Resolution of Generic Safety Issues: Issue 77: Flooding of Safety Equipment Compartments by Backflow Through Floor Drains (Rev. 1) ( NUREG-0933, Main Report with Supplements 1–34 )
On November 11, 1981, the DAILY REPORT-REGION I carried a "prompt report" from Calvert Cliffs Units 1 and 2 indicating the licensee had been notified that the water tight integrity of the service water pump rooms in both units could be impaired because check valves had not been installed in the floor drain system which drains by gravity to the turbine condenser pit in the turbine building. Without these check valves, the operability of the service water pumps for both units could not be assured in the event of a circulating water conduit break in the turbine building of one unit. This event was subsequently reported as LERs 81-79 and 81-47 for Units 1 and 2, respectively.
This matter was presented in an AEOD report525 in which an evaluation was performed on the generic implications of these events. It was noted that the Systematic Evaluation Program, begun in 1978, did not specifically review the matter of backflow flooding protection through drain lines in safety-related equipment compartments. In addition, AEOD reviewed other programs to establish whether this issue had been treated elsewhere. It was established that a generic review entitled, "Flood of Equipment Important to Safety," was tracked as Topic 3-18 in the Regulatory Licensing-Status Summary (NUREG-0328) and was applicable to all operating plants as of March 1974. Topic 3-18 was not concluded successfully, however, and the problem was assigned to NRR Generic Technical Issue B-11, "SubCompartment Standard Problems." A review by AEOD led to the conclusion that the drain line problems and the matter of backflow flooding protection had not been addressed adequately. The most relevant ongoing work that had been identified by AEOD was USI A-17, "Systems Interactions in Nuclear Power Plants," and an adjunct TMI Action Plan Item, II.C.3, "Systems Interaction." However, it was concluded that these activities did not explicitly address the issue of improperly-designed floor drains system and Issue 77 was prioritized separately. An IE Information Notice527 concerning the potential generic implications of this issue was published on July 1, 1983.
The service water systems at Calvert Cliffs Units 1 and 2 each have three pumps and serve both safety and nonsafety equipment. The three service water pumps for each unit are located in a single room and Units 1 and 2 service water systems can be cross-connected by spool pieces to allow the Unit 1 system to backup Unit 2 and vice-versa. However, Units 1 and 2 share a common turbine building, so both of the service water pump rooms would be simultaneously affected by a circulating water conduit break in the turbine building if backflow flooding protection was not provided. Additional specific details concerning the Calvert Cliffs plants are presented in AEOD/E304.525
The safety significance of the loss of the service water pumps lies in the fact that the service water system serves as the ultimate heat sink in nuclear plants. In addition to being the AFW pump emergency suction supply, the service water provides cooling, either directly or indirectly, for the following plant components: component cooling water heat exchangers, containment fan coolers, diesel-generator coolers, control-room air-conditioning system condensers, computer room air-conditioning system condensers, auxiliary building ventilation system cooling coils, containment spray pump diesel engine coolers, and auxiliary building room coolers. The component cooling water, in turn, is required for the proper operation of essential pumps and heat exchangers required for the safe shutdown of a nuclear plant. Without these essential systems, the probability of core-melt becomes unacceptable.
This issue does not apply to plants reviewed and licensed in accordance with the SRP because SRP11 Sections 9.3.3, "Equipment and Floor Drainage Systems," and 10.4.5, "Circulating Water System," adequately deal with the concern. The safety significance is limited to older plants that were licensed some time prior to the formalization of the SRP, but the extent of possible design deficiencies in these older plants is unknown at present.
In addition, it is noted that the fundamental problem of backflow flooding of safety systems through drains is a potential problem with implications that are much broader than those related to the specific situation at Calvert Cliffs, used for the purposes of analysis herein. Safety components other than service water pumps may be affected in either BWR or PWR systems and the flooding may be from sources other than circulating water conduits and the turbine condenser pit. An example illustrating this point is the flooding incident which occurred at the Oconee Nuclear Station resulting from the inadvertent opening of a main condenser isolation valve.
A temporary preventive measure is the installation of inflatable drain plugs, but this is of limited value as the drains are prevented from functioning by these plugs. A permanent solution is the installation of check valves in the drain lines to prevent backflow flooding and permit proper drain operation. Both of these solutions have been employed at the Calvert Cliffs Nuclear Plant, Units 1 and 2.
Inasmuch as the possible design defects which could lead to backflow flooding through floor drains are plant-specific and the details are not known at this time, the prioritization will be based on the circumstances and events as noted for the Calvert Cliffs plants and generalized as needed.
Based on a review of the LERs performed by AEOD,525 it was noted that Quad Cities Unit 1 had experienced a rupture of an expansion bellows in the circulating water system in 1972. The resultant flooding caused some degradation of engineered safety feature equipment. No other similar event has been noted in the operating experience of nuclear plants. Therefore, based on this one event in (72 plants)(12 years) = 864 plant-years, the internal flooding frequency is estimated to be approximately 10-3 event/plant-year. This is an overestimate because plants have been previously reviewed to assess the potential for internal flooding and corrective actions have been taken as a result of this incident.
The consequences for this event are assumed to result from the following scenario. As a result of the flooding of the turbine condenser pit and the service water compartment, it is assumed that the reactor would be tripped.
Inasmuch as the component cooling water (CCW) system would fail following the failure of the service water pumps, essentially all of the ESFs would be unavailable because of their dependency on CCW for cooling. In addition, primary pump seal failure would follow within a short time after loss of CCW526 causing a small break in the primary system. Moreover, the containment spray and containment fan coolers would be inoperative following the loss of service water. The primary system would be depressurizing through the small break associated with the pump seal failure without the capability of make-up available because of the failure of ECCS. Natural convection cooling would be available for a short period of time inasmuch as the auxiliary feedwater pumps would still be operative. However, as the primary system depressurizes without the availability of the charging pumps, a void will form in the vessel head which will eventually interface with continued natural convection flow. Simultaneously, the containment continues to be pressurized because of the unavailability of containment sprays and heat removal capacity. Eventually the core will uncover and melt. The molten core will slump into the lower vessel head presenting a distinct possibility of a steam explosion on contact of the molten core with coolant that may still be contained in the lower vessel head. Containment failure will occur as a result of overpressurization and/or the steam explosion. This sequence of events is closely approximated by PWR Release Category 3.16 For this category, the release is estimated to result in an exposure of 5.4 x 106 man-rem.
This estimate is also an overestimation of the conditional probability and consequences of a core-melt resulting from an internal flooding incident. The location of ESFs relative to the location of the flooding can greatly reduce or eliminate the probability of core-melt. For example, most plants have the service water pumps outside the plant at a crib house. The interaction of systems can also change the probability.
Based on the frequency of flooding resulting from a rupture of the circulating water pipe bellows of 10-3, the probability of failure of the containment due to overpressurization of 0.6, the PWR release of Category 3 estimated to be 5.4 x 106 man-rem/event,16 and 25 years of remaining average reactor life, the risk reduction is estimated to be (10-3)(5.4 x 106 man-rem/event)(25 yr) or 81,000 man-rem/reactor.
The costs associated with the resolution of this issue are difficult to assess in general because the deficiencies that may exist will be plant-specific. However, on the basis of informal contact with representatives of the Calvert Cliffs nuclear plant, it was established that the purchase and installation of ball-type check valves (13 in all) as well as expandable plugs in some of the additional drain lines and maintenance of these valves will not exceed a total cost of $10,000. This cost reflects easy access to the drain lines for the installation of the valves in the case of the Calvert Cliffs plant. Assuming that a typical plant may have greater difficulties installing similar valves, the cost for a typical plant is estimated to be approximately $100,000.
Based on an estimated risk reduction of 81,000 man-rem/reactor, the value/impact score is given by:
Based on the estimated core-melt frequency of 10-3 as well as the calculated risk reduction of 81,000 man-rem/reactor, this issue would have a high priority ranking. Even if the cost of the resolution of the issue is substantially greater, the risk alone justifies a high priority ranking. In addition, it is concluded that this issue has broader potential safety implications than the Calvert Cliffs situation and flooding can affect many safety systems in BWRs or PWRs and may occur from many sources. These risks estimates are conservative and, as noted, specifics of each plant design can affect the risk greatly. Without further detailed information, the degree of conservatism in these estimates cannot be known. Thus, a high priority was assigned to this issue to more accurately determine the risks involved and to develop a solution. However, in May 1986, this issue was integrated1075 into the resolution of USI A-17.