"Seawalls"... sorry for the mixup. Contaimnent "walls" would be a better term. A double row of sheet piles with a bentonite mix between them. Big job ringing the site with them.

What about fusing with electricty? Wasn't that done at Chernobyl... or at least considered? With directional drilling, it could be done even underneath but I don't know the logistics of supplying power.