Until recently, when engineers at Southern Company Engineering in Birmingham, Ala., designed an electric generating unit, they used AutoCAD in 2-D and communicated design information via drawings. The biggest drawback to this approach was that each discipline involved in the project could proceed independently. Although the drawings were routed throughout the engineering offices for review, they quickly became outdated, and interferences persisted in the construction phase.

Southern Company Engineering improved the coordination of its projects by representing an entire plant in software as a single 3-D CAD model.

Southern Company Engineering is a division of Southern Company Services, a large producer of electricity. The engineering unit has a staff of about 1,000. Approximately half of its work involves nuclear power plants, while the remainder deals with fossil and hydro power plants.

To improve the coordination of its projects, the engineering unit decided to try representing an entire plant in software as a single 3-D CAD model. The company adopted AutoPlant 97, a PC-based program from Rebis, of Walnut Creek, Calif. AutoPlant 97 runs as an add-on to AutoCAD, and uses the same menus and modeling conventions.

Now when the engineering unit designs a new power plant, an individual solid model is created for each piece of equipment and every run of pipe. Solid models representing piping and equipment are placed into appropriate work areas using AutoCAD's "xref" feature. All the pertinent information that a designer needs is visible in the AutoPlant model. The information is always up-to-date because when the software displays xref files, it shows current versions. Having up-to-date information has virtually eliminated inter-ferences caused by miscommunication between disciplines. The software's interference detection feature has also helped to reduce interferences. It recognizes both AutoPlant entities and solids, and highlights potential clashes. AutoPlant saves time in generating bills of materials and drawings.

Southern Company Engineering has used AutoPlant 97 in the recent design of two power plants. One, called the Olin project, is a cogeneration plant that will produce 112 MW in a relatively small site. One mechanical engineer and one mechanical designer handled all the piping design, including the production of 250 isometric drawings. The second plant, called the Barry project, is a combined-cycle plant that will produce 532 MW.

FEA Helps Keep Reactors in Check

In 1990, upon declaring its independence from Russia, Lithuania became the sole operator of the world's two largest, Chernobyl-style power reactors. Lithuania's lack of nuclear regulatory experience at the time and the Ignalina plants' production of more than 80 percent of Lithuania's electric power prompted numerous internationally funded programs to ensure the safety and continued operation of the reactors.

The Ignalina nuclear power plant has two 1,500-MW, water-cooled, graphite-moderated power reactors, each protected by an ACS.

A joint effort by the international community and the Lithuanian Energy Institute 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). The LEI is using finite element analysis software donated by Algor Inc. of Pittsburgh to ensure that radioactive releases are contained locally and do not escalate into catastrophic releases.

In 1991, Algirdas Marchertas, professor emeritus of mechanical engineering at Northern Illinois University in DeKalb, Ill., 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," he said. "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.

The Ignalina nuclear power plant contains a pair of 1,500-MW, water-cooled, graphite-moderated power reactors known by the Russian acronym RBMK (for Channeled Large Power Reactor). Each reactor is protected by an ACS, which consists of localized confinement systems for separate coolant circuits. Containment systems in Western countries are usually steel or reinforced concrete, semi-cylindrical buildings that completely enclose the reactor and its cooling circuits.

Ignalina's two ACS units enclose the reactors fully in several interconnected shells, but they enclose only 65 percent of the primary cooling circuits. 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.

LEI researchers have used Algor FEA software to design and analyze a model of an ACS section that contains the largest piping system and thus presents the greatest risk. Algor's 3-D thick-plate "sandwich" composite elements were used 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, they 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, they applied loading to the leak-tight 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. Initial results indicated a concentration of high stress in the reinforced leak-tight compartments bordering the location of the circulation pumps. Researchers modified the composition of the reinforcement structures and processed the model again. Maximum stresses dissipated and appeared instead on the opposite wall of the leak-tight compartments, while 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 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. Additional analysis is under way to determine whether stresses can be reduced if a gradual pressure history, simulating an accident event, is used instead of a constant pressure.


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