EXECUTIVE SUMMARY
"A Safety Assessment for Proposed Pump Mixing Operations
to Mitigate Episodic Gas Releases in Tank 241-SY-101: Hanford Site, Richland,
Washington"
F. Biehl, W. Mirchler, T. Butler, R. Davidson, J. Edwards, S.
Eisenhawer, R. Farman, S. Fischer, W. Kubic, R. Fujita, L. Goen, J. Hanson,
R. Johnson, D. MacFarlane, K. Pasamehmetoglu, R.T. Perry, E. Rodriguez,
D. Sanzo, J. Sapir, K. Sasser, N. Schnurr, D. Smith, J. Spore, R. Stevens,
H. Sullivan, L. Trujillo, J. White, T. Wilson, C. Brown, L. Shephard, J.
Teel, J. Holderness, S. King, G. Medford, C. Unal, J. Travis, D. Smith,
and J. Clark
This safety assessment (SA) addresses each of the elements required for
the proposed action to install, operate, and remove a mixer pump in Tank
241-SY-101, which is located within the Hanford Site, Richland, Washington.
The proposed action is required as part of an ongoing evaluation of various
mitigation concepts developed to eliminate episodic gas releases resulting
in hydrogen concentrations in the tank dome space that exceed the lower
flammability limit. In addition, the scope of this SA covers the installation,
operation, and removal of the water-lance assembly (used during mixer pump
installation) and the water wands (used during mixer pump removal). The
installation, operation, and removal of the multiport riser, multiport flange,
in situ viscometer, and voidmeter also are within the scope of this SA.
The pump was installed on July 3, 1993, and was tested through several phases
of a test plan designed to slowly increase the pump's operating speed and
duration. The test program was very effective in demonstrating that the
mixer pump will minimize gas suspended in the waste and maintain the waste
in the tank at a low level. The SA has been modified to take into account
information learned in the test program. This process continues. This SA
originally was written before installation of the mixer pump and contained
tight restrictions on operations to prevent making the tank conditions worse
than before mixer pump operations. As information has been gained through
continued operations, new problems have been identified and have been addressed
in this SA. This SA currently is in transition from covering the testing
over a limited time span to continued long-term operations to keep the tank
mitigated. We anticipate that in future revisions of the SA, the controls
for operations will continue to be relaxed to promote more routine operations.
Credit has been taken for the fact that the mixer pump appears to be effective
in keeping the tank mitigated; however, this requires that the pump be operated
routinely to mitigate the tank and prevent pump failures as a result of
plugging.
In this SA, the potential hazards associated with the proposed action were
identified and evaluated systematically. As new knowledge of the tank and
pump system behaviors was gained, the SA was revised to incorporate new
information. Several potential accident cases that could result in radiological
or toxic gas releases were identified and analyzed and their consequences
assessed. Administrative controls and procedures required to eliminate or
reduce the potential of hazards were identified.
The majority of the accident sequences were evaluated using deterministic
methods. The sequences were found not to result in radiological, toxic gas,
or structural consequences that would cause a breach of the tank below the
level of the waste. The load capacities of the pump structure, 42-in. tank
riser, tank bottom, tank wall, tank dome, and pump pit were computed. The
drop loads of the pump with the shock absorber installed were found to be
within the capacity limit of the structures, provided that the procedural
controls for pump lifting are maintained. The seismic and rollover loads
were found to be within the capacity limits of the pump and tank. The misalignment
loads associated with pump installation and removal were computed and compared
with the riser capacity; controls and procedures then were developed to
prevent unacceptable damage to the riser. The loads associated with missiles
from a disintegration of the pump assembly were found to be within the capacity
limits of the tank bottom and side wall. Criticality associated with the
concentration of fissile material in the tank was examined and found to
be impossible. The loads associated with a postulated hydrogen burn in the
pump support column were found to be within the capacity limits of the pump
support column. The structural loading associated with a hydrogen burn in
the tank was found to be within the capacity limits of the tank structure,
provided that the gas release is limited by procedural controls on pump
operation.
Accident sequences that could result in either radiological or toxic gas
releases were analyzed and found acceptable if activities were performed
in accordance with approved procedures and specified administrative controls.
These accident sequences consisted of the following:
gas releases and burns during pump installation or pump removal,
gas releases and burns during pump operation,
spills of radioactive material associated with contaminated pump removal,
release of radioactive material associated with a spray of waste outside
the tank resulting in failure of pressure sensing devices, and
release of radioactive material associated with drops of a contaminated
pump assembly.
Mitigation by mixing requires that the pump continue to be operated routinely
to prevent excessive gas accumulation and to prevent plugging. If the pump
fails and cannot be replaced, the tank could revert back to its unmitigated
condition. In this case, we could experience gas releases, if ignited, that
would result in tank failure. Mitigation of the tank using a mixer pump
is not without risk; however, we believe, as demonstrated by this SA, that
the risks associated with mixer pump operation are acceptable and are preferable
to allowing the tank to revert back to its unmitigated state.
LA-UR-92-3196, Rev. 12 (September 8, 1994)
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