# Thorium Salt Reactor: Efficient or not?

Molten Thorium Salt, when used as “Coolant”, massively boosts the Maximum stat of Fuel Rods. Presumably this greatly improves the efficiency. However, a Critical Thorium Salt Reactor produces Energy only by emitting Neutrons onto other Rods encased in adjacent Reactor Cores, meaning a disadvantage on efficiency. Also, Thorium is valuable itself for Energy Production, by making into Th-232 Fuel Rods, or even more, by breeding into U-233 Fuel Rods. And if I understand it right, the Molten Thorium Salt consumption is calculated based on “Heat” generated in the Core filled with Molten Thorium Salt. So I try to do some calculations:

For a Critical Fuel Rod with Factor 1/4, there’s no room for Absorber Rods if we want it to go critical, unless Moderator Rods are used. Thus we consider such moderated situation:

• Two sides of the Fuel Rod always face another Rod in the same Core, meaning no point to use Absorber Rods there. Thus we place Moderator Rods, easily achieving Criticality with a 2x2 checkerboard layout.
• For the other two sides we place Absorber Rods, each receiving 1/4 of the Neutrons on the Fuel Rod, generating 2HU per Neutron, summing up to peak HU output of 1x the Neutron count on the Rod, or 4x the nominal Maximum stat of the Rod.
• Such setup can be controlled by disabling one Moderator Rod, chopping the Neutron Flux to 1/2x. Thus the average output is ~3/4x the peak output. (both for Produced Energy and for Thorium Salt)
• The Duration gets divided by 4 (Moderated).
• Without Molten Thorium Salt, 4 Reflectors would be put adjacent to such a Fuel Rod, with no room for Absorber Rods, meaning peak HU output of 1x the Neutron count on the Rod, with nominal Duration. And the average HU output would be ~7/8x the peak output (by disabling one Reflector).
• So the overall effect on efficiency is that, 4*(3/4)/(7/8)*1/4=6/7x Energy is produced per Fuel Rod, at the expense of Thorium Salt consumption equivalent to 6/7 times the original Fuel Rod Energy Production. This means that with 1/4 Factor Fuel Rods, in addition to Thorium consumption, you actually gets a small penalty.

For a Critical Fuel Rod with Factor 1/3, we can go either moderated or unmoderated. For the unmoderated situation:

• Three sides of the Fuel Rod is adjacent to Reflectors or other Fuel Rods, achieving Criticality.
• One side adjacent to Absorber, receiving 1/3 of the Neutrons on the Fuel Rod, generating 2HU per Neutron, summing up to peak HU output of 2/3x the Neutron count on the Rod, or 8/3=~2.67x the nominal Maximum stat of the Rod.
• Such setup can be controlled by disabling one Reflector Rod, chopping the Neutron Flux to 2/3x. Thus the average output is ~5/6x the peak output. (both for Produced Energy and for Thorium Salt)
• The Duration stays the same.
• Without Molten Thorium Salt, the optimal layout stays the same, meaning peak HU output of 5/3x the Neutron count on the Rod, with nominal Duration. And the average HU output is also at ~5/6x the peak output (by disabling one Reflector).
• So the overall effect on efficiency is that, (8/3)/(5/3)=8/5 times Energy is produced per Fuel Rod, at the expense of Thorium Salt consumption equivalent to 4/(5/3)=12/5 times the original Fuel Rod Energy Production. Speaking another way, Each 1HU absorbed by Thorium Salt translates to (8/5-1)/(12/5)=0.25 additional HU produced.
Some quick calculations would show that, if we go moderated, the gain provided by an additional Absorber Rod would not cover the moderation penalty. Thus the exact calculations are not provided here.

Next, we estimate the Energy Production, would the Thorium be made into Rods instead of Molten Salt:

• 1L of Molten Thorium Salt is consumed for each 2_560_000HU absorbed.
• 20_736L of Molten Thorium Salt is produced for each 1 unit of Thorium.
• 1 Th-232 Fuel Rod consumes 0.5 unit of Thorium, equivalent to 26_542_080_000HU absorbed by Molten Thorium Salt.
• For maximal efficiency in a Stable Reactor, 1 Th-232 Fuel Rod is surrounded with 4 Absorber Rods inside a Reactor Core using Industrial Coolant. This outputs 8+8*4*2=72HU/t for a Duration of 100_000min, summing up to 8_640_000_000HU.

Summing up everything above, this shows that, 1 Rod worth of Thorium, When used as Molten Salt in a power generating Reactor, translates to at most 6_635_520_000 additional HU gained, which is even less than just using the Thorium in a Stable Reactor. Also this ignores the value in Depleted Fuel Rod extraction products. And breeding the Thorium into the more powerful U-233 would presumably gain much more for each unit of Thorium.
This however doesn’t mean that Molten Thorium Salt is completely useless at all. It’s actually indeed useful in Breeder Reactors, since breeding is limited by Neutrons emitted onto Breeder Rods, and higher Neutron Flux means vastly more efficient breeding.

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Whoops, this gets far more complicated than what I expected at first. Finally do I complete this one. And I hope I don’t get the calculations wrong.

I should perhaps @Erik3003 for review on this one?

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Glancing over these calculations, they seem very well thought through, however a very significant error is present at the very beginning of these calculations: Molten Thorium Salt increases the Factor of fuels inside it. A fuel with usually (i.e. in water, gas or molten chloride coolants) having a Factor of 1/3 will have a Factor of 1/2 in Molten Thorium Salt.

That is a quite significant boost and from my calculations I used to balance the coolant, should more than make up for the thorium used, but I don’t want to rob you of doing the calculations here yourself.

Molten Thorium Salt is an extremely powerful coolant, however only when used with the best (in terms of Factor) fuels. It has the advantage of burning thorium very efficiently and fast (fast being significant, since thorium at maximum efficiency burns really slow, requiring a lot of reactors to get a decent power output) and reactor designs being very compact. Another huge benefit of Molten Thorium Salt reactors is not even needing a heat exchanger and a boiler, since the reactors with the absorber rods can use distilled water directly outputting steam.

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You’re right. I didn’t even notice the Factor increase for Molten Thorium Salt. The ingame manual doesn’t mention it, but NEI does show it correctly.
So 1/4 becomes 1/3 and 1/3 becomes 1/2 in Molten Thorium Salt. This would definitely make a huge efficiency gain, both for 1/4 Rods and for 1/3 Rods.

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Also, this would enable vastly more efficient breeding, right? In addition to be able to breed 2 Rods simultaneously, the Neutron Flux can be quadrupled without Duration losses.

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Indeed, Molten Thorium Salt also makes for good breeder reactor designs or even for burning cobalt-60 into thorium, although I haven’t run the numbers on that one.

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…And I already built one. Was quite fun

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Then I’d like to remind that, the in-game Fission Reactor Manual mentions “Decreases divisor of factor stat of fuel rods by 1”, in the page about Industrial Coolant, instead of the one about Molten Thorium Salt. Seems to be a mistake?

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I’m enjoying doing calculations to predict Energy Production and planning for future designs. Although I actually have much more than needed Fission Fuel. Conversion to RU then EU, instead of HU production, is indeed the real bottleneck.
And thus not requring a Heat Exchanger and a Boiler would be a huge benefit. So much material is needed. Also, this would solve the problem of the highest tier Turbine not having an equally large Heat Exchanger, right?

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