Well, they're 3m tall and weigh 750-1000kg. Their energy consumption is 10-400kW thermal.cadbrowser wrote:Only a few grams of nuclear fuel a week? As big as these things are?
With a kg of Uranium providing 1.3TJ, we need 0.0047 to 0.187kg of fuel per week, slightly higher in practice because the fuel produced by the breeder cycle is less energy-dense than uranium. I detailed the nuclear fuel consumption rates for various communities in an earlier post.
I'll discuss my background more here, even if I repeat information posted earlier.Consider establishing an upper limit for how many "thousands of years" has passed since humans have left the earth. How rare is this commodity? How many Iron Giants are there in the world? What is their rate of procreation/infant (?) mortality rates...etc.
In 200 years, humanity will not find any more natural deposits of uranium. As the price per kg rises above 200USD/kg, they will be forced to recycle spent fuel using breeder reactors, but even if that holds out for 1000 years, they will need fresh fuel. The space exploration program is in full swing too, so nuclear fuel will leave in spacecraft drives and never return. The solution is a vast project of uranium extraction from seawater. As time passes, the process is automated. Scoops filter water and mud from the seafloor and process it into nuclear fuel. It is initially deposited in coastline installations with their own nuclear power supply.
Fusion never works out on Earth. By the time it is developped, the installations for gathering He3 have to tread on the uranium exctractors, so people just stay with fission.
10000 years pass and Earth has become unrecognizable. It is green, yes, and covered by plant and animal life; but most of it is engineered to survive the disasters humans have wrought over the years. By now, the population living in other solar systems exceeds that living in ours. Multiple restoration efforts fail, but people don't care, they have new homes now and an alien war to worry about.
20000 years later, the Sun's light is visibly dimmed by the solar power installations that power relativistic kinetic weapon launches. During the war, an alien race manages to divert part of a Gamma Ray burst's energy and sterilizes the solar system with it in a display of power.
Half of the Earth's oceans boil, the ozone layer is destroyed and the tons upon tons of uranium stockpiles meant to fuel the extraterrestrial colonization effort are flash-vaporized into the air.
When conditions settle down again, most of the Earth's water is locked up in huge polar caps around the poles, and extremely strong convection cells circle from the equator to the polar caps. They do a good job of grinding away the top soil and leaving a sandy belt around the planet. Between this and the polar caps is a dry, cold wasteland with flat, hard rock, rubbed down mountains, and very little water.
Onto the Iron Giants.
They are the last step of many unsuccessful efforts to return Earth to its former greenery. Capsules containing nanomachines are sent from the colonies on other solar systems, across interstellar space, to seed the earth with artificial bodies the returning humans can inhabit. They run on nuclear power, injesting practically any radioactive fuel and extracting whatever they can from it.
70000 years later, the humans do not return. The machines slowly overcome their inital programming, and the incessant degradation by the surrounding radioactivity allows them to mutate and gain consciousness. Civilization starts from the year +100000.
Not all of the nuclear fuel depots were destroyed during the gamma ray burst. Many of those placed underwater of underground survived. Over the millenia, they broke down, stopped functioning and were covered by wind-driven sedimentation. Of course, without ventilation or any sort of control, the containers broke down too, so some melted down and other heated up enough to leave a visible trace on the surface.
Natural hotspots caused by uranium concentrations on earth already. The giants had to detect these thermal signatures and began digging for them. What they extracted was an odd mix. On one side, we have high-quality refined fuel. In the midst, we have various products of radioactive decay of various usefullness, from as-good-as-uranium to poison-for-our-reactors.
The giants had to learn to separated them, and this is where my idea for centrifuges came up. Seems I was wrong. Only simple milling and sedimentation/filtering is needed to separate the lightest actinidesfrom the rest.
Oh it'll be millenia before they run out of fuel, but the amount they extract at any one time is very limited. There is competition for this rare resource.How long before they run out of fuel? In respects to this finit supply...where are you introducing the timeline? This is important, because if this is an established "race" and your introduction to them is "well estimates put fuel reserves expiring in a million years"...then it won't be as facinating. However, if there is less than a decade of fuel left...then hell yes you are going to have wars.
The giants' population is mostly limited by how much iron and other metals they can put into the birthing pools. The active nanites, once they detect enough of each metal that composes a giant, will produce a youngster. The adults don't know how much exactly is needed, or what exactly the difference between titanium and aluminium is (beyond weight, hardness and other 18th century analytical data), so they just pour in what they mine until there is enough. I can't really say what birth/death rates are, but I can say that there are few births, and few deaths, and the average life span is 100 years.
Small communities are in the hundreds, large one in the thousands, and the largest few in the ten thousands. I'm guessing the world population is about 50 million, scattered around the equator and along the ancient coastlines. They would naturally group in where current major cities are, because of the likelihood of finding a nuclear fuel deposit.
5 million on average needs 465 tons a week, or 16.9 thousand tons a year. Cosidering that we actually consume 68 thousand tons a year, and that the average nuclear power station holds 75 tons of pure nuclear fuel, this is entirely reasonable.
Of course, if all the hidden fuel deposits and buried fuel reactors in a region are used up, all the inhabitants will all die out unless they manage to co-ordinate the invasion of another region before their fuel reserves are used up. This is doubly dangerous because fighting and building war machines and whatnot will increase their fuel consumption by up to 40 times.
