feld said:
Edit:
Bryn…I think you should ignore or retcon that reference. The system described, while called “fission”, is apparently driven by “radioactive decay.” That’s not a fission power plant. It’s a radioisotope thermal generator. I can’t think of a fission fuel you’d actually ever want to use that decays that fast. I’ll go look at Thorium but it’s half life is in the billions of years. I’m 25 years from my reactor design courses..but I *did* take them. I think GDW goofed.
Turns out my copy of my reactor design text is at the office. I’ll try to check this tomorrow.
The numbers are about correct for a reactor using standard uranium fuel, like a commercial power plant. They are orders of magnitude off for an RTG. Forgive GDW's imprecision.
I can remember trying to explain why RTG's were a bad power source. Essentially, they need specific synthetic elements (typically 238Pu), which are extremely difficult expensive and time-consuming to make. The starting material (237Np) is a byproduct of 239Pu production. Oak Ridge is running multiple breeder reactors with the aim of making 1.5 kg of it per year - enough for 810 watts (thermal) of RTG's. RTG's are approximately 5% efficient, with a theoretical cap of around 15%. In round terms, to make a 1 MWe RTG would need require 37 tons of decaying 238Pu, or approximately 25,000 years of current US production.
RTG's are great for something that requires 100 watts for a few decades, but essentially useless as power plants.
Naval reactors often use different material than commercial power plants. American and British reactors use highly-enriched U (HEU, i.e. weapons-grade at 93%+), as French reactors used to as well. The Russians used a middling grade (30-40%), and the Chinese and French (now) use commercial low-enriched U (LEU, ca. 5% for the Chinese and 10% for the French). The advantages of a well-designed HEU reactor is that they can last upto ca. 30 years before a core change, whereas LEU reactor core will have much shorter lifes.
The French went from HEU to LEU. They found no difference in performance of their submarines. The only difference was core life. Commercial fuel in a power plant lasts about 2 years when constantly running at fairly high power. This is, of course, how our starships will be running. Submarines don't dash about at full power, and so the fuel lasts longer. A French SSN gets about 8-10 years out of one LEU core (so the plant is averaging about 20% output over this time). Whereas a British or US submarine will get 20+ years out of a HEU core. However, stick that same reactor on a merchant running the reactor at high power most of the time (like an electricity supplier) then core life would shorten to ca. 2 years.
A nuclear powered merchant will need refuelling about every two years. A warship that spends much less time underway may have a significantly longer core life on the same fuel, because they aren't burning it as fast.
Fusion plants are stated to be sealed. The life of a fusion plant would depend on how much use it has. The fusion plant has to constantly operate to maintain containment, and the PP rating is essentially the maximum excess after housekeeping. Housekeeping is around a third of the total output by some back of the envelope calculations a while back, and so a 150 MWe plant is actually producing 200 MWe and consuming 50 MWe itself (say). So, running the plant at a usable 150 MWe is consuming fuel at twice the rate of 50 MWe. The life of the ship is dependent upon the amount of use it has gone through. The earliest fusion ships, the French Ypres class, are nearly the end of their lives after ca. 40 years service. Over time, the output of the reactor is likely to start dropping as the fuel becomes less abundant, but the housekeeping overhead is the same or higher. Fusion plants will likely start having lower outputs over time, and the Ypres being sold off may well be into that period.
Fusion powered merchants are likely to have a shorter life than warships, because they will be using full power much of the time. If the Ypres is coming to an end to her useful life after 40 years, assuming hard war-service, merchants would have around 20 years or less useful life.