Westinghouse has just completed implementing a Simplified Head Assembly modification at the Seabrook nuclear plant. Westinghouse developed this head assembly upgrade to simplify problems in handling the reactor head during outages with the aim of reducing the outage time by several days. In the case of Seabrook, however, the main motivation was staff safety as the removal and replacement of the vessel head elements requires substantial manual activities involving safety harnessing for some of the work – this has proven to be a challenge to Seabrook’s worker safety programme in past refuelling outages.
In its more advanced reactors Westinghouse had already introduced an Integrated Head Assembly designed to simplify and speed up the procedures for disassembling and re-assembling the reactor head, which can consume several days of critical path time during refuelling outages. These have been in use at several of the latest generation Westinghouse plants for a number of years. To provide earlier PWRs with a similar capability, the company developed the Simplified Head Assembly, adapting many of the design features of the Integrated Head Assembly.
A Simplified Head Assembly upgrade can be installed as a back-fit into most operating PWR plants, allowing a reduction in disassembly/re-assembly time to hours rather than days. This helps utilities optimise plant outages and reduce costs, not only by reducing critical path time, but also by reducing maintenance personnel radiation exposure and demands on other critical outage resources.
According to Westinghouse’s Mike Ball, who is supervising the installation at Seabrook, the upgrade should take about three days off the total outage time. Furthermore, although Seabrook already has low staff exposure levels, it should result in an additional reduction of at least 3 rem. At many other plants, the savings in dose can be as high as 12 rem.
SIMPLIFIED HEAD ASSEMBLY
The Simplified Head Assembly integrates the Control Rod Drive Mechanism (CRDM) cooling system, missile shield, and head lifting rig as an integral assembly rather than individual components that require separate handling and assembly. Furthermore, with the Simplified Head Assembly, the power and instrumentation cabling that runs to the head assembly is rearranged to a simpler configuration that locates all of the connectors to one or two easily accessible common connector plates. The net result is a significantly simplified process to prepare the head for lifting, helping to decrease risks to personnel safety and reduce both dose exposure and time.
The main features of the Assembly are the following:
• CRDM cooling system. With the Simplified Head Assembly, the CRDM cooling fans are permanently mounted on the head assembly itself, and the ductwork that surrounds the head assemblies at most plants is eliminated. The existing upper and lower cooling shroud assemblies are used with new extension panels that extend the shroud to the underside of the seismic platform. However, the existing ductwork is removed from the shroud nozzles. Air flow direction and other considerations are evaluated and addressed in plant specific applications. Engineering analysis of the air flow through the head assembly and surrounding reactor cavity area confirms the effectiveness of this design. This not only eliminates the effort to disassemble and re-assemble ductwork, with the associated radiation dose and challenges to personnel safety, but also eliminates the need for laydown space and polar crane demand to handle and store the ductwork.
• CRDM missile shield and head lift rig. Integral with the Simplified Head Assembly are the CRDM missile shield and head lift rig. The integral shield replaces the existing external missile shield and eliminates the need to disassemble, lift and store it during the refuelling outage. The existing head lift rig tripod is permanently mounted on the head assembly, thus eliminating the effort that would be required to assemble and remove it from the head assembly during each refuelling outage. This also reduces polar crane and laydown space demand.
• Power and instrumentation cabling. CRDM and Rod Position Indicator (RPI) cables are routed over a cable bridge to easily accessible cable terminations at the operating deck. This allows for the electrical disconnects and reconnects to be performed at one location at the end of the cable bridge rather than at several locations on the head assembly itself. This again helps to reduce both dose exposure and time. The cable bridge is hinged, allowing it to easily swing clear to allow for head lift. A winch is provided so that the polar crane is not required to raise or lower the bridge.
Significant savings in time and exposure are achievable with the Simplified Head Assembly as is shown by the list of the tasks associated with the head disassembly process which must be completed prior to lifting the head. Those tasks which are eliminated or significantly reduced with the Simplified Head Assembly are given in bold.
In addition to the potential time and dose savings associated with the elimination and reduction of these activities, other potential benefits include reduced need for laydown space and polar crane demand, as well as elimination of personnel safety risks associated with these activities.
