Sure, the future canal would be mostly safe, especially after decades. But there's a rather big problem in that making the canal very much wouldn't be.

Using nukes for mining was actually tried, it left a very impressive crater you can visit today: https://en.wikipedia.org/wiki/Sedan_(nuclear_test)

It also caused more fallout than any nuclear bomb ever. And the crater is "only" about 450m. So you're going to be blasting a LOT of highly radioactive dust into the surrounding several countries. Even with more efficient nukes causing less fallout, that's probably not going to be very popular.

The material doesn't go away by magic. It gets ejected and scattered over a wide area, after being in a nuclear explosion and getting highly irradiated. The blasting a canal would throw an entire canal's worth of fallout around the area. That will go away mostly eventually, but it won't be fun living within several hundreds of kilometers for a for decades.

And of course, even with the decades old Sedan crater, there's a giant radiative ball underneath.