Reactor core safety

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. 

SG-D14 Design for reactor core safety in nuclear power plants 1986... 

The past safety record of nuclear reactors, other than the Soviet Chernobyl-type RBMK reactors, is excellent Excluding RBMK reactors, there had been about 9000 reactor-years of operation in the world by the end of 1999, including about 2450 in the United States.1 In this time there was only one accident involving damage to the reactor core, the 1979 Three Mile Island accident, and even at TMI there was very little release of radionuclides to the outside environment. 

Significant advances have also been made in reactor safety. Earlier reactors rely on a series of active measures, such as water pumps, that come into play to keep the reactor core cool in the event of an accident. A major drawback is that these safety devices are subject to failure, thereby requiring backups and, in some cases, backups to the backups The Generation IV reactor designs provide for what is called passive stability, in which natural processes, such as evaporation, are used to keep the reactor core cool. Furthermore, the core has a negative temperature coefficient, which means the reactor shuts itself down as its temperature rises owing to a number of physical effects, such as any swelling of the control rods. 

During normal operation, the main circulator transports hot helium at 1266°F (686°C) from the bottom of the core to the steam generator which, in turn, produces superheated steam at I005°F (541 °C) and 2500 psia. The cold helium at 496°F (258°C) is returned to the top of the reactor core. During normal shutdown and refueling, the non-safety auxiliary shutdown heat removal system removes core afterheat if the main heat transport system is not operational. 

Future nuclear reactors are expected to be further progressed in terms of safety and reliability, proliferation resistance and physical protection, economics, sustainability (GIF, 2002). One of the most promising nuclear reactor concepts of the next generation (Gen-IV) is the VHTR. Characteristic features are a helium-cooled, graphite-moderated thermal neutron spectrum reactor core with a reference thermal power production of 400-600 MW. Coolant outlet temperatures of 900-1 000°C or higher are ideally suited for a wide spectrum of high temperature process heat applications. 

Nuclear and Radiation Safety in Case of Water Ingress into Reactor Core of KM-1 Stand... 

Implementation of the method of Spent Nuclear Fuel (SNF) unloading from unwatered NS reactors represents a crucial solution of the nuclear safety challenge because removal of moderator eliminates the risk for reactor core to attain critical condition under any design-basis/beyond-the design-basis operations with reactor control systems. 

Despite the safety regulations, accidents have occurred with nuclear reactors and reprocessing plants, primarily due to mistakes of the operators. By these accidents parts of the radioactive inventory have entered the environment. Mainly gaseous fission products and aerosols have been emitted, but solutions have also been given off. In the Chernobyl accident, gaseous fission products and aerosols were transported through the air over large distances. Even molten particles from the reactor core were carried with the air over distances of several hundred kilometres. 

The high cost of constructing a modem nuclear power plant— three to four billion dollars, in the U.S.— reflects in part the wide range of safety features needed to protect against various possible mishaps, especially those which could release to the environment any of the plant s inventory of radioactive substances. (Small special-purpose reactors, such as those used to power nuclear submarines or aircraft carriers, have different costs and technical features from the large, land-based reactors used to supply electrical grids.) Some of those features are incorporated into the reactor core itself. Eor example, all of the fuel in a reactor is sealed in a protective coating... 

Every nuclear plant is also required to have an elaborate safety system to protect against the most serious potential problem of all, loss of coolant. If such an accident were to occur, the reactor core might melt itself down, possibly breaching the structures which contain it and releasing radioactive materials to the rest of the plant and, perhaps, to the outside environment. To prevent such an accident, the pipes carrying the coolant to and from the reactor are required to be very thick and strong. In addition, back-up supplies of the coolant must be available to replace losses in case of a leak. 

Even if terrorists succeeded in detonating an explosive at a reactor site, the health consequences would be limited. The reactor accident at the Three Mile Island, Pennsylvania nuclear power plant caused a small release of radiation, insufficient to cause any radiation injuries. Bypassing several safety systems caused the Chernobyl reactor incident, involving two explosions, fires and reactor core meltdown. This accident caused the following early phase health effects (1) ... 

M. Richards, A. Shenoy, Y. Kiso, N. Tsuji, N. Kodochigov, and S. Shepelev Thermal Hydraulic Design of a Modular Helium Reactor Core Operating at 1 000°C Coolant Outlet Temperature, Proceedings of the 6 International Conference on Nuclear Thermal Hydraulics, Operations and Safety (NUTHOS-6), October 4-8, 2004, Nara, Japan, Atomic Energy Society of Japan, Tokyo, Japan (2004). 

Tire reactor core, composed of graphite blocks, is so designed as to keep all specific safety features. In the cooling system, the intermediate heat exchanger (HEX) is equipped to supply high-temperature helium gas to some process heat application system being coupled to the HTTR in the future. 

The worst nuclear power accident in the U.S. occurred at the Three Mile Island plant in Pennsylvania. In this accident no one was killed and no one was directly injured. The event at Three Mile Island occurred from faulty instrumentation that gave erroneous readings for the reactor vessel environment. A series of equipment failures and human errors along with inadequate instrumentation allowed the reactor core to be compromised and go into a partial melt. The radioactive water that was released from the core was confined within the containment building and very little radiation was released. In the Three Mile Island incident, the safety devices worked as planned and prevented any serious injury. This accident resulted in improved procedures, instrumentation, and safety systems being implemented. 

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