9. Inherently Safe Reactor Designs

3 ease the reactor siting restrictions away from

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Unformatted text preview: siting restrictions away from population centers, and make possible the combined production of electricity and heat for densely populated areas. 4. Build economical nuclear power plants in smaller unit plant sizes, in an environment where the economies of scale favor large size units. 5. Simplify the design to create an intrinsically safe water power reactor which eliminates the need for add on engineered safety systems and the associated quality assurance measures. It is recognized that as long as the core of a reactor remains unharmed, no incidents in a light water reactor will result in the release of radioactive matter with major environmental impact. Accordingly the design philosophy here is to protect the core against melting or overheating under all credible circumstances. Table 1: Comparison of the main features of 860 MWe MHTGR and PWR. Operating Characteristic MHTGR PWR Steam temperature (degrees F) Net plant thermal efficiency (percent) Reactor core power density (Watts/cm3) Makeup cooling water required with wet cooling towers with forced draft (gallons/min) Fuel required per billion Watts of electricity during 30 years of plant operation (short tons of U3O8) PWR: Low Enrichment Uranium, MHTGR: Medium enrichment, once through fuel cycle. PWR: Low enriched Uranium with Pu recycle, MHTGR: High enrichment Uranium with U233 recycle. Radioactive wastes Liquid (Curies/year) Solid (m3/year) Gaseous (Curies/year) 1,000 38.5 7.1 14,400 540 32 100 19,900 4,510 6,000 2,280 4,100 134 61 450 310 180 450 The core will to remain unharmed when two design criteria are met: 1. The core shall remain submerged in water at all times. 2. The power level of the submerged core shall not rise to a level where the cooling capability of the submerging water is exceeded. Dryout of the core must be avoided. The designers of the PIUS concept adopted the design principle that the process of heat extraction from the core shall be such that fulfillment of the two design criteria will be ensured on the basis of the laws of thermodynamics alone, following any credible incidents, with the primary system intact, or subject to foreseeable damage. Following this design principle, makes the plant independent from mechanical or electrical components and systems, defeating Murphy's law, where if something can go wrong, it will. The plant becomes insensitive to human error by the operators. It becomes immune to destructive human intervention. The concept was developed in Sweden by ASEA/ATM. The design principle depends on immersing the core, as shown in Fig. 11, in a large pool of cold borated water. The pool of water is contained at full reactor pressure within a large Prestressed Concrete Pressure Vessel (PCRV). The steam generators and the primary cooling system are also located in the pool. All piping and instruments penetrations are at the top of the PCRV. Such a configuration makes impossible for any type of leak to lead to the uncovery of the reactor core, and it remains submerged under water under all credible conditions. Fig. 12: The primary system of a 500 MWe PIUS plant. Under normal operation, the primary coolant water is pumped through the core to the steam generators where steam is produced and sent to the electrical generators and turbines. A stagnant connection of the primary circuit to the reactor pool is provided in terms of the lower and upper density locks. The interface there is pressure balanced so that the two water circuits do not mix under normal operational conditions. If the normal flow of coolant in the primary circuit is interrupted for any reason, such as a loss of power to the circulating pumps, borated water water from the pool would naturally enter the primary system. The boron in pool's water is a strong absorber for neutrons, and would shut down the chain reaction and the power generation in the core. In addition, the water ingress would cool the reactor from the remaining decay heat by establishing natural circulation of the water through the core and the pool. The water in the pool is sufficient to keep the core cooled for at least a week, in the absence of other sources of water. Over this time, the decay heat would have declined to low levels, and makeup water could then be added. The design of the core is a typical PWR 16x 16 square fuel assembly without channels. The fuel rods have a larger diameter than normal PWrs at 0.5 in in diameter. This reflectcs the reduced specific power of the fuel at 23 kW/ kg U. The core height is is reduced to about 6.5 feet. This, combined with the reduced specific power, decreases drastically the core pressure drop and permits the use of a pressure balance system. Each fuel assembly can accommodate 4 shutdown neutron absorber rods. These rods are more of a licensing requirement for early plants, but may not be necessary for later plants sinvce shutdown can be achieved by boron injection. Each fuel assembly has 4 rod positions for gadolinia neutron absorbers which are replaced each year. For longer fuel cycles the number c...
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