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."
|
