The sunk cost fallacy is about continuing to incur additional sunk costs. The parent is arguing that we're going to have to pay $X/year to manage the nuclear waste we've already incurred and that the cost is fixed--adding more nuclear waste isn't going to increase the cost. Whether those claims are true may be up for debate, but it's certainly not a sunk cost fallacy.
Do you have any data that says the cost (ie. money or security) is materially increased from this point forward given the space to store is truly small.
