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Evaluation of Hydrogen Explosion and

Other Chemical Hazardsto the AP1000 Reactor Plant

Westinghouse Electric CompanyApril 15, 2010

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Agenda●Review evaluation of design certification explosion

hazards (RAI SRP2.2-RSAC-01).●Design Control Document changes and Combined

Operating License Applicant (COLA) actions.●Review evaluation of chemical hazards by

Westinghouse and COLAs.

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Purpose and Objective●Discuss the response to RAI-SRP2.2-RSAC-01● Explain the AP1000 Design Certification hydrogen

hazard evaluation approach ● Explain the interface with site specific evaluations

of chemical hazards● Provide opportunity for questions

●Clarify the types and scope of evaluations conducted for standard plant chemicals

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RAI-SRP2.2-RSAC-01 ●NRC requested potential explosion hazard for on-

site chemicals included as part of the standard design

●Westinghouse evaluated hydrogen storage for explosion and delayed vapor cloud ignition.

●Hydrogen is also evaluated for flammability and control room habitability.

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RAI-SRP2.2-RSAC-01● Evaluation of hydrogen is part of larger evaluation

of all on-site chemicals.●DCD changes are identified to address hydrogen

explosion and vapor cloud ignition.– DCD Sections 2.2, 3.5, 6.4, and 9.3.

●No changes to COL Information Items

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On Site Hydrogen Is Evaluated for Four Hazards● Explosions (Reg. Guide 1.91)● Flammable Vapor Cloud Ignition● Toxicity and Asphyxiation (Control room

habitability)● Fires

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Hydrogen Explosion Hazard

● Plant Gas System includes storage of 1500 gallons of liquid hydrogen

● H2 explosion evaluation method is per Reg. Guide 1.91

● Reg. Guide safe distance is where explosive over pressure is 1 psi or less at safety-related SSCs

● AP1000 Evaluation shows acceptable results for standard design PGS storage location

– Safe Distance = 577 feet to Shield Building– AP1000 Distance = 635 feet to Shield Building

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Hydrogen Storage Location

635 FeetH2 Storage Area

Shield Building

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Hydrogen Explosion Hazard

Reg. Guide 1.91 method is extremely conservative:

● Assumes all hydrogen is released instantaneously and uniformly which is non-physical behavior

● Assumes all hydrogen instantly vaporizes where experimental tests show formation of boiling pool

● Assumes all hydrogen participates in explosion where actual accidents involve 1% to 2% of hydrogen.

● Assumes hydrogen / air mixture is at stoichiometric ratio where diffusion and air turbulence will rapidly dilute gas

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Hydrogen Vapor Cloud Hazard Evaluation● Evaluation per Reg. Guides 1.206 and 1.70● Assumptions:

– Least favorable (i.e. most dangerous) weather conditions

– Flammable, unconfined vapor cloud moving toward SSCs

– Delayed ignition– Over-pressure limit 1 psi for acceptance– Includes evaluation of missile generation if

probability of event exceeds 10-7 per year

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Hydrogen Vapor Cloud Hazard Evaluation●Westinghouse Postulated H2 Release Event

Evaluation– Storage Tank completely full, 1500 gallons– Bottom nozzle at lowest is point sheared off, flush with

tank wall– Tank initial pressure at design pressure 150 psig

(compared to normal operating pressure ~100 psi)

● Storage Tank is empty at 444 seconds (7.4 minutes)

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Hydrogen Vapor Cloud Hazard EvaluationAP1000 postulated unconfined hydrogen cloud

delayed ignition analysis is based upon:

EPA Report EPA420-R-04-016 “ Safety and Security Analysis: Investigative Report by NASA on Proposed EPA Hydrogen-Powered Vehicle Fueling Station”, October 2004

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Hydrogen Vapor Cloud Hazard EvaluationEPA420-R-04-016 considers:

1. 1500 gallon cryogenic hydrogen storage, the same as AP1000

2. 2 Scenarios - Equipment failures and intentional sabotage

3. Least favorable (i.e. most dangerous) meteorological conditions

4. Delayed ignition of gas cloud

5. Most likely injury to people and damage to structures

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Hydrogen Vapor Cloud Hazard EvaluationEPA420-R-04-016 work scope included:

1. Multiple tests of 1500 gallon hydrogen releases

Test release times were 35 to 40 seconds, considerably faster than AP1000 postulated release. This produces a larger gas cloud

2. Tests were performed at varying weather conditions; still air, different wind speeds

3. Hydrogen releases were modeled using TRACETM

Computer Code.

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Hydrogen Vapor Cloud Hazard EvaluationEPA420-R-04-016 Main Results:1. Modeling correlates well with experimental results2. Worst case cloud development is at wind speed of 8 mph.

Faster wind disperses and dilutes hydrogen. Slower wind permits hydrogen buoyancy to rapidly move cloud upward.

3. Ignition of vapor cloud results in flash-fire threat, not explosion. Free air over-pressure is less than 0.1 psi.

4. Recommendation - 175 foot exclusion zone for pedestrian or vehicular traffic and no structures

5. Missile generation is of no concern

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Chemical Hazards EvaluationOVERVIEW

Evaluation of chemical hazards focuses on control room habitability.Westinghouse provides the evaluation of main control room habitability for the chemicals that are in the standard design.

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Main Control Room Habitability

AP1000 Main Control Room habitability evaluated per Reg. Guide 1.78

● Chemicals evaluated in the AP1000 standard design are tabulated in DCD Table 6.4-1.

● Analysis uses HABIT code. Release of all chemicals assumes maximum storage tank capacity. Chemicals are in a circular pool 1 cm deep even though there is a containment dike. Diffusion is straight line with no consideration of intervening walls, floors or equipment. Worst case meteorological conditions apply, even though chemicals in turbine building do not have wind, sunlight and high ambient temperature.

● Chemicals enter MCR through the air inlet on the roof.

● The chemicals are evaluated for toxicity and asphyxiation effect in the main control room.

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Main Control Room Habitability

AP1000 Main Control Room Habitability Evaluation Results● Maximum concentration at MCR ventilation air intake

occurs within two minutes of release● Maximum MCR concentrations do not exceed toxicity limits

based upon National Institute of Occupational Safety and Health (NIOSH) exposure limits as immediately dangerous to life and health (IDLH). All are less than 10% of IDLH.

● Maximum MCR concentrations do not exceed asphyxiation limits based upon Occupational Safety and Health Administration (OSHA) definition of an oxygen deficient atmosphere. All are less than 10% of permissible oxygen dilution.

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Chemical Fire Evaluation

● Fires are evaluated for heat, combustion products and smoke in the main control room

● A fire at the hydrogen storage produces less smoke and combustion products than a diesel oil fire.

● The diesel oil tanks are closer to the main control room air inlet than the hydrogen storage. Diesel oil fire assumes entire containment dike area is covered with burning oil.

● Maximum MCR combustion products concentrations due to diesel oilfires do not exceed toxicity limits based upon National institute of Occupational Safety and Health (NIOSH) exposure limits as immediately dangerous to life and health (IDLH). All are less than 10% of IDLH.

● Fires near other chemicals do not produce fumes that challenge MCR habitability.

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Design Control Document Changes

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Changes to the Design Control Document● Section 2.2

– Add a discussion of the hydrogen storage in the discussion of explosion potential

– Add a discussion of the hydrogen vapor cloud ignition to the discussion of flammable vapor clouds

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Changes to the Design Control Document● Section 3.5 Missiles

– Revise the discussion in Subsection 3.5.1.1.2.2 for a gaseous hydrogen explosion.

– Revise the discussion in Subsection 3.5.1.1.2.2 for the hydrogen storage tank.

– Add a discussion in Subsection 3.5.1.1.2.2 for a flammable vapor cloud.

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Changes to the Design Control Document● Section 6.4

– Add asphyxiation and flammability to hazards considered in control room.

– Add liquid hydrogen to Table 6.4-1● Section 9.3

– In Subsection 9.3.2.3 add discussion of hydrogen explosion and the flammability and asphyxiation potential for the main control room

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Changes to the Design Control Document●COL Information items in DCD Section 2.2, 3.5,

and 6.4 are not altered by the RAI response.

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Calculations and Analyses

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Supporting CalculationsAPP-VES-M3C-006 R0, Main Control Room Emergency Habitability from Toxic Chemical EffectsAPP-PGS-M3C-010 R0, AP1000 Hydrogen Gas Explosion Evaluation per Reg. Guide 1.91APP-PGS-M3C-102 R0, AP1000 Hydrogen Gas Cloud Ignition Evaluation per Reg. Guide 1.206

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Questions