Spallation Neutron SourceOak Ridge December 5-9, 2005 Impact of BL 13 NPDγ Experiment H 2 Target on SNS Safety Basis: I. Introduction II. USID III. Hazard Analysis IV. Accident analysis Mike Harrington, SNS/ES&H June 9, 2008

Spallation Neutron SourceOak Ridge December 5-9, Part I, Introduction: Safety Basis SNS safety basis documents –FSAD for Proton Facilities –FSAD for Neutron Facilities Unreviewed Safety Issue (USI) process for proposed modifications and new experiments: –6 questions/criteria help with the USI Determination (yes/no) –If “yes,” then safety reviews include DOE approval. Basic USID consideration for NPDγ H 2 at SNS: –Does it have potential to affect confinement of the irradiated target mercury in a significant way?

Spallation Neutron SourceOak Ridge December 5-9, Standard Industrial Hazards & Safety Basis H2 is a standard industrial hazard NPDγ experiment design protects workers by incorporating NPDγ-specific safety features and by complying with applicable standards This presentation considers potential impacts of NPDγ H2 on mercury confinement.

Spallation Neutron SourceOak Ridge December 5-9, Part II, USID Status of NPDγ H 2 Target A Jan 2007 USID categorized NPDγ H 2 target as a USI. –Possible increased medium fire frequency (Question 1) –Possible failure of credited fire suppression system in instrument hall (Questions 3 & 6) –Possible new accident (Question 5) Design and hazard analysis since Jan 2007 –Based on what we know now, a USID would conclude NPDγ H 2 Target is not a USI.

Spallation Neutron SourceOak Ridge December 5-9, Current Answers to USID Questions Jan 2007 answer to Q1: H 2 cylinder handling or crane load handling accident could move the Medium Fire frequency from increase “Unlikely” to “Anticipated” frequency category. Current answer: No significant increased risk of NPDγ H2 starting a medium or large fire capable of affecting the confinement of mercury. –Outdoor location of H 2 cylinders & a flow restriction orifices prevent uncontrolled loss of gaseous H2 inside the building –H2 system presents low profile for hits by crane load –Cryogenic form and limited quantity of H 2 (1.2 kg) –Limited BL-13 combustibles inventory

Spallation Neutron SourceOak Ridge December 5-9, Current Answers to USID Questions (cont’d) Jan 2007 answer to Q3 & Q6 H 2 combustion event in BL 13 enclosure could possibly fail the enclosure sprinklers while starting fire. Current answer: Enclosure sprinklers not essential for preventing large/medium fire that could cause mercury release. –No large quantity of hydrocarbons in BL 13 (e.g., neutron shielding) –Factors listed on previous viewgraph eliminate the possibility of a large-scale combustion event that could threaten the sprinklers above the 30-ton crane, which are a CEC

Spallation Neutron SourceOak Ridge December 5-9, Current Answers to USID Questions (cont’d) Jan 2007 answer to Q5: H 2 combustion inside neutron beam line may be feasible. Current answer: H 2 combustion inside neutron beam line not credible. –Buoyancy of leaking H2 causes it to flow to enclosure ceiling, away from the neutron beam line. –Leaking H2 does not fill the instrument enclosure – it flows out of the enclosure thru ceiling slots and PPS-controlled access gate.

Spallation Neutron SourceOak Ridge December 5-9, Summary, Conclusions for NPDγ USI NPDγ H 2 target declared to be USI in Jan 2007 –Based on work since 1/2007, the NPDγ would not be a USI USI requires DOE approval NPDγ team are working on a report that documents the hazard and accident analysis of the NPDγ H 2 The balance of this presentation looks at hazard and accident analysis.

Spallation Neutron SourceOak Ridge December 5-9, Part III, Hazard Analysis: Initiating Events Loss of cryogenic refrigeration Loss of cryogenic insulating vacuum Failure of H2 supply line during charging Large/medium fire in north instrument hall Crane load drop or mishandling Seismic Event

Spallation Neutron SourceOak Ridge December 5-9, Loss of Refrigeration Scenario: H2 heats up very slowly, may eventually require pressure relief. Frequency: Anticipated Concern: H2 release inside instrument hall Mitigation: Two relief devices on H2 side. If these fail and H2 leaks to the vacuum, there are 2 relief devices on vacuum space. New CEC designation required?: No.

Spallation Neutron SourceOak Ridge December 5-9, Loss of Cryogenic Insulating Vacuum Scenario: Air, He, or H2 leaks into vacuum space. Liquid H2 heats up rapidly. H2 pressure builds, requires relief. Frequency: Anticipated Concern: H2 release inside instrument hall Mitigation: Early warning from RG analyzer. Two relief devices on H2 side. If these fail and H2 leaks to the vacuum, the 2 vacuum burst discs can deploy. New CEC designation required? No.

Spallation Neutron SourceOak Ridge December 5-9, Failure of H2 Supply Line during Charging Scenario: During charging the NPDγ is connected to one or more H2 cylinders outside the building. Failure of the charging line releases H2 inside the instrument hall. Frequency: Unlikely Concern: Rapid H2 release inside instrument hall. Mitigation: A flow limiting orifice (<25 cylinder pressure) controls the rate of potential releases, preventing significant chance of combustion. New CEC designation required? The flow limiting orifice(s) should be treated as a CEC.

Spallation Neutron SourceOak Ridge December 5-9, Fire in Instrument Hall Scenario: Medium or large fire in the vicinity of BL 13. The fire could impact NPDγ infrastructure (see Events 1 and 2). Frequency: Unlikely/Extremely Unlikely Concern: Release of gaseous H2 inside the instrument hall during a fire. Mitigation: H2 relief devices prevent release inside the building (see Events 1 & 2). Existing target building fire sprinklers, fire department, target service bay fire barriers mitigate the fire. New CEC designation required? No.

Spallation Neutron SourceOak Ridge December 5-9, Load Drop onto BL 13 Instrument Enclosure Scenario: A 30-ton crane load is dropped or mishandled onto cryogenic NPDγ, causing failure of H2 and vacuum boundaries. H2 leaks into instrument hall. Frequency: Unlikely Concern: H2 combustion event could start a fire and fail the credited fire sprinklers. Mitigation: Inherent factors prevent combustion event that could fail the credited fire sprinklers: –Initial cryogenic state, limited quantity of H2 (≤1.2 kg) –Unconfined geometry, buoyant behavior of vaporized H2 –Limited combustibles in BL13 New CEC designation required? No.

Spallation Neutron SourceOak Ridge December 5-9, Seismic Event Scenario: Seismic event with NPDγ in cryogenic mode. BL 13 enclosure survives but NPDγ fails. H2 vaporized and released inside BL 13 enclosure over >10 minutes. Frequency: Unlikely Concern: Release of H2 into enclosure causes severe combustion event. Mitigation: Inherent factors prevent severe combustion event: –Initial cryogenic state, limited quantity of H2 (≤1.2 kg) –Buoyant leaking H2 flows out of the enclosure New CEC designation required? No.

Spallation Neutron SourceOak Ridge December 5-9, HA Conclusion Design features of NPDγ H2 target ensure that NPDγ cannot cause failure of mercury confinement: –Cryogenic state of H2 inventory inside the building –Limited quantity of H2 inside the building (1.2 kg) –Flow limiting orifice between outdoor H2 cylinders and inside of building –Multiple and diverse H2 and vacuum region pressure relief devices

Spallation Neutron SourceOak Ridge December 5-9, Part IV, Accident Analysis Assumptions Accident analyses were done for worst case seismic event BL 13 instrument enclosure does not fail (designed for PC- 3 seismic event) NPDγ H2 system not seismically qualified, releases H2 inside enclosure (worst case) –Rate = 1.8 g H2/s Normal enclosure ventilation fails Buoyant H2/air flows (1) out the PPS gate (wire mesh) and (2) through slots in ceiling.

Spallation Neutron SourceOak Ridge December 5-9, H2 Flammability vs. Concentration Flame propagation vs. Vol% H2 in air –4% < Upward Flame Propagation < 74% –6% < Horizontal propagation < 74% –9% Downward propagation < 74% Ignition energy –At 29%  mJ (stoichiometric)  Amt of energy in clothing related static spark –At 8.5%  0.29 mJ  Same as min ignition energy of methane-air Combustion efficiency –4% < Incomplete/poor < 8% –>8% increasingly efficient

Spallation Neutron SourceOak Ridge December 5-9, Spreadsheet-based Parameter Study Model: Conservation of mass & energy, Bernoulli law balance to find natural circulation flow Input: H2 mass flow rate (1.8 g H2/s) Parameters varied: –Air entrainment in buoyant plume (%H2 in air at ceiling), varied between 8.4% and 29% H2 –Thickness of H2-air mixture at ceiling adjusted until discharge rate = 1.8 g H2/s Desired to find: (1) inventory of escaped H2 in enclosure, (2) pressure pulse if combustion occurs inside enclosure.

Spallation Neutron SourceOak Ridge December 5-9, H2 & H2-Air Flows in Parameter Study H2 target 1.8 g H2/s leak rate Air entrainment an input parameter Ceiling flow toward Open door Flow thru ceiling (slots) Enclosure ceiling Layer thickness an input parameter

Spallation Neutron SourceOak Ridge December 5-9, H2-Air Flow to PPS Gate Buoyant of H2-air from the enclosure H2 leaking from cryostat Schematic of BL 13 Instrument Enclosure Illustrating Buoyant Ceiling Flow of H2-air mixture (looking down on enclosure) PPS-controlled chain link gate – Entrance to the BL 13 Instrument Enclosure Dashed lines indicate ceiling T-beams. H2-air also escapes through the slot between adjacent T- beams.

Spallation Neutron SourceOak Ridge December 5-9, H2-Air Flow thru Ceiling Slots H2-air mixture at enclosure ceiling Half section of concrete T- beam Concrete filled shim tube (8-in by 12-in) 9” Partial Cross-section of BL 13 Enclosure Ceiling

Spallation Neutron SourceOak Ridge December 5-9, Results: H2 Inventory versus %H2 in Ceiling Layer

Spallation Neutron SourceOak Ridge December 5-9, Results: Peak Press with 100% Combustion and No Venting (Edeskuty,1996, Fig. 8.8) Stoichiometric NPDγ Worst Case (value ~1.6 atm)

Spallation Neutron SourceOak Ridge December 5-9, CFD Analyses Confirm Behavior after Release of Cryogenic Gases CFX simulation for SNS cryogenic helium release in superconducting linac tunnel –Strong buoyant effect with ceiling flow layer away from release point FLUENT calcs. done at LANL for NPDγ –10X release rate showed strong buoyant escape effect and rapid clearing of released H2 from the instrument enclosure –1X release rate calculation not yet complete.

Spallation Neutron SourceOak Ridge December 5-9, Conclusions: H2 Release Analysis Buoyant behavior of released H2 prevents accumulation of a significant fraction of H2 inside the instrument enclosure More air entrainment in plume leads to higher H2 mass inside enclosure –Worst case is about 240 g H2 for 8.4% H2 Combustion of worst case H2-air buildup inside enclosure does not cause a severe pressure pulse.