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**UNITED STATESNUCLEAR REGULATORY COMMISSIONOFFICE OF NUCLEAR REACTOR REGULATIONWASHINGTON, D.C. 20555-0001**

November 20, 1998

**Addressees**

All holders of operating licenses for nuclear power reactors, except for those who have ceased operations and have certifiedthat fuel has been permanently removed from the reactor vessel.

**Purpose**

The U.S. Nuclear Regulatory Commission (NRC) is issuing this information notice to alert addressees to recent inspectionfindings related to a design deficiency that leads to an automatic shutdown of the emergency diesel generator (EDG) whenthe starting air supply pressure depletes. It is expected that recipients will review the information for applicability to theirfacilities and consider actions, as appropriate, to avoid similar problems. However, suggestions contained in thisinformation notice are not NRC requirements; therefore, no specific action or written response is required.

**Background**

The Riverbend Nuclear station has three emergency diesel generators. Two of these EDGs are built by TansamericaDelaval, and are individually supported by non-safety related air compressors, powered by non-class1E power supply, thatmaintain sufficient inventory of high pressure starting air in the safety related accumulators. This air provides the motiveforce for the engine starting system and also supplies a pneumatic control logic system that trips the EDG when certainnon-emergency engine parameters are sensed to exceed its set limits. The air accumulators are maintained at 235-250 psig,and control air pressure for the logic system is maintained by a regulator at approximately 60 psig. The control airmaintains a static air pressure on engine sensors during engine operation. Air pressure loss, following an engine start, maybe due to system leakage or through a tripped engine protection sensor that reached an unacceptable level. When the EDGis operating in the emergency mode, the declining control air pressure first unbypasses the protective trips that werebypassed for the emergency mode, and further decrease in pressure results in an engine trip. The starting air system isequipped with a pressure switch that actuates an alarm in the main control room if the air accumulator pressure drops below210 psig. The alarm response procedure requires that an operator investigate the problem at the engine control panel. Theoperating procedure requires a supplemental air source if EDG operation is required for more than 2 hours while the offsitepower is unavailable to power the associated air compressor.

**Description of Circumstances**

On November 14, 1989, at River Bend Nuclear Station, the Division 1 EDG tripped within one minute after starting, as aresult of a failed temperature sensor for the No. 7 main bearing. As designed, the sensor bled control air pressure downfrom 60 psig to 10 psig, and the diesel control logic card, sensing the low pressure, tripped the EDG. During a telephoneconversation with the NRC on January 3, 1990, the NRC staff questioned long-term operability of the emergency diesels,since sufficient leakage from the control air system, with the compressors unavailable (in the absence of non-1E power),would eventually cause the non-emergency diesel trips to become unbypassed and could trip the EDG. The NRC staffquestioned the availability of a long term supply of control air. In response, the licensee reviewed procedures and madechanges to ensure availability of a long-term air supply.

On June 24, 1998, as a result of questions from NRC inspectors, the licensee discovered that they had failed to completenecessary measures to ensure a long-term air supply for EDG control air. Although operations had written procedures forinstalling a backup air supply, plant staff had neither assembled and staged the materials nor demonstrated the ability tosupply control air within 2 hours as stated in the procedure. On June 25, 1998, the licensee completed staging the materials,including charged air bottles, and demonstrated the ability to connect the backup air supplies within approximately 30minutes. The licensee is evaluating the appropriate permanent resolution to ensure a long-term supply of control air.

**Discussion**

The problems described emphasize the need for licensees to ensure reliability of systems that perform critical supportfunctions for safety-related systems. Licensees are responsible for ensuring the capability of safety related systems toperform their functions during design events. In some EDG designs, the starting air system also performs a critical controlfunction that may necessitate ensuring availability of the starting air system throughout the duration of certain analyzedevents, such as station blackout. The same problem existed at Comanche Peak Nuclear station and it was resolved byimproving the design with safety grade power supply and air compressor for continued supply of air when the EDG isoperating. Other support systems may exist that perform critical support functions for long-term reliability of safety relatedsystems. In addition, relying on operator action as a corrective measure for an inadequate design is inappropriate withouttraining the personnel involved in procedures that implement the compensatory measures.

This information notice requires no specific action or written response. However, recipients are reminded that they arerequired to consider industry-wide operating experience (including NRC information notices) where practical, when settinggoals and performing periodic evaluations under Section 10 CFR 50.65, "Requirement for Monitoring the Effectiveness ofMaintenance at Nuclear Power Plants." If you have any questions about the information in this notice, please contact one ofthe technical contacts listed below or the appropriate Office of Nuclear Reactor Regulation (NRR) project manager.

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