I found something relevant for you.
I hope it helps.

"capacity, once installed, is sunk but where fixed costs are not sunk yet..."

Reference,
Sunk Capacity Costs, Long-Run Fixed Costs, and Entry Deterrence under Complete and Incomplete Information
Larry Arvan
RAND Journal of Economics, 1986, vol. 17, issue 1, pages 105-121

Take a look at the upper paper may give you more insight.

sunk capacity cost
分解开来，sunk cost是指的沉没成本，沉没成本是指由于过去的决策已经发生了的，而不能由现在或将来的任何决策改变的成本
capacity cost指的能力成本，容量成本，结合在一起应理解为沉没容量陈本