Which of the following is a BWR indication of a DBA?

• A. Decrease in Reactor Water Level
• B. Rapid Rise in Dry Well (Primary Containment) Pressure
• C. All ECCS Pumps Start
• D. Reactor Empty and Depressurized (Inventory to Suppression Pool)

Answer: (B)

When a design-basis accident happens in a BWR, the most telling early signal is a rapid rise in the drywell (primary containment) pressure. The drywell is the space surrounding the reactor coolant system inside the containment. If the boundary is breached and steam and gases vent into the containment during a LOCA or similar event, the amount of steam entering the drywell causes its pressure to climb quickly. This pressure spike is monitored by instrumentation and serves as a direct indication that the primary boundary is under accident conditions, triggering containment isolation and the safety system responses.

A decrease in reactor water level can occur during a loss of coolant, but it’s a downstream symptom and not a direct indicator of the containment boundary being pressurized in an accident. All ECCS pumps starting reflects the safety system taking action, not the specific sign that an accident has begun. Reactor inventory ending up in the suppression pool describes a later progression of severe loss of cooling, not the immediate DBA indication.