**\[DOE LETTEHEAD\]**

May 21, 2004

The Honorable John T. Conway

Chairman

Defense Nuclear Facilities Safety I3oard

625 Indiana Avenue, N W. Suite 700

Washington, D.C. 20004-2901

Dear Mr. Chairman:

This letter provides the U.S. Department of Energy (DOE) report
(Enclosures 1 and 2) in response to the Defense Nuclear Facilities
Safety Board (DNFSB) concerns raised in the DNFSB letter dated March 24,
2004. The DNFSB notes that Bechtel National, Inc. (BNI) is attempting to
build a technical basis for addressing hydrogen hazards related to
non-Newtonian high-level wastes experimental Research and Technology
(R&T) program using surrogate materials. The DNFSB's concern is that the
use of preliminary data so heavily based on experimental testing with
surrogate materials, which has not undergone a thorough quality review,
increases the chances of introducing errors into the design that may be
irreversible. The DNFSB believes that decisions to proceed with the
final mixing tank system design(s) and development of operating
strategies to prevent hydrogen deflagrations/explosions are: premature,
given the degree of uncertainty that presently exists.

The DNFSB also reviewed the report of the DOE's Office of River
Protection (ORP) on the adequacy of the "black cell" design concept. The
DNFSB is concerned that open items in the ORP report, like the one on
BNI's material selection basis appear to be sufficiently significant to
require resolution before proceeding with certain design activities.

*With respect to the Board's concern that use of preliminary data based
on surrogate materials experimental testing without a thorough quality
review may introduce irreversible errors into the design:*

Thorough quality reviews of the experimental testing have been completed
and the surrogate materials used in testing have been endorsed by
independent mixing experts. The design of the hydrogen mixing and
control (HMC) system is based on conservative assumptions of waste
behavior used in the testing program (described further in Enclosure 1).

On April 2, 2004, BNI completed its initial technical and quality
verification of

the R&T program test results for mixing system designs for vessels in
the Pre-treatment facility containing non-Newtonian high-level wastes.
The material in this report (Enclosure 3) has undergone quality reviews
by the Battelle Pacific Northwest Laboratory (PNL) and Savannah River
Technology Center (SRTC) which carried out the testing. The results of
the reviews are documented in Enclosure 3 of this letter. The enclosure
provides the scaled prototypic test platform data, the mixing scale-up
approach and basis, sparging zone-of-influence (ZOI) development, and
demonstration that the mixing systems will both keep retained gas at low
levels during normal operations and following a design basis event (DBE)
for the March 2004 base design, thereby preventing hydrogen
deflagrations or explosions. The quality review of the Enclosure 3 test
report for the Pretreatment (PT) facility confirmed that the design and
operating strategies specified in Enclosure 4 remain consistent with the
test data.

A Peer Review meeting with an external panel comprised of experienced
safety personnel from Savannah River, Oak Ridge, and other areas was
held March 31, 2004, and April 1, 2004. The panel evaluated the safety
strategy of the base design and provided recommendations for potential
modifications. The panel observed (in their exit briefing) that the WTP
hydrogen correlations are adequately conservative.

The acceptability of using surrogate materials to bound in-situ waste
behavior was established through the analysis of actual wastes, tests of
simulant materials, consultant input, and comparison with data of other
high-level wastes. The simulant had higher weight percent solids than
the expected waste, a high yield stress value, and very conservative
consistency for physical modeling. An external panel of international
mixing experts met on October 30, 2003, to November 1, 2003, and
concurred with the WTP simulant selection. Additional details of the
acceptability of the simulant are discussed in Enclosure 1, item 4.

*Design Conservatism, Margins, and Flexibility*

As a result of the experimental testing with surrogate materials,
several mixing systems have been identified, which will accomplish the
needed mixing and hydrogen control. These include combinations of pulse
jet mixers (PJMs), recirculation systems, and spargers. PJMs will be
used in all pretreatment inaccessible (black cell) tanks for mixing and
suspension of solids off the tank floor. Spargers and/or recirculation
pumps provide the mixing for the upper elevations of the tanks. The
current pretreatment operational design for the Lag Storage and Blend
vessels allows for mixing via all methods: PJMs, recirculation pumps,
and spargers.

Conservatisms have been included in both the design and safety strategy.
Conservative approaches taken at each stage of the HMC testing program
are outlined below. The safety margin in the mixing systems results
from: 1) the conservative simulant selected for rheologic properties: 2)
the scaled testing approach; 3) the methodology used to develop the
sparging correlation; and 4) the techniques employed to evaluate gas
retention and release behavior models. In order to put the conservatism
of the design into perspective, scaling of test results to date
indicates that normal operation gas hold-up in the full scale vessels
will be less than 1 percent by volume. Instantaneous gas releases were
not observed during testing until more than 20 percent gas by volume
accumulates in the tank. Even then, in the test of 7 Pascal simulant,
where this was observed, the release occurred over a 20-second period.
Because these hold-ups are so low, an instantaneous uncontrolled release
does not appear to be credible.

As a further step to ensure the operational safety, hydrogen monitors
will be provided in the vessel vent system (tank head space or vent
pipe). While these systems will most likely not be safety class systems,
the ability to monitor H~2~ in actual plant operations is viewed as
beneficial to confirm design assumptions.

In addition, safety class hardware to control the fill level of the
tanks has been added to the proposed design, and the radionuclide
inventory will be controlled by technical safety requirements (TSRs). By
controlling dome volume and radionuclide inventory, the estimated time
to LFL can be extended beyond the current 6- to 8-hour minimum. As an
operational conservatism, the Pretreatment operation limits, including
rheology, for each waste tank will be defined using the samples
delivered by the tank farm contractor prior to WTP processing. This
additional TSR will assure that the waste slurries remain well within
the rheologic limits supported by the testing program.

A current review of mixing systems redundancy in the pretreatment lag
storage and blend vessels indicates that mixing may be accomplished with
solely PJM operation and sparger operation in *both* normal operations
and post DBE. An initiative to eliminate the recirculation pumps in
these tanks depends on current testing, which is evaluating the
effectiveness of the spargers to rapidly establish a ZOI and to maintain
a low gas hold-up with intermittent sparging. If this is not
demonstrated, then the recirculation pumps will be retained.

The current non-Newtonian fluid mixing design requirements and operating
strategies are provided in Attachment 2 for the Pretreatment facility.
More complete design requirements and how they are achieved will be
contained in a system description document under preparation.

Current designs and planning schedules allow for system refinement
without compromises to safety. The ability to modify the design before
the construction is "irreversible" ties to the placement of the ceiling
over the black cells, which is

projected to be February 2005 for Pretreatment.

*With respect to the Board's concern that the Black Cell report open
items appear sufficiently significant to require resolution before
proceeding with certain final design activities:*

BNI has developed the closure plan for each recommendation open item
identified in the Black Cell Review report and has entered them into the
BNI Recommendation and Issue Tracking System for formal tracking and
closure. ORP has reviewed and concurred with the closure plan
activities, deliverables, and schedule. DOE expects that completion of
the closure plan activities on the identified schedule will ensure that
the recommendations and open items are resolved before proceeding with
associated final design activities. The closure plan is statused and
updated on a weekly basis and these updates are provided to the DNFSB
staff. Enclosure 2 provides the April 19, 2004, update of the closure
plan.

Regarding the Board's specific example of an inadequate basis for
materials selection and wear rates, DOE has conducted a preliminary
review of the technical basis for corrosion allowances, and determined
that the likelihood of change to the material selection is minimal, once
the basis for material selection is more rigorously defined.

This preliminary review found that the primary waste constituents of
concern were the chlorides, fluorides, and sulfates. These constituents,
along with the possible pH values expected in the tanks during
operations were considered in the review. In this preliminary
assessment, BNI concluded (and ORP concurred) that there was a wide
margin before the maximum concentrations for the constituents would
impact the current materials selected. This margin was greater than the
expected variation in waste concentration of these constituents.
Therefore, the risk of change is minimal.

ORP will verify: (1) that waste feed hardness properties used by BNI in
erosion evaluations are representative of tank farm waste information;
(2) that BNI has evaluated the erosivity of the process waste streams;
(3) that the corrosion evaluations have been adequately updated to
account for erosion; and (4) ORP will review the need for modifications
to the design required to accommodate changes in erosion allowances, if
any. These actions are scheduled to be completed by July 30, 2004.
Therefore, based on these reviews, both the erosion and corrosion
attributes of the materials selected will be confirmed.

Based on the information provided in this letter and enclosures, ORP
judges that the programmatic risk associated with continuation of the
black cell vessel and piping design is acceptable.

Thank you for meeting with us on May 18. I realize that the Board has
continuing concerns regarding the WTP and we have scheduled a follow-on
meeting on June 2. As a result, we may need to provide refinements to
this response.

If you have further questions, please call me at (202) 586-7709 or
Patrice Bubar, Deputy Assistant Secretary for Integrated Safety
Management and Operations Oversight, at (202) 586-5151.

Sincerely,

***Jessie Hill Roberson***

***Assistant Secretary for***

***Environmental Management***

4 enclosures

[Enclosure 1](../AttachedFile/TB04Y21E_enc.pdf)

[Enclosure 2](../AttachedFile/TB04Y21E_enc2.pdf)

[Enclosure 3](../AttachedFile/TB04Y21E_enc3.pdf)

[Enclosure 4](../AttachedFile/TB04Y21E_enc4.pdf)

cc:

M. Whitaker, DR/DOE

P. Bubar, EM/DOE

I. Triay, EM/DOE

C. O'Dell, EM/DOE

C. Fetto, ORP/DOE

R. Schepens, ORP/DOE

S. Hahn, RL/DOE

M. Sautman, DNFSB