In my opinion, while these accidents can be examined from a historical point of view, you'll face many difficulties in your research. First of all, I agree with your professors that you should have a more focused research question. As you've mentioned, there are historical studies about the events but it's not a very good option to just summarize them in your study. You should find an issue which was researched less. Maybe reading the earlier studies can give you some inspiration. I have some topic suggestions where there might still be questions to discuss but I also warn you about the problems with them. First, there have been some studies examining the difference between the two disasters but there might be room to study further the handling of these disasters by the the authorities or in the media. Seeing that the events are quite recent, they are still surrounded with secrets and controversaries and sometimes it's really hard to differentiate between facts, assumptions and lies. For a history thesis, I'd concentrate on Chernobyl rather than Fukushima. It raises problems too, mainly because several sources are still not accessible for research. For doing a more original examination, you should be able to speak Russian (and maybe Japanese too) and have access to documents in these languages. If you do, it can enable you to write about the events and mistakes leading to the disasters although I think that this topic has been thoroughly researched. An other option could be to write a study on the political effects of the Chernobyl disaster, e.g. if it had any role in the collapse of the Soviet Union or how it affected international relationships (e.g. between the Soviet Union and its satellite states or between USSR and USA). Language knowledge would help you with these questions as well but several of the documents were translated into English. If you don't speak Russian and you don't have anyone to help you, you still can find a topic to research. I think something along the lines of the influence of these disasters on the anti-nuclear movement or the difference in presenting the accidents in the mass media. About these topics, you can base your research on sources in English. I hope you find your field soon. Good luck.

I am late to this discussion but I will say: nuclear history is definitely a thing, as is the history of science and technology, as is the study of historical accidents, and so on. I work on the history of nuclear weapons; I got to it through an interest in the history of science and technology, but there are other approaches. You might look into the field of Science and Technology Studies, as well, which has more accommodation for present-day topics than pure history alone today (I identify as an historian but am employed by an STS program and am heavily influenced by its methods).

If you are interested in a few readings to get you started in this area, I might recommend:

• Charles Perrow, Normal Accidents (sociology of accidents)

• Adriana Petryna, Life Exposed (a historical/anthropological approach to Chernobyl)

• Eric Schlosser, Command and Control (popular history of Cold War nuclear accidents, very well done and covers a lot of ground)

For general history of science, Helge Kragh's Quantum Generations is an excellent overview of history and historiography on the history of physics. For general history of technology, Thomas Hughes' American Genesis is a good place to start. One could also be interested in the environmental history angle (Jake Hamblin's Arming Mother Nature would be maybe a useful book here). On the history of nuclear power, the work of J. Samuel Walker is good, though I admit that I have not yet found that one "history of nuclear power" book that I would recommend un-hesistantly above all others. His book on Three Mile Island is worth checking out, though.

Anyway, there are a lot of possibilities here. If you have any further questions feel free to PM me. On my blog I do an annual "nuclear history bibliography" which tries to list as much scholarship as I can find that is published each year on nuclear historical matters, as a way to get a survey of what kind of work is being currently done, which might also be useful.