GLOBAL CONCERN FOR NUCLEAR SAFETY SPURS ANALYSIS
OF REACTOR ACCIDENT CONTAINMENT SYSTEM
FEA software from ALGOR, Inc. used in safety analysis of Lithuania's
Ignalina Nuclear Power Plant.
 International experts and researchers from the
Lithuanian Energy Institute (LEI) have combined efforts to establish
the analytical basis for structural safety analyses of the Ignalina
Nuclear Power Plant near Ignalina, Lithuania. The analysts at
LEI used linear static stress analysis software donated by Pittsburgh-based
ALGOR, Inc. to study the accident containment systems of the world's
largest nuclear reactors.
November 6, 1998, Pittsburgh, Pennsylvania - The 1986
nuclear disaster at Chernobyl forever altered international consciousness
about the risks of nuclear power generation. Therefore, when Lithuania
won independence from Russia five years later and became the sole
operator of the world's two largest, Chernobyl-style power reactors,
the international community took action. Lithuania's lack of nuclear
regulatory experience at the time and Ignalina's production of
more than 80 percent of Lithuania's electric power incited numerous
internationally-funded programs to ensure the safety and continued
operation of the Ignalina reactors.
A joint effort by the international community and the Lithuanian
Energy Institute (LEI) has enabled western experts and Lithuanian
scientists and engineers to establish the analytical basis for
structural safety analyses of sections of two reactor accident
containment systems (ACS). Finite element analysis (FEA) software
donated by Pittsburgh-based ALGOR, Inc. is the backbone of efforts
by the LEI to ensure that radioactive releases are contained locally
and do not escalate into catastrophic releases with global consequences.
"The objective at LEI is to study the behavior of the entire 3-D
ACS structures using the latest analytical tools," said Dr. Algirdas
Marchertas, emeritus professor of mechanical engineering at Northern
Illinois University. "These tools were not available to the original
designers."
Ignalina's Accident Containment Systems
The Ignalina Nuclear Power Plant contains two RMBK, Russian acronym
for Channeled Large Power Reactor, 1500 megawatt, water-cooled,
graphite-moderated power reactors. Each reactor is protected by
an ACS, which consists of localized confinement systems for separate
coolant circuits. Containment systems in western countries are largely
steel or reinforced concrete, semi-cylindrical buildings that completely
enclose the reactor and its cooling circuits. While Ignalina's two
ACS's fully enclose the reactors in several interconnected shells,
they only enclose 65 percent of the primary cooling circuits.
The reactors are cooled by pressurized water within the cooling
circuit. During operation, most of the heat is located in the cooling
circuit. A break or rupture of the circuit could release a high-pressure
mix of steam and water. If such an accident occurs, special pools
of water condense part of the steam released to decrease peak pressures;
therefore, the risk of release of radioactive material to the environment
is reduced.
"In the case of an accident, such as the rupture of a cooling pipe,
the reactor cannot be properly cooled. This could cause a meltdown
and the release of radioactive substances," said Dr. Marchertas.
"Our collaboration with the LEI provides Lithuania with the capability
to study the containment capabilities for certain postulated accidents."
The LEI researchers used ALGOR FEA software to design and analyze
a model of a section of the ACS that contains the largest piping
system and thus presents the greatest risk.
Safety Design and Analysis
In 1991, Dr. Marchertas traveled to his native Lithuania to begin
the massive task of collecting data about the structure of the ACS
for finite element modeling and analysis. "There was an amazing
lack of technical data about the Ignalina reactors," said Dr. Marchertas.
"Originally, neither the Ignalina power plant nor the government
of Lithuania claimed any knowledge of structural drawings of the
nuclear facility." German researchers finally provided the structural
plans, on which the finite element model was based.
While the LEI researchers originally planned to use a CAD system
to model the ACS, they decided instead to use the modeling capabilities
of Superdraw III, ALGOR's precision finite element model-building
tool. They used ALGOR's 3-D thick-plate "sandwich" composite elements
to represent the reinforced concrete walls by specifying smeared
uniaxial layers of steel and adjacent layers of concrete. The researchers
disregarded the tensile strength of concrete by estimating a neutral
surface.
Next, the analysts applied translational boundary conditions to
all external nodes that in actuality are connected to adjacent structures.
Material properties for steel and concrete were adjusted for a temperature
of 143° C to account for the steam that would be released during
an accident. The analysts applied static internal design pressure
loading to the reinforced containment structures that house the
circulation pumps and high-pressure piping, the steam-receiving
channel, the connecting channel of the ACS and the steam reception
chamber. Furthermore, the analysts applied loading to the leaktight
compartments to simulate the total weight of four circulation pumps.
LEI engineers analyzed the model using ALGOR's linear static stress
processor to determine the maximum stresses and deflections that
result when pressurized compartment walls expand outward. The initial
results indicated a concentration of high stress in the reinforced
leaktight compartments bordering the location of the circulation
pumps. The researchers modified the composition of the reinforcement
structures and processed the model again. The maximum stresses dissipated
and appeared instead on the opposite wall of the leaktight compartments.
Maximum deflections corresponded to the largest unsupported wall
area of the ACS.
Overall, the modified design exhibited a rather uniform stress distribution.
"A uniform stress distribution is an indication of a good design,"
said Dr. Marchertas. "The uniformity observed in the preliminary
test data of the concrete reinforcements attests to the quality
of the structural design." While uniformity was a positive result,
the analysts found that when the design pressure was used, the stress
of the reinforcement at certain locations of the ACS in the ALGOR
model was excessive. This finding has led to additional analyses.
Implications and Further Studies
According to Dr. Marchertas, further analysis is underway to determine
whether stresses can be reduced if a gradual pressure history, simulating
an accident event, is used instead of a constant pressure. By examining
the areas of maximum stress within the ACS model, the analysts have
built detailed models of these sections to determine how to improve
the actual ACS structure. To this date, only one part of the ACS
has been exposed to this analytical scrutiny and even this analysis
is not complete. The other ACS structures encompassing smaller reactor
coolant piping also will be examined.
The analyses continue as the Ignalina Nuclear Power Plant operates
at near capacity. According to Dr. Marchertas, LEI analysts will
continue to refine the finite element models of the ACS, using ALGOR's
visualization capabilities to determine the integrity of the designs.
"ALGOR's visualization capabilities have been extremely important
to this design," Dr. Marchertas said. "Superview provides a detailed
representation of our FEA results and lets us see how we can improve
our models."
|
电力/公用事业---应用实例