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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
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
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
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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- Spring '08
- The Land