There is a picture there of what appear to be ordinary goods vans in formation with the flasks.... are these simply barrier vans or was there sometimes other freight traffic (presumably between Sellafield and the relevent nuclear sites) that was "tacked on", and if so, why does this not occur now?
They're just being used as barrier vehicles AFAIK. I do recall however remember seeing a photo of a steam hauled nuclear flask in the sixties behind, IIRC, an 8F that appeared to be in the consist of a much longer van train. That could have been a brand new nuclear flask being delivered rather than a bonefide nuclear working though, given the time period.
As for why other freight traffic could not be conveyed in the consist of the nuclear workings, the nuclear flask trains are obviously a very sensitive form of rail traffic and need particularly attentive care to be paid to them and their consist, which of course would be more difficult if they were part of a longer formation. There would probably also be limitations on what other types of goods could be conveyed in such close proximity to (admittedly well protected) radioactive material; both for the safety of the goods themselves and also in the risk that could be posed in the case of a collision or accident. Plus, the nuclear trains tend to run direct from terminal to terminal (from power station to nuclear processing plant, or possibly in the less environmentally conscious 1980s to the nearest rail served pier for it to be chucked into the sea
Note: Just kidding!), and don't generally call in at any major marshalling yards en route, which is the only way that additional traffic could join or leave the train. Layovers at marshalling yards and the increased quantity of shunting required to attach and uncouple additional freight vehicles would pose a greater number of hazards.
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as regards the nuclear flask train, in the early 90's, one used to come into tyne yard around 9.30pm, run round and go back out within 20mins, it always had a grey 31 on it, i only seen it about 3 times if i was lucky and twice it was, 31276 Calder Hall Power Station.
http://www.flickr.com/photos/ingythewingy/6122962137/
Not the working but thats what livery the 31 had that 3 times i saw that flask working
Yep, in the early nineties, and probably the late eighties too, nuclear workings were allocated to pairs of coal sector class 31s, and as the OP says, class 20s too.