nuscale doe-smr slides for 2011-april-26

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

2

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


Large Pool of Water Holds Reactor Modules

3

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.


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

4


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

5


Stable Long Term Cooling of Reactor and Nuclear Fuel without Pumps or Power

6

WATER COOLING BOILING AIR COOLING


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)

1

2

34

5

6

7

Ground level

7


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

8


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

9


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

10


However PRA Typically Ignores the “Unknown Unknowns”

11

> 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.


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?

12


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?

13

1000 NE Circle Blvd, Suite 10310Corvallis, OR 97330+1-541-207-3931www.nuscalepower.com

Bill [email protected]

14

nuscale doe-smr slides for 2011-april-26

Documents