Resolution of Generic Safety Issues: Issue 50: Reactor Vessel Level Instrumentation in BWRs (Rev. 1) ( NUREG-0933, Main Report with Supplements 1–34 )
In January 1982, AEOD published a report (AEOD/C201322) on safety concerns associated with reactor vessel level instrumentation in BWRs. The report was forwared to NRR for further action.
BWRs use reactor level instrumentation to perform a number of functions including control functions, such as feedwater control, and protective functions, such as automatic scram and autostart of emergency core cooling systems. AEOD concluded that, depending on specific plant instrumentation configurations, there could be the potential for adverse interactions between the control systems and the protection systems. As an example, the interactions may lead to loss of reactor water level due to automatic termination of normal feedwater (control) and failure to automatically start the emergency feedwater source (protection).
The AEOD report322 made three recommendations which were believed to be necessary to resolve the safety concern. The recommendations were:
(1) Action should be implemented to assure that automatic and manual safety-related low-low level start and high pressure injection functions of HPCI and RCIC turbines are not prevented or delayed by the non-safety-related high level trip. For example, the control system of HPCI and RCIC turbines could be modified to provide a low-low level start signal which overrides the high-level trip signal.
(2) Action should be implemented to assure that protective functions are provided in spite of any adverse control system/protection system interaction in the narrow-range level instrumentation. For example, the protective functions provided by the narrow-range level sensors could also be provided by the wide-range level sensors (in employing the wide-range level instrumentation, the desired output signal quality in terms of sensitivity, resolution, accuracy, and repeatability must be considered to assure that the initiating signals achieve the required protection function.) This approach would be consistent with the concept of "alternate channels" as defined in paragraph 126.96.36.199 of IEEE 279-1971.397
(3) Control room operators should be trained to recognize spurious vessel level indications and procedures should be provided for corrective actions to mitigate the consequences of potential transients that may be caused by level instrumentation malfunctions. We believe that the BWR emergency procedure guidelines provide the best vehicle for the definition of appropriate corrective actions in the event of level instrumentation malfunctions.
NRR responded to the three AEOD recommendations by describing a set of ongoing actions.323 The ongoing actions, although related to the concerns, did not specifically address the AEOD recommendations.
In response to the concerns of this issue, the BWR Owners Group commissioned S. Levy, Inc. to study the reactor water level systems. As a result of this study, SLI-8211411 was prepared in July 1982 and submitted to the staff for review. This report identified the three basic areas that water level instrumentation could be improved:
(1) Temperature effects causing decalibration and flashing,
(2) Failures and malfunctions of mechanical level indication equipment,
(3) Break in an instrument line combined with a single failure.
Although the staff believed that improvements in all three areas would be prudent, only the first two were found necessary to satisfy TMI Action Plan Item II.F.2 and produce substantial improvement in the water level instrumentation.695 The affected licensees voluntarily agreed to make the necessary improvements and plans for implementing them were requested from the licensees by DL in Generic Letter No. 84-23.696 Licensee actions in response to this request will be tracked under MPA F-26. The third area of improvement will be addressed in Issue 101.697 Thus, this issue was RESOLVED and no new requirements were established.