The Chernobyl accident dominates the energy accidents sub-category of most disastrous nuclear power plant accident in history, both in terms of cost and casualties. It is one of only two nuclear energy accidents classified as a level 7 event (the maximum classification) on the International Nuclear Event Scale, the other being the Fukushima Daiichi nuclear disaster in Japan in 2011. The struggle to safeguard against scenarios which were perceived, in many cases falsely, as having the potential for greater catastrophe, together with later decontamination efforts of the surroundings, ultimately involved over 500,000 workers and cost an estimated 18 billion rubles.

Where did the largest nuclear accident take place? Okie... think carefully first, then make a decision:
The relevant information to answer the above question is: The Chernobyl accident dominates the energy accidents sub-category of most disastrous nuclear power plant accident in history, both in terms of cost and casualties. So, the answer is The Chernobyl accident.