ORNL-1816 - the Molten Salt Energy Technologies Web Site

ANP QUARTERLY PROGRESS REPORT
standpoint of both operational and corrosion prob-
lems but is efficient and rapid in the high-tempera-
liquid medium of fused fluoride salts.
ircraft Reactor Fuels
he recovery and reprocessing schedule for
fluid-fuel aircraft reactors will probably be dictated
by aircroft reactor and turbine maintenance sched-
ules rather than by the rate of formation of neutron
poisons. It is anticipated that operation will
follow a schedule such as: (1) one day of oper-
ation and one day of downtime for an accumulation
n operatingdays, (2) seven days of downtime
for minor maintenance, (3) repetition of this sched-
ule until 1000 operating hours have been accumu-
lated. After 1000 hr of operation, the entire reactor
will be dumped and the fuel will be reprocessed.
icipated cooling period before repro-
II be 10 days to allow decay of short-
activities. The minimum decontamination
factor required for the process would be approxi-
mately 100 for poison removal only. The other
steps in fuel makeupcan easily be handled remotely.
The following essential steps are used in the
chemical processing (Fig. 11.2). First, the UF,
in the molten fuel mixture is fluorinated to volatile
UF, by introducing a 10-fold excess of elemental
fluorine to achieve separation and partial decon-
tamination from the other fuel components and
fission products; second, the UF, is reduced to
UF, with hydrogen in the gas phase in the Y
reactor, as designed by K-25; and third, the re-
sulting UF, is added to 2 moles of NaF to prepare
a fuel concentrate for subsequent return to the
reactor. Present knowledge of this system indicates
that all steps are adaptable for radioactive remote
operati on. Considerable engineering development
and operational experience have been obtained
with the second and third steps, while extended
laboratory development has indicated the feasibility
of the direct fluorination of the molten fuel mixture.
geneous Reactor Fuels
The separation of U235 from zirconium alloy
fuel elements is currently the most promising
application of the fluoride process to fixed fuel
element reactors. The dissolution rate of zirconium
with HF in the NaF-ZrF, fused salt is very high,
that is, 22 to 35 milshr (Table 11.4). The range
138
probably due to metallurgical differences.
Although the ratio of zirconium to uranium is very
high in STR elements, the cost of HF to dissolve
zirconium will be only 54 per gram of U235. A
probable method for zirconium separation as a
guide for process development is indicated by the
flow diagram shown in Fig. 11.3. This flow sheet
is similar to that for fluid-fuel processing, but it
includes hydrofluorination of the fuel element in a
TABLE 11.4. RATES OF HF PENETRATION OF
VARIOUS METALS AND ALLOYS IN A
TYPICAL FUSED FLUORIDE
SALT BATH
Bath composition: NaF-KF-ZrF, (7-48.5-44.5 mole %)
HF flow rate:
3
250 cm /min
Temperature: 675'C
Nitrogen or argon blanket incases ofopen test vessels
Material
Vanadium
Si I icon
Nickel
Monel
Molybdenum
Tungsten
Silicon carbide
Type 304 stainless steel
Type 347 Nb stainless steel
Niobium
Tantalum
Manganese
Mild steel (Unistrut)
1 .
Thorium, h-tn. plate
Uranium
Zirconium
C hrom i um
Titanium
Zircaloy-2
95 wt % uranium-5 wt %
zircon i urn
Tin
Zinc
Penetration Rate
(mi I s /h r)
Not detected
Not detected
0.0001
0.02
0.03
0.06
2*
4
7
7
8
10
13
14
17
22 to 35**
31
31
22 to 46**
50
Sampl e dissolved
instantly
Sample dissolved
instantly
*Material disintegrated and left suspended particles.
**Range is due to metallurgical differences in indivi
du a1 specimens.
1
*