A nuclear plant has to conform to strict release limitations set by the AEC in its Code of Federal Regulations outlined in the previous chapters. Moreover, this compliance is not measured against actual releases, as in the case of the fossil-fueled plants but against hypothetically bad conditions described as the design basis for the plant.

Thus Table 6.5 compares the annual doses which are recommended as maxima by the Federal Radiation Council against those that are calculated to occur if 1% of the fuel were leaking and also against those that are actually expected on the basis of present operating experience in PWR plants.

The table shows that the design basis conditions are about a hundredth of the recommended maxima or better at the various exclusion zone bound­aries, while the actual expected values are again a further factor of a hundred lower {8).

The assumptions in this table are that a 45-day hold-up system is used to eliminate all short-lived isotopes and that the gaseous releases are those resulting from pessimistic wind dispersal conditions. The liquid releases here are calculated from the intake of aquatic food as well as drinking water. The fish and mollusks account for about 5% of the total.

If multiple plants in one location are considered, then sample calculations showed that, for pessimistic conditions downstream, a set of three plants resulted in about a 50% increase in dose (8). Combined doses are not ad­ditive, unless the plants are in identical positions in relation to winds and ground water flows.

It is worth noting that the FRC limits apply only at the boundary of each zone, and the doses will be significantly less elsewhere farther away. Figure 6.7 shows that while the design basis release is much lower than the FRC exposure limit at the fence line, as one moves away the dose becomes exponentially less. In fact, even with a complete failure of the fuel in this example, where the FRC exposure limit would be exceeded at the fence line, doses drop below the limitation very rapidly as the distance is increased.