Patent Number: 055704018
Section: description

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS With reference to FIG. 4, the preferred embodiment of the present invention comprises a wetwell airspace which has been divided into a multiplicity of chambers through the use of wetwell airspace divider partitions or equivalent structure. The number of chambers resulting from incorporation of airspace divider partitions would, in the preferred embodiment, be matched one-for-one to the number of PCC heat exchangers provided for the PCCS. A multiplicity of single-disk vacuum breakers of ordinary design (or of developed high-reliability design) - generally using a minimum of one vacuum breaker per chamber but using more than one per chamber if economically advantaged - are provided to give a pressure-relieving pathway between each respective chamber and the (common) drywell airspace, which is not compartmentalized. For ease of discussion, FIG. 4 depicts a partition 78 which divides the wetwell airspace into two chambers 26A and 26B, which can communicate individually with the drywell 20 via respective vacuum breakers 36A and 36B. However, it is understood that the present invention encompasses the use of one or more partitions to form two or more wetwell airspace chambers. Each wetwell airspace divider partition 78 is designed to provide leaktight structure (with, perhaps, steel plate bounding liners) spanning radially across the entire original SBWR wetwell airspace and extending from the diaphragm floor to partially (preferred embodiment) or fully (alternative embodiment) down into the suppression pool 24. The submergence into the pool by the airspace divider partition must be, as a minimum, the submergence designed for the PCC heat exchanger vent pipes 66A and 66B plus some margin. As can be seen from examination of FIG. 4, the compartmentalized wetwell airspace is thus made able to withstand the loss of any single chamber's driving .DELTA.P - and therefore the loss of driving pressure causing flow through the respective PCC heat exchanger - while enabling all other chambers and their respective PCC heat exchangers to continue passive throughflow operation nominally unaffected by the event, whether this be a single active failed-open vacuum breaker condition or a high drywell/wetwell bypass leakage condition in the impaired chamber. As seen in FIG. 4, a small portion of pool water inventory in the impaired chamber 26A will be displaced. This will cause a modest rise in the surface elevation of the pool in each of the other chambers (e.g., 26B). This will produce a sightly higher operating .DELTA.P across the PCC heat exchanger/vent pipe combination, as the vent pipe operational submergence has been increased (modestly). However, these conditions do not significantly affect the operations of the PCC heat exchanger 54B, as the higher .DELTA.P is offset by the slightly longer water column now being expelled from the PCC heat exchanger vent pipes 66B. Noncondensable gases originally in the impaired chamber's airspace will migrate (slowly) back to the drywell airspace once the vacuum breaker failure takes place. The noncondensable gases will then be swept up by the operating PCC heat exchangers and discharged into their respective connected chambers. Therefore a pro rata higher partial pressure for noncondensable gases will also develop in the unfaulted chambers. While containment peak pressure (at 72 hr post-LOCA) will thus be higher for the case of one vacuum breaker failed open, the containment/PCCS designer can minimize the extent of this incrementally increased pressure condition by utilizing either or both of the following means: (a) using more than three PCC heat exchangers (which is the number in the conventional SBWR) and therefore obtaining more than three wetwell airspace chambers; and (b) using individual PCC heat exchanger vent pipes, one per PCC heat exchanger lower drum (of which there are conventionally two such lower drums in each currently designed PCC heat exchanger), instead of using a combined PCC heat exchanger vent pipe (which, as now designed, accepts and merges noncondensable gases from both lower drums), and allocating an individual wetwell airspace chamber to each such PCC heat exchanger vent pipe. As should be apparent from the foregoing disclosure, the present invention provides a containment system design for plants of the SBWR type characterized by their use of passive decay heat rejection systems which can meet design-for-licensability goals given either the consequence of a vacuum breaker in the failed open state or the consequence of a high drywell/wetwell bypass leakage state being present. The foregoing disclosed preferred embodiment incorporating at least one wetwell airspace divider partitions is an example of a containment configuration which accomplishes this goal. Other variations and modifications will be apparent to persons skilled in the design of passive pressure systems for boiling water reactors. All such variations and modifications are intended to be encompassed by the claims set forth hereinafter.