Transmitted to M. Whitaker, DOE, 12/08/95

DEFENSE NUCLEAR FACILITIES SAFETY BOARD

                                          June 14, 1995

MEMORANDUM FOR:    G. W. Cunningham, Technical Director

COPIES:            Board Members

FROM:              David T. Moyle

SUBJECT:           Rocky Flats Solution Tanks Safety Review - Trip Report
                   (May 23-25, 1995)

1. Purpose:  This report documents a follow-up review of radiolytic hydrogen
   generation in Rocky Flats solution tanks.  This review was conducted by
   Defense Nuclear Facilities Safety Board (Board) technical staff members D.
   Moyle and R. Zavadoski on May 23-25, 1995.

2. Summary:  The current managing and operating contractor (EG&G) at the Rocky
   Flats Environmental Technology Site (RFETS) has declared a positive
   Unreviewed Safety Question (USQ) for hydrogen buildup in plutonium solution
   tanks in buildings 371 and 771.  A tank vapor space sampling program in
   Building 771 has confirmed that explosive hydrogen concentrations exist in
   at least three tanks.  EG&G plans to mitigate the hydrogen hazard by purging
   the tanks.  However, the purging process has not been well thought out to
   ensure that the hydrogen is adequately swept out of the interconnected tanks
   and vent line system.  Furthermore, it appears that potential hydrogen
   buildup is not being given an appropriate priority in Building 371, where
   sampling and mitigation plans have not been expedited.  In light of the high
   hydrogen concentrations measured in Building 771 tanks, a comprehensive
   site-wide program Has not been implemented to investigate and mitigate
   hydrogen buildup in actinide solution tanks and process lines.

3. Background:  Prior to 1989, when operations were terminated at RFETS,
   plutonium solutions were frequently transferred and processed.  Since then,
   the solutions have remained stagnant and hydrogen buildup from radiolytic
   destruction of water has become a concern.  The Los Alamos Technology Office
   (LATO) investigated this issue in 1993 and concluded that hydrogen would not
   build up in tank headspaces if ventilation was maintained on the tanks. 
   EG&G verified that all vent line valves were in the open position and
   measured negligible hydrogen concentrations at the vent line outlets into
   their respective glove boxes.  EG&G and LATO then concluded that there was
   no significant hydrogen buildup in tanks.  However, they neglected to
   account for the fact that the tank vent line systems had no active
   ventilation and were dead-ended extensions of the glove box system.  Thus,
   concentration gradients could develop in the vent lines such that outlet
   concentrations would not accurately reflect the tank headspace conditions.


   The Board's staff conducted reviews on this issue in November 1994  and
   January 1995.  In January 1995, EG&G drafted a negative Unreviewed Safety
   Question Determination. The Board's staff  predicted hydrogen buildup
   approaching 67 volume percent in several tanks.  EG&G later performed
   separate calculations that gave comparable results.

4. Discussion:  Recent analyses have convinced EG&G of the potential for
   significant radiolytic hydrogen buildup in tank headspaces and vent lines
   even if vent lines are open.  A positive USQ has been declared because the
   consequence of a hydrogen explosion in a tank have not been analazed. EG&G
   has begun a tank sampling program in Building 771.  Mitigation plans,
   however, are not well thought out and EG&G has not extended its
   investigation of hydrogen buildup to other tanks across the site.

   a. Tank Sampling:  As of May 23, 1995, EG&G had sampled nine tanks in
      Building 771 out of ten identified  as potentially containing high
      hydrogen concentrations.  Three of the tanks sampled, D550, D931, and
      D933, contained approximately 50, 50, and 40 volume percent hydrogen,
      respectively.  These concentrations are well within the explosive range
      and 40-50 times higher than the hydrogen level allowed by Code 69 of the
      National Fire Protection Association (NFPA 69).  Tank D452 contained two
      volume percent hydrogen, which also exceeds the NFPA 69 limit but is not
      flammable.

      The other five tanks sampled are operationally empty, but potentially
      contain a significant amount of residual solution below the sight glass
      level.  Assuming large solution heels, calculations predicted high
      hydrogen concentrations in these tanks.  Measured hydrogen
      concentrations, however, were much less than one volume percent, which
      may indicate that the tanks actually contain very little solution in the
      tank heel.  These results, however, do not discount the possibility for
      other operationally empty tanks to contain significant residual solution
      and to generate significant amounts of hydrogen.

      Tank D1810 was not sampled initially because connecting the sampling
      apparatus would require opening a direct ignition path into the tank. 
      Thus, EG&G stated that Tank D1810 would be purged first through a
      calibration line before attaching sampling equipment.  Based on
      calculations, explosive hydrogen concentrations are expected in Tank
      D1810, and pre-sample purging is desirable as a safety precaution.

      Potential hydrogen buildup in Building 371 tanks is not being treated
      with the same sense of urgency as Building 771 tanks.  The tanks have
      not been sampled, and no effort has been made to verify open vent lines
      on the four tanks of concern in that building.  Three tank samples from
      Building 771 came out reasonably close to the predicted concentrations
      from a conservative model.  This model has predicted explosive
      concentrations in Building 371 tanks as well, and indicates that
      sampling and mitigation of Building 371 tanks are also warranted.

   b. Mitigation Approach:  The high hydrogen tanks will be purged with inert
      argon gas and resampled to verify effective purging.  Then, after some
      time,  the tanks will be sampled to understand the hydrogen buildup
      rates.  For the longer term, it has not been decided if a continuous or
      periodic purge will be implemented.  EG&G recognizes the implications of
      evaporative solution loss during purging and will account for this in
      the final decision.

   c. Common Vent Lines:  During a tour of Building 771, the Board's staff
      noticed that tanks are often interconnected through a common vent line
      to a given glove box.  Since all vent lines are open, hydrogen generated
      in one tank may diffuse into other tanks common to its vent system.  The
      staff believes that tanks common to high hydrogen generators should be
      sampled to determine if there is significant diffusion into other tanks. 
      Furthermore, tank purging must be carefully planned to assure that all
      hydrogen is swept out of the interconnected headspaces and vent system.

   d. Screening of Tanks for Analysis:  The staff believes that inadequate
      criteria have been used to screen the tanks that could potentially build
      up explosive hydrogen concentrations.  Out of over 100 tanks in
      buildings 371 and 771, only 14 were determined to be a concern for
      hydrogen buildup.  The tanks were screened based on their potential to
      generate a specified amount of hydrogen gas over five years, but not on
      the potential for hydrogen to build up to explosive concentrations. 
      Therefore, tanks that have flammable concentrations of hydrogen and pose
      an explosive hazard may have been overlooked.

      A comprehensive site-wide investigation could identify all actinide
      solution tanks that may contain explosive hydrogen concentrations. 
      Uncertainty exists in solution levels and actinide concentrations; and
      vent lines may connect tanks that would normally not be considered a
      concern for hydrogen buildup to other tanks that contain high hydrogen
      concentrations.  Therefore, strict analytical treatment of many of the
      tanks may not be adequate to conclude that they pose no hydrogen hazard. 
      Extensive vapor space sampling may be needed to identify tanks with
      significant hydrogen buildup.  In addition to tanks, there are several
      thousand liters of actinide solutions in process lines at Rocky Flats. 
      Hydrogen is being generated in these lines and the implications of
      flammable gas buildup in lines may also warrant attention.