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9
Summary and Responses to Questions in
Statement of Task
The Statement of Task for this study (Box ~ . T) asks the pane! "to
review the current state of the Department's (DOE's) evaluations of
alternatives" (presented in Peretz, 1996c) "for removal, separation, and
stabilization of MSRE salts to determine the extent to which (~)
appropriate technologies and options have been identified and evaluated;
(2) evaluations are sufficiently complete to form a basis for decision-
making; and (3) potential hazards associated with fuel and flush salt
removal have been identified and addressed." Following a technical
summary and some general perspectives that summarize pane! views,
specific answers to these questions are presented.
TECHNICAL SUMMARY
In brief, the Molten Salt Reactor Experiment (MSRE) salts
contain uranium, transuranium elements, and fission product
radioactivity, as well as fluorides of lithium, beryllium, and zirconium,
and should not remain indefinitely in the drain tank cell where they have
been kept for more than 25 years. Major reasons for this conclusion
include the continuing migration of fluorine and uranium from the bulk
salt, its location below the natural water table, and the potential critical
configuration upon the intrusion of water. The permanent disposition of
transuranics well in excess of 100 nanocuries per gram at shallow depths,
just below grade, is also likely to be disallowed.
Removal alternatives for the salt are
1. melting, fluorination to form uranium hexafluoride (UF6),
and removal of the molten salt with its plutonium and fission products; or
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particles.
AN EVALUATION OF DOE ALTERNATIVES FOR MSRE
2. breaking up the bulk fee! salt and removing it as solid
The first alternative is preferred, with or without preliminary uranium
removal in situ, with option 2 as a backup procedure.
Radiation effects continue to move fluorine and uranium from
the bulk salt. In October of 1996, there was nearly one atmosphere of
excess gas pressure over the fuel drain tanks, almost twice that observed
in 1994. The pressure is contributed largely by molecular fluorine (F2)
and the saturation pressure of UFO which suggests solid deposits in the
piping of UFO and its radiation-induced lower fluorides. Any removal
alternative must start with the gases, including whatever uranium can be
removed by pumping. The uranium material balance is poorly known,
especially the amounts in and the locations of nonvolatile forms. Partial
decomposition of solid UFO by the effects of alpha radiation (from 233U
and daughters) is expected to form nonvolatile uranium fluoride deposits
essentially throughout the equipment. Because fluorine is more effective
in reforming UFO when such locations can be heated, alternate reagents
for UFO formation (e.g., BrFs [bromine pentafluoride] and KrF2 Krypton
difluoridel; see Appendix B) should be considered to treat nonvolatile
plugs in unheated regions.
Removal of uranium from the molten Ale} salt by F2 sparging to
produce UFO was accomplished routinely in the past. Hence, this
approach would normally be favored. However, melting of an irradiated]
surrogate salt sample did not yield a clear melt but yielded at least two
phases, one a metallic-appearing precipitate. To date, it has yet to be
established that refluorination treatment of the salt containing such
precipitates will be successful in reestablishing a homogeneous melt.
Experiments should be performed on additional irradiated
surrogate salt samples in attempts to redissolve the precipitate and restore
the salt melt to a homogeneous condition so that transfer to another
container or uranium removal by fluorination could ensue. It should be
emphasized that the MSRE fuel salt, probably in a highly reducing
chemical condition, remains a major problem, which will only worsen
with time.
Even if uranium removal is accomplished, the residual salt will
still contain activity from 0.7 kg of plutonium and from fission products.
Formation of fluorine and volatile fluorides will continue. Plans to
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SUMMARY
81
"getter" the fluorine formed from the interim stored fuel salt should also
provide for the possible formation of some plutonium hexafluoride
(PuF6), although a significant quantity of volatile PuF6 is not expected
(Mills, 1996~.
STRATEGY FOR REMEDIATION
There is insufficient knowledge in hand to outline, in detail, a
series of processes that, if followed, would lead clearly to a satisfactory
outcome of nonhazardous removal, separation, and stabilization of
MSRE salts. Information gathering in the near future should allow better
decisions concerning these alternative actions. Indeed, relevant
alternatives have been identified in Peretz (1996c), but final evaluation
awaits further information. No important options or technologies seem to
have been omitted from Peretz (1996c).
There is a preferred approach within which some options exist.
Of the several alternative processes, fluorination to extract UFO from the
molten salt is the leading option. However, choices are available for
specifics of the processes for salt pretreatment, salt melting, and
fluorination.
Although Peretz (1996c) contains a baseline hazard section that
identifies present hazards, evaluations are not yet sufficiently complete to
allow for thorough knowledge of the extent of hazards associated with
various alternative processes. However, the information required is well
defined and can be obtained by available methods. Three information
needs are (1) locating the uranium within the system, (2) further work in
assessing the condition of the fuel salts and their containment, (storage
tanks, piping, and associated hardware), and (3) bench-scale testing of
possible remediation processes.
Where Is the Uranium?
As the charcoal trap and the piping system are cleaned up, some
related information will be obtained. As the volatile components are
taken from the system, identified, and quantified, more information will
follow. The amount of uranium removed from the piping as the
hexafluoride plus that in the charcoal trap will help determine the
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ANEVALUATION OF DOE ALTERNATIVES FOR MSRE
remaining total inventory. This inventory could be present in lower
oxidation states, either in the salt or deposited in the headspace and
piping. If there is much uranium remaining in the system external to the
tanks, it might be feasible to map its distribution by radiation surveys.
The distribution of uranium-containing species in the salt could be
determined with gamma monitoring supplemented by neutron
monitoring. Of possible relevance to a remediation strategy is whether
inhomogeneities in uranium concentration exceed the variations in
distribution that were observed in surrogate salt samples. Core and/or
radial samples could also provide this information, but gaseous
radioactive contamination is a significant hazard associated with
sampling the salt.
How Can a Condition Assessment Affect Remediation Plans?
Information related to the condition of the system, obtained as
work has progressed (ORAL, 1996a), has established that plugs from
deposits in the piping remain and interfere with the removal of volatile
substances by pumping.
Fluorination may remove lower oxidation state uranium that
could remain in the plugs and headspace. Although fluorine, hydrogen
fluoride (HF), and BrF5 are the preferred fluorination agents, corrosion
and temperature concerns and the extent to which uranium-containing
deposits resist reaction may lead to consideration of other fluoridating
agents, such as atomic fluorine or KrF2 (see Appendix B). Plutonium is
present in such low concentrations (approximately 155 ppm tparts per
million]) that it is not likely to form PuF6 readily with any of these
possible fluorinating agents.
Although it seems unlikely since gases above atmospheric
pressures remain in the system, there may be cracks or pinholes in the
apparatus and piping above the salt. These would not be revealed if they
were plugger! by the uranium-containing species. If it were determiner!
that corrosion damage is not a problem, consideration could be given to
plugging the thimble tubes physically and fluorinating the gradually
melting salt in the storage tanks. The panel recommends, prior to
'Fluorine is a better oxidizer at elevated temperature. If the pipes and valves cannot be
heated, fluorinating agents more active than F2 at ambient temperature may be required.
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SUMAl4RY
83
fluorination, consideration of an annealing procedure of heating the solid
salt in an atmosphere with a gradually increasing concentration of
fluorine or HF. The panel recommends an initial atmosphere with an HF-
inert gas mixture, rather than the proposed HF-H2 (hydrogen) mixture.
This would be succeeded by a ~-F2-inert gas mixture to ensure that all
uranium was oxidized at least to uranium(IV). Omitting hydrogen from
the hydrofluorination step would reduce the hazards associated with the
explosive reactivity of hydrogen gas.2
The Panel's Preferred Alternative
Because one does not know at this time what options will be
available for final disposition of the actinides, fission and decay products,
and the salt, the panel considers that interim storage of separated uranium
and of the residual salts is the only realistic option available at this time.
Interim storage of the separated uranium as an oxide in existing sites at
Oak Ridge seems acceptable, as does interim storage of the fluoride salts
(containing radioactive species other than uranium) with a getter for
radiation-produced fluorine gas that would be expected to form and
migrate out of the solid salt residue.
in short, the pane! finds that the general approach given in Peretz
(1996c) is acceptable but backup alternatives must be available. The
scheme should have hold points to confirm that the expected process
behavior is being realized. Given adequate preparations, the intended
processes are likely to perform as planned. However, it would be prudent
to add faliback and contingency plans to the intended operational
scenario.
Hazards
The pane! considered various hazards. Criticality, in particular,
was given considerable attention. The panel finds that, in the absence of
water, there is no appreciable likelihood of the occurrence of a critical
excursion. As stated in Chapter 6, the pane! believes that the probability
2The HF-F2-inert gas mixture would produce hydrogen via the reaction 2UF3 + 2HF ~
2UF4 + H2. This evolved hydrogen would then be consumed by F2 to form HF. The
addition of F2 thus serves to reduce the hazard of excessive hydrogen concentration.
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AN EVALUATION OF DOE ALTE~ATIVES FOR MS~
of such an excursion during remediation of the MSRE salt is extremely
low and, even if such an event were to occur, the safety and technical
consequences would be insignificant.
The pane! finds that the available documentation (Peretz, 1996c)
shows an awareness of the hazards and how to deal with them. The panel
has some concern over the use of hydrogen as a diluent and anticorrosion
agent in the proposed treatment with mixtures of hydrogen and hydrogen
fluoride. An evaluation of the specific hazards associated with each step
of the planned work, and of the branches in the scheme to be developed,
should be laid out in further detail.
RESPONSES TO QUESTIONS IN STATEMENT OF TASK
The Statement of Task for this study (Box A) asked the panel to
address three questions concerning the scoping activities of the
Department of Energy (DOE) and DOE contractors to date on the
remediation techniques and strategy that could be applied to MSRE fuel
and flush salts. These questions (and answers) are intended to assist the
decision-making process involving DOE and regulatory agencies. Of
course, selection of a proper cleanup approach is of interest to others,
such as workers and the general public.
The material in preceding chapters provides the necessary
background for answers to these questions. The panel provides brief
answers to each question below. More detailed discussions can be found
elsewhere in the report.
Question 1
To what extent have the appropriate technologies and options
been id~entif ed and evaluated?
The overall technical alternatives seem to have been identified,
but the actual details and steps are not clear enough for final evaluation.
in some cases, alternatives exist within a technology: for
instance, if fluorination in the drain tanks is not feasible by using
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SUMMARY
85
fluorine, fluorination with another agent such as BrFs or fluorination in a
separate vessel may be possible.
Because the source of volatile uranium fluoride was the drain
tank and uranium migrated through the system to the carbon trap, there is
a high probability that substantial amounts of uranium compounds are
located on the cover and walls in the freeboard area of the drain tank.
This issue may be important in assessing technical options.
The alternative of solid salt removal by mechanical
fragmentation and particle evacuation was conceptual and not specific. If
there are data or experience to support the concept, they were not
presented to the panel.
Question 2
To what extent are the evaluations stuff cien fly complete to form a
basis for decision making?
The development of additional information, now in progress, is
needed to support decisions about alternatives. The final selection of one
method for example, direct fluorination with F2 is subject to the
acquisition of additional information about the condition of the system
and the hazards, which appears to be feasible given adequate resources.
The literature documents the procedure, and personnel at Oak Ridge
National Laboratory (ORNL) have had extensive experience in the direct
fluorination of uranium tetrafluoride (UF4) to produce UFO. Nevertheless
it is important to be aware of alternative possibilities.
The Consequences of Failure to Complete According to Plan
Additional assessment is needed to address an important hazard,
the hazard offailure, to explore whether the selected remediation method
precludes a desirable backup option in the event that unexpected or
undesirable behavior is encountered. For instance, could the failure of
one technological alternative preclude using another? Could failure to
reoxidize the reduced metals sufficiently result in a sludge in the melted
salt such that liquid removal becomes impractical?
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Cost Estimates
AN EVALUATION OF DOE ALTERNATIVES FOR MSRE
Because of the lack of details it is not possible for the panel to
assess the credibility of cost estimates. For example, it is not obvious
how to compare the cost of an as-yet-undefined mechanical process with
the cost of liquid removal.
A Possible Strategy
In ~eretz t1996c), the strategy being used by the project is not
defined. Each strategy should have a primary alternative and one or more
backup alternatives to cover the hazard of failure of the primary
alternative. A preliminary cost estimate should be done for each case.
The decision maker can then optimize the choice of strategies based on
probable success, initial costs, and possible ultimate costs. The decision
maker can then choose between strategies of lowest base cost but higher
potential total cost versus strategies with higher base costs but lower
potential overall costs. An alternative technology that is primary in one
case might be the backup in another case.
Question 3
To what extent have the pofenlial hazards associated with fuel
andf ush sad removal been adequately iden~if ed and addressed ?
The Peretz (1996c) report contains a baseline hazard section that
identifies the present hazards, but there is almost no discussion of the
hazards associated with various alternatives. A preliminary hazards
screening has also been made (ORNE, 1995~. The term hazard is used
instead of risk because the probability of a hazard becoming a risk is best
determined only after operation steps are detailed for execution.
Identification of the hazards associated with various process alternatives
awaits further development of those alternatives.
The hazards associated with radioactivity and criticality are not
unique to this project. However, the complex mechanical and chemical
questions elevate the importance of the hazard of failure, a concept not
developed in detail in Peretz (1996c).
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SUMAL4RY
87
The Peretz (1996c) report contains a cursory risk analysis of
potential future human exposure in which there are indications that
unduly conservative (i.e., beyond credible) assumptions may have been
used. One stated assumption is a scenario in which a component of the
now pressurized off-gas system suddenly fails and releases the entire
radioactive inventory within the building. It is further assumed that 50
percent of this inventory is retained, and 50 percent is lost as a ground-
leve! release under typical conservative atmospheric conditions (Peretz,
1996c, p. 1-36~.
These assumptions are unduly pessimistic; at ambient
temperature, only a small fraction of the uranium is in the gas phase, and
exposure to moist air rapidly converts most of the uranium to nonvolatile
oxides. In addition, all system components are in sealed cells inside a
building. Any highly exothermic process with carbon (e.g.,
decomposition of CFX to form carbon compounds and CF4 Carbon
tetrafluoride]) that could volatilize a large amount of radioactive fluoride
salts is a potential hazard that has already been mitigated by isolation of
the activated charcoal bed from the majority of the radioactive material.
A major overestimate of a hazard is not a conservative strategy
when using the results to choose a line of action. It easily can result in
selecting a course of action more dangerous than the discarded action. In
the absence of more complete data (e.g., realistic probability distribution
functions for every uncertain parameter), and for present decisions, the
panel believes that estimates of risk that provide the best basis for
decision making should be on an expected value basis, bracketed by an
uncertainty range.
PANEL PERSPECTIVE
These findings and recommendations are offered to enable the
parties involved (DOE, contractors, and regulatory agencies) to clean up
the MSRE fluoride salts safely and expeditiously, with due regard for the
hazardous materials (such as reactive F2 and UFO gases and Missile 233U).
Fluorination procedures, subject to important caveats and further
information-gathering activities advocated in this report, appear at this
time to be a preferred technical approach worthy of further consideration.
However, without the additional information that comes from testing and
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AN EVALUATION OF DOE ALTERNATIVES FOR MSRE
experimentation, it is too early to make a final, sound decision. Thus, it is
still too soon to eliminate backup remediation methods.
A hazard management strategy is recommended by the pane} as
an appropriate way to develop a decision-making process that is phased,
or "gated." A period of further testing and information-gathering activity
would provide for a more informed decision that could be deferred until
the relevant information is received. Candidate technologies can be in
view during this time and, subject to the receipt of confirming evidence,
could be used in the MSRE salt cleanup. The aim of acquiring more
information with additional scoping studies is to affect the choice of
technical alternatives and enable project personnel to gain increased
confidence in the success of the chosen technical approach. As new
information becomes available on the fuel and flush salts and on the
status of the rest of the MSRE system, additional reviews of the major
issues may be warranted.
OVERALL CONCLUSION
After reviewing Peretz (1996c) and discussing the problems with
responsible personnel, the pane} concludes that the evaluations done to
date are adequate for proceeding with the remediation program and that
work needs to go forward on a timely schedule. As discussed in the
report, the pane} recommends a phased program with each phase
designed to do those things and acquire the information that will make
going on with the next phase an action with acceptable risks. The
technical capabilities of Oak Ridge National Laboratory are quite
adequate to ensure that the measures needed to prevent or control all
likely hazards can be implemented practically and safely.
Representative terms from entire chapter:
interim storage
