Experimental Investigation of Reactor Core Isolation Cooling System Exhaust Design in Boiling Water Reactor Suppression Pools and Reactor Safety Implications

Abstract

Steam is exhausted from a turbine into the BWR suppression pool as a means to remove energy from the primary loop during operation of the Reactor Core Isolation Cooling (RCIC) system. If most of the energy is deposited in the upper layer of the water pool, thermal stratification results, causing a higher pool surface temperature and rate of containment pressure increase. Conversely, if the pool is well mixed, the RCIC pump at the bottom of the Suppression Pool may experience a high enough suction temperature to cause cavitation. The energy distribution is largely affected by the design of the steam exhaust line. The RCIC system was simulated in the Nuclear Heat Transfer Systems lab at Texas A&M University, using a steam generator, a ZS-1 Terry turbine, and a 1400-gallon pressure vessel acting as the suppression chamber. The vertical perforated sparger class of RCIC exhaust was tested for its effect on thermal stratification in the suppression pool for comparison with the vertical open-ended pipe class of design. These results are important for RCIC System operation. One RCIC exhaust line design is shown to allow for longer operation before the operational limit of the RCIC pump is reached (Vertical Perforated Sparger) while the other design promotes longer RCIC operation before the containment pressure limit is reached (Open-ended pipe).

It is recommended that RCIC systems be outfitted with a secondary flow path into the lower part of the suppression pool. This would give operators control over whether or how severe stratification is allowed to occur in the pool. For this reason, it is also recommended that RCIC systems have their exhaust into the SP be of the vertical perforated sparger class of designs. Empirical data show the pool temperature operating band that will experience thermal stratification resulting from RCIC operation with the perforated sparger exhaust design is much wider than with the open-ended pipe directed vertically downward.

This thesis investigated for the first time the effect of a non-condensable gas fraction in the steam flow through the RCIC sparger on thermal stratification in a suppression chamber analog. This was to simulate the presence of hydrogen from cladding metal-water reactions in a hypothetical core overheat and delayed-RCIC start scenario. All air mass fractions tested, as low as 0.20% nominally, completely inhibited chugging and any air fraction above this same level caused stratification to be non-existent.