nuscale doe-smr slides for 2011-april-26
TRANSCRIPT
NuScale SMR Preliminary Design and Risk Insights
Bill GalyeanSenior PRA Engr.
Ed WallaceSr. VP- Reg. Affairs
April 26, 2011
© 2011 NuScale Power, Inc.
Passively Safe Reactor Modules• Natural Convection for Cooling
– Passively safe natural circulation of water over the fuel driven by gravity
– No pumps, no need for emergency generators
• Seismically Robust– System is submerged in a pool of
water below ground in an earthquake resistant building
– Reactor pool attenuates ground motion and dissipates energy
• Simple and Small– Reactor is 1/20th the size of large
reactors – Integrated reactor design, no large-
break loss-of-coolant accidents• Defense-in-Depth
– Multiple additional barriers to protect against the release of radiation to the environment
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High-strength stainless steel containment 10 times stronger than typical PWR
Water volume to thermal power ratio is 4 times larger resulting in better cooling
Reactor core has only 5% of the fuel of a large reactor
45 MWe Reactor Module
© 2011 NuScale Power, Inc.
Large Pool of Water Holds Reactor Modules
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NuScale nuclear power reactors are housed inside high strength steel containment vessels and submerged in 4 million gallons of water below ground level inside the Reactor Building.
The Reactor Building is designed to withstand earthquakes, floods, tornados, hurricane force winds, and aircraft impacts.
12-module, 540 MWe NuScale Plant
Reactor and containment are submerged in underground steel-lined concrete pool with 30-day supply of cooling water.
Any hydrogen released is trapped in containment vessel with little to no oxygen available to create a combustible mixture.
© 2011 NuScale Power, Inc.
Decay Heat Removal System (DHR)• Main steam and main
feedwater isolated
• Decay Heat Removal Isolation Valves (DHRIVs) opened
• Decay heat passively removed via the steam generators and DHR heat exchangers to the Reactor Pool
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© 2011 NuScale Power, Inc.
Containment Heat Removal (CHR)• Reactor Vent Valves (RVVs)
Opened on safety signal
• On sufficient containment liquid level, Reactor Recirculation Valves (RRVs) opened to provide recirculation path through the core
• Decay heat removed via condensing steam on inside surface of reactor and containment vessels as well as convection and conduction through liquid and both vessel walls
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© 2011 NuScale Power, Inc.
Stable Long Term Cooling of Reactor and Nuclear Fuel without Pumps or Power
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WATER COOLING BOILING AIR COOLING
© 2011 NuScale Power, Inc.
Added Barriers Between Fuel and Environment
Conventional Designs1. Fuel Pellet and Cladding2. Reactor Vessel3. Containment
Additional Barriers in NuScale Design
4. Water in Reactor Pool (4 million gallons)
5. Stainless Steel Lined Concrete Reactor Pool
6. Biological Shield Covers Each Reactor
7. Reactor Building (Seismic Category I)
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Ground level
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© 2011 NuScale Power, Inc.
Spent Fuel Pool SafetyIncreased Cooling Capacity • More water volume for cooling per fuel assembly than current
designs• Low Density Spent Fuel Racks permit air cooling in the event of
loss of coolant • Redundant, cross-connected reactor and refueling pool heat
exchangers provide full back-up cooling to spent fuel pool.• Stainless steel refueling pool liners are independent from
concrete structure to retain integrity External Coolant Supply Connections• Auxiliary external water supply connections are easily
accessible to plant personnel and away from potential high radiation zones
Below Ground, Robust Deep-Earth Structure• Below ground spent fuel pool is housed in a seismically robust
reactor building• Pool wall located underground is shielded from tsunami wave
impact and damage• Construction of structure below ground in engineered soil limits
the potential for leakage
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© 2011 NuScale Power, Inc.
Draft PRA of Conceptual Design Estimates Very Low CDF
• CDF estimated <1E-7/module-year– Internal events expected to be small contributor to total risk– High conservatisms in external events analyses expected to
drive risk results• No specific site – ALL traditional hazards enveloped
• Very-small single-module source term– Core full-power ~160MWt– Additional barriers to environmental release– Is a “large” release even possible?
• Plan to perform level-3 analysis to quantify consequences of potential release– Dose at site boundary– Expected to be very low
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© 2011 NuScale Power, Inc.
Low CDF Implications• Renders unproductive quantitative risk-informing
approaches– Little room for improving design (risk-wise)– No “cost-benefit” basis for making design changes
• Defense-in-depth issues are the only reasonable use of PRA during design when CDF is low– PRA identifies barriers to core damage– Qualitative not quantitative insights
• What will be the benefit of low CDF in design certification process? – Depends on NRC review philosophy
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© 2011 NuScale Power, Inc.
However PRA Typically Ignores the “Unknown Unknowns”
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> 1E-3/year
< 1E-3/year to 1E-6/year
< 1E-6/year
Known unknowns (more events to consider, trying to predict occurrence of never seen events)
Knowns (relatively few events at this frequency, events easily accommodated in plant design)
Unknown unknowns (“once in a million years,” virtually an infinite number of possible events, borders on fantasy)
Reactor trip, LOSP, etc.
Loss of all SW, total SBO, Large LOCA, etc.
Meteor impact, volcanic eruption, global events, etc.
© 2011 NuScale Power, Inc.
Screening-out Potential Risk Drivers Affects PRA Results and Risk-Informing Process
• Does low CDF “lower the bar” for hazard screening?– For a 1E-5/yr CDF, a 1E-7/yr hazard is screened-
out• Does this hold for a CDF <1E-7/yr?
– How to treat human errors of commission?• Do incredible HFEs now need to be identified and
quantified?
• Does a safer plant now need to spend more time and resources on an enhanced and more comprehensive PRA?
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© 2011 NuScale Power, Inc.
NuScale Approach to Risk-Informing Design Focuses on Defense-in-Depth
• Quantitative approach is not productive• Defense-in-depth provides protection
against the “Unknown unknowns”• PRA sequences and cut sets highlight
where the lower redundancy levels exist– Question to ask: How can existing systems
be better utilized (with minor design changes) to enhance redundancy and diversity?
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1000 NE Circle Blvd, Suite 10310Corvallis, OR 97330+1-541-207-3931www.nuscalepower.com
Bill [email protected]
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