U-233
Statistics
Yield	1,000,000,000
Reaction function	${\sqrt {x}}*25$
Function type	Square root
Function danger	Medium
Heat per flux	7.5 TU
Hazards
Start radiation	5.0 RAD/s
End radiation	50.0 RAD/s

HEU-233 is another average PWR fuel with a strong function, somewhat stronger than HEU-235, but with the same heat output of 7.5 TU.

Graph

Assembly

Recycling

3 U-235 nuggets
3 Pu-238 nuggets
1 Tc-99 nugget
5 tiny piles of nuclear waste

RBMK

Highly Enriched Uranium-233 Fuel
Composition

Fissile material	U-233
Statistics
Yield	100,000,000
Self-igniting?	No
Splits with	Slow neutrons
Splits into	Fast neutrons
Flux function	$x/100*27.5$
Flux function type	Linear
Flux function danger	Dangerous
Depletion function	Gentle slope
Xenon generation function	$x*0.5$
Xenon burn function	$x^{2}*50$
Heat at 100 flux	1.25°C
Diffusion	0.02 1/2
Melting point	2,865°C
Starting radiation	20.0 RAD/s

HEU-233 is similar to HEU-235, but it uses a linear function, which makes it unstable and requires driver/stabilizer fuels. Additionally, while it is considered stronger and outputs even more heat per flux, its function is actually fairly weaker than even MEU-235 at inbound flux levels ~65 and below. Linear functions also make fuels difficult to use, as if its receives too much flux, it can easily go supercritical, requiring a weaker or more stable fuel to keep it stable, and if it receives too little, it produces too little output flux, requiring a driver to consistently provide it input flux.

As with all highly enriched fuels, especially in the RBMK, it can almost completely burn through during a fuel cycle to full depletion, resulting in a lot of nuclear waste and their byproducts.

Graph

Assembly

Recycling

Requires a SILEX with an infrared laser from the associated FEL.

(Xenon poison reduces U-233 by 1%)

Brand New
- 90.0% U-233
- 4.0% long-lived nuclear waste (U-233)
- 6.0% short-lived nuclear waste (U-233)
Barely Depleted
- 70.0% U-233
- 12.0% long-lived nuclear waste
- 18.0% short-lived nuclear waste
Moderately Depleted
- 50.0% U-233
- 20.0% long-lived nuclear waste
- 30.0% short-lived nuclear waste
Highly Depleted
- 30.0% U-233
- 28.0% long-lived nuclear waste
- 42.0% short-lived nuclear waste
Fully Depleted
- 10.0% U-233
- 36.0% long-lived nuclear waste
- 54.0% short-lived nuclear waste

Overall

Combined Plots

Trivia

Realistically, HEU-235 and HEU-233 would likely be opposite than their behavior in NTM. U-233 has a far lower critical mass than U-235, but the former has less mass, meaning the latter would (marginally) produce more energy upon fission.