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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.
Gaseous (Curies/year) 1,000
19,900 4,510 6,000 2,280 4,100 134
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
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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- Spring '08
- The Land