welding or rewelding cabinets to steel embeds. Some cabinets were stiffened by addition of internal bracing. Not all essential cabinets were upgraded and for those that were, in almost all cases, the upgrades were inadequate to provide the required stiffness for screening or to justify reasonable amplification factors for which existing component tests could be used for seismic adequacy verification. The inadequacy resulted primarily from inconsistent installation of internal bracing, poor or unknown quality of welding and expansion anchor installation, prying action on expansion anchors and incomplete evaluation of load path. For electrical cabinets, the most effective way to alleviate all potential problems was to top brace them to adjacent structural members to increase stiffness and to resist overturning. The fix at the base then only required resistance to base shear.
In several rooms that contained circuit breaker and control cabinets, the cabinets were welded to raised steel floors constructed of a gridwork of channel. All of these steel floors needed to be upgraded to increase the stiffness. The lack of stiffness in many cases resulted from the steel floors not being built in accordance with the drawings. Also, in many cases,
the steel floor gridwork was only supported for dead weight whereas the drawings showedpositive attachments of the vertical supports to the floor. Upgrading of the steel flooring
required addition of horizontal cross bracing and some diagonal bracing to the concrete floor.In the 6kv switchgear, some stiffening of the panels to which relays mount had been done in earlier modifications. In this case the modifications were found adequate to justify
lower amplification factors than suggested in the GIF. In this case, relays are not mounted on
the front door panel as seen in many U.S. manufactured switchgear and the stiffened internal panels resulted in significantly lower amplification. Relays in electrical power equipment and instrumentation and control equipment were evaluated separately as described in a following subsection.The 6kv transformers required bracing of the internal coil assembly. The top of the coils were connected to the metal enclosure which first appeared to result in top bracing of the coils but upon a close examination it was determined that the coils were actually supporting the enclosure. An internal A-frame was designed to stabilize the upper portion of the coil assembly and enclosure and to reanchor the transformer to the concrete floor.
Piping: During the small reconstruction program, some of safety related piping was upgraded to resist seismic forces. GERBS dampers were added to the primary coolant system piping and components to stabilize the six primary loops. Other piping connecting to the primary loop system was also reinforced by the addition of GERBS dampers. In the REKON
project, all safety piping within the scope of reconstruction is being reassessed and upgraded
if necessary.67
Some of the analyses from the small reconstruction project were initially reviewed.
These analyses were conducted for a different earthquake than is currently defined so the objective was to see if current loads exceed the loads used in the small reconstruction and also to verify the adequacy of the small reconstruction modeling. In some cases the reviewers disagreed with the existing modeling and that work will be revisited using the most current definition of the Review Level Earthquake. This may result in modification to the supports for the primary system. For most of the piping, whether it was included in the small reconstruction or not, the simplified walkdown and screening criteria were applied and, where warranted upgrades were recommended.
The walkdowns and screening revealed many issues of poor construction of pipe supports, improper supporting of heavy motor operated valves and a general lack of supports for seismic loading. In some instances, it was observed that GERBS dampers had been placed in illogical locations and had been used when rigid struts would have been acceptable.
Several instances were noted where pipe guides, which were supposed to allow thermal growth in one direction, were binding and not allowing thermal movement.
The solution to most of the issues identified by walkdown and screening, using chart methods, was to add simple supports or fix existing supports. In most cases, the upgrade designs were accomplished in the field without computer modeling of the piping system or
without conducting detailed analysis of supports. New supports were generally selected from standard configurations contained in the pipe routing guidelines. Screening by experienced
Western contractors and preparation of detailed fabrication and erection drawings by localcontractors proved to be very efficient to resolve piping seismic issues.
Cable Raceways: Because of fire separation issues and the addition of new I&C, many new cable raceway systems are being added. These new systems were designed for
seismic loading by classical stress analysis methods. There were still a large amount of cable
raceways that were to remain in service, which for the most part, required upgrading. TheGIF with some modification was used for initial screening and design of upgrades. Typical
discrepancies found during the walkdowns and screening were:• Trays not attached to raceway supports
• Non-ductile connection of raceway supports to structures
• Floor to ceiling columns in the cable spreading room lacked sufficient
flexibility to accommodate vertical differential movement of floors• Overloading of raceways
• Unacceptable welding quality
The upgrade designs were performed primarily in the field. In general, the upgrades focused on altering the details to meet GIF requirements rather than to meet classic structural strength criteria. In this manner, the upgrade designs could usually be accomplished without
detailed mathematical modeling or detailed calculation of strength for supports. New supports were attached to the existing structure by welding where possible. Expansion
anchors were used to attach supports to concrete only if welding to steel structures was not practical.HVAC Ducting: The initial screening was done by walkdown and comparison to allowable span charts. The span charts were based upon ducting capacities derived from test
data. In almost all cases, existing HVAC ducting required resupporting. The typical detail for existing support of the ducting was by rod hangers where the rods were attached to the
edge of structural I beams by poor quality welding. The only lateral support provided to ducting was at wall penetrations, but most of these interior walls were of unreinforced masonry and required stabilization measures as well.Most of the new supports for ducting were attached to existing structural steel by
welding. Use of expansion anchors into concrete was avoided if possible. Just as for piping
and cable raceways, the support designs were primarily done in the field using standard configurations contained in the routing guidelines. Very little detailed analysis of supports was required.Relay Evaluation of Electrical Distribution and Control Systems: The distribution systems of Bohunice and the relay I&C-cubicles consisted in the past exclusively of conventional switchgear cabinet types; no motor control centers were used. The
mechanical design varied slightly depending on the voltage level, function, date of manufacturing and manufacturer. Variation is partly as a result of the available equipment at
the time of construction of the plant and/or of the responsibility of the different supplier and partly as a result of the historical development. The design is dominantly of Czech origin.Some parts, like the trip breakers, are from the former Soviet Union. But, fortunately, for
physical reasons the basic construction of the feeders and relay I&C constituting the systems turned out to be quite comparable. Generally, the same design is used for safety and non-safety systems.
As mentioned in Section 3, during the course of the reconstruction of the Bohunice plant, major parts of the safety systems have to be replaced. The new equipment to be installed will be qualified by conventional shake table testing. Because of almost total replacement of I&C, the group of remaining equipment containing relays could be reduced exclusively to the distribution systems for the emergency power supply. For reasons, which
will be explained in the following, within the emergency power supply we did not distinguish, whether a piece of equipment is necessary for the mitigation of the seismic event
or not (including all supporting functions and consequential functions). Regardless of function and level of safety, with few exceptions, switchgears and I&C-cabinets are generally equipped with devices which may vary in size and power dependent on the special task, but types, manufacturer, and functional arrangements of the constituting parts necessary for the active functions are in most cases very similar.Medium Voltage Distributions: The 6kV medium voltage emergency power distribution systems are exclusively made up of bus bars and feeders; they contain:
• Circuit breakers
• Interfaces (relays) to I&C (automation and control)
• Protective relays (overload, short circuit, electric arc, etc.)
• Time relays
• Mini circuit breakers for the power supply of the protection and control circuitry
• Relays for electrical interlocks (if any)
69
Low Voltage Distributions (AC and DC): These 400v AC and 220v DC systems are exclusively made up of bus bars and:
• Circuit breakers
• Load-break switches
• Break switches
• Contactors
• Manually operated switches
• Time relays
• Protective relays (overload, short circuit, etc.)
• Relays for electrical interlocks (if any)
I&C Cubicles Including Diesel Generator Control: These cubicles contain:
• Relays for automation and control
• Time relays
• Memory relays