| If a fusion plant is just like a fission plant, but replaces the fission reactor with a fusion reactor, then it is entirely reasonable to compare the cost of the reactors. That a fission reactor itself is a small part of the cost of a fission plant doesn't mean the same must be true of a fusion plant. And indeed, if you look at the cost of conceptual DT fusion power plants the reactor is a significant part of the total cost of the plant. You are right that cost is not JUST a function of size. It's also a function of how exotic the materials are and how intricate the device is. By those metrics, fusion will do even worse. A fission reactor is a rather simple thing, in comparison. Fusion's costs will be further increased by reliability concerns. The part of a nuclear plant that's too radioactive for hands-on maintenance must be extremely reliable. In a fission power plant, this part is rather small and simple. Multiple fuel rods in a fission plant can leak without necessarily shutting down the plant; a single leak of coolant into the vacuum vessel of a fusion plant will likely prevent it from operating. Fusion power plants will almost certainly need containment buildings. The cryogens of ITER, for example, would (if fully vaporized) present a larger pressure x volume load than the steam from a fission reactor meltdown. Containing this gas is not cheap. In any case, tritium must be kept from leaking, which will imply expensive hermetically sealed buildings and seals (tritium will permeate through polymer seals.) Tritium will be everywhere inside the fusion reactor building. The tritium that cycles through a 1 GW(e) DT fusion reactor in 1 year is enough to contaminate two months of the flow of the Mississippi River above the legal limit for drinking water. Even small levels of leakage will be extremely vexing. For ARC specifically, the magnets are shielded by titanium hydride. This material will fully decompose to titanium and hydrogen at the temperature of the molten salt, so it must be assumed that in a serious accident it will all decompose. |
Instead of a $100 million reactor, you're looking at $1 billion in reactor spending, but instead of a $4 billion dollar plant that this reactor goes into, you're looking at a $2 Billion plant.
A fusion plant would be comparable to a very expensive fission reactor in a world where people weren't afraid of fission plants, but in that world a fission plant would be dirt cheap. In the real world, fission is way more expensive than the engineering challenges would imply. It's not the materials or the containment that is expensive, it's having all of your assets sit idle for years on end while yet another environmental impact study is conducted.
Also, some of your assumptions are unreasonable. For example the reason you need a containment building around a nuclear reactor is that you can't just vent to atmosphere, because the water contains large amounts of tritium. It's perfectly fine to just vent helium to atmosphere in case of an emergency as it's not radioactive. While a fusion reactor would use a lot of tritium over time, at any given moment the amount present is rather miniscule, it is being actively generated on site specifically and if anything the major technical issue is not having enough. A reactor the size of ITER would have approximately 0.6 g of Tritium in the reactor at any given time, losing all of that to atmosphere would be equivalent to approximately 2% of the annual tritium release from The Hague Nuclear Reprocessing plant. Decomposition of titanium hydride at the temperature of the molten salt is slow, while obviously undesirable, there is no danger in the magnets decomposing, the real issue is quenching, which is one of the few genuine safety concerns of a fusion reactor.