Nuclear Engineering Program Safety Systems for Pressurized Water Reactors Active Safety Systems AP1000 Passive Core Cooling System AP1000 Passive Containment Cooling System Larry Foulke Director of Nuclear Education Outreach University of Pittsburgh IAEA Workshop 2011 Selected slides courtesy of Westinghouse Electric Co.
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Safety Systems for Pressurized Water Reactors · Safety Systems for Pressurized Water Reactors Active Safety Systems AP1000 Passive Core Cooling System ... PWR rods - electromagnetic
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Nuclear Engineering Program
Safety Systems for Pressurized Water Reactors
Active Safety Systems AP1000 Passive Core Cooling System
AP1000 Passive Containment Cooling System
Larry Foulke
Director of Nuclear Education Outreach University of Pittsburgh IAEA Workshop 2011
Selected slides courtesy of Westinghouse Electric Co.
Nuclear Engineering Program 2
Learning Objectives Explain the primary objectives of reactor safety. Describe what passive safety means and how it differs from
active safety. Identify natural phenomena that support passive safety. Identify active and passive safety features for current reactors. Describe the background for and the design of the AP1000. Describe passive safety concepts for other advanced reactors Identify passive safety features for AP1000. Describe the passive system verification testing for AP1000. Relate how core damage frequency is used to infer whether a
design is safer.
Nuclear Engineering Program 3
Primary Objectives of Reactor Safety
The primary objectives of reactor safety systems are: Shutdown the reactor
Maintain it in a shutdown condition
Prevent release of radioactive material
How are these objectives accomplished in today’s reactors?
Nuclear Engineering Program 4
Active Safety Design Principles Multiple barriers
Defense-in-depth
Protection / safety functions Redundancy
Diversity
Physical separation
Fail-safe principle
Nuclear Engineering Program 5
Active Safety Design Principles Redundancy
More than minimum number of components
Design to tolerate component failure
Diversity Protects against common-mode failure
Control rods and boric acid injection
Aux feed pumps - both electric and steam driven
Nuclear Engineering Program 6
Active Safety Design Principles Physical separation
Protects against simultaneous loss
Distance / physical barriers
Combination redundant / diverse / separated Example: emergency electric power supply
Diesel generators
Multiple ties to off-site electrical grid
Storage battery systems
Nuclear Engineering Program 7
Active Safety Design Principles Fail-safe principle
Components / systems automatically into safest condition w/ failure or power loss
Examples: PWR rods - electromagnetic and gravity; control rods
drop if power supply fails
Protection circuit requiring 2 out of 4: instrument malfunction indicates trip condition
Nuclear Engineering Program 8
Active Safety Objectives These objectives are accomplished in today’s
reactors using a variety of equipment and concepts such as:
Defense in Depth
Reactor protection system
Emergency core cooling systems
Emergency electrical systems
Multiple barriers
Fail-safe systems
Physical separation
Diversity
Redundancy
Nuclear Engineering Program
Multiple Barriers
1st & 2nd Barriers Pellet & Cladding
4th Barrier Reactor Containment
3rd Barrier Primary-System Boundary
Close-up of steam generator showing the 3rd Barrier
Nuclear Engineering Program
Active Engineered Safety Features
Nuclear Engineering Program
Passive Approach to Safety
Passive safety-related systems Use “passive” processes only, no active pumps,
diesels, … Use the laws of physics to meet safety objectives Gravity Natural circulation Flow from high pressure to low pressure
One time alignment of valves No support systems required after actuation No ac power / cooling water / HVAC / I&C
Greatly reduced dependency on operator actions
Nuclear Engineering Program
Passive Safety Features on Existing Reactors
Examples of passive safety-related systems on existing reactors Pressure relief valves to manage overpressure Doppler broadening of U-238 neutron capture
resonances in nuclear fuel Natural circulation cooling Gravity assisted control rod insertion Design for negative water temperature coefficients of
reactivity
Nuclear Engineering Program
Background for AP600/1000 1980 - Westinghouse started AP600 design
and development AP600 - 600 MWe PWR with 2 loops Focus: passive safety systems and plant
channels • Slightly enriched uranium fuel • Reactor coolant system similar
to PWRs • Pre-application review
underway
Nuclear Engineering Program
ESBWR • “Economic and
Simplified BWR” • 1390 MWe GE reactor
based on Simplified BWR and Advanced BWR
• Natural circulation • Passive safety
systems • Pre-application review
in progress • Design Certification
Application submitted and staff review underway
Nuclear Engineering Program
PBMR (Pebble Bed Modular Reactor)
High Temperature Helium Cooled Reactor
165 MWe range per module 8 modules per common control
room Coated Particle Fuel Spherical Fuel Elements (as per
German reactors) 10 years fuel storage in plant Direct Cycle Gas Turbine (multi-
shaft) Has ‘passive safety with no “safety
systems” fuel integrity maintained under
most severe possible accident remains passively cool by
natural circulation Exelon decided not to continue with
pre-application PBMR Pty. planning pre-
application review
Nuclear Engineering Program
Toshiba 4S Reactor (Super Safe, Small, Simple)
• 10 MWe
• Sodium coolant
• Reactivity control – movable reflectors
• No refueling over 30 year lifetime
• Passive safety
• Pre-application review pending
Nuclear Engineering Program
Westinghouse AP1000 AP1000 is 1117 Mwe Two-Loop PWR
designed to ALWR Utility Requirements NSSS, fuel, & power generation
components same as in current plants Passive safety systems permit
simplification and improve safety Modularization reduces construction to 36
months (1st concrete to fuel load) NRC Design Certification provides
regulatory certainty AP600 12/99
AP1000 1/06
75729A.39
Nuclear Engineering Program
Passive Safety Features - AP1000
Passive safety functions Dedicated safety systems / not used for normal operation Mitigate design basis accidents without non-safety systems Meet NRC safety goals without use of non-safety systems
Passive safety design features Only passive processes; no active pumps, diesels, fans, … Reduced dependency on operator actions
Passive safety equipment design Reliable, experienced based, nuclear grade equipment ASME, Seismic I, full fire / flood / wind protection Availability controlled by Tech Specs with shutdown requirements Reliability controlled by ISI / IST / Maintenance Program
Nuclear Engineering Program
Passive Safety Features – AP1000 Passive Residual Heat Removal
Natural circulation HX connected to RCS Passive Safety Injection
Natural circulation / gravity drain core makeup tanks (at RCS press) N2 pressurized accumulators (700 psig) Gravity drain refueling water storage tank (at containment press) Gravity recirculation of sump water (at containment press) Automatic depressurization valves (from pzr and hot legs)
Passive Containment Cooling Natural circulation of air / evaporation of water on outside surface of steel
– Automatic RCS depressurization – Staged, controlled depressurization – Stages 1-3 to IRWST, Stage 4 to containment
Nuclear Engineering Program
Nuclear Engineering Program
Passive Core Cooling – AP1000 ● Uses passive safety systems
– Proven by extensive testing and analysis – Extensively reviewed by USNRC – No safety pumps, DGs, chillers
– No ac power required – One time valve alignment
– Most are fail safe ● Provides improved margins
– Transient DNBR margin > 15% – No core uncovery for SBLOCA
– Breaks up and including a DVI line (8”) break
– No operator actions required for SGTR
Nuclear Engineering Program
CMT Operation – AP1000
Standby conditions Filled with borated water, outlet isolated, inlet open to RCS
Line routed up from CL to top CMT and well insulated > water will be hot
Normal CMT conditions are <120oF (49oC) and 2,250 psig (155 bar) Non-LOCA operation
CL remains filled > hot water flows into CMT top Hot CL water and cold CMT water drives natural circulation injection CMT injects for ~45 minutes until CMT water replaced by hot CL water
Initial net injection is ~ 29 lb / sec (13.2 kg / sec) per CMT
Nuclear Engineering Program
CMT Operation (cont.) – AP1000
LOCA Operation Cold Leg drains > steam flows into CMT top Steam from Cold Leg and cold CMT water
drives stronger natural circulation CMT injects for ~25 minutes until CMT
empties Initial injection is ~ 135 lb / sec (61.2 kg / sec) per
CMT
Nuclear Engineering Program
AP1000 ADS Automatic Depressurization System (ADS) Stages 1, 2, 3
Six lines connected to top pressurizer, discharging into IRWST via two spargers Stage 1 lines are 4”, Stages 2 / 3 lines are 8”
Each line has two series closed MOVs, one gate and one globe Gate valve provides low leakage, is sequenced open
first Globe valve controls flow with specified opening time
Stage 1 opens in 25 seconds to minimize air clearing loads on IRWST
Nuclear Engineering Program
Nuclear Engineering Program
AP1000 ADS (cont.)
ADS Stage 4 Four lines are provided (14”), two on each HL
Discharge locally in their respective loop compartments
Squib valves are used to isolate these lines Eliminates possibility of leakage Provide highly reliable actuation; each valve has
three igniters -- two Protection and Safety Monitoring System (PMS), one Diverse Actuation System (DAS)
Nuclear Engineering Program
LOCA Long-Term Cooling
Nuclear Engineering Program
Passive Safety Injection Operation During a LOCA
Nuclear Engineering Program
AP1000 Increases Safety Margins
Typical Plant AP1000 Loss of Flow Margin to ~ 10-14% ~16% DNBR Limit
Feedline Break (oF) >0oF ~140oF Subcooling Margin
SG Tube Rupture Operator actions Operator actionsrequired in 10 min NOT required
Small LOCA 3" LOCA < 8" LOCA core uncovers NO corePCT <1500oF uncovery
Large LOCA PCT (oF) 1700 - 2000oF <1600oF with uncertainty (1) (1)