Nuclear System Isolation Function

The nuclear system protection system provides for the closing of valves to isolate the containment, thereby preventing the release of sfeam and process fluids. The logic and equipment required for fhose valves, which are required to be open during ECC are part of each of the separate ECCSs. 

The lines, which penetrate the containment and are required, to be isolated during emergency core cooling consist of fhree groups  

Reactor coolant pressure boundary isolation These are lines fhat connect directly to the reactor vessel and penetrate the dr3rwell and containment barrier. 

Containment isolation These are lines that do not connect to the reactor vessel but penetrate the drywell and containment atmosphere. 

Closed system isolation These are the lines that penetrate the containment. However, they are neither part of fhe reactor coolant pressure boundary nor are they connected directly to the containment atmosphere. 

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Nuclear system isolation by isolation valve closure in process lines penetrating the containment barrier is based on two-out-of-four logic for main steam isolation valves and a one-out-of-two taken twice logic for remainder of nuclear system isolation function. 

Such a separation is exact for atoms. For molecules, only the translational motion of the whole system can be rigorously separated, while their kinetic energy includes all kinds of motion, vibration and rotation as well as translation. First, as in the case of atoms, the translational motion of the molecule is isolated. Then a two-step approximation can be introduced. The first is the separation of the rotation of the molecule as a whole, and thus the remaining equation describes only the internal motion of the system. The second step is the application of the Born-Oppenheimer approximation, in order to separate the electronic and the nuclear motion. Since the relatively heavy nuclei move much more slowly than the electrons, the latter can be assumed to move about a fixed nuclear arrangement. Accordingly, not only the translation and rotation of the whole molecular system but also the internal motion of the nuclei is ignored. The molecular wave function is written as a product of the nuclear and electronic wave functions. The electronic wave function depends on the positions of both nuclei and electrons but it is solved for the motion of the electrons only. 

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