Operating since 2019/2020:

https://en.wikipedia.org/wiki/Akademik_Lomonosov

It's in the far north, I'm not sure how much fishing is going on around there, or how a potential radiological leak would spread around.

There are hundreds of nuclear reactors sailing around our oceans in ships and submarines and have been doing so for more than 70 years. Most estimates are above 400 total reactors though the actual count isn't really known since nearly all of these are military vessels.

What makes is it so expensive?

Decommissioning isn’t usually as bad as building reactors — it’s not unusual for decommissioning projects to come in under budget and early.

Indeed. There was a big push towards electrifying the offshore platforms and refineries using green hydro power. This was so our carbon budget didn't look so bad, as the actual end products are not counted.

However in recent years there's been strong pushback as people realized just how much power these platforms and refineries take, and what plugging them into our grid would do to electricity prices.

So this seems to be aimed at having your cake and eating it. We get "green" oil and gas, and no astronomical electricity prices.