[Federal Register Volume 61, Number 165 (Friday, August 23, 1996)]
[Notices]
[Pages 43534-43536]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: 96-21484]


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DEFENSE NUCLEAR FACILITIES SAFETY BOARD

[Recommendation 96-1]


In-Tank Precipitation System at the Savannah River Site

AGENCY: Defense Nuclear Facilities Safety Board.

ACTION: Notice; recommendation.

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SUMMARY: The Defense Nuclear Facilities Safety Board has made a 
recommendation to the Secretary of Energy pursuant to 42 U.S.C. 2286a 
concerning the In-Tank Precipitation System at the Savannah River Site. 
The Board requests public comments on this recommendation.

DATES: Comments, data, views, or arguments concerning this 
recommendation are due on September 23, 1996.

ADDRESSES: Send comments, data, views, or arguments concerning this 
recommendation to: Defense Nuclear Facilities Safety Board, 625 Indiana 
Avenue, NW, Suite 700, Washington, DC 20004-2901.

FOR FURTHER INFORMATION CONTACT:
Kenneth M. Pusateri or Andrew L. Thibadeau at the address above or 
telephone (202) 208-6400.

    Dated: August 19, 1996.
John T. Conway,
Chairman.

August 14, 1996.

    The Defense Nuclear Facilities Safety Board (Board) has devoted 
substantial attention to the planned use of the In-Tank Precipitation 
(ITP) System at the Savannah River Site, because of its importance to 
removal of high-level radioactive waste from storage tanks at that 
Site, and because certain unique hazards are associated with the ITP 
process.
    The hazards are a consequence of the volatile and flammable organic 
compound benzene that is released during the process in amounts that 
must not exceed safe limits. The benzene is generated through 
decomposition of tetraphenylborate (TPB) compounds. These compounds are 
added in the process with the objective to precipitate and remove 
radioactive cesium from solution in the waste water destined for the 
saltstone process. The concentrated slurry containing the precipitated 
cesium constitutes a much smaller volume than the original waste, and 
its feed to the vitrification process leads to production of a 
correspondingly smaller amount of glass ultimately to be disposed of in 
a repository.
    The proposed treatment process calls for addition of a quantity of 
TPB in excess of that theoretically required to precipitate the cesium 
as cesium TPB. That excess is required partly because the significant 
amount of potassium present is also precipitated as potassium TPB, and 
partly because an excess of TPB in solution ensures more effective 
scrubbing of the radioactive cesium through precipitation. However, the 
benefit of effective scrubbing is accompanied by the generation of the 
benzene, which presents hazards of a different sort, and which also 
requires safety controls.
    Westinghouse Savannah River Company is the Department of Energy 
contractor in charge of ITP. The Westinghouse staff at the Savannah 
River Site believed until recently that the principal cause of 
decomposition of TPB and generation of benzene is exposure of the TPB 
to the high level of radiation in the waste. That belief was based on 
results of full-scale tests conducted in 1983 that may have been 
misinterpreted, and on a decade of subsequent bench-scale tests using 
non-radioactive stimulants (almost exclusively) rather than actual 
waste. The first large-scale operations with actual waste since 1983 
were conducted recently in Tank 48, and they showed that the generation 
and release of benzene did not follow predictions. The generation of 
benzene in the waste under treatment in Tank 48 was unexpectedly rapid. 
A surprisingly large amount of the benzene remained captured in the 
waste, and that benzene was released through action of mixing pumps in 
the tank.
    The current view of the contractor staff is that benzene is 
produced principally through catalytic decomposition of TPB ions in 
solution. They believe the catalysts are potentially both soluble and 
insoluble species, one of which is soluble copper known to be present 
in the waste. They also believe that the cesium TPB precipitate and the 
potassium TPB precipitate are relatively immune to catalytic 
decomposition. The contractor proposes to conduct two Process 
Verification Tests (PVT), PVT-1 and PVT-2, to further establish the 
validity of these views and to demonstrate the accuracy of the model it 
has developed to predict the rate at which the captured benzene is 
released from solution. PVT-1 would be performed on the homogenized 
nuclear waste not in Tank 48, which has already been treated with TPB 
that subsequently has partly decomposed with the result that some 
cesium has returned to solution. Additional TPB would be added to this 
material to reprecipitate that cesium. The amount of TPB to be added 
would be strictly limited to a small amount as needed to reduce the 
concentration of cesium remaining in solution to a low radiation level 
acceptable for processing as low level waste in the saltstone process, 
and a large part of that solution would be sent to saltstone. The 
subsequent proposed experiment, PVT-2, will involve adding to the 
slurry remaining in Tank 48 a large amount of additional untreated 
waste and a substantial quantity of TPB as needed to precipitate the 
cesium in this new waste.
    The Board has been informed that the primary safety precaution for 
the proposed cesium removal activities is to maintain an inert 
atmosphere in the headspace of Tank 48. This is to be done through 
establishing a sufficient flow of nitrogen to the tank. Two nitrogen 
feed systems are available, a normal system and a supplemental 
emergency system. The nitrogen systems are present to keep the 
concentration of oxygen below the level that would support combustion 
of the benzene. Westinghouse staff members have

[[Page 43535]]

pointed out that these redundant inerting systems provided a sufficient 
safety factor for control of oxygen concentration in the headspace. 
They have further stated that the rate of buildup of oxygen 
concentration from air ingress into the tank headspace, if both 
inerting systems are simultaneously inoperable, would be slow enough to 
allow reestablishment of nitrogen flow before the bulk vapor in Tank 48 
reaches the minimum oxygen concentration that could support combustion 
of benzene.
    Operations since December 1995 indicate that for the current batch 
of waste, mixing pump operation increases the benzene release rate from 
the waste and that turning off the pumps essentially stops the release. 
The Board has been informed of the consequent belief that the actual 
rate of benzene release into the tank's headspace and its subsequent 
removal can be controlled through managing the action of the mixing 
pumps. This stratagem is to be followed in the tests as a means of 
maintaining the concentration of benzene in the headspace at a low 
enough level to prevent it from becoming flammable even if the oxygen 
concentration were to increase to an undesired level.
    Westinghouse representatives also plan to impose a temperature 
limit for PVT-1 which is expected to prevent decomposition of TPB or to 
reduce its rate. Finally, they state that for PVT-1 the addition of TPB 
will be limited to 200 gallons of fresh 0.5 Molar sodium TPB solution, 
and that any subsequent additions during this experiment would be 
subject to review and approval by the Department of Energy. 
Westinghouse believes that this, in turn, would limit the maximum 
amount of additional benzene that could be produced. In effect, the 
amount of TPB added will be treated as an Operating Limit.
    The Department and its contractor have brought substantial 
expertise to bear on understanding the science of the ITP process and 
the phenomena attending it. However, the Board is concerned that some 
important questions remain unanswered. First, the physical basis for 
holdup of large amounts of benzene in the waste and its removal through 
mixing pump operation is not yet well understood. Therefore, confidence 
in the ability to control its release is not as high as desired.
    The Board is also concerned with the results of a recent 
laboratory-scale experiment using Tank 48 solution and TPB additive. 
The results from this experiment indicate that the amount of TPB which 
decomposed exceeded that amount which had been added during the 
experiment, suggesting that the cesium and potassium TPB precipitates 
had also partially decomposed, presumably through catalytic attack. If 
the cesium and potassium TPB precipitates were subject to rapid and 
extensive attack by a catalyst, an enormous amount of benzene could be 
generated, and the rate of release could be rapid enough to overwhelm 
the removal capability of the purging system for Tank 48.
    The Board concurs with the view that ITP is of high value for 
subsequent vitrification of the nuclear waste in the tanks at the 
Savannah River Site, and that further testing is necessary to gain a 
better understanding of the science of the process to assure safety 
during and after precipitation of the cesium. The Board believes that 
if it were conducted according to the limitations stated above, PVT-1 
can be run safely and can help in leading to an improved understanding 
of the science and the mechanisms involved in the ITP process.
    The present plan for conduct of PVT-2 involves new and untested 
nuclear waste and a much larger addition of TPB. Furthermore, the 
liquid in Tank 49, which contains TPB from the previously mentioned 
1983 demonstration test, is to be used as the source of a significant 
part of the TPB to be added to Tank 48 during PVT-2. The Board 
understands that Tank 49 was also the source of TPB used in the one 
experiment which led to an apparent decomposition of precipitated 
cesium and potassium TPB. One very probable interpretation of that 
anomaly is that the material in Tank 49 contains an unknown catalyst 
which can attack the precipitated material and might also increase the 
rate of release of benzene by an amount that is unpredictable at 
present. Furthermore, waste from tanks not yet tested could contain 
unknown constituents that could also adversely affect the rate of 
production and release of benzene.
    The Board believes that the uncertainty in understanding of the 
science of ITP would make it imprudent to proceed from PVT-1 to PVT-2 
without substantial improvement in the level of understanding. Some 
such improvement may follow interpretation of the results of PVT-1. 
Better understanding of the anomalous experiment suggesting 
decomposition of TPB precipitates is also required.
    Therefore, the Board makes the following recommendations:

    1. Conduct of the planned test PVT-2 should not proceed without 
improved understanding of the mechanisms of formation of the benzene 
that it will generate, and the amount and rate of release that may 
be encountered for that benzene.
    2. The additional investigative effort should include further 
work to (a) uncover the reason for the apparent decomposition of 
precipitated TPB in the anomalous experiment, (b) identify the 
important catalysts that will be encountered in the course of ITP, 
and develop quantitative understanding of the action of these 
catalysts, (c) establish, convincingly, the chemical and physical 
mechanisms that determine how and to what extent benzene is retained 
in the waste slurry, why it is released during mixing pump 
operation, and any additional mechanisms that might lead to rapid 
release of benzene, and (d) affirm the adequacy of existing safety 
measures or devise such additions as may be needed.
John T. Conway,
Chairman.

Appendix--Transmittal Letter to Secretary of Energy

DEFENSE NUCLEAR FACILITIES SAFETY BOARD

625 Indiana Avenue, NW, Suite 700, Washington, D.C. 20004 (202) 208-
6400

August 14, 1996
The Honorable Hazel R. O'Leary,
Secretary of Energy, 1000 Independence Avenue, SW, Washington, DC 
20585-1000
    Dear Secretary O'Leary: On August 14, 1996, the Defense Nuclear 
Facilities Safety Board (Board), in accordance with 42 U.S.C. 
2286a(a)(5), unanimously approved Recommendation 96-1 which is 
enclosed for your consideration. Recommendation 96-1 deals with the 
In-Tank Precipitation System at the Savannah River Site.
    42 U.S.C. 2286d(a) requires the Board, after receipt by you, to 
promptly make this recommendation available to the public in the 
Department of Energy's regional public reading rooms. The Board 
believes the recommendation contains no information which is 
classified or otherwise restricted. To the extent this 
recommendation does not include information restricted by the 
Department of Energy under the Atomic Energy Act of 1954, 42 U.S.C. 
2161-68, as amended, please arrange to have this recommendation 
promptly placed on file in your regional public reading rooms.
    The Board will continue to review these preparations for routine 
activity in the In-Tank Precipitation System and will seek to ensure 
that Board actions do not delay this important program any more than 
may be needed for assurance of safety. Should the Secretary accept 
the recommendations, the Board is prepared to allocate priority 
resources in the form of Board members and staff to join in 
expedited development of a mutually acceptable Implementation Plan.
    The Board will publish this recommendation in the Federal 
Register.

    Sincerely,
John T. Conway,
Chairman.

Enclosure

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c: Mr. Mark B. Whitaker, Jr.
[FR Doc. 96-21484 Filed 8-22-96; 8:45 am]
BILLING CODE 3670-01-M