I've said this elsewhere, but, in brief, most major airports have web sites. Those web sites often include a statement about film scanning. Everyone I've seen says film at ISO 800 and below is safe in their carryon scanners. Everyone I've seen says no film is ever safe in their checked baggage scanners. Frankly, considering the potential blowback and litigation by institutions who lose serious money in fogged film, I suspect those pronouncements err on the conservative side.
The impact of x-rays is cumulative. I've had 100 and 400 film scanned as many as 6 times with no visible change.
Regardless of the law or regulations in a given country, asking for a handcheck puts you at the disposal of the local security staff. "They" say, for example, that no one gets film handchecked at Heathrow, period. If you do ask, make life easier by taking the film out of its boxes, and arrive 30-45 minutes earlier.