>why is nuclear waste such a big problem? in terms of m^3 the amount is rather small
The meme is that nuclear waste produced so far amounts to 'a football field worth' or some similar framing. But you could fit the entire earth into a football field if you're prepared to go quite deep. It's a deliberate trick to make it seem like a non-issue, and deflects from the serious challenge that is long-term handling of nuclear waste. Plus, this is the size of the problem if we stop tomorrow, just the amount produced in the 50 years nuclear power has been up and running. So you must multiply not only by the length of time to be stored but also future projections for continued operation. If you try and project that out to a rolling total, assumming you keep the level of waste production at it's current level and no greater, it's an enourmous quantity.
Then there's looking at how well we're progressing with permanent, safe sequestration of existing nuclear waste. Not very well - There have only ever been three sites in the world for permanent waste disposal and only one of them is operating today (WIPP in the USA). The other two in Germany closed many years ago, and are costing billions in ongoing remediation costs as it turns out 'permanent' didn't pan out as well as hoped. Of course, no nations are accepting waste produced by other nations as that would be a political nightmare, and so each country is left to work out a plan on it's own.
The USA has shut down something like 30 plants so far, but of those only about 10 have so far been decommissioned. Of those, about half were truly decommed and most of the rest were put in to the SAFSTOR programme, where they are left to decay for up to 60 years. Theory is they'll be a bit cheaper to decommission after that time, and the funds to decomm them up-front are not available. A handful opted for a third option of entombing the reactor in situ.