How does nuclear energy affect human tissue?

After the atomic bombs were dropped in Japan, the Atomic Bomb Casualty Commission was set up to study the effects of radiation on the population. This is an extract from Wikipedia:

The ABCC also drew on the work of Japanese scientists, who were already studying the survivors in the time before the ABCC arrived in Japan, so there was information from both American and Japanese officials. Masao Tsuzuki was the leading Japanese authority on the biological effects of radiation. He said there were four causes of injury in the bombed cities: heat, blast, primary radiation and radioactive poisonous gas.

In a report that was released by Tsuzuki, he answered the question, "What does strong radioactive energy do to the human body?" His answer was, "damage to blood, then hematopoetic organs such as bone marrow, spleen, and lymph nodes. All are destroyed or damaged severely. Lungs, intestines, liver, kidney etc are affected and their functions disturbed as a result." The damages were rated by severity. People suffering from severe damage were people who were in within a 1Km radius of the hypoenter. The severely affected people typically died within a few days, some living as long as two weeks. Moderate damage was seen in people living in a 1-2 km. radius from the hypoenter, and those people would live for 2-6 weeks. Those people living within a 2-4 km. radius had slight damage, and which would not cause death, but would cause some health problems during the several months after the exposure. If you want to read more the link below is worthwhile, and there are other links within Wikipedia

Since then of course the science of Health Physics has moved on a long way, and safe working limits for different types of radiation are now well understood. The first thing that seems to happen is the bone marrow being affected which affects the white cell count in the blood. If these immediate effects can be survived, there may still be long term development of cancers, but in studying a population for these effects it is always difficult to separate out what would be the normal rate of development of cancers, without the extra radiation (and we have a background from natural causes, which is always there and always has been in human history)