Friday, July 22, 2016

Nuclear Power

(Note: the following is one of various posts that were copied here from the forum for the sake of preserving the early days of the project. It was originally posted there on Dec 11th, 2015.)

Here i had figured nuclear plants of sufficient size didn't exist and might not for a long time because people are so nervous about nukes. And then i heard about the SAFE reactor today. There is a working prototype. 100 kW output in a reactor that weighs about 600 kg. Man, that is a real nice output for a nice light mass, all day and all night. I was so wrong.

I don't know how much power the heavy-duty work will take - the stuff the crane and the digger do. So much is being done directly with solar heat, it saves a lot on the electrical power the first missions need. But there is meant to be an awful lot of digging. And the crane is huge, no matter how efficient it can be. Getting a handle on that amount would be really helpful. But it is so much easier to be comfortable when the budget i'd penciled in for 135 kW of solar panels would be enough for 700 kW of nuclear power!

I note that Joe Strout asked me why not go with nuclear - i spoke before without knowing about this option and didn't give it much consideration. Solar turbines and energy storage as lifted mass still seem better choices long term. They are simple, reliable, and robust. That can come later though. The first missions can take their time setting up the towers and cables to make that possible. Until then, nukes all the way!

Response by Sigvart Brendberg:
An even more important reason to choose nuclear power for the moon-base is energy storage. Even though you can produce as much solar power as you want, for example using the parabola, the night is 14 days long. Having about 0.5 MJ of storage capacity per kilogram for a secondary cell, you are going to need two and a half tons of batteries per kilowatt of power you want to spend through the night... Better then with something continuously producing electricity.

PESS is actually kind of solving that problem, but the mass required is large. (750 million kg*m per kilowatt of night power). For a 1 km (vertical component only) cable, that is a mass of 750 tons per kilowatt.

(kind of a related, you say thorium can be used for NTRs in the first slide, but thorium is surely not fissile material on its own? It must be turned into U-233 before it is of any use.)

Anyway, I totally agree on your choice of using nuclear power for the base.

Cheers Sigvart!

750 tons per kilowatt, huh? That does indeed sound like an awful lot. Let's see, bulk powder regolith is about 1.5 g/cm3, so that's 1.5 tons per cubic meter, so 500 cubic meters to get the 750 tons for a kW, over that 1 km drop. That's a cube 8 m on a side. Basalt is 3 g/cm3 here on Earth. I'll low-ball a bit and give fused basalt produced by one of the fresnel lens setups a bulk density of 2.5 g/cm3, that's 300 m3 for 1 kW, a cube 6.8 m on a side.

I had played with the idea of making weights in bulk using the MIP beds which i think will be idle a fair bit on the first and second mission, or maybe during idle time with the rovers just scanning a linear fresnel lens (one that focuses to a line) across the open surface. I thought, melt a thin layer wait a bit for it to be almost congealed, roll it into a log. I'd figured they'd need to be big, but not that big. It would take 3 logs 3 m in diameter by 15 m long to provide that kW. Then i start thinking about the cables needed to support that over and over, given that to be worth it they'd need to have weights like that say every 10m constantly lowering, all night long, for it to be worth it. Let's say with losses a line like that could provide 25 kW of power if it was continuously loaded like that.

So, do you mean that while that 750 tons is turning the generator, it would be generating a kW? I guess that is a rough calculation that doesn't consider friction loss and such? And i don't know how that fits with time. How long should i consider that it supplies that kind of power, before it reaches the bottom of the line and is unloaded? If i take the lunar night as 350 hours, giving a little space at dawn and dusk to consider that the sun might be blocked for a bit by the landscape or panels might not work well at such high angles, then we're looking at 33 weights always going down the line, times 350 hours, that alone is 11,550 weights. Then there needs to be some multiplier for how long it would take for each weight to descend. Whatever that multiplier is, this deals with a lot more weights than i'd idly guesstimated.

If you had the weights made, certainly it could still be worth it. But god that's a lot. Stick a bunch of lenses somewhere on the face of Lalande crater, doing nothing but melting the crater face in such a way that the lava melts and puddles into masses of roughly the right size? Maybe. Maybe. It is awfully seductive for me to think about doing all kinds of things with lenses, considering how little payload they occupy and how much power they represent. 115, 000 tons of lava, give or take. Hm.

Okay - moving on to thorium refinement. You're right of course, the thorium would need to be processed into uranium 233. A long term goal. It isn't at all for the early days. It just is interesting because of the possibility of nuclear rockets and energy independence in a robust way. But where there is thorium, there may also be uranium. The thorium is what was tracked by the Lunar Prospector probe, but there was a later Japanese analysis by Kayuga found uranium. Still either one requires considerable processing, it is a long term thing.

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