In 2015-2016 I did some mechanical engineering work at BWXT, designing nuclear waste containers for the Bruce Power CANDU reactor refurbishment program.
I learned that the spent fuel storage is pretty trivial for a few reasons:
1. The amount of fuel waste produced by a reactor is volumetrically tiny. So it's not too expensive to just have a relatively small guarded yard of sealed storage casks next to the reactor, where the fuel can sit and decay until safe for offsite disposal (typically around 60 years).
2. Safe waste storage containers are relatively easy to make. Most containers only need to be a thick stainless steel inner liner and a stainless steel outer liner filled with a special mixture of concrete. The nuclear waste is dropped in, the lid is bolted shut, then the container can be safely left in a guarded open yard for >100 years. They are safe by virtue of just having ridiculously thick welded stainless steel walls. We designed them to fall off a train going over a bridge, fall 300ft onto solid rock, and not break.
3. Reactor technology keeps getting better at extracting more energy from fuel. So what was "waste" in a past decade, can be put back into a reactor that is better at pulling energy out of the fuel. For this reason, there hasn't actually been much actual "fuel waste" in the history of the nuclear industry. It's all just being stored for future use by better reactors.