A horrible accident at Los Alamos in 1958 illustrates what happens to a person's body when they stand next to plutonium during a chain reaction.
Cecil Kelley was an experienced and knowledgeable lab technician. This is one of the reasons why what happened to him is so inexplicable. He was sifting traces of plutonium from other elements in a plutonium processing plant at Los Alamos. The procedure he was running involved running a mixing tank. The tank forms a whirlpool, which concentrates the material at the center of the pool.
Under normal circumstances, this would not have been a problem — but on that day, the plutonium concentration in the tank was 200 times what it should have been. The plutonium briefly sustained a chain reaction, giving off neutrons and gamma radiation. Kelley, who had been watching through an observing window, was irradiated.
Gamma radiation is dangerous enough. The high energy rays pass all the way through the body. They can ionize elements in the body, or give electrons in the body enough energy to cause secondary ionizations. These rays are, usually, what cause radiation sickness after a major radiation exposure.
But Kelley was also exposed to a cavalcade of neutrons. Neutron bombardment is often used in physics experiments to change stable atoms into isotopes likely to decay and give off radiation. This process is called neutron activation. It doesn't just happen in a lab. It happens in a human, or anything else, exposed to a concentrated blast of neutrons. Sodium-23 in the human body, for example, becomes sodium-24, a radioactive isotope which decays inside the body.
Kelley was transported to a hospital, but died 35 hours after the accident. His death prompted a thorough investigation, but no member of the public knows why the plutonium concentration in the tank wasn't what it should have been. In the mid-1990s, his daughter initiated a class action lawsuit, not against Los Alamos but against the University of California, due to the discovery that doctors had been removing and studying tissue from Los Alamos workers to understand the long-term effects of working in plutonium processing. The suit was finally settled in 2007 for 10.3 million dollars.
Plutonium in Solution Image: The Chemical Complexities of Plutonium, Los Alamos, Second Image: Los Alamos Laboratory, Department of Energy