I'm not sure why this is surprising to anyone, but I will note that the discussion is muddled.

The paper notes

> The excess waste volume is attributed to the use of neutron reflectors and/or of chemically reactive fuels and coolants in SMR designs. That said, volume is not the most important evaluation metric; rather, geologic repository performance is driven by the decay heat power and the (radio-)chemistry of spent nuclear fuel, for which SMRs provide no benefit.

There should be no surprise that smaller reactors require more shielding when normalized to output energy. Scaling has advantages.

More importantly, the muddling comes from the non-nuanced discussion of waste that is ever so common. We cannot conflate high level waste with low/mid level waste. One is dangerous, the other isn't. Long lives mean lower danger. We should know this from first principles: radioactivity is shedding of mass, the faster you shed mass the shorter you live. And this is exponential too! Iodine-129, which has a half life of 1.57×10^7 yr has a decay energy of 0.189 MeV through beta- decay (electron). On the other hand, Sodium-24 has a half-life of 15hrs and has a 1.39MeV beta+ decay (10x) followed by a 2.76MeV gamma decay and a 1.38MeV gamma decay, both happening in a fraction of a fraction of a second and reaching a stable ground state (the most common form of magnesium, who's main danger is flammability).

At a 1cm distance, a 10uCi sample of I-129 results in a dosage of 57uSv/hr while 10uCi of Na-24 results in 1609uSv/hr, and almost 30x difference. Public dose limit is 1mSv/yr and occupational is 20mSv/yr. That's the difference of 17.5hrs (351hrs occupational) vs 37 minutes (12hrs occupational) to reach recommended limits. This is all before we even start talking about cancer risk (there's >an order of magnitude safety. 1Sv single does requires intervention but is non-fatal), and ease of shielding (you can assume that one is easier to shield) or how natural radiation exposure is around 2mSv/yr and a flight crew going NYC-Tokyo gets about 9mSv/yr. Conflating these is not helpful to the discussion. Our required storage techniques are vastly different for these components and so our perception also needs to be.

The article there is making the same mistake that they are criticizing. They criticize studies for only looking at spent fuel, saying that it offers little insight into danger and storage concerns. But then they only focus on mid and low-level waste (i.e. mass) which similarly offers little insight. Complex systems require complex discussions. This is why public conversations like we have on HN and Reddit are less than useless. It is a bunch of armchair experts who have convinced themselves that they understand an extremely complex subject and confidently discuss the issues in naive and inaccurate ways. At the end of the day we live in a specialized world and we have to accept that we need to learn on expert decisions because there is no possible way that we can have (high) expertise knowledge in more than one or two domains. This isn't a jab at your (anyone who is reading) intelligence, but a fact of our world. I don't want to discourage people from learning more about these complex subjects, but I do want to discourage confident discussions when one doesn't have domain expertise. Fruitful conversations can be had if we discuss under this premise but if we don't we just muddle the conversation and there's a good chance we actually decrease our domain knowledge.