TRANSMITTED TO DOE/M. WHITAKER 07/05/95

DEFENSE NUCLEAR FACILITIES SAFETY BOARD

March 27, 1995

MEMORANDUM FOR:     G. W. Cunningham, Technical Director

COPIES:        Board Members

FROM:               David T. Moyle

SUBJECT:       Radiolytic Hydrogen Generation in Rocky Flats Plutonium-
               Nitric Acid Solution Tanks

1.   Purpose:  This report documents an independent Defense Nuclear
     Facilities Safety Board (Board) staff review of hydrogen generation in
     actinide solution tanks at the Rocky Flats Environmental Technology Site
     (RFETS).  This is a follow-up action to concerns raised in a recent
     Board staff visit to Rocky Flats (November 28-December 1, 1994).  The
     staff intends to  evaluate whether passive venting of tanks is
     sufficient to prevent hydrogen accumulation and potential risk of
     hydrogen explosions in the tanks.

2.   Summary:  Assuming diffusion limited transport as a bounding
     calculation, analysis shows that even though vent lines to the actinide
     solution tanks are open, hydrogen and oxygen will accumulate in tank
     headspaces.  Without headspace sampling, all tanks may be assumed to
     contain explosive mixtures of hydrogen and oxygen.  Detonations and/or
     deflagrations may occur in vent lines and tanks if ignition sources
     arise.  Detonation effects are similar for a wide range of hydrogen
     concentrations where the maximum reflected pressures at tank walls could
     exceed the failure pressure by a factor of two.  Even if the tank wall
     remains intact, fittings and sight glasses may likely fail, breaching
     containment.  The analysis indicates that within relatively short times
     hydrogen gas can build up to explosive concentrations.  Due to the
     relatively long stagnant storage of these solutions, a hydrogen
     detonation in a tank is believed to be a credible hazard.

3.   Background:  Alpha decay of plutonium isotopes causes radiolysis
     reactions in solutions which produce hydrogen and oxygen gases as major
     products.  Build up of this flammable gas mixture in tank void spaces
     poses several hazards to the facility, resulting from possible
     explosions, including:

          a.   Loss of containment which could cause spills and airborne
               releases from vaporization.
          b.   Missiles / Shrapnel.
          c.   Criticality from settling of broken Raschig rings, geometry
               changes.

     The Board trip report dated December 8, 1994, discussed Rocky Flats'
     past efforts to resolve this issue.[1]  In 1993, Los Alamos Technology
     Office at Rocky Flats (LATO) performed a safety study of plutonium and
     uranium solutions at RFETS, which concluded that in unvented high
     plutonium concentration tanks, sufficient hydrogen could be generated to
     reach the lower flammability limit (LFL) in about 12 hours.  Further,
     LATO recommended that it was "extremely important that ventilation be
     maintain on all solutions in tanks."  At Department of Energy / Rocky
     Flats Office (DOE/RFO) request, vent line outlet hydrogen concentrations
     were measured for tanks in building 771.  All readings were zero except
     one at 17% of LFL.  LATO assumed that outlet concentrations through
     several meters of vent line reflected the status of the tank void
     spaces, and concluded, "these measurements indicate that the potential
     for an explosion in a tank is extremely low."  Furthermore, they
     concluded that the consequences of a tank explosion would be minimal,
     possibly blowing out gaskets and causing a leak, but not breaching the
     tank itself.[2]  In response to Board staff questions about the hydrogen
     explosion scenario, EG&G representatives stated that there would be no
     off site consequences, and since only workers would be affected, EG&G
     considered it was not necessary to pursue.[1]

     Occurrence report numbers RFO-EGGR-371OPS-1995-0037 and RFO-EGGR-771OPS-
     1995-0064 referenced a potential unreviewed safety question regarding
     hydrogen gas generation and buildup in stagnant actinide solution tanks
     in buildings 371 and 771.[3][4]  More details were offered in Operating
     Experience Weekly Summary Report 95-09, reporting the completion of a
     draft Unreviewed Safety Question Determination (USQD) on February 28,
     1995.  The study concluded that a detonation could rupture the tanks if
     they were not vented and an ignition source was present.  If vented,
     however, hydrogen accumulation was determined to be a manageable hazard. 
     Further, an ignition source could not be identified.[5]  

     The Board staff is concerned that this draft USQD does not adequately
     address the hydrogen accumulation and explosion issue, and this paper
     summarizes  results of an independent staff analysis.  Diffusion
     calculations, presented in Appendix A, determine theoretical worst case
     concentrations of hydrogen in tanks and vent lines, while the explosion
     analysis in Appendix B estimates maximum pressures that could be
     experienced by containment in event of an explosion.

4.   Discussion:

     a.   Industry Standard Design and Operating Requirements:  National
          Fire Protection Association (NFPA) 69, Standard on Explosion
          Prevention Systems, states that, "The combustible concentration
          shall be maintained below 25 percent of the lower flammability
          limit," when no automatic safety interlocks are provided.[6] 
          Implicit in this requirement is the general assumption that an
          ignition source will be present.  It is generally not acceptable
          practice to rely on the lack of an obvious ignition source unless
          a safety system is in place to assure suppression of ignition
          sources.

     b.   Identification of Tanks:  This analysis assesses hydrogen
          generation in a total of 14 actinide solution tanks in buildings
          371 and 771.  Building 371 contains four of these tanks (D49B,
          D49C, D55A, D134C), while building 771 houses the remaining ten
          tanks (D452, D472, D550, D931, D933, D971, D972, D974, D1007,
          D1810).   

     c.   Hydrogen Generation Rates:  Hydrogen generation rates (G-values)
          for the alpha radiolysis of nitric acid solutions are taken from
          N.E. Bibler's experimental work at Savannah River.[7],[8]  Oxygen
          G-values are estimated by relative trends observed at Rocky Flats
          in Kazanjian's research with actual plutonium nitric acid
          solutions.[9]  See Appendix A for more details.

     d.   Diffusion Analysis:  A Rocky Flats report has established that all
          solution tanks are vented, i.e. the vent lines are not
          blocked.[10]  However, the vents are "passive", and in the absence
          of pressure variations, the escape of hydrogen from tank vapor
          spaces is limited by diffusion down the vent line.  Concentration
          measurements at the vent line outlet are not representative of the
          tank vapor concentration, because diffusion limitations will cause
          a concentration gradient to develop down the length of the vent
          line.

          Appendix A develops a three component, one dimensional model for
          radiolytic hydrogen and oxygen diffusing through non-diffusing air
          in a horizontal vent line.  This model predicts that vent lines
          longer than approximately 1 meter will result in a flammable tank
          atmosphere (4 volume % hydrogen).  For realistic vent line
          lengths, equilibrium concentrations in several tanks may approach
          the stoichiometric ratio of hydrogen and oxygen.  Hydrogen is
          quite easy to ignite over a wide range of concentrations,
          requiring less energy than most other flammable gases.  In oxygen,
          hydrogen concentrations between 4 and 94 volume % are flammable,
          with a detonable range between 15 and 90 volume%.[11a]  All tanks
          are expected to have equilibrium concentrations in the detonable
          range for hydrogen-oxygen mixtures.

          Appendix A also gives diffusion results and hydrogen buildup times
          for Kazanjian's hydrogen generation rate data (less conservative
          than Bibler's), as estimated in the Rocky Flats draft USQD
          calculation.[12]  With these generation rates, explosive mixtures
          are still expected in all solution tanks.

     e.   Flammable Gas Buildup Time:  Using the hydrogen generation rates
          from Bibler's research, worst case hydrogen gas build-up times
          were approximated, neglecting diffusion.  Results in Appendix A
          show that LFL (4 volume %) can be reached in 1 to 10 days in some
          tanks, and equilibrium concentrations are reached in 1 month to 8
          years.  Based on the number of years that the tanks have remained
          idle, it could be assumed that all tanks are currently at
          explosive concentrations.

     f.   Explosion Analysis:  The method followed for the explosion
          analysis in Appendix B primarily came from references used in a
          seminar on the calculation and evaluation of fire and explosion
          hazards sponsored by the American Institute of Chemical Engineers. 
          For confined gas explosions, the deflagration pressure wave is
          generally assumed to be 10 times the initial pressure. 
          Calculations for a stoichiometric hydrogen-oxygen mixture show the
          deflagration pressure is approximately 143psia.  When a pressure
          wave strikes a surface, a reflected pressure wave is developed. 
          This reflected pressure is greater than the incident pressure and
          results from a momentum change, due to a change in direction when
          the moving air strikes a dense surface.  The reflected pressure of
          a deflagration wave striking a surface normal to the incident
          pressure wave is approximately twice the deflagration pressure or,
          285psia.[11],[13]

          The detonation pressure for a confined gas can be estimated as
          twice the deflagration pressure or, 285psia in this case.  The
          maximum reflected pressure from the detonation shockwave striking
          a surface, such as the inner tank wall, normal to the direction of
          propagation, will be approximately 1800psia.[11],[13a]  This worst
          case reflected pressure results in a tensile stress nearly twice
          the ultimate stress of a 42 inch diameter, 1/4 inch thick wall,
          304L stainless steel cylindrical tank.  Due to the ductility of
          304L stainless steel,[14] it is difficult to determine if the
          impulse of a reflected detonation pressure will rupture the tank,
          but it is likely to cause deformation and blow out fittings.

          The calculated detonation pressure for a stoichiometric hydrogen-
          oxygen ratio corresponds to that reported in Bureau of Mines
          Bulletin 627.  Further, a graph of detonation pressures shows that
          the effects of a hydrogen detonation will be essentially the same
          for much of the explosive range (20 to 80 volume %).[15]

     g.   Deflagration Versus Detonation:  By definition, deflagrations
          propagate at subsonic velocities, and detonations propagate at
          supersonic velocities.  Deflagrations transition to detonations
          when the reaction front accelerates to the speed of sound.  If a
          deflagration were to occur in a storage tank, it is possible that
          pressures could be vented enough to avoid extensive damage to
          system components.  However, 90 degree elbows in vent lines and
          large length to diameter ratios will limit the effectiveness of
          venting.[11],[13]  Therefore, deflagrations may cause some
          structural damage.

          Based on current analyses, the possibility of a detonation can not
          be ruled out.  Acceleration to detonation in horizontal pipes
          generally occurs in distances proportional to the square root of
          the pipe diameter.  However, depending on the strength of the
          ignition source, detonation can be almost instantaneous.[11],[13] 
          Therefore, a hydrogen detonation in an actinide solution tank or
          vent line can not be ruled out.  Furthermore, deflagrations and/or
          detonations are likely to affect other tanks connected through
          common vent lines.  See appendix B for further discussion.

5.   References
     
     [1]       Bamdad, F., memorandum for G. W. Cunningham, "Nuclear and
               Criticality Safety at Rocky Flats", Trip Report (November
               28-December 1, 1994), December 8, 1994.

     [2]       Los Alamos Technology Office at Rocky Flats, "Plutonium and
               Uranium Solutions Safety Study", October 14, 1993.  DNFSB
               Control # RF: 94-3523.

     [3]       Occurrence Report RFO-EGGR-371OPS-1995-0037.

     [4]       Occurrence Report RFO-EGGR-771OPS-1995-0064.

     [5]       "Operating Experience Weekly Summary 95-09", February 24
               through March 2, 1995.

     [6]       "NFPA 69, Standard on Explosion Prevention Systems", 1992
               Edition.

     [7]       Bibler, N.E., "Curium-244 Alpha Radiolysis of Nitric Acid. 
               Oxygen Production from Direct Radiolysis of Nitrate Ions",
               Savannah River Laboratory, E.I. duPont Nemours & Co., DP-MS-
               72-68.  DNFSB Control # SR:94-257.

     [8]       Bibler, N.E., memo to R. Maher (plus attachments),
               "Radiolytic Hydrogen Production from Process Vessels in HB
               Line - Production Rates Compared to Evolution Rates and
               Discussion of LASL Reviews", November 12, 1992, WSRC-RP-92-
               1312. DNFSB Control # SR: 94-258.

     [9]       Kazanjian, A.R., & D.R. Horrell, "Radiologically Generated
               Gases in Plutonium-Nitric Acid Solutions",  Radiation
               Effects 1972, Vol 13, pp. 277-280, Gordon and Breach Science
               and Publishers Ltd., Glasgow, Scotland.

     [10]      Adams, A.B., memo to C. Sprain, "Potential for Line Blockage
               Resulting in Hydrogen Pressurization of Actinide Solution
               Holding Tanks in Building 771 - ABA-001-95".

     [11]      Tunkel, Steven J., "Methods for the Calculation of Fire and
               Explosion Hazards", AIChE Today Series.  Course notes and
               excerpts, including:

     [11a]     Handbook of Compressed Gases, Third Edition, Compressed Gas Assn.,
               Inc., Van Nostrand Reinhold, NY, 1990.

     [11b]     Kuchta, Joseph M., Investigation of Fire and Explosion Accidents
               in the Chemical, Mining, and Fuel-Related Industries - A Manual,
               U.S. Department of the Interior, Bureau of Mines, Bulletin 680,
               1985.

     [12]      Colwell, R.G., "Analysis of Hydrogen Generation, Explosivity
               and Pressure Rise in Unvented Pu-HNO3 Solution Tanks Due to
               Radiolysis", (DRAFT) Calc. No. CALC-RFP-95.0386-RGC-USQD.

     [13]      Grelecki, Dr. Chester, "Fundamentals of Fire and Explosion
               Analysis", AIChE Today Series.  Course notes and excerpts,
               including:

     [13a]     Glasstone, Samuel (Editor), The Effects of Nuclear Weapons,
               Chapter III: "Air Blast Phenomena", United States Atomic Energy
               Commission, April, 1962.

     [13b]     Cook, Melvin A., The Science of High Explosives, Appendix II,
               American Chemical Society Monograph Series. Reinhold Publishing
               Corp., New York, 1958.

     [14]      Perry, Robert H., & Don Green (Editors), Perry's Chemical
               Engineers' Handbook, Sixth Edition, McGraw-Hill Book
               Company, New York, 1984.

     [15]      Zabetakis, Michael G., Flammability Characteristics of
               Combustible Gases and Vapors, U.S. Department of the
               Interior, Bureau of Mines, Bulletin 627, 1965.

     [16]      Bird, Stewart, & Lightfoot, Transport Phenomena, John Wiley
               & Sons, Inc., 1960.
NOTE:  Please contact the Defense Nuclear Facilities Safety Board directly for
Appendix A and B which contains charts and equations.