Haven't read the linky but what of dirty bombs made up from the nuclear waste. A terrorist's or blackmailer's dream?
This is indeed a danger but if you put yourself in the terrorists' shoes the waste from sub-critical plants is not really dirty enough for your purposes. The danger here would be from a sub-critical plant which was using waste from a critical plant as fuel, having its fuel stolen. After use, the fuel would be even more degraded and less interesting to terrorists. The spent fuel danger for dirty bombs is principally a problem for existing critical plants. A greater danger than terrorists is probably rogue states trying to re-engineer sub-critical fuels into uranium compounds. Discussion of this danger here:
https://www.nature.com/articles/492031a
However, it is almost certainly far easier for a rogue state to go down the Iranian-style conventional nuclear pathway than to be reliant on sub-critical fuels as a weapons input source.
Of course, using existing nuclear waste as a fuel should, over time, become obsolete. When sub-critical nuclear becomes the norm it would be hoped that all the waste would be reprocessed and current critical stations would not be renewed at the end of their lives. The spent sub-critical fuel would still be radio-active and need protected storage but it would be many magnitudes less dangerous than current nuclear waste.
Other than existing waste, the main candidate for fuel in sub-critical plants is thorium. This is a naturally occurring mineral with deposits in many parts of the world. There is probably enough thorium in Scotland and Wales to provide all the UK's needs. Having said that, thorium is currently produced as a by-product of mining for other minerals in several parts of the world, so there may well be supplies, even ready stocks, available more cheaply on the world market. The UK's most probable supplier is likely to be Scandinavia. Unused thorium fuel would be useless as a component of dirty bombs.
More about thorium here:
https://www.world-nuclear.org/information-library/current-and-future-generation/thorium.aspx
Keep informed. In a world serious about tackling climate change, sub-critical nuclear is bound to play a role.