Reactor lower head penetration CO2 cleaning robot

The reactor pressure vessel(RPV) heads of PWRs have penetrations for control rod drive mechanisms and instrumentation systems. The bottom heads of PWRs also have penetrations for instrumentation systems. The bottom heads of PWRs also have penetrations for instrumentation. Nickel based alloys(e. g., Alloy 600) are used in the penetration nozzles and related weld. Primary coolant water and operating condition of PWR plant can cause cracking(PWSCC). The susceptibility of RPV head and bottom head to PWSCC appears to be strongly linked to the operating time and temperature of RPV head. After circumferential cracking of the nozzles above the J-groove weld of the RPV heads was reported by the NRC in 2001, licensees in Korea started bare metal visual inspection on the outside surface of the RPV heads and the bottom heads. While performing inspection, many domestic licensees found boric acid crystal deposited around the penetrations of the RPV heads and the bottom heads. The source of the boric acid crystals on the RPV heads and the bottom heads. The source of the boric acid crystals on the RPV heads was the primary water spilled during dynamic venting and the boric acid crystals on the bottom heads were the refueling water leaked through the cavity seal ring while the refueling cavity was filled. Because the identified boric acid crystals affect the reliability of the bare metal inspection, Korea Institute of Nuclear Safety (KINS) recommended that it is required to clean the boric acid deposits for future inspection. In response to the recommendation, we developed the dry ice cleaning robot which is remotely controlled and performs the dry ice blasting. In this paper we will reflect on the bare metal inspections and cleaning method for removal of the boric acid crystal deposited on the bottom heads and the RPV heads. The dry ice cleaning robot successfully cleaned the white residues on the lower heads repeatedly during outage in plants of different designs