Passive safety features

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

The development of computer capabiUties in hardware and software, related instmmentation and control, and telecommunication technology represent an opportunity for improvement in safety (see COMPUTER TECHNOLOGY). Plant operators can be provided with a variety of user-friendly diagnostic aids to assist in plant operations and incipient failure detection. Communications can be more rapid and dependable. The safety control systems can be made even more rehable and maintenance-free. Moreover, passive safety features to provide emergency cooling for both the reactor system and the containment building are being developed. 

Passive safety features for the MHR include ceramic, coated-particle fuel and an annular graphite core with high heat capacity and low power density. Recently, INL has used the ATHENA thermal hydraulic code to model the response of the MHR during loss-of-flow and loss-of-coolant accidents and has confirmed these passivity safety features work to maintain fuel temperatures well below failure thresholds [8]. 

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. 

It is also nuclear power that will be of utmost importance in the energy supply of many countries over the next few decades. Nearly 500 nuclear power plants are currently being operated or are under construction around the world. The development of new, innovative reactor concepts utilizing passive safety features for process heat and electricity generation applications are considered by many to play a substantial role in the world s energy future in helping to reduce greenhouse gas emissions. 

The breakage of all heat transfer tubes in the steam generator was assumed, and protective actions like water steam dumping were neglected to find the passive safety features against this type of accident. 

Controlled state In DBC 2 (incident conditions), or DBC 3 4 conditions (accident eonditions) or in complex sequences, the plant is in a controlled state if the following conditions are ensured by operator actions or by the active or passive safety features ... 

These building structures are passive safety features and therefore perform passive safety functions. The only SSC whose mis-operation can directly affect the safety function of these structures is the shield door hydraulic system [a non-safety-reiated (NSR) system], which is used to lower and raise the massive shield doors connecting Rooms 108 and 109, and Room 109 and the Zone 2A canyon. Mis-opeiation of this system with the Room 109/Zone 2A shield door down and radioactive waste in Room 109, can present a radiation hazard to workers at the north end of Room 112. The Zone 2A canyon structures, including the associated shielding windows, and the Room 109 structures are the only HCF SSCs that are required to maintain their safety functions following an earthquake. 

The SCBs are passive safety features and therefore perform a passive safety function. Table 4.4-3 summarizes the interfaces of the SCBs with other safety-related and non-safety-related SSCs in the HCF. 

Safety evaluation studies have been conducted for confirming the physical phenomena and integrity of the reactor fiiel elements and the structures in the primary system during the normal operation, scram transients, and the early stage of postulated accidents. Recently major emphasis has been placed on an evaluation of passive safety features such as decay heat removal by natural circulation. 

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. 

In either option, the reference plant has a 1,700-MWe power level, an operating pressure of 25 MPa, and a reactor outlet temperature of 550°C. Passive safety features similar to those of the simplified boiling water reactor are incorporated. Owing to the low density of supercritical water, additional moderator is added to thermaKze the core in the thermal option. Note that the balance-of-plant is considerably simplified because the coolant does not change phase in the reactor. 

The very low power range <12 MW, includes NHR-5, SCORE, DRX, GAMMA, ELENA, ABV-1.5 and SHR. All of these small IPWR designs use natural circulation of the primary coolant without pumps and incorporate passive safety features such that some degree of limited autonomous reactor operation may be achievable. 

The core structure is similar to the CANDU core structure, but for this reactor system, the core is vertical, t heie the absorber material can be inserted by gravi force and passive safety features can be explored as much as possible. 

TECHNICAL OUTLINE OF A HIGH TEMPERATURE POOL REACTOR WITH INHERENT PASSIVE SAFETY FEATURES... 

Reactors based on this concept, and incorporating inherent shutdown and passive safety features are called LEADIR-PS, an acronym for LEAD-cooled Integral Reactor - Passively Safe. A possible configuration for LEADIR-PS, discussed in more detail in Section 3 and Section 4, is presented in Figures 1 and Figure 2. 

Passive safety features simplify the design and attain the required safety objective in a different way compared to large plants with more active safety systems. This could reduce cost and facilitate the presentation of the safety of the reactor to both regulatory authorities and the public. 

The purpose of this report is to provide up-to-date balanced technical information to engineers and scientists involved in the development, design or licensing of SMRs. It brings out the design approaches and features of SMRs, in particular their simplicity, their larger flexibility for a variety of applications and the use of passive safety features as fundamental... 

CANDU 6 Incorporates a number of passive safety features. These include ... 

Each GT-MHR power unit has a design rating of 600 MWt/286 MWe. (The rating of initial units is planned to be limited to 550 MWt/262 MWe to provide design margin). One or more standardized power units are used to form plants with ratings up to 1150 MWe while maintaining the passive safety features of the MHR. 

The passive safety features have been described in section 7.5.2.1. They comprise ... 

---