Resolution of Generic Safety Issues: Issue 24: Automatic ECCS Switchover to Recirculation (Rev. 3) ( NUREG-0933, Main Report with Supplements 1–34 )
This issue was raised by the staff following a review28,29 of operating events that indicated a significant number of ECCS spurious actuations, particularly, the four events that occurred at Davis-Besse during 1980.
ECCS operation has two different phases in PWRs: injection and recirculation. The injection phase involves initial cooling of the reactor core and replenishment of the primary coolant following a LOCA. In this phase, the ECCS pumps typically take suction from the refueling water storage tank (RWST). The recirculation phase provides long-term cooling during the accident recovery period. The ECCS is realigned in the recirculation phase to take suction from the containment sump.
Switchover from injection to recirculation phase involves realignment of several valves and may be accomplished by: (1) manual operations to realign the valves; (2) fully automatic realignment of the valves; or (3) automatic realignment of some valves, followed by manual completion of the process (semi-automatic). Each option is vulnerable in varying degrees to human errors, hardware failures, and common cause failures.
During a LOCA, ECCS pump suction must be switched from the RWST to the containment sump before RWST inventory is lost, or loss of the ECCS pumps will occur. Switching to the sump early could adversely affect the accident because the containment sump may not have enough inventory to provide pump suction. The automatic and the semi-automatic switchovers reduce the risk of human error but have a slight increase in risk for inadvertent actuations. This issue affects PWRs only.
The two possible solutions to this issue are alternate cases requiring fully-automatic or semi-automatic switchover to the containment sump. The fully-automatic switchover could be implemented by a system that would monitor the water level in the RWST and, at a preset level, automatically realign the ECCS to take suction from the containment sump. The semi-automatic switchover could be implemented by a system that would involve automatic alignment of some valves and manual completion of the switchover process.
All operating or proposed PWRs may be affected by this issue and the Oconee 3 PRA was assumed to be representative of PWRs. LERs between 1987 and 1990 were used to calculate the potential risk from spurious actuations. The spurious actuation probability was then used for the automatic switchover and modified for the semi-automatic switchover.
The base case of the possible solutions assumes manual switchover. Some PWRs are already fully automatic and some are semi-automatic. It was assumed that all PWRs with manual switchover could benefit from the possible solutions; since some PWRs are already automatic, some (fixed) costs will be spread over fewer reactors than calculated.
The issue to be addressed is the failure of an operator to open containment sump suction valves at the start of recirculation. New parameters were introduced to provide estimates of recirculation system unavailability corresponding to manual, automated, and semi-automated switchover options.29 The new parameters were then updated for human error rate estimates given in NUREG/CR-4639.1327 The updated parameters were then factored into the core-melt frequency.
The frequencies of the affected release categories were summed for each case to give the total core-melt frequency for the three cases considered.64
|Base Case (Manual):||3.1 x 10-6/RY|
|Semi-Automatic Switchover:||1.6 x 10-6/RY|
|Fully-Automatic Switchover:||1.3 x 10-6/RY|
The adjusted case core-melt frequencies were calculated by substituting the adjusted probabilities into the failure scenarios which require sump suction valves to be opened for success.64 Thus, the potential reduction in core-melt frequency was estimated to be 1.5 x 10-6/RY and 1.8 x 10-6/RY for the semi-automatic and the fully-automatic switchover options, respectively.
Multiplying the affected release categories by the estimated public dose, the total affected public risk for the three cases were as follows:
|Base Case (Manual):||7.5 man-rem/RY|
|Semi-Automatic Switchover:||3.2 man-rem/RY|
|Fully-Automatic Switchover:||3.0 man-rem/RY|
Thus, the estimated risk reduction was 4.3 man-rem/RY and 4.5 man-rem/RY for the semi-automatic and fully-automatic switchover, respectively. Based on an average remaining operating life of 28.8 years for PWRs, this reduction was estimated to be 125 man-rem/reactor and 130 man-rem/reactor for the semi-automatic and fully automatic switchover, respectively.64
Installing automatic or semi-automatic systems reduces human error. However, the estimated risk reduction from installing actuation systems that are less prone to human error was offset somewhat by an increased risk due to spurious actuations.
Industry Cost: The cost was estimated to be the same for both semi-automatic and fully-automatic switchover at all affected PWRs. The estimates for TS, maintenance procedure, and operating procedure changes were taken from NUREG/CR-4627.961 The implementation costs were calculated as follows:
|Design/QA||= 8 man-weeks|
|Install/Calibrate/Test Equipment||= 1 man-week|
|Safety Analysis||= 8 man-weeks|
|TS Changes||= 16 man-weeks|
|Training||= 8 man-weeks|
|Hardware (New Controller/Logic Module)||= $5,000|
|Revise Operating and Maintenance Procedures||= $7,800|
Thus, the total estimated cost was $110,000/plant, based on 41 man-weeks at $2,270/week and a fixed cost of $12,800.
Operation and maintenance of the possible solutions were estimated to require an additional 1 man-week/RY. Over the average remaining operating life of 28.8 years, and at a discount rate of 5%, this cost was estimated to be $34,000/ reactor.
NRC Cost: It was estimated that 1 man-year of contractor effort would be required to research potential design changes and prepare a regulatory analysis. A project manager would be required at 10% of the contractor cost. At an estimated cost of $100,000/man-year, the contractor and project manager cost was estimated to be $110,000.
Eight man-weeks would be required to review and evaluate each plant's design, safety analyses, QA documentation, TS changes, and procedure changes. With an assumed labor cost of $2,270/week, this cost was estimated to be $18,000/reactor.
Total Cost: The total industry and NRC cost associated with the possible solutions was estimated to be $272,000/reactor.
Separate value/impact scores were calculated for the semi-automatic switchover and the fully-automatic switchover possible solutions.
Since much of the work will be in radiation zones, a significant occupational dose will occur. The dose rate was assumed to be 2.5 millirem/hr for work outside containment.64 The occupational dose was assumed to be the same for both possible solutions. The implementation dose was calculated at 0.5 man-rem/reactor and total operation and maintenance dose at 0.6 man-rem/reactor. This resulted in a total ORE of 1 man-rem/reactor.
Based on the value/impact score and the potential risk reduction for PWRs with manual switchover, this issue was given a medium priority ranking (See Appendix C). Because the uncertainties in the assumptions and analysis were very large, it was believed that a more extensive study would be required to resolve this issue with reasonable confidence in the conclusion, including more reliable estimates of equipment reliability, human error rates, and competing risks. The resolution of this issue addressed the safety concern of Issue 156.3.5 as well as the impact of a license renewal period of 20 years.1564
An evaluation of the PRAs for four PWRs indicated that the conversion from manual to semi-automatic ECCS switchover to recirculation would produce a small reduction in CDF. However, the regulatory analysis, published in NUREG/CR-6432,1666 indicated that this solution was not justified on a cost/benefit basis. Consideration of a license renewal period of 20 years did not affect this conclusion. Thus, the issue was resolved with no new requirements.1667