Knowledge of Nuclear detectors, acceleration, fission and fusion is vary important in understanding the behaviour of nuclear radiations as well as particles of cosmic rays. This in turn gives us an opportunity to understand the mystery of the origin of this universe and may other related phenomena This book has been written to help students of physics in their understanding of detectors, accelerations of nuclear radiations and the fission as well as fusion of nuclei that is again a great source of energy. Fusion is the source of energy of stars. A brief introduction to nuclear reactors is also given. The gives introductory concept about working of detectors, accelerators and nuclear reactors. Every effort has been made to keep that language simple so that the book may be useful for casual reader as well as students of physics. Several books and reference materials have been referred in preparation of the manuscript and the author is grateful to all them who propagated scientific temper through different means. The author also hopes that this book will wish to explore the subject.