Safety Characteristics

Advanced reactors of tomorrow can be even safer than the current generation of nuclear power plants. This additional margin of safety can be achieved by designing future plants to enhance "passive" safety characteristics, supplanting the comples, active safety systems in use today. In technical jargon, a passive safety characteristic is determined by the laws of nature, and does not rely on the proper functioning of humam operators or special equipment.

Reactor accidents can only result from failures in which more heat is produced in the fuel than is carried away by the coolant. If the fuel gets hot enough, radioactive fision products will be released from the fuel because the cladding ruptures or melts. Radioactivity would still be kept away from the environment by the steel reactor vessel and the large containment dome. This was the case in the Three Mile Island accident--only a small amount of inert radioactive gas was released. But the psychological effect of the accident on the local population was traumatic, and the cost has been staggering. Clearly, there is a strong incentive to design power stations such that upsets in the plant are much less likely to lead to a reactor accident. This incentive has led to substantial improvements in today's nuclear plants, but exciting work is also being done to optimize the inherent safey features of future reactors.

(more to come here)

These passive safety characteristics of the IFR were tested in EBR-II on April 3, 1986, against two of the most severe accident events postulated for nuclear power plants. The first test (the Loss of Flow Test) simulated a complete station blackout, so that power was lost to all cooling systems. The secont test (the Loss of Heat Sink Test) simulated the loss of ability to remove heat from the plant by shutting off power to the secondary cooling sytesm. In both of these tests, the normal safety systems were not allowed to function and the operators did not interfere. The tests were run with the reactor initially at full power. In both tests, the passive safety features simply shut down the reactor with no damage.

As shown in the figures, the fuel and coolant remained within safe temperature limits as the reactor quickly shut itself down in both cases. Relying only on passive characteristics, EBR-II smoothly returned to a safe condition without activation of any control rods and without action by the reactor operators. The same features responsible for this remarkable performance in EBR-II will be incorporated into the design of future IFR plants, regardless of how large they may be.

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