3. Nuclear Reactor Safety Systems

It was mentioned in Section 1.1 that all nuclear reactors generate “decay heat” even after “shutdown.” In order to remove this decay heat, the reactors are equipped with safety systems or Emergency Core Cooling Systems (ECCS). Determination of this decay heat as a ratio of the reactor power after shutdown to the initial or rated reactor power has undergone a progressive series of refinements through the publication of ANSI/ANS standards. Figure 3-1 shows the result derived from the 2005 ANS standards for a UO2 – fueled PWR with 4.2% enrichment after a long period of operation as fitted by Shwageraus and Fridman and as depicted in Figure 3-11 of Reference 112.