9. Inherently Safe Reactor Designs

This reduction was achieved by shortening the sbwr

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Unformatted text preview: on was achieved by shortening the SBWR fuel bundles from 3.7 meters to 2.7 meters. In addition the core's chimney, which is the annular region above the top of the core has been lengthened from 3 to 6 meters, enhancing the natural circulation process. The vessel of the SBWR is made of forged rings with a diameter of 6 meters, and a height of 24 meters. The most attractive feature of the SBWR is its passive safety system shown in Fig. 8. In the case of Loss of Coolant Accident (LOCA), the water level in the reactor's core would drop to a level at which the safety system is initiated. A Depressurization Valve (DPV), ahead of the Main Steam Isolation Valve (MSIV) to the turbine, opens and the reactor's core is rapidly depressurized. Upon reaching a pressure of 30 psig, water from a Gravity Driven Cooling System (GDCS) pool, flows into the core. There are 3 independent pools of this type situated 12 meters above the core to provide enough head of water to overcome the reactor's pressure. Isolation Condensers (ICs), situated in water pools on top of the reactor building replenish the water in the GDCS pools. These ICs are essentially heat exchangers, and were used in earlier BWRs. Steam in the drywell portion of the containment structure is diverted by the pressure in the drywell into the IC where it is condensed and returned to the GDCS pool and, then to the reactor core. The operation of the ICs leads to a cooling of the containment. Heat is first transferred from the IC to the surrounding IC water pool. As the temperature of the pool rises, boiling ensues, and steam is released. A vent releases the steam to the atmosphere, making the atmosphere the ultimate heat sink. This approach to cooling the containment is designate as the Passive Containment Cooling System (PCCS). It eliminates the need for safety grade core cooling , for heat removal pumps, and for the supporting diesel generator units. The water available in the pools can support heat removal for up to 72 hours without any operator's intervention. The IC pool is outside the containment structure so that any escaping steam does not contain any radioactivity. For long term cooling, water is also available from the pressure suppression pool. Thus dependence is on the Gravity Driven Cooling System pool, the Isolation Condenser pool and the pressure suppression pool as sources of cooling water, as shown in Fig. 9. This provides three redundant passive cooling systems, each one being capable to independently mitigate the consequences of a LOCA. THE MHTGR, MODULAR HIGH TEMPERATURE GAS COOLED REACTOR This reactor design comprises a 100 MWe graphite core, gas cooled reactor. Its small size and low power density achieve inherent safety. For the MHTGR the power density is 3 [W/cm3], compared with PWR at 100 [W/cm3]. The graphite core offers a high thermal inertia capable of absorbing a great amount of heat under accident conditions. The core is cooled with an inert gas: Helium. The design possesses a high negative temperature coefficient of reactivity, which would terminate the accident after a modest temperature rise without a radioactive release from its encapsulated fuel particles. The fuel particles themselves act as miniature pressure vessels containing the fission products. Figure 10 shows a multilayered TRISO pyrolytic graphite and silicon carbide coated MHTGR fuel particle. As shown in Fig.11, the core of the MHTGR is limited in diameter, permitting the decay heat to be conducted and radiated to the environment without overheating the fuel to the point where the fission products would be released. A steel vessel now replaces the prestressed concrete pressure vessel of previous HTGR designs. The radiative cooling property is here gained without the active intervention of the operators. Many of the safety features of the MHTGR have been demonstrated in Germany on a 15 Mwe reactor: the Arbeitsgemeinschaft Versuchs Reactor (AVR) which was started in 1968. Fig. 9: Plant Layout for the SBWR showing the three independent passive cooling water pools. Fig. 10: Pyrolytic carbon and Silicon carbide coated TRISO particle for the MHTGR. Fig. 11: Pressure Vessel and Steam Generator for the Modular High Temperature Gas cooled Reactor (MHTGR. The MHTGR offers some perceived advantages compared to the PWR concept as outlined in Table 1. Chief among them are operation at high temperature resulting in higher overall thermal efficiencies, and the ability to produce process heat for industrial applications such as high temperature electrolysis of water to produce hydrogen as a future fuel. THE PIUS, PROCESS INHERENT ULTIMATE SAFETY REACTOR The PIUS concept was conceived with the following objectives: 1. Alleviate the public concern by relying on laws of nature, particularly natural convection, rather than the failure prone equipment and human intervention in the operation of a nuclear power plant. 2. Improve the safety margin to operate under more adverse conditions than exist in current PWRs, such as in third world countries. 3. Ease the reactor...
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