Skip to content
flowss
Start freeChecking your session
Fault tree (FTA) templates

Fault Tree — Reactor Decay Heat Removal

Loss of decay heat removal after a reactor trip: two redundant paths, a two-of-three auxiliary feedwater vote and an INHIBIT gate for the fouled service-water strainer, quantified over a one-year mission.

Fault tree (FTA)Risk & ReliabilityFault tree (FTA) syntax docs
Template previewFault tree (FTA)
Loss of reactor decay heat removalFault tree (IEC 61025) · 8 basic events · 5 gates · depth 3topCore cooling lostafter trip1.14e-4rhr_failsResidual heatremoval fails0.06059rhr_pumpsBoth RHR pumpsunavailable0.053409rhr_pump_aRHR pump A fails torun0.231104rhr_pump_bRHR pump B fails torun0.231104service_waterService watercooling lostsw_strainer0.120.0036sw_pump_tripService water pumptrips0.03dc_bus125 V DC bus 1Afails0.004feed_failsAuxiliary feedwaterunavailable2/30.00188afw_motor_1Motor-driven AFWpump 1 fails0.021afw_motor_2Motor-driven AFWpump 2 fails0.021afw_turbineTurbine-driven AFWpump fails0.035Top event probability (exact)1.14e-4Minimal cut sets9Smallest cut set order3Single points of failurenoneMission time8760 hExact top-event probability 1.14e-4 · rare-event sum ΣP(cut set) 1.17e-4The usual approximation overstates this tree by 2.35% — that gap is the overlap between cut sets, which only the exact evaluation removes.Minimal cut sets — by order, then probability1. {dc_bus, afw_motor_1, afw_turbine} order 3 P 2.94e-6 2.6% of the top2. {dc_bus, afw_motor_2, afw_turbine} order 3 P 2.94e-6 2.6% of the top3. {dc_bus, afw_motor_1, afw_motor_2} order 3 P 1.76e-6 1.5% of the top4. {rhr_pump_a, rhr_pump_b, afw_motor_1, afw_turbine} order 4 P 3.93e-5 34.5% of the top5. {rhr_pump_a, rhr_pump_b, afw_motor_2, afw_turbine} order 4 P 3.93e-5 34.5% of the top6. {rhr_pump_a, rhr_pump_b, afw_motor_1, afw_motor_2} order 4 P 2.36e-5 20.7% of the top7. {sw_pump_trip, sw_strainer, afw_motor_1, afw_turbine} order 4 P 2.65e-6 2.3% of the top8. {sw_pump_trip, sw_strainer, afw_motor_2, afw_turbine} order 4 P 2.65e-6 2.3% of the top9. {sw_pump_trip, sw_strainer, afw_motor_1, afw_motor_2} order 4 P 1.59e-6 1.4% of the topImportance — Fussell–Vesely (share of current risk) and Birnbaum (potential swing)rhr_pump_a — RHR pump A fails to run q 0.231104 FV 88.1% B 4.31e-4 CR 87.5%rhr_pump_b — RHR pump B fails to run q 0.231104 FV 88.1% B 4.31e-4 CR 87.5%afw_turbine — Turbine-driven AFW pump fails q 0.035 FV 77.4% B 0.002491 CR 76.5%afw_motor_1 — Motor-driven AFW pump 1 fails q 0.021 FV 61.7% B 0.003304 CR 60.9%afw_motor_2 — Motor-driven AFW pump 2 fails q 0.021 FV 61.7% B 0.003304 CR 60.9%dc_bus — 125 V DC bus 1A fails q 0.004 FV 6.6% B 0.001773 CR 6.2%sw_pump_trip — Service water pump trips q 0.03 FV 5.9% B 2.13e-4 CR 5.6%sw_strainer — Intake strainer fouled by weed q 0.12 FV 5.9% B 5.32e-5 CR 5.6%Rectangle intermediate event · dome AND · shield OR · shield with k/n voting · hexagon INHIBIT with its oval condition · circle basic · diamond undeveloped · triangle transfer

Make it your own.

title "Loss of reactor decay heat removal"
mission 8760h

top "Core cooling lost after trip" = AND(rhr_fails, feed_fails)

gate rhr_fails "Residual heat removal fails" = OR(rhr_pumps, service_water, dc_bus)
gate rhr_pumps "Both RHR pumps unavailable" = AND(rhr_pump_a, rhr_pump_b)
gate service_water "Service water cooling lost" = INHIBIT(sw_pump_trip, condition: sw_strainer)
gate feed_fails "Auxiliary feedwater unavailable" = KOFN(2, afw_motor_1, afw_motor_2, afw_turbine)

event rhr_pump_a "RHR pump A fails to run" rate=3.0e-5
event rhr_pump_b "RHR pump B fails to run" rate=3.0e-5
event dc_bus "125 V DC bus 1A fails" p=0.004
event sw_pump_trip "Service water pump trips" p=0.03
condition sw_strainer "Intake strainer fouled by weed" p=0.12
event afw_motor_1 "Motor-driven AFW pump 1 fails" p=0.021
event afw_motor_2 "Motor-driven AFW pump 2 fails" p=0.021
undeveloped afw_turbine "Turbine-driven AFW pump fails" p=0.035