Costs that rely on net electric

[timothy-nunn]

I've spoken with quite a few people about this, some of these comments and concerns are written in the original issue.

The model is clearly flawed because it uses net electric to try and estimate the O&M cost. This is done because all O&M models in the literature are fits on data from existing fission/gas plants. I suspect net electric is used here as a loose proxy for plant complexity.

My suggestion is to accept and merge this PR because it will fix the negative square root, and does no harm: the cost model will produce a NaN if given a negative number and so producing a 0 instead is no worse.

An issue should be created where we can discuss and plan how to produce more realistic O&M costs that rely on something a bit more meaningful that net electric.

@geograham @j-a-foster how does this sound to you? If we all agree, I will merge this PR, create an issue and we can discuss our concerns further there and plan if/how we want to fix it.

Originally posted by @timothy-nunn in #4065 (review)

The cost calculations for annwst (cost of waste disposal) and annoam (cost of operations and maintenance) use the square root of the net electric power (scaled to a 1.2GW reference, Starfire). This is common in the costing literature for both fusion plants and fission/gas plants. This is because the formula comes from fitting a model to data from fission/gas plants (I assume because net electric is a reasonably good proxy for plant complexity).

There are a couple of problems with this:

1. In PROCESS, we often encounter designs with negative net electric on the path to a converged solution. On these designs we have to fudge the cost calculation to stop negative square roots (Fixed negativity in sqrt in costs.py by np.clip. #4065).
2. This method of costing just does not make a whole lot of sense. It implies that a plant making 0 net electric will have 0 operations and maintenance costs, despite the fact this plant will still need to employ staff, pay insurance, and maintain the device. A better variable for calculating the maintenance cost would surely be gross electric, and most of the operating costs will likely be fixed. It also means that when optimising for minimum cost of electricity, the optimiser will aim to minimise the net electric.

I think for these two reasons we should seriously investigate replacing these two costs with more representative calculation that does not rely on net electric.