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OCR for page 1
Executive Summary
The Molten Salt Reactor Experiment (MSRE) is a nuclear facility
that is no longer operational and now poses a cleanup challenge to the
U.S. Department of Energy (DOE). This report comments on several
alternative cleanup strategies that are under consideration (Peretz,
1996a,b,c) for remediation of radioactive fluoride salts stored at the site.
A brief description of the facility provides a useful context in which to
understand its present status and the important issues that affect
remediation plans.
TEIE MSRE FACILITY AND CURRENT REMEDIATION PLANS
The MSRE was built at Oak Ridge National Laboratory (ORNL)
In the 1960s to explore the possibility of thermal breeding, using the
thorium-232 (232Th) and uranium-233 (233U) fuel cycle. For this
experiment, the fissile fuel (initially 23sUF4 [uranium tetrafluoride], and
later 233UF4 and 239PuF4 [plutonium tetrafluoride]) was dissolved in a
molten salt mixture and circulated at 650°C through a metal (Hastelloy
N) vessel to achieve a controlled nuclear chain reaction, moderated by
graphite rods. This novel nuclear reactor configuration produced a
thermal power of g MW. However, after the facility was shut down in
1969, no further work was conducted on this reactor or on any similar
reactor, except for design studies of a breeder reactor for which a
prototype was never built (Weinberg, 19941.
At the time of shutdown, the molten fused salt was drained from
the reactor vessel into three metal drain tanks provided as part of the
original installation. Two of the drain tanks store the now solidified
fluoride salts containing most of the uranium and plutonium fuel. A third
"flush" drain tank stores the fluoride salt mixture used to flush the system
piping, an operation that imparted to the flush salt a small inventory of
radioactive species.
From the 1960s (Peretz, 1996c, p. 1-1 1; Thoma, 1971, p. 59) it
was recognized that radiolysis of the solid fluoride salts produced
OCR for page 2
2
ANEVALUATION OF DOE ALTERNATIVES FOR MSRE
fluorine, but uranium hexafluoride (UFO) gas was not observed. Aside
from periodic maintenance checks and reheating to submersing
temperatures, the facility essentially lay dormant until recent years, when
migration away from the drain tanks of both fluorine and UFO
(representing more than 10 percent of the total uranium) was detected in
the system. Without remediation, these gases form continually due to the
instability of fluoride salts in the presence of ionizing radiation and
increasingly pressurize the system.
The condition of the facility is such that eventual environmental
or human exposures to some of the toxic materials in the facility cannot
be ruled out (NRC, 19851. Health hazards may arise from radioactive
materials such as actinides (especially gaseous UFO) and fission products
and from chemical substances such as hydrogen fluoride (HF) and
fluorine (F2) gases. Accordingly, the facility is now a cleanup priority of
DOE. Both the U.S. Environmental Protection Agency (EPA) and the
State of Tennessee have regulatory authority over cleanup activities.
One of the cleanup objectives is to reduce the hazard associated
with fluoride salts that are now stored in the three unheated drain tanks.
These tanks contain 4650 kg (4.65 metric tons, or 5.13 English short tons,
where one short ton is 2000 pounds) of solidified fluoride salt that was a
fused solution of lithium fluoride (LiF), beryllium fluoride (BeF2), and
zirconium fluoride (ZrF4~. The fuel salt contains approximately 0.7
percent (by weight) radioactive compounds specifically, uranium
fluorides (the compounds UF3, UF4, UFs, and UFO are all believed to be
present), plutonium tetrafluoride, fission products, and alpha decay
products. The flush salt contains much smaller amounts of the same
materials.
This cleanup project is at a stage where DOE is assessing several
technical approaches to salt removal, conditioning, and processing, as
summarized by Peretz (1996c), and is beginning the regulatory approval
process. Safe cleanup of the fluoride salts in this one-of-a-kind facility
poses unusual challenges.
More than one way to process the salts is possible in principle.
One option under consideration is to remelt the salts in the drain tanks
where they are now stored, redissolve any radiation-induced precipitates,
and transport the fused salt (in liquid form) through the system pipes to
an external vessel for further processing. A second option is to extract the
OCR for page 3
EXECUTIVE SUMMARY
3
uranium content of the fuel salt (through direct fluorination in the drain
tanks after partial or complete remelting of the filet salt, in order to
convert the uranium fluorides into volatile UFO gas) prior to transport of
the melted salt to external storage containers. This option would leave
fission products and plutonium fluorides in the residual salt. A third
option is to chip or blast away solid pieces of the salt without applying
heat to remelt it and then remove the salt as solid particles. In all cases,
the radioactive waste would require storage at an approved site, with
provision to "getter" any released fluorine and other volatile fluorides.
Which technical approach should be used and why? In order to
comply with regulatory requirements, several alternatives must be
considered. Relevant EPA Comprehensive Environmental Response,
Compensation, and Liability Act (CERCLA) criteria include the
protection of human health and the environment, compliance with
regulatory requirements, effectiveness (in both the long and the short
terms), reduction of toxicity, implementability, and cost. The approach
that satisfies these criteria most favorably should be the remediation
method of choice.
The challenge for project personnel is to select a remecliation
approach that deals appropriately with all the potential chemical and
nuclear problems that could arise in the handling of the fluoride salts.
Examples of such concerns are release of UFO, exothermic chemical
reactions of fluorine, and a nuclear criticality excursion due to the
quantity of Missile isotopes (e.g., 233U and 239Pu) present in the system.
PURPOSE OF THIS REPORT
The Peretz (1996c) report, published in August, identifies various
technical alternatives for the treatment and disposition of the radioactive
fluoride salts presently stored in three drain tanks. At the request of DOE,
the National Research Council (NRC) has undertaken the present study to
review current evaluations of these different cleanup approaches.
Specifically, the pane! was asked to review the proposed
alternatives for removal, separation, and stabilization of the salts to
determine the extent to which
OCR for page 4
4
ANEVALUATION OF DOE ALTERNATIVES FOR MSRE
I. 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.
Topics relevant to this determination include nuclear criticality
safety, radiolysis, nuclear reactions associated with the radioactive
species in the facility, and the chemistry and partitioning of the fluorine
salts and fluorinated compounds that exist in various phases and
compositions in the facility.
A summary of the findings on these three enumerated issues,
posed as responses to questions, is presented below.
SUMMARY OF FINDINGS ON THREE MAJOR ISSUES
To what exter'' have appropriate technologies andt
options been id~enfif edt and evaluated(?
The pane! finds that the relevant alternatives have been
identified. The pane} does not find that any important options have been
omitted, although some of the information required for a final choice is
still to be developed. Nevertheless, with the present state of knowledge,
the pane} considers that fluorination to extract UFO from the salt is the
most promising approach for isolation of the uranium. Fluorination has
been utilized successfully in the facility in the past. For in-tank
fluorination to succeed, major issues need to be resolved, among them
the potential for effective redissolution of all the solid phases in the drain
tanks and evaluation of the extent of corrosion damage to the tank walls
due to the effects of the complex interactions of ionizing radiation and
fluorine compounds.
Some alternative approaches exist within this favored
technology. These include fluorination within the drain tanks, transfer of
the salt to a new fluorination vessel, or use of the existing fluorination
vessel in the process cell. Other options are to use alternative fluoridating
OCR for page 5
EXECUTIVE SUMMARY
agents other than F2, such as bromine pentafluoride (BrFs), chlorine
trifluoride (CIF3,) dioxygen difluoride (O2F2), krypton difluoride (KrF2),
or xenon difluoride (XeF24.
To what extent are the evaluations suff ciently complete
fit form a basisfor decision making?
The pane} finds that the final waste disposal objectives in the
proposed (Peretz, 1996c) alternatives 1-6 are presently insufficient to
lead to a sound remediation strategy and concludes that interim waste
storage (alternative 7) is the only practical approach at present.
Additional information about the system is needed to support
decisions about remediation options. The final selection of an approach
must be based on additional information about the system and the
hazards.
A crucial part of molten salt processing will be the initial
melting, which will demonstrate whether the precipitated phases are
manageable and the tank confinement is intact. Positive results would
show that direct fluorination on a liquid system could proceed.
In Peretz (1996c), the remediation strategy being used by the
project is not defined. Each strategy developed for remediation of the
drain tank salts should have a primary alternative and one or more
backup alternatives to cover the hazard of failure of the primary
alternative. A comparative cost estimate should be completed for each
case. The decision maker can then optimize the choice of strategies based
on probable success, initial costs and risks, and possible ultimate costs.
To what extent have the potential hazards associated
with fuel and flush salt removal beer' adequately
identity ed arid addressed?
The term hazard is used here instead of risk because the probability of occurrence of
the hazard has not been defined. Assessment of a risk can be made only when the
likelihood of a hazard scenario has been assessed and applied to the measure of the
importance (or size) of the possible consequences. In other words, risk is quantitatively
defined as the product of probability of occurrence and consequence of a hazard scenario;
in the absence of numerical calculations, one may refer to hazards, not risks. Both
concepts (hazards and risks) are used in this report.
OCR for page 6
6
AN EVALUATION OF DOE ALTERNATIVES FOR MSRE
The Peretz (1996c) report contains a baseline hazard section that
identifies the present hazards. A preliminary hazards screening has also
been made (ORNE, 1995~. Evaluation of the possible hazards associated
with the various process alternatives awaits the further development of
those alternatives.
Once the pertinent assessments of likelihood and magnitude of
hazard scenarios are available, present safety standards and risk analysis
methodologies, properly applied, can provide adequate delineation of
hazards and their potential for becoming risks. In a risk analysis of the
MSRE drain tanks, an overly conservative, upper-bound estimate of any
particular risk is undesirable if the quantitative results are used to choose
among various remediation alternatives, because this approach might
then fail to identify the course of action with the least risk. 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.
The panel believes that the probability of an inadvertent
criticality hazard during mitigation of the MSRE salt is extremely low
and that, even if such an event were to occur, the safety and technical
consequences would be minor. However, because the public concern and
political consequences could be very large, the pane! has addressed the
question in some detail.
Regarding radiation hazards, high radiation fields external to the
drain tanks necessitate remote operations, with workers outside the
shielded drain tank cell except for very brief episodic access. For
example, the gamma radiation levels at the surface of the fuel salt tanks
due to fission products are approximately 640 roentgens (R) per hour
(Williams, 1995), primarily due to the 0.66-MeV (million electron volt)
gamma rays from cesium- 137.
MAJOR CONCLUSIONS
The pane} concludes that cleanup of the MSRE can be
accomplished with risks to the public, operations personnel, and the
OCR for page 7
EXECUTIVE SUMMARY
7
environment that can be well managed to quantitative levels at or below
those values that form standard safety guidelines (such as an annual
probability of occurrence of ~ o~6~.
Additional information is needed to define and select the most
favorable processes for the removal of fissionable materials, the
associated removal of the salt, and their interim disposition. However,
present information is good enough, and overall safety well enough
ensured, to be used as a basis for making the decision to proceed with the
overall project.
The responses given above to questions posed in the Statement of
Task are generally favorable judgments subject to important cautionary
caveats, for example, that some of the specific parameters and procedures
required for processing steps are confirmed by further information
gathering, testing, and analysis. Examples and suggestions of this kind
are contained in the report, particularly in Chapter ~ and Appendix E.
These technical options, offered as suggestions for consideration to
MSRE project personnel, are secondary to the basic recommendation that
MSRE remediation work is ready to proceed, with the fluorination
approach the most promising way to remove uranium, given the present
state of knowledge. The panel is not chartered to recommend specific
parameters or procedures, and these selections are best done by project
personnel, as new information is obtained.
What is missing and what could be collected in the first stages of
the remediation project are current data on the fuel salts and on the
condition of the piping, vent lines, and equipment of the MSRE, which has
been shut down since 1969. Final selection of processes and the necessary
backup systems will depend on the crucial aforementioned data and on
laboratory and engineering developments. Irradiated samples from the
period of MSRE operation are available for simulation and mockup of the
remediation processes to be used. The project would proceed ideally in
stages dete~ined in part by what is learned as the work progresses.
While the drain tanks and their contents were well characterized
at reactor shutdown, more than 25 years of radiolysis and corrosion have
produced changes in chemistry and may have produced changes in the
integrity of components. As a result, the panel recommends a cautious,
stepwise approach to all operations.
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8
ANEVALUATION OF DOE ALTERNATIVES FOR MSRE
Fluorination procedures, subject to important caveats and to
further information gathered on the system, as noted in this report, appear
to be the technical approach most worthy of Further consideration.
Storage of the separated fissile 233U as the uranium oxide U3Os in an
existing 233U storage site at ORNE represents a suitable interim
disposition. A stabilized or glittered salt residue after fluorination
treatment also is a reasonable form for interim storage at ORNL.
MAJOR RECOMMENDATIONS
The Peretz (1996c) report on remediation alternatives is only a
preliminary document related to the CERCLA process. Further analyses
and development of specific plans leading to a final decision will occur
over the next several years. 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.
The pane} recommends that cleanup strategies for the MSRE
project provide one or more sets of workable approaches for the safe
removal, processing, and interim storage of fuels and flush salts that take
into account the need for alternative and backup strategies. All relevant
factors should be considered, including cost, equipment failure, criticality
potential, remediation effectiveness and implementability, risk
management, uncertainties, trade-offs, and duration of actions. Of equal
importance is the need to fully consider possible process perturbations,
failures, contingencies, resource requirements, and other factors that may
warrant backup support or alternative approaches to offset them.
Plans and work in progress at ORNE are addressing these issues.
Sound procedures, such as stepwise processes for the acquisition of
information and remediation, are a necessary strategy because not enough
is known about the system (e.g., distribution of uranium in the system) to
make a fully informed decision at the onset.
Because the final resolution of disposal alternatives may take
considerable time, the panel suggests that DOE use a phased decision
strategy focusing on interim storage, with the flexibility it provides,
rather than trying to make a final disposition alternative determination in
the near future. It is premature at this stage to derive treatment
OCR for page 9
adhesives Mom requirements far ukim~e d~poshion in geologic
repositories o~she, because these long-te~ storage options are not
developed sufficiently to define Anal waste acceptance crheris~ It
appears that resolution of uldm~e (geologic) disposal she criteria and
characteristics probably lies beyond the time horizon of the ASH
cleanup pr~ec1
The panel ~ ate 1h~ any processing of 1he Mel sags ma prevent them cl=~0c~ion
a.
Representative terms from entire chapter:
fluoride salts
