Acute exposures would be important only in the event of a major disaster in which the reactor was essentially destroyed and its contents of radioactive material released to the environment. This would almost certainly involve forces external to the installation, such as a major earth­quake, total failure of all safety features, or hostile forces either from without or within, including an internal conspiracy. Recovery of the or­ganism from these acute effects would be almost complete if the individual survived, although there is a finite risk of a small residuum of irreversible damage.*

The immediate or early effects of such acute exposure would involve primarily the blood-forming organs, the gastrointestinal tract, skin, and if the dose were very high (>1,000 rads total body exposure) the central nervous system. These are threshold responses so far as can be ascer­tained, and recovery is complete except for the small fraction of irrepa­rable injury, f There is a fourth — or fifth-order risk of a long-term delayed effect after such an acute exposure. The long-term risk is the development of leukemia or other forms of cancer, cataracts, and possibly accelerated incidence of the degenerative diseases of old age.

The doses at which these acute effects occur are many times higher than those associated with maximum permissible population or occupa­tional exposure. They are mentioned here because any general discussion of the problems of nuclear power generation must consider them, however * The presence of an irreversible component in radiation exposure may be basi­cally different from the biological effects of most other agents in our environment. The irreversible component may range from a few per cent or less with X or gamma irradiation to as much as 50 per cent for neutrons and 70-80 per cent of the total effect with alpha particles and other high linear energy transfer radi­ations.

tThe risk of long-term effects might be based on the fraction of irreversible in­jury, but this is not usually done except in considering nonspecific effects such as shortening of the life-span. Processes with long latent periods may or not be linked to the “irreversible” component.

unlikely the maximum credible accident. It is not my role as a biologist to estimate the probability or improbability of such an event. The reactor engineers do this and relate it to the probability of other major catastro — phies in our technological civilization.