|
Items in cart [1] |
Questions? Call 888-624-8373 |
| Copyright © 2009. National Academy of Sciences. All rights reserved. Terms of Use and Privacy Statement |
Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter.
Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.
Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.
OCR for page 58
6
Nuclear Criticality Considerations
As discussed in this chapter, the pane! finds that the probability
of a critical excursion during the processing of the Molten Salt Reactor
Experiment (MSRE) salt is extremely low. Even if such an event were to
occur, the safety and technical consequences would be insignificant.
Additionally, certain process options (e.g., see Appendix D) can reduce
the likelihood of criticality even further. However, the public concerns
and political consequences could be very large; thus, the pane} has
addressed the question in some detail.
CRITICALITY ISSUES IN PROCESSING
Nuclear criticality safety considerations provide significant
restrictions to the process designs to be implemented for each of the
processing options. Nuclear criticality safety issues are determined by the
chemical and physical behavior of the constituents, for example, the
potential for uranium hexafluoride (UFO) evolution and condensation, the
lower uranium fluorides produced, uranium reduction to metal, and zone
refining as a result of melting. Table 6.1 outlines some specific concerns
for each remediation operation.
A more general nuclear criticality issue that is not specific to any
one processing option is the potential consequence of a nuclear criticality
excursion. Two elements of relevance are the shutdown mechanism that
would limit the consequences of such an excursion in the salt medium
and the range of energy releases from such an event.
Pruvost and Paxton (1996) describe initiating mechanisms and
consequences for historical and other postulated nuclear criticality
accidents. Augmented neutron flux, heat generation, and production of
new fission products would be signs of an excursion were it to happen.
If the MSRE salt is inhomogeneous, it is not evident that a
mechanism exists that would vary the composition to concentrate
58
OCR for page 59
NUCLEAR CRITICALITY CONSIDERATIONS
TABLE 6.1 Nuclear Criticality Safety Implications: A Sample of
Concerns Considered by the Pane}
59
Remedial Operation
General: Verify current
subcritical configuration
Criticality Hazard/Concern
Baseline:
Establish (best-estimate)
critical mass values for
potential compositions of
233u
in dry salt
in (unrestricted) aqueous
solution
with effects of water and
other moderators
Compensatory
Measures
Maintain
configurations and
compositions
Monitor for changes
Prevent additions of
moderators
Apply adequately
conservative safety factors
(e.g., based on assumptions
that bound uncertainties in
supporting data, which are
known to be inadequate,
arid calculations)
1. Remove reactive gases Ingress of water or other Strict control of
moderator to drain tanks moderator sources
Favorable geometry in
NaF traps, alumina, and
zeolite bed
2. Remove solid UFO Redistribute existing Apply batch/mass
deposits material to critical limits
configuration
Accumulate external
critical mass
Ingress of water or other
moderator
Remove to favorable
geometry containers
Strict control of
moderator sources
OCR for page 60
60
AN EVALUATION OF DOE ALTERNATIVES FOR MSRE
3. Sample solid salt Ingress of water or other Strict control of
moderator moderator sources
Collapse of salt
Mechanical reconfiguration
of salt
4. Remove residual Ingress of water or other Strict control of
uranium in piping moderator moderator sources
5. Fluorinate in place to
UFO
6. Remove salt as liquid
for fluorination
elsewhere
7. Remove salt as solid
Accumulate external
critical mass
Ingress of water or other
moderator
Redistribution or
precipitation of uranium or
plutonium
Uranium redistribution,
segregation, or
precipitation to form
critical mass
Ingress of water or other
moderator
Ingress of water or other
moderator
Collapse of solid
Remove to favorable
geometry containers
Strict control of
moderator sources
Fluorinate carefully
with HE, followed by
F2
Apply batch/mass
limits
Remove to favorable
geometry containers
Analyze and establish
controls for new
external systems
Strict control of
moderator sources
Control geometry
and/or mass of
removed liquid material
Strict control of
moderator sources
Salt removal
procedures to stabilize
geometry and prevent
collapse
OCR for page 61
NUCLEAR CRITICALITY CONSIDERATIONS
8. Convert UFO to U3O8
Accumulate critical mass
Ingress of water or other
moderator
Note: This conversion step
is well established, having
been conducted frequently
at ORNL (for 235U-bearing
materials as well as for
U-bearing materials)
over the last several
decades
61
Strict control of sources
of water and other
moderators (or apply
limits and/or
geometries that
accommodate optimum
moderation)
Apply batch/mass
limits and/or use
favorable geometry
containers
NOTE: F2 = molecular fluorine; HE = hydrogen fluoride; NaF = sodium fluoride; 0RNL = Oak Ridge
National Laboratory; U = uranium; UFO = uranium hexafluoride; U3O~ = uranium oxide.
fissile material at the rapid rate required to sustain a nuclear chain
reaction (i.e., to go critical). Without a rapid assembly or concentration
mechanism, even if a chain reaction is achieved the resulting energy
release would be very small and a breech of the drain tank containment
would not occur. Further, the configuration of the MSRE system includes
both distance and heavy shielding, which were enough to provide
protection during operations at a steady power level of ~ MW. The
radiation effects of a potential critical excursion would be unlikely to
spread beyond the enclosed vessels; evacuation of surrounding rooms and
buildings would be unnecessary. Thus, the consequences of a criticality
excursion within the drain tanks are manageable. The pane! concludes
that, even if such an event were to occur, the safety and technical
consequences would be insignificant.
CRITICALITY HAZARD OF REMELTING THE FLUORIDE
SALTS IN THE DRAIN TANKS
Aside from these criticality issues for various process
alternatives, an assessment can be provided on the low nuclear criticality
hazard when remelting the salt. The salt mixture is less effective as a
moderator than water. However, three of its four constituents are "good"
OCR for page 62
62
AN EVALUATION OF DOE ALTERNATIVES FOR MSRE
moderators. Specifically, the effectiveness of lithium and beryllium is
explained because they are lower in atomic number than the carbon
(graphite) used to provide extra moderation for the MSRE core during
operation. Fluorine comes after carbon in the periodic table, but it is still
of low enough mass to be considered a moderator. Zirconium has a
relatively high mass, with reduced moderating effectiveness. The overall
combination of the elemental salt constituents provides a somewhat less
effective moderator than graphite, which is one reason graphite was
added as a moderator to the MSRE core design (at some substantial cost
and inconvenience).
The composition and concentrations of the salt mixture are such
that even in the optimum configuration, criticality could not be achieved
in the MSRE reactor vessel without the addition of a graphite moderator.
Without as good moderation as in the graphite core, and with reduced
uranium content (each drain tank originally contained only
approximately half the uranium inventory, and more than 10 percent of
that has migrated out of the salt), the probability of a criticality
excursion in each drain tank is further reduced. Additionally, the pane}
notes that the shielding was clesigned to be adequate for sustained
criticality during reactor operations.
One approach to assessing criticality potential is to use model
calculations that generate a keff value for a particular geometry of fissile
i ~ there 233U 23su and 239Pu tplutonium-2393~; neutron-
moderating material (here, water, lithium, beryllium, and fluorine), and
reflecting material (here, salt and concrete). Presentations to the panel
(Rushton et al., 1996a,b) have included such calculations. Criticality is
possible for the case of water intrusion into the salt (interior to the tanks),
and for this reason, moderator controls are important. However, these
calculations have shown (i.e., have generated a ken value much less than
one) that external moderation alone cannot cause criticality.
The nominal uranium-in-salt concentration (i.e., approximately
~3 g of uranium per liter, if half of the original uranium content is
assumed to be distributed equally among the two tanks) is insufficient to
be critical in a drain tank. A local increase in concentration could lead to
a critical configuration in a tank. Should the concentration increase by a
factor of two in a large enough, sphere-like configuration, criticality
might be possible. Conversely, if the current uranium concentration in the
drain tank remains relatively uniform (or, as is possible, is well below the
OCR for page 63
NUCLEA R CRI TI CA LI TY CONSIDERS TI ONS
63
nominal value), a well-controlled melt is unlikely to lead to a critical
configuration.
RATIONALE FOR TECHNICAL INSIGNIFICANCE OF A
CRITICALITY EXCURSION
The pane} believes that a criticality excursion, even were it to
occur, would have no important technical consequence.
Any operation would be done by remote means; that is, no
human would be inside the drain tank cell. The drain tank cell provides
massive concrete shielding (several feet thick; see Figure 1.3) and a gas-
tight steel liner. Any criticality excursion event would be totally
contained and well shielded. The building ventilation system provides the
necessary secondary containment. This containment system was designed
for an operating reactor and thus should contain any plausible criticality
excursion in the drain tanks.
A comparison of the MSRE drain tank system to that of previous
criticality accidents provides perspective. The latter involved
mechanisms to concentrate fissile material, for example, by a continuous
feed to a reaction. From a historical and practical point of view (Stratton
and Smith, 1989; Frolov et al., 1995; Knief, 1985; Pruvost and Paxton,
1996) a configuration similar to the MSRE reactor vessel is required for
this. Such a configuration is absent in the case of the MSRE drain tanks.
For example, were the tank wall to leak or to rupture (as a worst-case
scenario), the material would disperse inside the drain tank cell and be
subcritical in a slab geometry configuration. The cleanup problem would
be transferred from the tanks to the cell.
Even the rupture of the half-inch-thick Hastelloy N drain tank
wall (which, if it did occur, would still pose no significant risk to the
public) is a conservative scenario for a criticality excursion, with the
following rationale: any criticality burst could take place only if the salt
were liquid (i.e., at a temperature greater than 460°C), but beryllium
fluoride tBeF2] sublimes at 800°C (at one atmosphere). Therefore, a
temperature rise of 340C, obtained from any fission burst, would
automatically disassemble the salt system, just as it would in an aqueous
system (the BeF2 gas void would be similar to the steam void in a water
OCR for page 64
64
AN EVALUATION OF DOE ALTERNATIVES FOR MSRE
system). In both systems the nuclear reaction automatically shuts down
after an initial burst.
Without a breach of vessel walls, a criticality event may be
detectable only by the presence of fresh fission products.
CONCLUDING COMMENTS
The panel had neither the resources nor the charter to perform a
quantitative risk assessment (of criticality or of any other hazard) and
notes that a rigorous effort is premature until the specific event scenarios
and process conditions are defined. Nevertheless, the basis for the panel's
position is found in the approach suggested in Appendix E to evaluate the
criticality probability and to limit it to less than lob per year. The
likelihood of a significant dose to someone at the site boundary is several
orders of magnitude lower, due to the small fission product inventory, the
relatively low energy release (on the order of megawatt-seconds) that is
hypothetically possible from criticality (as a worst case), and the
presence of significant containment barriers.
Detailed analyses of credible uranium and salt configurations are
continuing at Oak Ridge National Laboratory (ORNL). Based on a
review of previous nuclear criticality safety evaluations, the pane! has
confidence in the ORNL analysis and evaluation capabilities. Further, it
is assumed that with whatever processing option is employed, the
associated nuclear criticality safety measures will include (1)
requirements that all activities proceed with caution; (2) measures to
prevent intrusion of water moderator into the drain tank, which Crume
(1994) reported could make the current configuration critical; and (3)
appropriate consideration of the potential value of monitoring neutron
multiplication or of adding neutron poison either for normal operation or
in response to an upset condition. One potential poison, gadolinium
(described further in Appendix D), is a uniquely powerful neutron
absorber. The stable compound gadolinium trifluoride (G6F3) would be
soluble in, and heavier than, the molten salt and would behave
chemically very much like uranium trifluoride (USA. The management
of nuclear criticality safety hazards during processing is addressed further
in Chapter 8 and Appendix E.
Representative terms from entire chapter:
drain tank
