There's an amusing or serious (depending on your views of nuclear power) bit of campaigning and info from 1999 at
http://www.kare-uk.org/coreguide/trainguide.htm which includes a rough map of routes used for flasks with flask design sketches too.
(The flask design has since been revised and all flasks replaced).
In addition to hauling fuel and waste, these containers might be called into remove reactor material during de-commissioning of our older reactors in the coming years so will appear on other routes.
From a distance, some of these DRS movements might be confused with MPVs on leaf-jetting duty! Both travelling slowly in short formations with odd shaped loads.
Sometimes they seem to be double headed and sometimes powered at both ends - I guess it depends on the shunting requirements & facilities at each destination.
There's no great mystery about the movement of fuel to and from nuclear power stations but unlike coal stations which need an almost constant flow of fuel, nuclear rods last for days or weeks and are stored locally to cool down before going off to Sellafield for processing so there will be a sporadic element to timetabling them. DRS do run regular flask services though, and there are people posting pics of them on the 'net regularly, some knowing when to be ready for the pic.
Frankly, I feel fine about these movements and I did a little work for the nuclear generating industry. They don't move fast, are much safer on rail than road and they seem to get very clear paths (travelling at those speeds, there's not much chance of running into a red anway!)
Anyway, what possible abuse can occur? Even if someone got onto one of these waggons what could they do? And what would anyone want with the radioactive mush that comes out of a reactor after a lively time burning itself out other than the painstaking processes of separating out the components which can only be done in massive plants such as at Sellafield? Most of the mush is pretty low-level radioactive and wouldn't be much worse than the mush that's poured into supermarket food!
There's no secret about about the processing/re-processing of fuel either - plenty info on the web for those interested.
Now, if we had nuclear-powered trains as were discussed in the 60's . . . . that WOULD be different! (No catenery, no clag, constant heating, running for weeks on a re-fuel).
Hmm - maybe not