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OCR for page 111
Appendix E
Hazard Scoping of Major Actions for
Remediation
This appendix provides a more in-depth view of the detailed
technical considerations for each of the process steps outlined in Chapter
4 in the development of a preferred approach. The execution of any
remediation program involves many detailed selections in chemistry,
engineering, and safety and monitoring provisions. A typical sequence
(not necessarily optimal or comprehensive) of such selections is outlined
here but is not intended to provide a definitive recommendation for any
step in the process. Such decisions are best made by project personnel as
new information is obtained.
The suggested actions, some of which are already being planned
or are under way at Oak Ridge National Laboratory (ORNL), include the
following:
Action 1: Removal (principally from the piping and the freeboard
of the tanks) and analysis of the reactive gases f uorine IFS and uranium
hexafI~oride LUFF from the vent lines of the salt drain tanks, now under
pressure.
The amount and composition of the gases can provide important
but partial information about conditions of the stored fuel salts and some
clues as to the condition of equipment (including possible leakage or
exposure to water via a pressure of hydrogen fluoride tHF] and oxygen
t023~.
Action 2: Removal of UFO gas specifically from the piping and
tankfreeboard.
The gas should be collected and its amount and composition
determined. These determinations will improve the estimates of the
degree of reduction of the salt relative to the oxidation state while the
E.1
OCR for page 112
E.2
AN EVALUATION OF DOE ALTERNATIVE FOR MSRE
reactor was in operation. These data would aid a uranium mass balance
calculation, and significant uranium recovery would reduce the criticality
potential associated with the drain tank remediation alternatives.
Action 3: Removal of nonvolatile uranium (if it existsJ from the
piping by fluorination.
If any aggregate amount remaining is small relative to criticality
(e.g., less than 250 g), it may be left for the last step after the tank
contents are processed unless the residue results in plugged lines. Final
removal may be either by fluorination or by dissolution.
If simpler measures (such as addition of any of the fluorinating
agents of Appendix B as gases) fail, another possible approach for
removing nonvolatile plugs in the lines is to introduce an aggressive
oxidizer such as krypton difluoride (KrF2) in anhydrous HE, to carry
KrF2 to the plugs in a liquid solution. This approach is not without
difficulty it would require cooling of the lines below the 19°C boiling
point of HE (or working at greater than atmospheric pressure), and in the
presence of a base such as lithium fluoride (LiF), it would enhance the
attack of KrF2 on nickel surfaces.
Action4: Obtain information about the distribution and
segregation of uranium in the salt, by one or both of the following
methods:
Option 4A. Gamma spectroscopy
Option 4B. Collection of a salt sample
Obtaining a solid sample of the fuel salt in the drain tank appears
to be an operation with hazards that result from gaseous radon
contamination, as discussed in Chapter 4 and Appendix C. The sampling
method considered here by the panel is a simple core drilling operation,
with the drill and contained sample removed from the drain tank to a hot
cell for sample handling and analysis, conducted after the reactive gases
F2 and UFO have been rem overt from the Coining ~nr1 the drain tank
freeboard.
r -or ---=
Obtaining fully representative samples of the drain tank salt in its
present solid condition is highly desirable but may not be possible
without significant spreading of radioactive contamination. Any bulk
OCR for page 113
APPENDIXE HAZARD SCOPING FOR REMEDIATION
E.3
sample provides some information on the extent of segregation present,
some of which might be expected due to zone refining effects during
initial solidification. The ideal goal of such characterization is to map the
spatial distribution of uranium-233 (233U) and plutonium within the
tanks, and their chemical states. If practical, several full-length core
samples, at different radial distances from the center of the tanks, can
provide a more precise measure of possible inhomogeneity in elemental
composition and chemical state and, therefore, of the potential for
segregation (and possible degree of precipitation) on melting. Because of
solubility uncertainties, melting the salt may not by itself ensure
homogeneity. Even though it may not be truly representative of the entire
salt mass, one full-length core sample can provide significant data on the
extent of segregation.
Gamma spectroscopy and mapping are mentioned as actions that
are less hazardous than core sampling. They can provide information on
uranium distribution that will help inform a decision on remediation
strategy.
Action 5: Determination of the structural integrity of the file! and
flush salt tanks and their potential for leakage during subsequent
operations (see options 6A, 6B, and 6C).
Several hazards to tank integrity must be evaluated. One hazard is
that the tank may have experienced some degree of corrosion during
operation or during exposure to fluorine (and possibly to nascent fluorine
species and fluorine radicals formed by radiolysis) and to ionizing
radiation over the extended storage period. Another hazard of tank failure
is associated with the melting of the salt. This hazard includes possible
clifferential thermal expansion effects, depending on the directionality
and overall pattern of the melting process. These effects can be
monitored during melting and the heat inputs arranged so as to minimize
differential effects by relieving most volumetric effects on the free
surface of the salt bed. A third hazard is that tanks may leak due to
progressive corrosion during fluorination operations. A water leak could
seriously complicate these problems.
It is recommended that all available techniques for condition
assessment be considered. Analyses of cell gas, tank gas, and any salt
samples can provide bounding information on tank condition and
possible general corrosion and on possible existing leaks. Ultrasonic
OCR for page 114
E.4
AN EVALUATION OF DOE ALTERNATIVE FOR MSRE
testing, where feasible, could provide more direct evidence of general
wall thinning. None of these methods is able to provide reliable evidence
that there is not some localized pitting or cracking that may progress to
leaks. However, if the measured rate of general corrosion is low and in
accord with prior laboratory results, the likelihood of unexpected severe
localized corrosion is reduced.
The most likely failure mode is projected to be corrosion of the
thinnest members, the cooling thimbles, and the more highly stressed
region in and near the welds. This hazard can be eliminated by removing
the bayonet coolers or by plugging thimble penetrations at the vessel
wall. Corrosion rates might be reduced by using a lower-temperature
fluorinating agent (such as bromine pentafluoride tBrFs]; see Appendix
B) than fluorine. The relative corrosion rates for different time-
temperature-reagent compositions for alternative fluorination processes
should be tested and confirmed, and the process conditions optimized, by
using small-scale laboratory tests.
In risk terms, this hazard appears likely to remain seriously
uncertain unless creative inspection methods are found or developed.
This fact suggests that procedures having minimum reliance on tank
integrity be given priority; these include the following:
.
FI?~orination at annealing temperatures without melting.
Because some fluorine species have been able to diffuse out of the solid
salt near room temperature, it is reasonable to test whether fluorine (or
HF) will diffuse into the solid salt and react at useful rates at annealing
temperatures lower than the melting point.
· Controlled zone melting and simultaneous fluorination, musing
the unmelted" portion of the salt bed as a "skull" or frozen crust for most
of the melting and fluorination operation. With proper monitoring, this
can be used to bound the possible extent of segregation and precipitation
of reduced species.
.
Contingency plans for coping with leaks of reagent gas and
UFO.
· Contingency plans for stopping melting anal fluorination and
reverting to removal of salt as a solid, if unavoidable. As mentioned in
Chapter 4, solid removal by carbon dioxide (CO2) blasting is a
contingency that it is preferable to avoid, because of the potential
spreading of contamination of gaseous radioactive radon.
OCR for page 115
APPENDIX E—HAZARD SCOPING FOR REMEDIATION
E.5
Action 6: In-tank fluorination for extraction and collection of
~ U. At least three methods can be identified:
.
Option 6A. Preliminarily conduct hydrofluorination without
melting to recover normal "oxidation" states of UF4 and PuF4 (uranium
and plutonium tetrafluorides) (e.g., using HE or fluorine-helium mixtures
at annealing temperatures), followed by options 6B or 6C to extract
uranium.
Option 6B. Zone melt the salt in place progressively and
fluorinate the uranium content to UFO using an appropriate fluorination
gas mixture at elevated temperature (e.g., fluorine at 500°C).
· Option 6C. Zone melt the salt in place progressively and
fluorinate the uranium content to UFO in place using alternative
fluoridating agents (e.g., BrFs or KrF2; see Appendix B).
.
There are two noteworthy hazards to this operation. The first is
failure of the vessel due to preexisting corrosion damage and the effect of
accelerated corrosion during fluorination. Analysis of the hazard of
vessel failure requires information about the present condition of the
vessel and the projected corrosion rates for fluorinating conditions.
The second hazard is the possibility of criticality due to possible
segregation of fissionable material. The criticality hazard can be limited
by using a low salt melting rate and by progressively fluoridating out the
uranium content of the melt zone before further melting occurs. By
operating in a quasi-batch size mode, it should be possible to monitor the
uranium removed from the melted volume and avoid having a critical
mass of uranium accumulating in the melt and subject to precipitation.
This does not necessarily address the question of the behavior of
possible insoluble plutonium species. The amount of plutonium appears
small enough (650 g divided among the two drain tanks, as reported in
Peretz, 1996c) that by itself it would be safely subcritical in any
configuration in the absence of moderator. However, the case of
incomplete dissolution and fluorination of uranium leaves open the
possible scenario of a mixture of plutonium and uranium segregating in
the drain tank vessel. Only if an effective fissile concentration of tens of
grams per liter were achieved in a large enough, sphere-like
configuration could criticality be possible. The detailed analyses under
OCR for page 116
E.6
ANEVALUATION OF DOE ALTERNATIVE FOR MSRE
way at ORNE address the presence of plutonium along with the credible
uranium and salt configurations.
Regarding option 6A, even if recovery of the normal oxidation
state is only partial, when combined with zone melting and monitoring it
appears likely to eliminate the possibility of criticality caused by
precipitation. Remaining uncertainties in the conditions required for this
process can be determined by a series of laboratory melting tests with
simulated "reduced" uranium in salt so as to permit measurement of
diffusion and reaction rates.
A continuous monitor of the degree of subcriticality of the system,
such as sensitive neutron monitoring, would provide a measurement to
support calculations and provide additional assurance of safety. The (a,n)
reactions with fluorine and beryllium provide an internal neutron source
that is augmented slightly by the subcritical neutron flux (ken
approximately 0.85) from fission events. Significant increases in neutron
flux might be readily observable far from the critical configuration. The
sensitivity of such a measurement could be enhanced further, if
necessary, by introducing a stronger external neutron source (e.g., a
pulsed neutron source).
If the processes of melting and fluorination to separate and
recover the uranium and transfer it to another vessel are acceptable for
selection on the basis of other criteria, means to ensure large margins of
criticality safety appear feasible, subject mainly to further detailing of the
process steps.
To support this position, a series of conditions that the panel
considers unlikely would all have to occur simultaneously without the
exercise of detection or control measures. These conditions are
1. substantial segregation of the fissile species, on the order of a
large fraction of the total inventory in the drain tanks;
inventory;
2. attainment of high density of the fissile material;
3. absence of continuous tracking of the location of the
4. absence of monitoring of possible increased neutron
multiplication; and
5. failure to take available corrective actions if trends toward any
of these conditions were to occur.
OCR for page 117
APPENDIXE HAZARD SCOPING FOR REMEDIATION
E.7
Given the normal disciplines of carrying out the intended types of
operations with intensive planning and training and with the usual
multiple levels of internal and external review and monitoring, the pane!
believes that the risk of criticality can be kept acceptably small.
Specifically, it should be readily feasible to maintain less than ~ percent
chance each for conditions 3, 4, and 5 above. Condition 2 may be
physically impossible and is subject to experimental verification. The
combined chance of the occurrence of criticality can be estimated
reasonably to be substantially below the target levels generally
considered satisfactory by nuclear safety authorities worldwide. This
preliminary estimate is assumed to be subject to documentation and
verification by a Safety Analysis Report, or its equivalent, normally
required for all operations involving the processing or transport of Missile
materials. Accordingly, the pane! believes that criticality poses negligible
risk to the planned operations.
The hazards of uranium-depleted salt removal appear to be
substantially smaller than the hazards of "loaded" (i.e., fissile material-
bearing) salt removal discussed in the next action.
Action 7: Processing office! andflush salt external to the present
drain tanks. Here two options have been identified:
.
Option 7A. Melt and transfer the salt to a separate vessel for
fluorination of uranium to UFO, preferably using option 6C or well-
monitored zone melting to prevent undetected segregation, and
subsequent recovery of uranium. The following procedural comments are
offered:
I. Conduct a series of trial runs with melting of simulated
reduced-state salt to establish possible segregation behavior.
2. Consider a possible option for further evaluation: run a full-
scate pilot melting test with the flush tank salt, preceded by an annealing
treatment with a gas atmosphere to simulate the partially reduced
conditions of the fuel salt.
Option 7B. Excavate the salt as a solid from the drain tanks
(e.g., by CO2 blasting) and transfer to a new vessel for
OCR for page 118
E.8
AN EVALUATION OF DOE ALTERNATIVE FOR MERE
1. conventional fluorination of the uranium to UFO with options
similar to options 6A, 6B, and 6C above;
2. electrorefining according to methods proposed by Argonne
National Laboratory; or
3. no further treatment operations beyond stabilization for
storage.
The following principal hazards in option 7B are noted:
1. It may be impractical to effect complete removal of the salt by
CO2 blasting due to internal tank structures or to impractically low rates
of salt removal.
2. Carbon dioxide could react with F2 to form carbony! fluoride
(COF2) or even carbon tetrafluoride (CF4) in an exothermic reaction.
3. Containment of radioactive species (in particular, dispersal of
radon, as discussed in Chapter 4 and Appendix C) would seem to be
aggravated by a technique such as CO2 blasting.
4. The large volumes of gas to be handled could be subject to
leaks or mechanical blowouts and subsequent material releases.
Secondary containment may be required to limit this hazard.
5. This procedure may not entirely eliminate the hazards of
segregation. in fact, segregation may be caused by density differences of
solid particulates.
6. Dry runs with simulated salts can establish optimum removal
conditions and likely rates of removal.
7. A full-scale test can be done by using the flush tank and salt,
avoiding criticality hazards, and minimizing corrosion and leakage
hazards. However, the flush tank may not be a good surrogate for the
physical removal test because of the absence of thimbles.
The mitigation measures already taken are recognized as useful,
but the pane} believes that preventive measures are also needed because
an unexplained event, even if well mitigated, can raise concerns of
institutional credibility that could render it difficult for the Department of
Energy to proceed with fuel and flush salt remediation on an orderly
schedule and budget.
OCR for page 119
APPENDIX E HAZARD SCOPING FOR REMEDIATION
E.9
Action S: Interim storage of the separated 233 U.
Transport of UFO to the existing on-site 233U storage facility
without further chemical conversion is a convenient option with well-
def~ned and experienced protocols and generally well-controlled hazards.
Radiation decomposition would be expected if the uranium were left as a
fluoride. This could be avoided by conversion to the uranium oxide
(UPON), a standard operation.
The option of converting UFO to U3O~ for storage is also a
practical one with experienced conversion and transport techniques,
generally well-def~ned procedures and equipment, and well-controlled
hazards. Stabilization of the salt residues after fluorination by chemical
Bettering is a less practiced operation but with less significant hazards.
OCR for page 120
Representative terms from entire chapter:
drain tanks
