I had a look at that post, and yes there was a spike, but the maximum reading according to the graph was 0.095 microsieverts per hour. It's about the same level of background radiation that New York had on May 31st 2012. The average global background radiation level is 0.27 microsieverts per hour (I'm not great at math but that 200 times higher, right?). The average background radiation level at cruising altitude in an aircraft is 2.7 microsieverts per hour (my calculation says that's 2000 times higher than Osaka's background level). Hope that gives the "spike" some context and perspective.
It's entirely up to you to decide what level of background radiation is acceptable for your circumstances, but personally I'd have no problem with that "spike" if I were planning on going to Osaka.
Phil from the safety hub about 6 years ago