There are several travel agent in Ukraine which organise Chernobyl tour and visited the Closed Area.
Is there any travel agents in Japan organising similar trips to Fukushima Closed Area?
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Officially or even semi officially no. The plant is still owned by TEPCO so arranging it with them would probably give you a better access and safer tour if they choose to allow it.
However, there are other options if you choose to consider them.
There is a boat tour that approaches the plant from sea or a Fairewinds virtual tour. Or finally you can try to get a local to give you a tour of the area but there are no contacts given so you may want to consider the repercussions of doing so and what kind of access you will be able to get remains a mystery.