[scythe]

From Lidsky:

>Fusion will almost certainly have a lower power density than fission and therefore will require a larger plant to produce the same output. Suppose a fusion plant had to be ten times as big and therefore likely ten times as costly — as a present-day fission plant to produce the same amount of power.

Fission is currently not cost-competitive due to the expense of ensuring that fission reactors do not pose an unacceptable risk of radioactive contamination in their vicinity. However, fusion is not subject to this constraint, or anyway suffers from it much less. There are no long-lived radioactive byproducts, and judicious selection of the construction materials (already implemented) can ensure that neutron activation of the walls is not a problem either. Furthermore, the inherently unfavorable nature of fusion reactions mean that criticality accidents ('meltdowns'; a la Chernobyl) are not possible.

From Pfirsch and Schmitter:

>It is shown that the claims made therein for the economic prospects of pure fusion with tokamaks, when discussed on the basis of the present-day technology, do not stand up to critical examination.

The analysis in the fulltext relies on a variety of plasma parameters estimated based on technology available in 1987. I cannot immediately determine if it generalizes to designs using HTS, but the comments on pp 1473-4 about the achievable B field strengths and corresponding betas suggests that they do not. Cf. this paragraph:

>Another possibility is to use higher magnetic fields: 6 T instead of 5 T would increase fw to values between 1.3 and 2.0 MW/m2, which are still very low. The latter comes close to the value of 3 MW/m2 obtained in Sec. IV.A.l from thermal wall load constraints. Higher fields would, of course, again increase the cost.

Overall I don't think that these links provide nearly as strong an argument as you suggest they do.

[pfdietz]

Fission plants are expensive for a couple of reasons. One is that they need additional layers of heat exchangers. Another is that their parts must be very reliable, to reduce the probability of serious accidents.

Fusion reactors will also require layers of heat exchangers, to isolate the tritium. They will also require very reliable parts: not because of public safety, but because fusion reactors will have so many parts in the hot area where hands-on maintenance is impossible. And this reliability will be expensive, even though the requirement for it is more to avoid a financial meltdown rather than a physical one.

> The analysis in the fulltext relies on a variety of plasma parameters estimated based on technology available in 1987.

The point of these arguments is that beyond a certain power, the plasma parameters become irrelevant. The limit is imposed by what the first wall can withstand, not what the plasma can put out.

If you look at areal power densities of fusion reactor concepts, the older studies had HIGHER areal power densities. But those higher power densities were found to be unrealistic.

Lidsky concluded DT fusion reactors would be an order of magnitude worse (in volumetric power density) compared to fission reactors. In this, he was being too generous: ARC is 40x worse than a PWR; ITER is 400x worse (and DEMO almost as bad).

The arguments there were farseeing, and experience since then has buttressed them, not contradicted them.

[scythe]

>Fusion reactors will also require layers of heat exchangers, to isolate the tritium. They will also require very reliable parts: not because of public safety, but because fusion reactors will have so many parts in the hot area where hands-on maintenance is impossible. And this reliability will be expensive, even though the requirement for it is more to avoid a financial meltdown rather than a physical one.

All of this applies to fission. Radiation equipment is expensive, period. We pay four figures for a block of plastic. A very, very accurate piece of plastic. The components in a fission reactor are not easy to replace either; the cost of a fusion meltdown is the reactor, while the cost of a fission meltdown is the reactor + up to several square miles of the area around it, the latter being so large that we typically ignore the very expensive reactor cost!

But more simply, you're underestimating concrete. Fission facilities are critically dependent on the stuff, wall after wall, being the only material that can be assembled thick enough to guarantee the safety of radiation workers who sit in the plant all day. Lowering the intrinsic radiation burden reduces the use of concrete, which is one of the most expensive parts of nuclear plant construction:

https://www.forbes.com/sites/jeffmcmahon/2018/10/02/4-ways-t...

While some of this applies to fusion reactors, it doesn't seem appropriate to compare only the power-generating components of fusion vs. fission reactors while ignoring the safety components when the primary advantage of fusion is safety. Regardless, I've made a note to read more about it.

>Lidsky concluded DT fusion reactors would be an order of magnitude worse (in volumetric power density) compared to fission reactors. In this, he was being too generous: ARC is 40x worse than a PWR; ITER is 400x worse (and DEMO almost as bad).

If the real power densities are available, arguments about the theoretical power density are irrelevant. I can probably build a warehouse ten times the size of a nuclear reactor for a tenth the cost of said reactor. ARC's true power density -- or that of any other reactor -- must obviously be factored into any cost projections. The power density of a particular design is not usually something you need to read a paper about!

[ncmncm]

A fission plant can be extremely cheap in certain environments. For example, in the cloud tops of Venus, Saturn, Uranus, Neptune, Ganymede, Titan, or Triton, a fission reactor is as simple as a big fabric tube suspended from a balloon, with a naked atomic pile hanging near the bottom, and a wind turbine at the top. Radiation is absorbed by the air inside the (sufficiently broad) tube, which rises through the turbine at the top. You could dispense with the balloon if you constrict the exit aperture just right.

[ncmncm]

Oops, not Ganymede or Triton. Not enough atmosphere.

But the planets beyond Jupiter have surprisingly gentle gravity.