Reactor operation, safety features

A second, more devastating, nuclear accident occurred in Chernobyl, USSR, on April 26, 1986. In this incident, reactor operators were conducting an experiment to lower maintenance costs. Many of the reactor core safety features were turned off to conduct the experiment. The experiment failed, and the fission reaction spiraled out of control. The heat that evolved blew the 1000-ton lid off the reactor, and the graphite core began to bum, scattering radioactive debris into the atmosphere. As stated earlier, 31 people died in the immediate aftermath of the accident, 230 people were hospitalized, and countless others were exposed to high levels of radiation. 

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. 

To reduce the risk of container failure, the pressure vessels are equipped with several safety features. These can include an effective self-venting system where unforeseen overpressure is released by a quick open-resealing step, or the use of safety disks which rupture when their pressure limit is reached. The small vials (0.2-20 mL) of some monomode reactors are protected by the pressure limit (20 bar) of the caps used, which is significantly lower than the operating limit of the vials themselves (40-50 bar). 

Operating with chemicals and pressurized containers always carries a certain risk, but the safety features and the precise reaction control of the commercially available microwave reactors protect the users from accidents, perhaps more so than with any classical heating source. The use of domestic microwave ovens in conjunction with flammable organic solvents is hazardous and must be strictly avoided as these instruments are not designed to withstand the resulting conditions when performing chemical transformations. 

Fig. 35. Sectional view of the 600 MWe pressurized water reactor (PWR) expected to be operational by 1995. Considered the PWR of the future, the plant is designed for a minimum useful life span of 60 years and features numerous economic and safety features, including passive systems for ultimate protection. (Joint project of Westinghou.se, the Electric Power Research Institute, and the (J.S. Department of Energy)...

It should also be remembered that most evolving technologies, whether boilers during the 19th century, airplanes in this century, or nuclear plants, entail some accidents from which lessons are learned. Both the Three Mile Island accident, from which only limited radioactivity escaped to the environment, and the Chernobyl disaster, have led to the introduction of new safety features in nuclear reactors, in plant operating procedures, and in regulations. 

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 broad functions of the safety systems are common to most reactors. In the event of an abnormal condition they should shut down the reactor, isure a suffici t supply of coolant for the fuel, and contain any fission products which might escape from the fuel elemrats. Such safety features can be active (requiring some action from a control system, involving mechanical devices, and relying on an external power source in order to operate) or passive (built-in physical fail-safe features whose operation is not dq)end t on any control system, mechanical device or external power source). 

The reactor cooling system is composed of the MCS, ACS and VCS as schematically shown in Fig. 1. The MCS is operated in normal operation condition to remove heat from the core and send it into the environment. The ACS and VCS have incorporated safety features. The ACS is initiated to operate in case of a reactor scram. Besides one out of two components of VCS has sufficient capacity to remove residual heat, the ACS is provided to cool down the core and core support structure. A helically coiled intermediate heat exchanger (IHX) whose heat-resistant material is Hastelloy-XR developed by the JAERI has been installed in S tember 1994. Nuclear heat application tests using the HTTR, are planned to be carried out, and accordingly a heat utilizaticxi system will be connected to the IHX. The fuel fabricaticm started in June 1995 and will complete in 1997. 

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. 

For example, suppose that cars are powered with hydrogen or fuel cells instead of gasoline and that carbon dioxide reduction laws are enacted requiring all vehicles operated in the city to be carbon dioxide free. The current annual consumption of gasoline in Tokyo is equivalent to about 600,000 tons of hydrogen. In order to provide this quantity of hydrogen, about 60 small reactors with a thermal output of 100 MW will be required. Owing to the safety features of... 

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. 

Consequently, small reactor designs having features that allow operation with reduced crew size are favoured. Such operation requires a greater level of safety autonomy, that is, the reactor would have to achieve a safe, stable end state by passive means for all realistic upset scenarios without operator assistance and it would have to provide a sufficient response time for any eventual intervention using off-site resources. 

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 choice of the fuel meat is determined by the conditions imposed to the system in the aspect of reactor operation, maintenance and safety. These conditions oblige the fuel element to have the following features ... 

Safety Features Experiment of NHR-5 In the course of conunissiooing and operation, a number of experiments ha C been carried out to demostrate the feasibiliQ and safct> of the vessel upe heating reactor concept In these experiments there are no any external interference of ther rators. 

Dunng four winters of NHR-5 heaung operation, the reactor has been known as a valuable tool for a number of e.xperiments on operation behaviors and safety features The operational and experimental results have successfully demonstrated the inherent and passive safety characteristics of NHR-5 It was pro en tiiat the design concept and technical measures of HR are suitable to meet the requirements for disinc heating in nortliem cities, congeneration and air condition in the middle aties of China, as well as the seawater desalination... 

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