Passive design feature

First, the philosophy requires that control of radionuclides be accomplished with minimal reliance on active systems or operator actions. By minimizing. the need to rely on active systems or operator actions, the safety case centers on the behavior of the laws of physics and on the integrity of passive design features. Arguments need not center on an assessment of the reliability of pumps, valves and their associated services or on the probability of an operator taking various actions, given the associated uncertainties involved in such assessments. 

Human factors incorporate both active and passive safety. For example, placing a valve so that it is easy to identify and operate is a passive design feature however, an operator control panel is more of an active safety system. 

Passive design features should be chosen over active systems. 

Passive design features Built-in design features (e.g., dividers, holders, spacers) that restrict operations to implement a control... 

Passive design features of the previous levels avoid any significant release of radioactive materials and necessity for evacuation or relocation measures outside the plant site. 

Control of accidents within the design basis. Achieve fundamental safety functions by optimized combination of active and passive design features limit fuel failures increase grace period to several hours. 

Fig. 13. Proposed advanced PWR design having passive safety features (79). Courtesy of Westiaghouse Electric Corp.

Passive—Minimizing the hazard by process and equipment design features which reduce either the frequency or consequence of the hazard without the active functioning of any device e.g., the use of equipment rated for higher pressure. 

Secondary containment systems are best described as passive protective systems. They do not eliminate or prevent a spill or leak, but they can significantly moderate the impact without the need for any active device. Also, containment systems can be defeated by manual or active design features. For example, a dike may have a drain valve to remove rain water, and the valve could be left open. A door in a containment building could be left open. 

Intrinsic and passive safety design features are by nature more reliable than design features that depend on a consistent performance of a physical or human system.(2) Passive designs reduce the probability of an occurrence occurring or minimize the severity of the adverse consequences without the requirement for successful operation of devices, con-... 

The Advanced Pressurized Reactor AP-600 (Westinghouse) final design certification by NRC is expected for 1998. A larger unit of an advanced PWR, the System 80+, with 1300 MW(e) is being developed by ABB/Construction Eng. [86]. These designs with enhanced passive safety features are pursued in the USA to provide additional electricity and replace retired plants. 

Some passive controls will live outside the user interface and may not be apparent to day-to-day operators. For example, HIT systems typically need to exhibit resiliency in their architecture whether brought about through redundancy or other systematic means. These design features represent active engineered controls. However it is common for this to be supported by other more passive controls which require some degree of human intervention. The platforms on which systems reside can often be monitored for availability and performance. In some cases systems may be specifically instrumented to provide metrics on the execution of specific functions or the success of database transactions. Similarly systems may log errors or failed messages which are then made available for inspection by service management personnel. 

Among the safety-related design features it is important to point out the extensive use of passive systems and safety features whose operation is based on natural processes with no need for external power supply. 

This section identifies those design features from the hazard or accident anaiysis which are considered passive in nature, but which provide a safety function to either prevent or mitigate the consequences of postulated accident scenarios. 

A design example is a system of 1,700 MWe with an operating pressure of 25 MPa and a reactor outlet temperature of 510°C, which is expected to range up to 550°C (O Fig. 58.5). This feature enables ca. 44% of thermal efficiency, which is about one third higher than current LWRs. Passive safety features are incorporated similar to those of simplified BWRs. 

In Japan, the self-actuated shutdown system (SASS) has been developed as a passive safety feature (Takamatsu et al. 2007 Nakanishi et al. 2008) using the phenomena that electromagnetic force of the control rod latch will be lost when alloy temperature exceeds the Curie point. Several kinds of in-sodium transient testing have been carried out together with in-pile experiments in the experimental fast reactor Joyo. This mechanism is effective to the robust restraint core, which is designed for seisnuc requirements. 

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