ORNL-2374.txt

 

 

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AEC RESEARCH AND DEVELOPMENT REPORT 9%y 1434

Special Features of Aircraft Reactors
MARTIN MARIETTA ENERGY SYSTEMS LIBRARIES
IR
AT

3 445k 0350527 7

   
 

0

 

 

 

 

    
   
 

SOME ASPECTS OF THE BEHAVIOR OF

  
 

FISSION PRODUCTS IN MOLTEN

  
 

FLUORIDE REACTOR FUELS

M. T. Robinson

W. A, Brooksbank, Jr.
H. W. Wright
T. H. Handley

 
  
      

OAK RIDGE NATIONAL LABORATORY

OPERATED BY
UNION CARBIDE NUCLEAR COMPANY

A Division of Union Carbide and Carbon Corporation
UCC

POST OFFICE BOX X * OAK RIDGE, TENNESSEE

 

     
ERRATA for ORNL-2373

NOTICE

The Molten Fluoride Reactor Experiment (MFRE) referred to in this
report is officially designated as the Alircraft Reactor Experiment

(ARE)
 

¥
&

_ ORNL-237h4
J This document consists of 20 pages.

Copy /23 of 251 copies. Series A.

Contract No. W-T4O5-eng-26

SOLID STATE AND ANALYTICAL CHEMISTRY DIVISIONS

SOME ASPECTS OF THE BEHAVIOR OF FISSION PRODUCTS IN
MOLTEN FLUORIDE REACTOR FUELS

M. T. Robinson

W. A. Brooksbank, Jr.
S. A. Reynolds

H. W. Wright

T. H. Handley

DATE ISSUED

AUG £ § 1957

OAK RIDGE NATIONAL LABORATORY
Operated by
UNION CARBIDE NUCLEAR COMPANY
A Division of Union Carbide and Carbon Corporation
Post Office Box X
Oak Ridge, Tennessee

1ES
MARTIN MARIETTA ENERGY SYSTEMS LIBRAR

M

u 3 445k 0350527 7

 
 

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C-84 - Reactors-Special
Featuges of Aircrafti Reactors
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QUUDEG Q>
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137.
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142-14Y,
145,
146,
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148,
149,
150-155,
156.
157.
158-159,
160.
161.
162,
163.
164,
165.

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BAR, Glgi
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Chicgl
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114-116

117-1

 
 

123

 
 
  
   
  
   
  
   
   
   
   
  
 

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106,
107.
108.
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110.
111.
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M. Weinberg
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K Zobel
JRNL - Y-12 Technical Library,

 

  
    
     
      
  
   
   
 
 
 
 

fDocument Reference Section

Laboratory Records Department
Laboratory Records, ORNL R.C.

. Central Research Library

ommand (RDGN)

pJect, Sandia
pject, Washington
orce.

Bureau offFfronautics General Reflesentative

hratories
 

w ~iv-

173. General Nuclear Endineering Corporation,
17h. Glenn L. Martin Confany /il
175. Hartford Area Qfficé 'fi¢
176-177. Headquarters, Air F- Fce Special Weavj s Center
178. Idaho Operations Off B,
179. Knolls Atomic Power ] boratory
180. Lockland Area Office ; i
181. Los Alamos Scientifil j Laboratory
182. Marquardt Aircraft C ~pany 4
183. National Advisery Coffiittee ford
184, Natiocnal Advisory Confllittee f'ol
185. Naval Air DevelopmentWenter
186. Naval Air Material Ceffker |
187. Naval Air Turbine TesfiStatd
188. Naval Research Laboratfry 2%
189. New York Operations Offlcdifg
190. Nuclear Development Corf§dition of America
191. Office cf Naval Researcillh
192. Office of the Chief of
193. Patent Branch, Washingj$t
194, Patterson-Moos 57? R
195-198. Pratt and Whitney Air .-ft Division
199, San Francisco Operatj Bhs oy 1ce
200. Sandia Corporation J L
201. School of Aviation Pédicine &
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206. U. S. Naval Radigllogical Defen

 
   
      
   
   
    
   
     
 
 
   
 
   
  
  
   
 
 
 

  

    
  

R

g Laboratory

ieronautics, Cleveland
Aeronautics, Washington

B
721 Operations (OP-361)

207. University of Cajifornia Radiati fon Laboratory, Livermore

208-225. Wright Air Deve)bpment Center (1 JOST - 3)
226-250. Technical Infcrfation Service EX
251. Division of Regearch and Develop

    
 

t, AEC, ORO

. &
;
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nsion, Oak Ridge

 
Observations are reported on the behavior of several fission
product elements in fused NaF-ZrFA- 4 fuels, irradiated in capsule
experiments, forced-convection in-pile loop experiments, and in the
Molten Fluoride Reactor Experiment (MFRE). .The rare pases have been ob-
served to escape readily from the fuels in dynamic tests, although
in static tests the rate of escape is very low. Ruthenium and
niobium depogit on the Inconel walls of the fuel container, probably
as metals. Other fission products studied (Sr, Zr, la, Ce) appear to
remain in the %uel. The unsatisfactory nature of Cs137 as a fission ..
monitor in such fuels is reported ang-the use of Zr95‘as a substitute is

discussed. . The hypothesis 'is propesed that fission product deposition

may serve to reduce corrosion of metals by molten fluoride fuels.

e
-D-

The chemical behavior of the fission product elements is of great
importance in any fluid-fueled nuclear reactor, as well as in the re-
processineg of nuclear fuels of any sort. Observations are reported here
on the tehavior of several important elements in fused fluoride fuels (1)
of the type employed in the Molten Fluoride Reactor Experiment (MFRE)(2). Most
of the fuels examined have been of the NaF-ZrFA—UF4 type, with various
compositions. The samples examined were taken from three different types
of experiment:

l. Static fluoride irradiations: Observations are reported on
samples of fuel from two in-pile static corrosion .tests (3). Two experiments
are also reported on the removal of %6125 from static fluorides.

2. Dynamic fluoride irradiations: Observations are reported on
fuel samples from two in-pile forced-convection loop tests and on metal
samples from one of these (4, 5).

3. The MFRE: Observations are reported on a fuel sample and on

a metal sample from the MFRE (6).

Behavior of the Rare Gases
The fission monitorine technique based on 09137, developed at the

Argonne National Laboratory (Z), was applied to two samples of NaF~ZrE4-
U23fiflflk(50n46n4 mole %, respectively) which had been irradiated in the
MTR fdr 116 hrs and 325 hrs, respectively, at about 800°C, at a thermal
neutron flux of (2.3640,16) x 1014 neutrons c@iRsec=d. The results are
shown in Table I. It will be observed that although ascreement between
the measured and calculated numbers of fissions occurring in the sample

is good in the shorter irradiations, in the loneger omne it is very poor.
-3

A portion of the capsule which was exposed to vapors from the molten salt
was dissolved in each case and a Cs137 determination was performed on the
resulting solution. The results (last column of Table I) show appreciéble
amounts of Cs237 to have been present on these surfaces. These results
are taken as evidence of the escape from the fuel of 3.9 minute Xel37,
the parent of the cesium isotope. |

An attempt was made to study directly the evolution of X3135 from
irradiated fluorides. Two runs were made under identical conditioms,
except that in one case the fuel was sparged by bubbling He throush it,
while in the other case, the carrier gas merely swept over the surface of
the melt. The helium, purified by passaee over hot copper turnines and
marnesium perchlorate, was conducted to and from the capsule through
0.036 in. 0.d. stainless steel capillary tubing. The off=-ocas was
passed throuch two Dry Ice acetone—cooled traps, the secéond filled with
activated charcoal to hold the xenon. A helium eras flow rate of 15 ml/min
was used in each experiment. The fuel sample in each case was 1 em of
NaF—KF-UF4 (4605-26,0=27.5 mole #,meltine point 530°C), containing
normal uranium. It was irradiated in the ORNL rraphite Reactor at 650

11 neutrons cm“z,Sec’l, for

to 750°C, at a thermal neutron flux of 7 x 10
3] minutes. After waiting 4.5 hours for short~lived activities to decay,
helium flow was started and continued for 6.5 hours. The capsules re-
mained in the reactor durineg this periodo The thermal neutron dose was

monitored with a ¢lip of Al-Mn~Co alloy, removed and counted immediately

after the irradiation was completed. The amount of Xe135 was determined

 
.

Table I

cs137 Analyses in MIR-Irradiated Static Fluorides

Flux 2(2.36%0,16) x 104 neutrons cm=2ses~l Temp.= 800°C

 

18 03137 recovered from
Time of cs137 (fissions/em x 10~18) Capsule tops 18
Irradiation (hrs) Observed (a) Calculated(b) (fission x 107
116 0,085 £ 0.005 0.11%0.01 0,001
325 0.091% 0,010 0,28+ 0,03 0,013

(a) Based on ANL calibration of Cs1?7 f£lux monitorine method (7).
(b) Based on flux determined by Co activation; corrected for flux

depression,

 
_5-
by transferring the contents of the charcoal trap to an appropriate vessel
and counting in a 4= | peometry hich-pressure ionization chamber. The
results are shown in Table ITin terms of the response of the instrument
used. No absolute calibration was made. It may be said, however, that
the amount of Xe13® recovered in the sparging expsriment was approximately
that expected from the fission history of the sample. It is clear from
the results of Table II that the rare cases do not aiffuse extremely
readily from static fused fluorides umder the conditions of these experiments.
Their removal is easily accomplished, however, by éfficient sparging of
the fuel with helium,

As one part of the operation of the MFRE (6), a so-called xenon
experiment was performed. The control rods were calibrated during the period
when the reactor was beine brought to criticality by measured additions of
NazUF6 to the originally uranium-free salt. In the "xenon experiment", the
rod positiqn was recorded as a function of time during a 20-hour run at a
nominal power of 1.5 meeawatts. The rod position data were converted to
A k/x values usine the previously estatlished calibration. When these
results were corrected for Smi4? polsonine and for the decrease in
reactivity due to <35 burnup; it was apparent that virtually all of the
%6135 had been removed from the fuel. Whils no certain quantitative
interpretation can be given of the poisoning remaining after correction
for Sm14Y and burn-up effects, it appeared that no more than about 2%
of the expected Xe135 remained in the reactor fuel during the period in

question.

During operatiqpfg{ the MFRE, an s¢cidential leak of gases
occurred from the reactor .imto the pit im whigh. it was installed (6).

This pas was dispersed by drawing it into an emercency off-gas line
 

Table II

Evolution of X9135 from Irradiated Static Fluorides

1

Flux =7 x 1011 neutrons em~?sec” Temp = 650 to 750°C

Thermal Neutron Dose(b) Amount of Xel135 (t) )
Observed ~ Calcuiated ‘2

o —————

 

 

 

Fuel sparged with He 0.117 1.44 oo eam

Fuel surface swept with He 0.097 0,032 1.22

(a) Based on results obtained in sparging experiment; corrected for
slioht difference in uranium content of the two capsulss.

" (b) Arbitrary units

 
-7
inserted into the pit. A sample of the off-cas from this line, adsorbed on
cooled charcoal, was examined by Bell, et al. (8), primarily by camma-ray
scintillation spectrometry. They established the presence of bS8 (daushter
of 2.8 hr. Krss), Xe135, and cs138 (daughter of 17 min.Xel38), but were
unable to identify many of the observed peaks in the samma-ray spectrum.

Determination of the amounts of cs?37 1n the fuel of the MFRE and of
one of the in-pile loops indicated the escape of less than about 20% of
the X9137 from these systems.

The data obtained on both static and dynamic systems demonstrates
that the rare eases are evolved readily from fused fluorides, although
in static systems, the rate of evolution is very low. The fraction of
any rare gas isotope which will be removed from a fluid fuel may be
estimated using a theory developed for Xel35 poisoning kineties (9).
This fraction depends on the geometry and flow conditions of the specific
reactor, as well as on the radioactive half-life of the nuclide in question.
Llonger-lived nuclides will be removed to a ecreater extent than shorter~
lived ones, very crudely in proportion to their half-lives. More detailed
discussion of the matter is deferred here, since it is treated in another
place (9).
Bohavior of Ruthenium and Niobium

Samples of fluoride fuel removed from two in=pile forced-convection
loops and a sample from the MBRE were examined for the presence of Ru103
by radiochemical techniques. The results are shown in Table IlI. The very
marked reduction below the expected levels of the R0 gontent of the fuel,

especially in the LITR loop and in the FSRE; indicated the existence of an
_8-

Table IIY

Rulo3 Analyses of Irradiated Fluoride Fuel from Dynamic Experiments

LITR MFRE MTR
loop —_— loop
Fuel Composition
(mole% NaF-ZrFA—U235F4) 62.5-12.5-25.0 53.5-40.0-6.5 53,5-40.0-6.5
Fissions/cm3 of fuella) x 10~16 12.9 8.7 . 655
Calculated Ru103 concn. in fusl
(atoms/ecm3 x 10-15) 3.9 2.5 - 190
Observed Rul®3 conen. in fuel
(atoms/em3 x 10-15) 0,001 0.00003 104,
Ratio, surface/volume (em~—1) 3.5 1 5
Averace Ru103 surface

concn. (atoms x 10“15) 1.1 205 17

(a) Estimated from hest generation for LITR loop and MFRE;
estimated from Zr?> analyses for MIR loop.
_9_
an efficlent means of ruthenium removal from molten fluorides. It was
possitle to obtain salt-free sections of Inconel pipe from the LITR loop
(4) and from the reactor. These sections were selected from parts of each
system which were not exposed to high thermal neutron fluxes, thus avoiding
activation of the cobalt content of the Inconel.

One pipe section was selected from a reeion of the LITR loop upstream
from the hish-flux region, another from a re-ion an equal distance down-
stream. famma-ray spectrometry of these samples showed the presence of Ru103
activity and of Zr?5-Np?5 activity. The latter activity occurred to the same
extent in each section, but the Ru103 éctivity in the downstream section was
40% greater than that in the upstream section. After a delay of 53 days,
the two sections were re-examined. The Ru103 in both samples decayed with
an apparent half-life of about 42 days, in eood acreement with published data
(1Q). The Zr?9-Nb?2 activity, however, decayed with an apparent half-life of
40 to 43 days. This indicates that the active deposit must have contained
~ 95% Nb?> (35 days) and only ~ 5% Zr95 (65 days) at the time of reactor
shutdown. The relative amount of Nb7° expected if no segrecation of the
element occurred is about 5% of the total activity.

The pipe section from the MFRE was a rineg cut out of the fuel inlst
line to the reactor core. Three samples cut from this ring showed the
presence of RulOB, Ru106, and Zr95—Nb95. Two of the samples were re-
examined after a delay of 130 days. The apparent half-life of the
Zr95-Nb95 activity was 50 days in each case, acain suggesting that the

deposit was very largely Nb959 An autoradioeraph of the third pipe

 
-10-
sample showed the radiocactive deposit to be well localized at the fuel-metal
interface, within the rather poor resolution obtainable with beta radiation.
A pipe eltow, which served as the inlet end of the MFRE emergency off-
eas line, was examined for radioactivity. A very small amount of Ru103
was detected, which was shown by chemical treatment to be entirely on the
outside of the pipe. It appears likely that a small amount of RuFs or of
RuOA (from reaction with air that may have been introduced into the reactor
when the leak occurred) volatilized from the fuel. In view of the larpe
amount of ruthenium found on fuel container surfaces, it is felt that
volatilization of this element is of very little importance in its removal
from the fuel. This view is supported ty experience to date with the
fluoride volatility process (1l) for recovering uranium from spent fuel.
It is evident from the results reported above that ruthenium is
rapidly and efficiantly removed from fused fluoride fuels bty Inconel
container surfaces. However, the data obtained for the MIR loop experiment :
indicate that saturation of the walls with ruthenium was approached in that
case. If this interpretation of the data is indeed correct, it seems
reasonable to suecest that deposition of fission product metals may well
interfere with the course of the ordinary corrosion process, (1, 12) and
that long-term in-pile corrosion of metals by fluoride fuels may be signi~
flecantly less than predicted from comparable out-of-pile tests. Short-term
in-pile corrosion tests to date are not in disacreement with this hypothesis (13).
Niobium appears to deposit on Inconel alone with ruthenium. It appears
likely that molybdenum also deposits, but there has not yet been an opportunity

to examine samples soon enough after irradiation to observe 67 hroMb999 the
-11-

loneest-lived radiocactive isotope of this element which is known in fission.

It is also possitle that zirconium may deposit from fuels not containing

ZrF
4’

Behevior of Other Fission Products

Samples of fuel drawn from the MFRE dump tank were examined by radio-

but no experiments have yet been comducted on such materials.

chemical methods. In order to eétimate the efficiency of retention of some
typical fission products, these analyses were compared with similar results
obtained on a sample of NaF—ZrF4=UF4 (50=46-4 mole 4) irradiated in the
solid state in the ORNL fraphite Reactor. The irradiation time was matched
approximately to the high ~power operating time of the MFRE. The comparative
analyses of the MFRE fuel and the standard are shown in Table IV. It is -
clear that, with the exception of RuloB, there 1s no gross loss of the
fission product nuclides listed. The ratio obtained for SJ:°89 could be
interpreted to show partial loss of its parent, 2.6 min. Kr89, but no
explanation can be offered for the value obtained for zr??, It is likely
that no loss occurred of any of these fission product elements from the

fuel of the MFRE, and that the variation from 0.3 to 1.6 is a raflection

of experimental errors, such as inhomogeneous samples, chemical difficulties
in the complex fluoride system, etc. A determination was also made of the
ratios of activities of 03136 and 09137_1n.the two samples. The result
indicated the loss of less than 20% of the Xel37 parent of the latter
nuclide.

Analysis of the oross camma~-ray spectrum of a 7 mg. sample of fuel
from the MFRE was continued throuch the period from 31 to 81 days after
shutdown of the reactor. The total activity of the sample was determined in
a high-pressure ionization chamber. These results were combined with gamma-

ray spectral data to yield both total phOton emission rates and differential

 
-12-
decay data. The only activities detected were Ba140-La140, 091419 and
Zr95~Nb95o Neither Ru103 nor 1131 were observed. The specific camma
activity of the sample was.estimated as 16 mc/em 31 days after reactor
shutdown and 3.5 mc/em 79 days after shutdown. The averace gamma-ray
energies were 0,96 and 0.73 Msv, respectively.

Bell and his coworkers (8) weré unable to establish the presence qf
iodine and bromine in the sample of MFRE off-gas which they examined.
Since 3 day 1431 could not be detected in the analysis of the eross
ramma~-ray spectrum of the MERE fuel, the question of the fate of the
halosen elements in molten fluoride fuels must be left open.
Use of Zr?? as a Figsion Monitor

Uncertainties as to the applicability of 05137 as a fission monitor
in fluid reactor fuels, led to the adoption of Zr?? as a substitute,; at
least for fuels containine macroscopic amounts of normal zirconium. In
order. to calibrate the use of this nuclide, two samples of enriched uranium
were irradiated as solutions for 2.375 days in the ORNL fraphite Reactor.
One solution was prepared from U3089 the other from a typical NaF=ZrF4wUFA
fuel, After 10 days decay, radiochemical determinations were made of~Gs137
and ZE95 o GComparison of these results, using the ANL 68137 standards,(7b),
rave a fission yield for Zr’° of 0.0664 t 0,0013 atom/fission. Usine the
inte~rated neutron dose, measured with a cobalt monitor, the yield is 0.0632
* 0.0021 atom/fission. The discrepancy between the two results is removed
when the cobalt activation flux is corrected for the cadmium ratio
prevailine in the irradiation facility (~ 30). The vyield recommended for

use in fission monitorine with Zr95 is 0.0664 =+ 0.0013 atoms/Pission.

 
-13-

Table IV

Fission Product Analyses in MFRE Fuel, Compared

to a Standard Sample

Nuclide
sr89
2535
Ry103
1240
Colél

Msan (Omitting Ru103)

Activity Ratio, MFRE/Standard
0.6

0.3
1.6 x 102
1.5
1.6

1.0 £0.5

-~
-1k

Conelugio

While most fission product elements remain in solution in molten fluoride
fuels, two important classes of elements have a sfirong tendency to escapes
the rare cases by volatilization and the noble metals, Ru and Nb, by
deposition on the walls of the container. Experiments are currently in
proeress, which will reveal the behavior of other elements, particularly the
helocens, Mo, and Zr, the latter in fuels not containing ZrF4 as a constituent,
Additional information is required concerning the extent to which metallic
deposition can occur on various metals and the desree of protection which
the deposited coatineg affords acasinst corrosion of the container ty the fuel.
It must be emplasized that in choosing suitable fission monitors for use in
fluid fuels, dlose attention must be given to the chemistry of the fission
product elements. While cs137 ig very satisfactory for this application in
so0lid fuel elements, the escape of its parent, X5137, makes it unreliable
in fluid fuels., Similarly, Zr?3 may be unsuitatle in fused fluorides

which do not contain ZrFA as & major constituent.

Acknoyledeements
The data reported here could not have been obtained without the

assistance of a oreat many members of the staff of the Oak Ridee National
Laboratory. C. C. Webster, W. 5. Piper, and R. H. Shields otteined many
of the samples. J. H. Oliver assisted in many of the experiments. E. I.
Wyatt's oroup in the Analytical Chemistry Division performed many of the
radiochemical ahalyses. We wish to thank these people for their assistance

in the work reported.
-15-

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