[off-topic]
why Harwell's P106 still exists
I can understand the question (like as if a little exclusion zone is going to prevent any unpleasantness).
There is still a lot of "old material" there, going back to the very earliest experiments. It is undergoing re-siting (within the Harwell complex) to protect it for a little longer than the original canisters could offer. The cost of this is enormous.
Unfortunately, a lot of this "material" is of unknown type and/or indeterminate radioactivity, from old gloves & suits to particles of unknown isotopes. Some has a half-life >100,000 years (so I am led to believe)...how dreadful that we impart this legacy to future generations.
[/off-topic]
Russ