Fourth Barrier Containment

The main objective of a containment building is to keep the release of radioactive substances to the environment within acceptable limits under accidental conditions [226,227]. 

The containment structure should also be designed to protect the public and plant personnel from undue exposure to direct radiation arising from the radioactivity present in the containment or in the systems within it. If this consideration is included in the design basis for the containment system, it may determine the thickness of the concrete of the containment envelope. 

The functional requirement for the overall containment system derives from its major objective, which is, in conjunction with other safety systems, to restrict the release of radionuclides resulting from accidental conditions to acceptable limits. The basic functional requirement that stems directly from this objective is to envelop (and thus isolate from the environment) certain systems, as the failure of these particular systems could cause an unacceptable release of radionuclides. This applies to all components of the reactor pressure boundary that cannot be safely isolated from the reactor core. 

The degree of containment tightness is usually defined in terms of the leak rate of the containment envelope. The leak rate is affected by factors such as  

The release of radionuclides from the containment envelope is determined by the following factors  

---

The flue gas passes through a number of small diameter high-efhciency cyclonic elements arranged in parallel and contained with the separator vessel. The UOP design uses an axial flow cyclone. After the catalyst particles are removed, the clean flue gas leaves the separator. A small stream of gas, called the underflow, exits the separator through the bottom of the TSS. In an environmental application, the underflow is diverted to a fourth stage separator (FSS) that is typically a barrier filter. The underflow rate is typically 2-5% of the total flue gas rate and is set by use of a critical flow nozzle. 

The containment is basically an envelope around the reactor and other nuclear systems. In normal condition, it acts as a housing. In case of an accident involving a break in the primary heat transport system, it acts as a barrier (fourth and final) against the release of fission products to the environment. This is a safety system comprising of many subsystems which are briefly outlined below. 

---