You know the joke about Chernobyl? It wasn't an accident, the Politburo decided it was an X-ray examination for the whole nation
The joke about Chernobyl is threefold:
1) It was built without a concrete dome in order to save money.
2) The powerplant staff at the time of the incident was new and inexperienced and failed to read its instruments correctly.
3) The same staff was tasked with carrying out a security experiment, testing the effectiveness of one single security measure in the event all others failed (which were shut off on purpose on that day). Turns out it didn't work if you failed to operate it correctly.
If one of these three things hadn't been the case, Chernobyl wouldn't have been the disaster it turned out to be. It's less a tale of the dangers of nuclear powerplants than a tale about the dangers of human incompetence.
And the joke is of course, that because of Chernobyl the entire western civilization now rather continues to pollute its climate with fossil powerplants instead of building more perfectly safe nuclear ones.
That being said, Sirveri, you have to consider that radiation poisoning is definitely not a process which starts only at the threshold you mentioned. It's a statistical process where at the mentioned threshold your likelihood to show symptoms gets higher than some imaginary number.
The imaginery number being 50%.
Provided the initial data responsible for the treshold was correct to begin with. Which is a realistic concern since our overall knowledge about radiation poisoning isn't the most reliable for obvious reasons. Scientists in this timeline probably already know much more about it than we do.
Considering that several million people were affected and then the dust was blown all across Europe, this means you can't extrapolate from the guidelines for individual exposure to the effects on large populations.
How so? The tresholds are the same for a whole population as they are for a single individual. If an entire population were given the same amount of radiation, 50% of it would show the appropriate symptoms, 50% not. Of course, the starving, sick, young and elderly are likely more affected than healty people.
If anything, the overwhelming difference would be the fear instilled into the population about a repeat of such an incident, which leads to the irrational scaremongering of today and ideological indoctrination against anything that has the label nuclear on it.
I dunno, people still go to and live in hiroshima and nagasaki, if anything the weapons exchange between you and the Axis probably used cleaner bombs. They're cleaner because they're more efficient, and thus burn more material upon detonation. That said, a major nuclear site would likely cause quite a bit of fallout due to production and refinement of Plutonium 239 and the resultant nuclear waste slurries created.
It's basically up to Firestorm how much damage the nuclear attack on London really did.
1) Does a nuclear attack ingame, which destroys the productive capability of an entire province, represent just one or multiple nuclear explosions?
2) What technology did the Axis use at this point? Was the bomb already an efficient Cold War-design (and therefore cleaner)? Or was it something in between the first extremely dirty bombs and optimated later ones?
3) How was the bomb delivered? Was it a groundburst (very dirty due to fallout) or an airburst (basically clean)? Note that against expectations an airburst is the attack method that does more physical damage to the target, however that first had to be found out.
4) Did the bomb target London (the city) or the nuclear facility? If the city was attacked, was the containment of the nuclear facility even breached? Presumably it was provided with at least a minimal security against axis attacks and was located somewhere on the provincial outskirts of London.
5) How much material was even left in the nuclear facility after Britain had used up all avaibable material for nuclear attacks?
etc.