Station Blackout and Loss-of-Feedwater Emergency Response
Objective. Students manage simulated station blackout and loss-of-feedwater events simultaneously, prioritizing safety function restoration actions per emergency procedure while coordinating with simulated plant staff.
This lesson, its worked examples and its practice questions has not yet been verified by a licensed professional engineer or master tradesperson or by a certification or approval body. It is a study draft and may contain errors. Do not use it as instructions for work on a job site or on live equipment. Always follow your instructor, your supervising professional, and current official references and regulations.
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Station Blackout and Loss-of-Feedwater: Emergency Response
Two of the most demanding emergency scenarios a nuclear plant crew can face are a station blackout (SBO) and a loss of feedwater (LOFW). When they occur simultaneously, the crew must manage multiple failing safety functions at once, follow multiple emergency operating procedures (EOPs), and communicate clearly under pressure. This lesson prepares you to do exactly that on the plant simulator.
What Is a Station Blackout?
A station blackout is the loss of all alternating current (AC) power to the plant — both from the offsite transmission grid (preferred AC) and from the onsite emergency diesel generators (EDGs). The U.S. Nuclear Regulatory Commission (NRC) requires every licensed reactor facility to demonstrate the ability to maintain core cooling and plant safety for a defined SBO coping duration using only battery-backed DC systems and any alternate AC sources available.
During an SBO, loads that are not essential are shed automatically in a predetermined sequence to protect the battery banks. Instrument buses, control room lighting, critical instrumentation, and certain valve actuators remain powered by DC. Operators must restore an AC source — either preferred AC from the grid or an alternate AC source — before batteries are depleted. Your one-line electrical diagram shows which buses are battery-backed and in what order non-essential loads drop off.
What Is a Loss of Feedwater?
Feedwater supplies the secondary side of the steam generators (in a pressurized water reactor) or the reactor vessel (in a boiling water reactor) with water. If feedwater flow is lost, secondary-side water inventory drops. As steam generator or reactor vessel level falls, the ability to remove decay heat from the reactor core is threatened. Auxiliary feedwater (AFW) systems, or reactor core isolation cooling (RCIC) in BWRs, are designed to provide backup water supply. However, many of these systems depend on AC power for pumps or valve operators, creating a direct link between the SBO condition and the LOFW response.
Why Simultaneous Events Are Especially Challenging
When SBO and LOFW happen together, operators face competing demands:
- Restoring AC power requires actions on electrical switchgear, which may be in areas away from the main control room.
- Maintaining secondary-side water inventory requires monitoring and operating AFW, which may depend on the very AC power that has been lost.
- Battery life is finite. Every minute without AC power reduces the coping margin.
EOPs are written to prioritize safety functions in a specific order. Regardless of which event drives entry into an EOP, crews must always confirm the priority hierarchy: reactivity control → reactor coolant inventory → reactor coolant pressure → heat removal → containment integrity. Both EOPs must be entered and tracked simultaneously, with one operator designated as the procedure controller for each.
Reading the One-Line Electrical Diagram
A one-line (single-line) diagram is a simplified schematic of the plant's electrical distribution system. For SBO response, you must be able to identify:
- Offsite power supply paths and the transformers that step down transmission voltage to plant distribution voltage.
- Emergency diesel generator output breakers and the buses they feed.
- Battery-backed buses (DC buses and their associated inverters that supply vital AC instrument buses).
- Load-shedding sequence: which non-essential loads drop first, second, and so on, protecting battery capacity for critical functions.
During the pre-simulator activity, you will label each of these elements on a training-version diagram. This is not busywork — in a real SBO, you may need to trace power paths manually from memory while communications are degraded.
Safety Functions and EOP Entry Conditions
Emergency operating procedures are symptom-based: you enter them based on observed plant conditions, not on a diagnosed cause. For an SBO, the typical entry condition is loss of all AC power confirmed by bus voltage indication. For LOFW, the entry condition is feedwater flow below a specified value combined with falling secondary-side level. Once entered, each EOP guides the crew through verifying and restoring each safety function in priority order.
Critical safety behaviors during this exercise:
- Always complete a task verification step before moving to the next action. Skipping verification is a common error under stress.
- Call out every action before you take it and announce the result. This maintains shared situational awareness for the entire crew.
- If a required action cannot be completed (equipment unavailable), transition to the contingency step in the EOP immediately — do not improvise.
- Lockout/tagout is not bypassed in an emergency; however, specific EOPs may authorize defeating certain interlocks under defined conditions and with shift supervisor authorization only.
Communicating as a Crew
The shift supervisor (SS) directs overall plant response. Reactor operators (ROs) and non-licensed operators (NLOs) carry out field actions and report back. All communications during an emergency use three-part communication: the sender states the message, the receiver repeats it back, and the sender confirms accuracy. On your communication log form, record every action directed, every status update received, and the time of each. These logs become part of the post-event documentation.
Post-Exercise Trend Analysis
Simulator trend strips show how plant parameters changed over time during the exercise. After the scenario, you will annotate your trend printout to identify the moment each safety function was restored. For example, if secondary steam generator level stabilizes and begins rising, mark that point and explain in one to two sentences what restored the level and why stabilization of that parameter matters for core cooling. This analysis reinforces the connection between your actions at the controls and the plant's physical response.
Summary
SBO and LOFW scenarios test your technical knowledge of plant systems, your ability to use EOPs correctly under pressure, your electrical diagram literacy, and your team communication skills. Passing this lab requires every crew member to perform safety-critical tasks correctly — there is no partial credit for a missed EOP step when core cooling is at stake.
Key terms10
Materials: Simulator with multi-failure injection, one-line electrical diagrams (training version), station blackout EOP and loss-of-feedwater EOP, simulator trend printouts, scenario analysis worksheets, communication log forms
Check: Crew evaluation checklist: correct entry into both EOPs and correct priority assignment of safety functions (pass requires all safety-critical tasks completed correctly); individual trend-strip annotation scored on a rubric—80% of identified restoration points correctly labeled and explained.
Practice check8
Answer each question, then check it. Nothing is recorded; this is your own practice.
