An Advanced RCP Seal Design for Coping With Extended Loss of AC Power Event

Abstract

The extended loss of AC power (ELAP) event can pose a significant challenge to the integrity of the Reactor Coolant System (RCS) in a Pressurized Water Reactor (PWR). To mitigate the consequences of this event, it is important to maintain adequate coolant inventory and the associated heat removal capability of the RCS. A critical RCS component in maintaining RCS inventory is the Reactor Coolant Pump (RCP) seals. To ensure seal integrity, the RCP seals are either independently cooled by back-up systems or are sufficiently robust to maintain their integrity without cooling until such time when cooling may be restored. A number of new RCP seal designs have been proposed by seal manufacturers to meet this challenge. This paper focuses on one such design, namely, the KSB Station Blackout (SBO) seal package. KSB RCP seals were first introduced in the nuclear industry in the 1970s. The original KSB RCP seals, designated as HDD-254 Type A, consisted of a three stage hydrodynamic design with each stage capable of retaining full RCS pressure. The Type A seals were initially installed in the Combustion Engineering (CE) Palo Verde units and in several reactors in Germany. In the 1980s, these seals were replaced with the Type C seals that incorporated several design improvements. These seals have been successfully installed and operated in over 100 RCPs in Europe, South America, South Korea, and China. These seals continue to demonstrate excellent operating experience with typical replacement interval of 4 to 6 years. Although the reliability of the Type C seals has been excellent based on operating experience, the seal was not specifically designed for coping with an ELAP event that might ensue following an SBO. In order to support the emerging needs of nuclear industry in response to Fukushima, KSB embarked on a seal design improvement program directed towards retaining the high operational reliability of the Type C seal while enhancing the RCP seal package’s high temperature coping capability. This activity resulted in a new seal package design that includes: (1) an advanced high temperature resistant, three stage, Type F seal, (2) a passive thermal check valve (PTCV) intended to passively isolate the RCP controlled bleed-off (CBO) line to maintain RCS inventory, and (3) a back-up fourth stage shaft seal called the “Stand-still” seal. This paper discusses the design and post-accident performance of KSB’s Type F hydrodynamic RCP seal package with emphasis on seal performance capabilities under ELAP conditions. The report concludes that the capability of the primary three stage hydrodynamic seals and the fourth stage “Stand-still” seal to function at high temperatures coupled with the reliable RCP CBO line closure capability and subsequent RCS depressurization ensures that any potential seal leakage would be held to a minimum (less than 2 gpm) for the duration of a 72 hour ELAP scenario. The ELAP coping strategies and their outcomes were validated by a seal test program conducted at KSB’s testing laboratories in Frankenthal, Germany. The results of the tests confirmed that the Type F seals exhibited extremely low leakage rates (∼0.01 gpm) at conditions representative of an ELAP scenario (120 hours up to 300 °C [572 °F] temperature and 160 barg [2320 psig] pressure).