- Email: [email protected]
Fuel fabrication processes, design and experimental conditions for the joint US-Swiss mixed carbide test in FFTF (AC-3 test)
Journal of Nuclear Materials 204 (1993) 39-49 North-Holland
Fuel fabrication processes, design and experimental conditions for the joint US-Swiss mixed carbide test in FFTF (AC-3 test) R.W. Stratton, G. Ledergerber
and F. Ingold
Paul Scherrer Institute, Switzerland
T.W. Latimer and KM. Chidester Los Alamos National Laboratory, USA
The preparation of mixed carbide fuel for a joint (US-Swiss) irradiation test in the US Fast Flux Test Facility (FFTF) is described, together with the experiment design and the irradiation conditions. Two fabrication routes were compared. The US produced 66 fuel pins containing pellet fuel via the powder-pellet (dry) route, and the Swiss group produced 25 sphere pat pins of mixed carbide using the internal gelation (wet) route. Both sets of fuel met all the requirements of the specifications concerning stoichiometry, chemical composition and structure. The pin designs were as similar as possible. The test operated successfully in the FFFF for 620 effective full power days until October 1988 and reached over 8% burn up with peak powers of around 80 kW/m. The conclusions were that the choice of sphere pat or pellet fuel for reactor application is dependent on preferred differences in fabrication (e.g. economics and en~ronmental factors) and not on differences in irradiation behaviour.
1. Introduction The AC-3 test in the Fast Flux Test Facility (FFTF) in Richland WA, USA was the result of many years of collaboration between the US-DOE and its delegated organizations and the Paul Scherrer Institute (PSI) (formerly EIR) in Switzerland on the subject of advanced (mixed carbide) fast reactor fuel. In September 1986 a fuel bundle ~ntaining 91, wire wrapped, helium bonded uranium-2~% plutonium carbide fuel pins began irradiation in the FFTF and irradiated until 16 October 1988 (620 effective full power days) to a peak burn up of just over 8% fima. This test, with the designation AC-3, contained 25 sphere pat fuel pins fabricated by PSI using the gelation (wet) route and 66 pelleted pins fabricated by Los Alamos National Laboratory (LANL) using the classical powder-pellet (dry) route. The aim was to demonstrate mature fabrication processes and to compare the performance of the two products and geometries under realistic fast reactor conditions to moderate bum up. This was the third, and final mixed carbide test to be irradiated in the FFTF. Following irradiation, post irradiation examina-
tions (PIE) were carried out at the FFTF (bundle inspection and dismantling), Argonne National Laboratory-west in Idaho Falls (non-destructive exams and fission gas release) and ANL-East (destructive exams). The PIE and final analyses of results are the subject of a second paper also prepared jointly by the US and Swiss partners.
2. Background Mixed (U, P&carbide fuel was for many years an advanced and alternative fuel to the mixed oxide being developed for fast breeder reactor use and, until the mid 1980’s, was the subject of work in many countries until the slow-down in FBR development forced a rethinking of priorities. Today, only Japan, France, and the Russian Federation retain small carbide activities with India having still a larger mixed carbide fuel programme. I-Iowever, as the AC-3 test shows, mixed carbide remains a technically valuable alternative fuel for a future large scale programme of liquid metal cooled reactors, rivalling the metal fuel concept.
0022-311.5/93/$06.00 0 1993 - Elsevier Science Publishers B.V. All rights reserved
The US interest in carbides was well described by Matthews and Herbst [ 11in 1983 in which it was stated again that the high thermal conductivity and density of carbide fuels permit superior breeding performance and high specific power operation. These advantages combine to increase plutonium production, reduce fuel cycle and power costs. The demonstration of good performance of pellet carbide fuels up to more than 15% fima and the establishment of a near industrial scale fabrication facility for mixed carbide pellets and fuel pins at LANL underscored the importance then given to this advanced fuel. Today carbide and more importantly nitride fuels are of interest for offering greater flexibility in fuel design, good fuel/coolant compatibility and, due to their high atomic densities, as candidatc fuels for the transmutation of actinides in
hard spectrum fast fluxes (Pu and minor actinide burner). Carbide has recently become supplanted here by its close alternative - nitride (also produced by carbothermic reduction of the oxide with carbon) (Prunicr et al. [2]). Matthews and Herbst also pointed out that in the quest for a semi-automatic fabrication process, compatiblc with remote fabrication facilities then king proposed, the gelation route has many attractions. This interest still remains high due to the potentially high levels of radio-activity of the fuels to be handled in future advanced systems. The gelation route has the potential of integrating the final nitrate to oxide conversion step in reprocessing by using a co-conversion process with colloidal carbon to produce the sphcrc pat product for direct loading into fuel pins. This
Fig. 1. Diagram of the US process for preparation of mixed carbide pellets.
R. W Stratton et al. / Joint US-Swiss mixed carbide test in FFTF
process thus has the advantage of eliminating the powder handling problems, reducing the number of unit operations and simplifying automation. After several years of promising development of the internal gelation process for mixed carbides at PSI and small scale demonstration of good fuel performance compared to pellet fuel in various materials test reactors,‘Stratton [3], PSI was invited to join the US groups then active, in a demonstration of the performance of pellet and sphere pat fuel in a large scale test in the FFTF. The final test design as it evolved is described below. A major feature of the work for PSI was to bring its laboratory scale fabrication route up to the full quality requirements of the FFTF and demonstrate stable production and homogeneous fuel and pin quality over many fuel batches totalling some 20 kg of carbide. In order to ease the comparison of performance, the two fuel pin designs were nearly identical using the same (US supplied) materials and components except
where the sphere pat fuel requirements wise.
41
dictated other-
3. General experiment description The 91, 9.4 mm diameter, fuel pins, were 2.37 m long and designed to be loaded into a cold worked D9 hexagonal duct of standard FFTF driver dimensions with wire-wrapped spacing. D9 is a titanium-stabilised, low-swelling stainless steel developed by the Westinghouse Hanford Company for the US-DOE. The cold worked D9 pin cladding had a wall thickness of 0.51 mm. The fuel column length was a nominal 914 mm and fuel smeared densities ranged from 78 to 80% theoretical for the sphere pat and from 75 to 79% for the pellet pins. Fuel enrichments were nominally 20% Pu,,, in U + Pu. Also for experimental comparison, the stoichiometry of the two fuels was held to similar limits - 10 + 5% (U, Pu),C, with oxygen levels below 2000
Fig. 2. Ceramograph of a typical mixed carbide AC-3 pellet.
R. W Stratton et al. / Joint US-Swiss mixed carbide test in FFTF
42
ppm. Pellet and sphere pat pins were filled with high purity helium at near ambient pressure (0.75 and 1.2 bar respectively). A tag-gas-capsule was loaded into
HN03 * Hz0 -
NH40H
each pin and after welding, punctured to release the specific krypton and xenon isotopic mixture to indicate, by the FFTF coolant sensing system, any premature
PLUTONIUM NITRATE, CONCENTRATION AND ADJUSTMENT
URANYL NITRATE, ADJUSTMENT
Pu-cont. = 1.3 molkg NOB/metal = 4.1 mol/mol
U-cont. = 1.8 molkg NOs/metal = 1.7 mol/mol
METAL SOLUTION, ADJUSTMENT
C/HMTA/UREA, MAKEUP
-
Hz0
HzQ (CHzkN4, H2NCONH2, C
HMTA-cont. = 35 %wt. UREA-cont. = 15 %wt. CARBON = 5.9 %wt.
Me-cone. = 1.5 molkg Pu/(U+Pu) = 20.7 %at.
Hz0
HN03 or UOs
FEED-SOLUTION,
(U+Pu)-cow = 0.75 molkg HMTA/(U+Pu) = 1.65 mol/mol UREA/(U+Pu) = 1.65mol/mol
SOLUTION ADJUSTMENT L___________________________________________________
AI------
r___-_______________________________________________
_______________-_______---_-
______--_____----____----____----~
f
i CONVERSION: : NITRATE + OXIDE
DROP FORMATION
I
LF: 7.4 drops/s FF: 6’000 drops/s
f GELATION temperature 114- 120°C
aqueous HMTA, NHz,NOa, NHz,OH, UREA, HCHO
WASHING of GEL
; THERMAL TREATMENT r
I
I
. DRYING Rotary Dryer, Nitrogen, 120°C, 15 h
SINTERING
CALCINING _)
Static bed, Argon/20%Hz, 7OO”C, 6 hours
_)
WC-Crucible, Argon a) 192OoC, 8 h b) 145OoC, 2 h
(UF’u)C-SPHERES (FF= 70pm, LF= SOO@;
off gas (HzO, CO, COz, AK)
m,
Fig. 3. Flow sheet of the PSI gelation
process
for mixed carbide
microspheres.
i
j
R. W. Stratton et al. / Joint US-Swiss mixed carbide test in FFTF
failure. No failures occurred. The nominal target operating conditions were a peak nominal power of 92 kW/m with a clad temperature of 650°C and operation to a bum up of 9 at%. This was not quite reached, due to last minute changes in the FFTF loading pattern, see below.
4. Fuel fabrication 4.1. Pelletfuel fabrication
followed by pelletisation, sintering, grinding and inspection of the finished pellets. In spite of many refinements of the conversion step from oxide to carbide developed over the years, a simple mechanical mixture of oxide powder with graphite, blended by ball milling was found to produce satisfactory results. Consolidation of the powders by briquetting before the carbothermic reduction stage promotes the formation of a homogeneous product. An equation for the conversion is: (1 - a)UO,+,
Pellet fuel fabrication was carried out in the then recently completed LANL advanced fuel fabrication line consisting of a complex of stainless steel glove boxes filled with high purity argon (> 10 ppm oxygen and moisture). It was used to fabricate fuel, at rates up to 1000 pellets per day, for testing in EBR-II and FFTF. Controlled variations of densities and microstructures could be achieved by controlling carbide powder characteristics, additives and sinter temperatures. A simplified flow sheet is shown in fig. 1. The first step was to prepare the carbide material, then powder treatment for the correct micros~cture
canversfon
m
+ aPuO,_,
+[3+X(l-a)+Yu++m]C + (1 - m)U,_,Pu,C
+ +r(U,_,Pu,),C,
+ [2 +X(1 - u) - Ya]CO For AC-3 the carbothermic reduction was carried out at 1600°C under vacuum. The resulting briquettes were friable and were reduced to a fine powder by crushing and vibratory ball milling. This powder was then pressed into pellets and sintered in flowing argon at 1550°C for two hours. Centerless grinding was used to adjust the pellet diameter. The semi-automatic process has been shown to be capable of producing high qual-
WET ROUTE
DRY ROUTE
0
43
Mcchanfcnl hnadltng
Heat treatment
Fig. 4. Comparison of the dry and wet processes for mixed carbide fabrication.
44
R, W. Stratton et ai. /Joint
\
\
US-Swi.ys mired cmhide test ie FFTF
f
\
0.4
\
\
toSESOUICARBIDE\\ 10 MZ C,) PERCEM \
IN FUEL
\ 20
\
\ \,
0. AC-3
FINE 0 AC -3 LARGE
\
\
\
\ I ‘. LEGEND
\ \
j \ 40
\ 30
\
Y
\
\
\ 0.2 AC-3\
‘\ \
\.
\
\ \
\
\
\
\ 5.b
4.bo CARBON
CONTENT
[weight percent]
Fig. 5. Diagram of the oxygen and carbon levels in the AC-3 sphere pat fuel charges.
ity pellets meeting the stringent quality assurance requirements for FFTF fuels. Five batches of pellets made up the fuel lot for the 66 AC-3 pins. Fig. 2 shows the structure of a typical fuel pellet. 4.2. Sphere pat fuel fabrication The mixed carbide fabrication process used by PSI was completely different to that of LANL. Several methods of gelation were developed by different groups in earlier years including the water extraction method, the external gelation method and the internal gelation method. PSI choose the latter process and developed it for plutonium fuels (oxide, carbide, nitride) (Ledergerber et al. [4]). A flow sheet is given in fig. 3. After preparing acid free, concentrated feed solutions of uranium and plutonium nitrates, these were mixed together with an aqueous solution of hexamethyline-tetramine (HMTA), urea and dispersed carbon black. The mixture was cooled and dropped into a column of hot silicon oil or injected into a stream of hot oil. The temperature rise in the droplets triggers off the decomposition of the HMTA to form ammonia which precipitates ammonia-d~uranate or the corresponding plutonate within the droplets, forming spheres which can be readily handled. Washing steps removed the silicon oil, solvents and reaction products and, after drying at 110°C in air, a calcining step in argon/ hydrogen drove off the volatiles and adjusted the uranium/plutonium to a known oxidation state. For the
high density carbide microspheres a combined reaction-sintering step was done at 1950°C in flowing argon for 8 h, producing cIean, dust-free spherical particles with densities of > 95% TD which could then be loaded directly into fuel pins. Two size fractions of 0.63-0.90 mm and the fine fraction with 0.045-0.106 mm dia were used to reach the required AC-3 smeared densities of approximately 80%TD. Thus the process flow sheet as used at PSI differs from the classical approach used in the US and elsewhere in that there is a reduction in process steps (from around 14 to 9 according to fig. 41, blending of the uranium and plutonium streams occurs in the liquid phase producing a very homogeneous solid solution of U in Pu, the dust producing mechanical milling, crushing, blending and grinding steps are eliminated and the two thermal treatment stages are combined into one reaction sintering step. There is no need to grind pellets to size and inspect, although the sphere pat fuel is normally sieved to remove wrongfy sized particles. These are the points which make it attractive for a remote facility to handle more active “recycled” fuels in a fast reactor fuel cycle. A summary comparison of the two flow sheets is shown in fig. 4. All the PSI processes were qualified to the stringent US specifications. Mixtures or “charges” of sinter batches of each fraction were made, enough for the filling of groups of five pins. Carbon, oxygen and plutonium analyses were performed and a ceramographic specimen prepared to qualify each batch be-
R. W. Stratton et al. / Joint US-Swiss mixed carbide test in FFTF
K-405
Fuel used for Pins:
3L117,3L118,3Ll 3L1153L116
Fuel charge sphere size LLm density g/cm3 av. amount loaded g
K-405 805
l!
K-404 45-90 13.22
12.97 407
136
fissile composition 235u 239m, 24opu
24’Pu
%at. %at. %at. %at.
high carbon
52% of spheres in the sample
0.723 88.22 10.90 0.680
chemical composition uranium %wt. plutonium %wt. thorium1 ) %wt. carbon %wt. oxygen %wt. sulphcrr) %wt. tungsten ppm silicon ppm chromium ppm iron ppm nickel PPm phase composition secondary phase ~01%
71.6 19.29
72.6 19.00 0.16
5.02 0.05 < 40 430 16 65 3.4
4.86 0.15 0.16 680 22 9 25 1.4
5.1 U&2)
K-405 medium carbon
9% of spheres in the sample
3.4 UC221
‘1 Nominal values taken from typical UC analysis. 2, nascent dicarbide nearly amorphous or very small crystallite. size
-J9OX
K-405 low :arbon
39% of rphcres in he sample
C-404
N 350x
Iischunc3.te.x WJ43l29.6.92
Fig. 6. Ceramographs of PSI fuel with key characteristics.
45
46
R. W Stratton et al. / Joint US-Swiss mixed carbide test in FFTF
fore mixing the charges containing some 2 kg of large or 750 g of fines. The charges were fully characterised at PSI with the exception of the trace elements and nitrogen analyses. which were performed at the European Institute for Transuranium Elements (EITE). Good reproducibility of the carbon and oxygen contents, the two most important characteristics of a carbide fuel, was achieved over the whole campaign as shown in fig. 5. An intercomparison of carbon, oxygen and uranium contents on three samples, between PSI, LANL and EITF! confirmed the accuracy of the analytical procedures and no differences could be identified between the laboratories. The maximum uncertainty in the equivalent carbon content, calculated from the error of carbon and oxygen determinations is of the order of 2%. The as-manufactured microspheres seemed to be multiphased in composition. The matrix phase was monocarbide. The identity of the second phase remained uncertain. Neutron diffractrometry at PSI suggested that it is a nascent dicarbide - nearly amorphous or of very small crystallite size, Ledergerber et al. [5]. It was readily transformed into sesquicarbide by an extra annealing thermal treatment at 1550°C for as little as four hours. The reasons for the differences identified were probably in the manufacturing procedures. The pellet manufacturing process included much mechanical working, i.e. crushing and milling operations. The thermal history of pellets also involved a longer heat treatment in the temperature range where mono- and sesquicarbide are known to be in equilibrium. The
difference observed in the phase compositions was due to the rate and degree of completion of the reaction forming sesquicarbide from a highly point defected mono-carbide or mono- and di-carbide mixtures. This rate is greater in the case of the highly strained powders and the sesquicarbide forming reaction was more nearly complete due to the thermal treatment used in pellet manufacture. Typical ceramographic sections of sphere pat coarse and fine fractions are shown in fig. 6.
5. Pin and assembly fabrication
5.1. Pellet pins Fabrication of the pellet pins followed the usual methods. The fuel pellets had a nominal length of 8.8 mm and each pin therefore contained about 104 pellets. The sequence for loading was, lower reflector (inconel), 2 bottom insulator pellets, fuel stack, two upper insulator pellets, upper reflector, plenum spring, plenum spacer tube and tag gas capsule. The final closure weld was made on the top end cap using tungsten inert gas (TIG) welding in a helium filled glove box. Precautions were taken to minimise contamination of the weld area when loading pellets but in spite of this some end cap replacement and rewelding was necessary, The pins were visually and dimensionally inspected and X-radiography, dye penetrant and He leak testing were applied.
PELLET PIN (USA)
SPHERE-PAC PIN (SWITZERLAND) Fig. 7. Comparison of the fuel pin designs used in the AC-3 test.
41
R. W. Strattonet al. /Joint US-Swks mixed carbide testin FFTF 5.2. Sphere-pat pins
the docking port was successful in preventing contamination of the weld region. The coarse fraction fuel was loaded first and vibro-compacted to a maximum stable packing density. Then the fines were infiltrated into the mechanically locked matrix of coarse particles and vibro-compacted until their maximum packing density was obtained. Following filling and temporary closure each pin was transferred to the horizontal weld box where it was evacuated and back filled with helium and the lower end cap welded by TIG under a helium/ argon mixture. Axial gamma scanning confirmed the excellent regular flat density distribution of the fuel over its whole length in all cases. Visual inspection, X-radiography, He-leak testing and dimensional controls confirmed fabrication to the specified standards.
Small changes had to be made to the sphere pat pin design, partly to allow for the characteristics of the particle fuel and also to take account of the special fabrication methods used at PSI. These were: (see also fig. 7) Modification of the tag gas capsule penetrator to prevent inadvertent penetration, and addition of a support for the TGC, during vibrofilling. Replacement of the spring by a compression tube assembly and relocation above the plenum spacer. Addition of molybdenum discs at each end of the fuel column to retain the fine microspheres. Modifications to the lower reflector to include a split mandrel/collet arrangement to retain the fuel column until a distance piece could be added and the lower end cap welded. All these components and fabrication procedures were developed and validated in extensive testing. The main difference was the filling of the pin from the bottom end and the final weld being made on the lower end cap. Filling took place with the pin docked vertically below the filling box and care in the design of
5.3. Assembly fabrication 25 sphere pat pins plus two archive pins were shipped to LANL where after further inspection the D9 wire wrapping was carried out with a 200 mm pitch. Ninety one pins were then shipped to Westinghouse Hanford at Richland for the assembly into the 20%
Table 1 Initial density and chemical composition of pins nondestructivly examined after irradiation Pin identification
Fuel density (g/cm31 Smear Carbon a Oxygen a density content impurity pellets spheres (%t.d.) (wt.%) (Ppm)
Nitrogen a Secondary impurity carbide (vol%) (ppm)
Fuel batch
3940A6 4718D18 4574E23 4574E23 4723H37 4723G32 4723632
Comments
Pelletpin.5 3M07 3M31 3Lo43 3M44 3M74 3M78 3L.079
Batch 1
Batch 2
10.55 10.95 10.86 10.86 11.00 10.90 10.90
10.68 _ _ 10.89 _ 11.00
coarse
fine
13.00 13.00 12.98 12.97 12.97 12.97 13.05 12.99
13.20 13.20 13.28 13.22 13.22 13.22 13.34 13.27
75.41 78.09 77.29 77.29 78.67 78.74 77.73
5.05 5.00 4.94 4.94 4.99 4.94 4.94
541 687 537 537 520 608 608
45 80 65 65 92 75 75
14.6 M,C, 12.5 M,C, 9.2 M,C, 9.2 M&s 12 MzC, 9.5 M,C, 9.5 M,C,
79.7 79.48 79.41 79.63 79.63 79.78 80.07 78.96
4.92 4.94 4.94 4.98 4.98 4.98 4.95 4.85
1090 949 940 760 760 760 990 1190
80 62 61 98 98 98 79 86
3.9 UC2 4.1 UC, 4.1 UC, 4.6 UC2 4.6 UC, 4.6 UC, 4.4 UC, 2.8 UC1
destructive exam.
Sphere pat pins
3L102 3L105 3L106 3L11.5 3L117 3L119 3L121 3L126
a For sphere pat pins: weighted mean according ratio fine/coarse spheres. b Nascent dicarbide - nearly amorphous or of very small crystallite size.
b b b b b b b b
K-402/K-403 K-401/K-400 K-401/K-400 K-405/K-404 K-405/K-404 K-405/K-404 K-406/K-407 K-408/K-409
destructive exam. destructive exam.
destructive exam.
48
R. W. Stratton
et al. /Joint
US-Swiss
mixed carbide test in FFTF
To Core
Centerline iPlN 3L 115 (sphere-pat]
PIN 3LO74
551 ’ 0
I 100
I
’
I
I 300
200 [Effective
’
1
Full Power
’
I 500
600
Days]
PIN 3L 074, (pellet)
-4--__
f5
’
400
(pellet)
/
//
520
0
E d
510
I
s
i-i_-__-
I 500’
I 0
Fig. 8. Diagram of the pin loading plan for the AC-3 fuel bundle. The pins selected for examination are underlined.
cold worked D9 hexagonal duct. These FFTF assemblies are 3.6 m long and are fitted with an orifice plate to regulate the sodium coolant flow to give the design clad operating temperatures with a bulk coolant inlet temperature of 360°C. The loading plan for the assembly is shown in fig. 8.
Table 2 Reactor and peak pin parameters, Cycle
9A 9B 9c 10A 10B
EFPD per cycle
137.7 106.4 97.7 151.8 126.7
a For peak pin 3M24.
I 100
1 300
200 [Effective
’
’ 400
Full Power
’
I 500
Days]
Fig. 9. Calculated pin peak power (a) and coolant outlet temperature (b) histories for the AC-3 test in FFTF for the pins 3L 074 (pellet) and 3L 115 (sphere pat), based on measured in-reactor data, corrected for fuel density.
Table 1 shows the principle characteristics of the fuel in those pins selected later for post irradiation examination.
per cycle Cumulative EFPD
137.7 244.1 341.8 493.6 620.3
1 600
Fluence
Burn up (at%) a Average
Peak
Total average
1.65 2.85 3.92 5.46 6.67
2.08 3.60 4.95 6.91 8.28
4.35 7.53 10.60 14.98 18.56
(X lo** n/cm’)
’
Fast (E > 0.1 MeV) Peak
Average
3.48 6.03 8.48 12.05 14.73
2.74 4.74 6.68 9.44 11.63
R. W. Stratton et al. / Joint US-Swiss mixed carbide test in FFTF
6. Summary of irradiation
conditions
The AC-3 assembly was loaded into FFIYF at the beginning of cycle 9 in August 1986 in a row 4 position. The choice of position was due to restrictions in the overall loading plan affected by the higher priority “Core Demonstration Experiment” assemblies and gave linear powers and burn ups somewhat lower than targeted. Maximum peak linear power (BOL) was estimated to be 84 kW/m (pin 3L 101) Peak cladding temperatures were calculated to be around 550°C at BOL. A summary of each cycle’s operating conditions is given in table 2. Fig. 9 shows the calculated power and coolant temperature history for two selected pins. The experiment operated without incident until it was unloaded in October 1988. After cooling at the reactor the assembly was transferred to the FFI’F Intermediate Examination and Maintenance (IEM) cell for inspection and dismantling. The assembly and individual fuel pins were shown to be in excellent condition with duct distortion within the limits for D9 material under these conditions and with all pins showing only moderate levels of growth and little bow or swelling as shown by the ease with which the duct could be removed. Details of the complete PIE results are given in another paper by Hoth et al. 161.
49
on carbides or nitrides should seek to explore these small differences at higher burn ups and ratings where the potential or the dif~culties are perhaps more evident. On the other hand comparisons could be made at the lower fuel smeared densities which the sphere pat route also offers (single fraction fuel), to take full advantage of a still simpler fabrication alternative. This experiment has shown that the choice of sphere-pat or pellet fuel for reactor application is dependant on preferred differences in fabrication (e.g economics and environmental factors) and not (at these burn ups and ratings) on differences in irradiation behaviour.
It is a particular wish of the authors to give thanks and recognition to the large number of people, too numerous to name, on both sides of the Atlantic, who have taken part in, supported and guided this programme over more than fifteen years of effort. The success of this collaboration is entirely due to their skiils and a determination to see the experiment through to its end. Working with you all was a pleasure.
7. Conclusions References
Fabrication of mixed carbide fuel by the dry, powder, pellet route and by the wet gelation, sphere pat route demonstrated that both are capable of giving a stable homogeneous product meeting all requirements of stoichiometry, density and chemical composition. The successful irradiation of full size fuel pins of both types in the prototypical FFTF conditions of bum up and fluence showed trouble free behaviour up to at least 8% fima with a large potential for higher burn ups and ratings. As shown in the later paper, only minor differences in behaviour were seen between the two fuels which can be attributed mainly to small differences in smeared density. Future work, whether
[I] R.B. Matthews and R.J. Herbst, Nucl. Tech. 63 (1983) 9. [2] C. Prunier, D. Warin, R.W. Stratton and G. Lederberber, presented at Int. Syrnp. on Fuels for Liquid Metal Reactors, Chicago, IL, 1992. [3] R.W. Stratton, Trans. Am. Nucl. Sot. 39 (1981) 421. [4] G. Ledergerber, H.P. Alder, F. Ingold and R.W. Stratton. ENC-86, Geneva (1986). [S] G. Ledergerber, R.J. Herbst, H.U. Zwicky, H. Kutter and P. Fischer, J. Nucl. Mater. 153 (3988) 189. 161 C.W. Hoth, R.E. Mason, B.J. Makenas, R. Wisner, F. Botta, R.W. Stratton, G. Ledergerber, F. Ingold, H.P. Alder, presented at Symp. on Fuels for Liquid Metal Reactors, Chicago, IL, 1992.
Fuel fabrication processes, design and experimental conditions for the joint US-Swiss mixed carbide test in FFTF (AC-3 test) R.W. Stratton, G. Ledergerber
and F. Ingold
Paul Scherrer Institute, Switzerland
T.W. Latimer and KM. Chidester Los Alamos National Laboratory, USA
The preparation of mixed carbide fuel for a joint (US-Swiss) irradiation test in the US Fast Flux Test Facility (FFTF) is described, together with the experiment design and the irradiation conditions. Two fabrication routes were compared. The US produced 66 fuel pins containing pellet fuel via the powder-pellet (dry) route, and the Swiss group produced 25 sphere pat pins of mixed carbide using the internal gelation (wet) route. Both sets of fuel met all the requirements of the specifications concerning stoichiometry, chemical composition and structure. The pin designs were as similar as possible. The test operated successfully in the FFFF for 620 effective full power days until October 1988 and reached over 8% burn up with peak powers of around 80 kW/m. The conclusions were that the choice of sphere pat or pellet fuel for reactor application is dependent on preferred differences in fabrication (e.g. economics and en~ronmental factors) and not on differences in irradiation behaviour.
1. Introduction The AC-3 test in the Fast Flux Test Facility (FFTF) in Richland WA, USA was the result of many years of collaboration between the US-DOE and its delegated organizations and the Paul Scherrer Institute (PSI) (formerly EIR) in Switzerland on the subject of advanced (mixed carbide) fast reactor fuel. In September 1986 a fuel bundle ~ntaining 91, wire wrapped, helium bonded uranium-2~% plutonium carbide fuel pins began irradiation in the FFTF and irradiated until 16 October 1988 (620 effective full power days) to a peak burn up of just over 8% fima. This test, with the designation AC-3, contained 25 sphere pat fuel pins fabricated by PSI using the gelation (wet) route and 66 pelleted pins fabricated by Los Alamos National Laboratory (LANL) using the classical powder-pellet (dry) route. The aim was to demonstrate mature fabrication processes and to compare the performance of the two products and geometries under realistic fast reactor conditions to moderate bum up. This was the third, and final mixed carbide test to be irradiated in the FFTF. Following irradiation, post irradiation examina-
tions (PIE) were carried out at the FFTF (bundle inspection and dismantling), Argonne National Laboratory-west in Idaho Falls (non-destructive exams and fission gas release) and ANL-East (destructive exams). The PIE and final analyses of results are the subject of a second paper also prepared jointly by the US and Swiss partners.
2. Background Mixed (U, P&carbide fuel was for many years an advanced and alternative fuel to the mixed oxide being developed for fast breeder reactor use and, until the mid 1980’s, was the subject of work in many countries until the slow-down in FBR development forced a rethinking of priorities. Today, only Japan, France, and the Russian Federation retain small carbide activities with India having still a larger mixed carbide fuel programme. I-Iowever, as the AC-3 test shows, mixed carbide remains a technically valuable alternative fuel for a future large scale programme of liquid metal cooled reactors, rivalling the metal fuel concept.
0022-311.5/93/$06.00 0 1993 - Elsevier Science Publishers B.V. All rights reserved
The US interest in carbides was well described by Matthews and Herbst [ 11in 1983 in which it was stated again that the high thermal conductivity and density of carbide fuels permit superior breeding performance and high specific power operation. These advantages combine to increase plutonium production, reduce fuel cycle and power costs. The demonstration of good performance of pellet carbide fuels up to more than 15% fima and the establishment of a near industrial scale fabrication facility for mixed carbide pellets and fuel pins at LANL underscored the importance then given to this advanced fuel. Today carbide and more importantly nitride fuels are of interest for offering greater flexibility in fuel design, good fuel/coolant compatibility and, due to their high atomic densities, as candidatc fuels for the transmutation of actinides in
hard spectrum fast fluxes (Pu and minor actinide burner). Carbide has recently become supplanted here by its close alternative - nitride (also produced by carbothermic reduction of the oxide with carbon) (Prunicr et al. [2]). Matthews and Herbst also pointed out that in the quest for a semi-automatic fabrication process, compatiblc with remote fabrication facilities then king proposed, the gelation route has many attractions. This interest still remains high due to the potentially high levels of radio-activity of the fuels to be handled in future advanced systems. The gelation route has the potential of integrating the final nitrate to oxide conversion step in reprocessing by using a co-conversion process with colloidal carbon to produce the sphcrc pat product for direct loading into fuel pins. This
Fig. 1. Diagram of the US process for preparation of mixed carbide pellets.
R. W Stratton et al. / Joint US-Swiss mixed carbide test in FFTF
process thus has the advantage of eliminating the powder handling problems, reducing the number of unit operations and simplifying automation. After several years of promising development of the internal gelation process for mixed carbides at PSI and small scale demonstration of good fuel performance compared to pellet fuel in various materials test reactors,‘Stratton [3], PSI was invited to join the US groups then active, in a demonstration of the performance of pellet and sphere pat fuel in a large scale test in the FFTF. The final test design as it evolved is described below. A major feature of the work for PSI was to bring its laboratory scale fabrication route up to the full quality requirements of the FFTF and demonstrate stable production and homogeneous fuel and pin quality over many fuel batches totalling some 20 kg of carbide. In order to ease the comparison of performance, the two fuel pin designs were nearly identical using the same (US supplied) materials and components except
where the sphere pat fuel requirements wise.
41
dictated other-
3. General experiment description The 91, 9.4 mm diameter, fuel pins, were 2.37 m long and designed to be loaded into a cold worked D9 hexagonal duct of standard FFTF driver dimensions with wire-wrapped spacing. D9 is a titanium-stabilised, low-swelling stainless steel developed by the Westinghouse Hanford Company for the US-DOE. The cold worked D9 pin cladding had a wall thickness of 0.51 mm. The fuel column length was a nominal 914 mm and fuel smeared densities ranged from 78 to 80% theoretical for the sphere pat and from 75 to 79% for the pellet pins. Fuel enrichments were nominally 20% Pu,,, in U + Pu. Also for experimental comparison, the stoichiometry of the two fuels was held to similar limits - 10 + 5% (U, Pu),C, with oxygen levels below 2000
Fig. 2. Ceramograph of a typical mixed carbide AC-3 pellet.
R. W Stratton et al. / Joint US-Swiss mixed carbide test in FFTF
42
ppm. Pellet and sphere pat pins were filled with high purity helium at near ambient pressure (0.75 and 1.2 bar respectively). A tag-gas-capsule was loaded into
HN03 * Hz0 -
NH40H
each pin and after welding, punctured to release the specific krypton and xenon isotopic mixture to indicate, by the FFTF coolant sensing system, any premature
PLUTONIUM NITRATE, CONCENTRATION AND ADJUSTMENT
URANYL NITRATE, ADJUSTMENT
Pu-cont. = 1.3 molkg NOB/metal = 4.1 mol/mol
U-cont. = 1.8 molkg NOs/metal = 1.7 mol/mol
METAL SOLUTION, ADJUSTMENT
C/HMTA/UREA, MAKEUP
-
Hz0
HzQ (CHzkN4, H2NCONH2, C
HMTA-cont. = 35 %wt. UREA-cont. = 15 %wt. CARBON = 5.9 %wt.
Me-cone. = 1.5 molkg Pu/(U+Pu) = 20.7 %at.
Hz0
HN03 or UOs
FEED-SOLUTION,
(U+Pu)-cow = 0.75 molkg HMTA/(U+Pu) = 1.65 mol/mol UREA/(U+Pu) = 1.65mol/mol
SOLUTION ADJUSTMENT L___________________________________________________
AI------
r___-_______________________________________________
_______________-_______---_-
______--_____----____----____----~
f
i CONVERSION: : NITRATE + OXIDE
DROP FORMATION
I
LF: 7.4 drops/s FF: 6’000 drops/s
f GELATION temperature 114- 120°C
aqueous HMTA, NHz,NOa, NHz,OH, UREA, HCHO
WASHING of GEL
; THERMAL TREATMENT r
I
I
. DRYING Rotary Dryer, Nitrogen, 120°C, 15 h
SINTERING
CALCINING _)
Static bed, Argon/20%Hz, 7OO”C, 6 hours
_)
WC-Crucible, Argon a) 192OoC, 8 h b) 145OoC, 2 h
(UF’u)C-SPHERES (FF= 70pm, LF= SOO@;
off gas (HzO, CO, COz, AK)
m,
Fig. 3. Flow sheet of the PSI gelation
process
for mixed carbide
microspheres.
i
j
R. W. Stratton et al. / Joint US-Swiss mixed carbide test in FFTF
failure. No failures occurred. The nominal target operating conditions were a peak nominal power of 92 kW/m with a clad temperature of 650°C and operation to a bum up of 9 at%. This was not quite reached, due to last minute changes in the FFTF loading pattern, see below.
4. Fuel fabrication 4.1. Pelletfuel fabrication
followed by pelletisation, sintering, grinding and inspection of the finished pellets. In spite of many refinements of the conversion step from oxide to carbide developed over the years, a simple mechanical mixture of oxide powder with graphite, blended by ball milling was found to produce satisfactory results. Consolidation of the powders by briquetting before the carbothermic reduction stage promotes the formation of a homogeneous product. An equation for the conversion is: (1 - a)UO,+,
Pellet fuel fabrication was carried out in the then recently completed LANL advanced fuel fabrication line consisting of a complex of stainless steel glove boxes filled with high purity argon (> 10 ppm oxygen and moisture). It was used to fabricate fuel, at rates up to 1000 pellets per day, for testing in EBR-II and FFTF. Controlled variations of densities and microstructures could be achieved by controlling carbide powder characteristics, additives and sinter temperatures. A simplified flow sheet is shown in fig. 1. The first step was to prepare the carbide material, then powder treatment for the correct micros~cture
canversfon
m
+ aPuO,_,
+[3+X(l-a)+Yu++m]C + (1 - m)U,_,Pu,C
+ +r(U,_,Pu,),C,
+ [2 +X(1 - u) - Ya]CO For AC-3 the carbothermic reduction was carried out at 1600°C under vacuum. The resulting briquettes were friable and were reduced to a fine powder by crushing and vibratory ball milling. This powder was then pressed into pellets and sintered in flowing argon at 1550°C for two hours. Centerless grinding was used to adjust the pellet diameter. The semi-automatic process has been shown to be capable of producing high qual-
WET ROUTE
DRY ROUTE
0
43
Mcchanfcnl hnadltng
Heat treatment
Fig. 4. Comparison of the dry and wet processes for mixed carbide fabrication.
44
R, W. Stratton et ai. /Joint
\
\
US-Swi.ys mired cmhide test ie FFTF
f
\
0.4
\
\
toSESOUICARBIDE\\ 10 MZ C,) PERCEM \
IN FUEL
\ 20
\
\ \,
0. AC-3
FINE 0 AC -3 LARGE
\
\
\
\ I ‘. LEGEND
\ \
j \ 40
\ 30
\
Y
\
\
\ 0.2 AC-3\
‘\ \
\.
\
\ \
\
\
\
\ 5.b
4.bo CARBON
CONTENT
[weight percent]
Fig. 5. Diagram of the oxygen and carbon levels in the AC-3 sphere pat fuel charges.
ity pellets meeting the stringent quality assurance requirements for FFTF fuels. Five batches of pellets made up the fuel lot for the 66 AC-3 pins. Fig. 2 shows the structure of a typical fuel pellet. 4.2. Sphere pat fuel fabrication The mixed carbide fabrication process used by PSI was completely different to that of LANL. Several methods of gelation were developed by different groups in earlier years including the water extraction method, the external gelation method and the internal gelation method. PSI choose the latter process and developed it for plutonium fuels (oxide, carbide, nitride) (Ledergerber et al. [4]). A flow sheet is given in fig. 3. After preparing acid free, concentrated feed solutions of uranium and plutonium nitrates, these were mixed together with an aqueous solution of hexamethyline-tetramine (HMTA), urea and dispersed carbon black. The mixture was cooled and dropped into a column of hot silicon oil or injected into a stream of hot oil. The temperature rise in the droplets triggers off the decomposition of the HMTA to form ammonia which precipitates ammonia-d~uranate or the corresponding plutonate within the droplets, forming spheres which can be readily handled. Washing steps removed the silicon oil, solvents and reaction products and, after drying at 110°C in air, a calcining step in argon/ hydrogen drove off the volatiles and adjusted the uranium/plutonium to a known oxidation state. For the
high density carbide microspheres a combined reaction-sintering step was done at 1950°C in flowing argon for 8 h, producing cIean, dust-free spherical particles with densities of > 95% TD which could then be loaded directly into fuel pins. Two size fractions of 0.63-0.90 mm and the fine fraction with 0.045-0.106 mm dia were used to reach the required AC-3 smeared densities of approximately 80%TD. Thus the process flow sheet as used at PSI differs from the classical approach used in the US and elsewhere in that there is a reduction in process steps (from around 14 to 9 according to fig. 41, blending of the uranium and plutonium streams occurs in the liquid phase producing a very homogeneous solid solution of U in Pu, the dust producing mechanical milling, crushing, blending and grinding steps are eliminated and the two thermal treatment stages are combined into one reaction sintering step. There is no need to grind pellets to size and inspect, although the sphere pat fuel is normally sieved to remove wrongfy sized particles. These are the points which make it attractive for a remote facility to handle more active “recycled” fuels in a fast reactor fuel cycle. A summary comparison of the two flow sheets is shown in fig. 4. All the PSI processes were qualified to the stringent US specifications. Mixtures or “charges” of sinter batches of each fraction were made, enough for the filling of groups of five pins. Carbon, oxygen and plutonium analyses were performed and a ceramographic specimen prepared to qualify each batch be-
R. W. Stratton et al. / Joint US-Swiss mixed carbide test in FFTF
K-405
Fuel used for Pins:
3L117,3L118,3Ll 3L1153L116
Fuel charge sphere size LLm density g/cm3 av. amount loaded g
K-405 805
l!
K-404 45-90 13.22
12.97 407
136
fissile composition 235u 239m, 24opu
24’Pu
%at. %at. %at. %at.
high carbon
52% of spheres in the sample
0.723 88.22 10.90 0.680
chemical composition uranium %wt. plutonium %wt. thorium1 ) %wt. carbon %wt. oxygen %wt. sulphcrr) %wt. tungsten ppm silicon ppm chromium ppm iron ppm nickel PPm phase composition secondary phase ~01%
71.6 19.29
72.6 19.00 0.16
5.02 0.05 < 40 430 16 65 3.4
4.86 0.15 0.16 680 22 9 25 1.4
5.1 U&2)
K-405 medium carbon
9% of spheres in the sample
3.4 UC221
‘1 Nominal values taken from typical UC analysis. 2, nascent dicarbide nearly amorphous or very small crystallite. size
-J9OX
K-405 low :arbon
39% of rphcres in he sample
C-404
N 350x
Iischunc3.te.x WJ43l29.6.92
Fig. 6. Ceramographs of PSI fuel with key characteristics.
45
46
R. W Stratton et al. / Joint US-Swiss mixed carbide test in FFTF
fore mixing the charges containing some 2 kg of large or 750 g of fines. The charges were fully characterised at PSI with the exception of the trace elements and nitrogen analyses. which were performed at the European Institute for Transuranium Elements (EITE). Good reproducibility of the carbon and oxygen contents, the two most important characteristics of a carbide fuel, was achieved over the whole campaign as shown in fig. 5. An intercomparison of carbon, oxygen and uranium contents on three samples, between PSI, LANL and EITF! confirmed the accuracy of the analytical procedures and no differences could be identified between the laboratories. The maximum uncertainty in the equivalent carbon content, calculated from the error of carbon and oxygen determinations is of the order of 2%. The as-manufactured microspheres seemed to be multiphased in composition. The matrix phase was monocarbide. The identity of the second phase remained uncertain. Neutron diffractrometry at PSI suggested that it is a nascent dicarbide - nearly amorphous or of very small crystallite size, Ledergerber et al. [5]. It was readily transformed into sesquicarbide by an extra annealing thermal treatment at 1550°C for as little as four hours. The reasons for the differences identified were probably in the manufacturing procedures. The pellet manufacturing process included much mechanical working, i.e. crushing and milling operations. The thermal history of pellets also involved a longer heat treatment in the temperature range where mono- and sesquicarbide are known to be in equilibrium. The
difference observed in the phase compositions was due to the rate and degree of completion of the reaction forming sesquicarbide from a highly point defected mono-carbide or mono- and di-carbide mixtures. This rate is greater in the case of the highly strained powders and the sesquicarbide forming reaction was more nearly complete due to the thermal treatment used in pellet manufacture. Typical ceramographic sections of sphere pat coarse and fine fractions are shown in fig. 6.
5. Pin and assembly fabrication
5.1. Pellet pins Fabrication of the pellet pins followed the usual methods. The fuel pellets had a nominal length of 8.8 mm and each pin therefore contained about 104 pellets. The sequence for loading was, lower reflector (inconel), 2 bottom insulator pellets, fuel stack, two upper insulator pellets, upper reflector, plenum spring, plenum spacer tube and tag gas capsule. The final closure weld was made on the top end cap using tungsten inert gas (TIG) welding in a helium filled glove box. Precautions were taken to minimise contamination of the weld area when loading pellets but in spite of this some end cap replacement and rewelding was necessary, The pins were visually and dimensionally inspected and X-radiography, dye penetrant and He leak testing were applied.
PELLET PIN (USA)
SPHERE-PAC PIN (SWITZERLAND) Fig. 7. Comparison of the fuel pin designs used in the AC-3 test.
41
R. W. Strattonet al. /Joint US-Swks mixed carbide testin FFTF 5.2. Sphere-pat pins
the docking port was successful in preventing contamination of the weld region. The coarse fraction fuel was loaded first and vibro-compacted to a maximum stable packing density. Then the fines were infiltrated into the mechanically locked matrix of coarse particles and vibro-compacted until their maximum packing density was obtained. Following filling and temporary closure each pin was transferred to the horizontal weld box where it was evacuated and back filled with helium and the lower end cap welded by TIG under a helium/ argon mixture. Axial gamma scanning confirmed the excellent regular flat density distribution of the fuel over its whole length in all cases. Visual inspection, X-radiography, He-leak testing and dimensional controls confirmed fabrication to the specified standards.
Small changes had to be made to the sphere pat pin design, partly to allow for the characteristics of the particle fuel and also to take account of the special fabrication methods used at PSI. These were: (see also fig. 7) Modification of the tag gas capsule penetrator to prevent inadvertent penetration, and addition of a support for the TGC, during vibrofilling. Replacement of the spring by a compression tube assembly and relocation above the plenum spacer. Addition of molybdenum discs at each end of the fuel column to retain the fine microspheres. Modifications to the lower reflector to include a split mandrel/collet arrangement to retain the fuel column until a distance piece could be added and the lower end cap welded. All these components and fabrication procedures were developed and validated in extensive testing. The main difference was the filling of the pin from the bottom end and the final weld being made on the lower end cap. Filling took place with the pin docked vertically below the filling box and care in the design of
5.3. Assembly fabrication 25 sphere pat pins plus two archive pins were shipped to LANL where after further inspection the D9 wire wrapping was carried out with a 200 mm pitch. Ninety one pins were then shipped to Westinghouse Hanford at Richland for the assembly into the 20%
Table 1 Initial density and chemical composition of pins nondestructivly examined after irradiation Pin identification
Fuel density (g/cm31 Smear Carbon a Oxygen a density content impurity pellets spheres (%t.d.) (wt.%) (Ppm)
Nitrogen a Secondary impurity carbide (vol%) (ppm)
Fuel batch
3940A6 4718D18 4574E23 4574E23 4723H37 4723G32 4723632
Comments
Pelletpin.5 3M07 3M31 3Lo43 3M44 3M74 3M78 3L.079
Batch 1
Batch 2
10.55 10.95 10.86 10.86 11.00 10.90 10.90
10.68 _ _ 10.89 _ 11.00
coarse
fine
13.00 13.00 12.98 12.97 12.97 12.97 13.05 12.99
13.20 13.20 13.28 13.22 13.22 13.22 13.34 13.27
75.41 78.09 77.29 77.29 78.67 78.74 77.73
5.05 5.00 4.94 4.94 4.99 4.94 4.94
541 687 537 537 520 608 608
45 80 65 65 92 75 75
14.6 M,C, 12.5 M,C, 9.2 M,C, 9.2 M&s 12 MzC, 9.5 M,C, 9.5 M,C,
79.7 79.48 79.41 79.63 79.63 79.78 80.07 78.96
4.92 4.94 4.94 4.98 4.98 4.98 4.95 4.85
1090 949 940 760 760 760 990 1190
80 62 61 98 98 98 79 86
3.9 UC2 4.1 UC, 4.1 UC, 4.6 UC2 4.6 UC, 4.6 UC, 4.4 UC, 2.8 UC1
destructive exam.
Sphere pat pins
3L102 3L105 3L106 3L11.5 3L117 3L119 3L121 3L126
a For sphere pat pins: weighted mean according ratio fine/coarse spheres. b Nascent dicarbide - nearly amorphous or of very small crystallite size.
b b b b b b b b
K-402/K-403 K-401/K-400 K-401/K-400 K-405/K-404 K-405/K-404 K-405/K-404 K-406/K-407 K-408/K-409
destructive exam. destructive exam.
destructive exam.
48
R. W. Stratton
et al. /Joint
US-Swiss
mixed carbide test in FFTF
To Core
Centerline iPlN 3L 115 (sphere-pat]
PIN 3LO74
551 ’ 0
I 100
I
’
I
I 300
200 [Effective
’
1
Full Power
’
I 500
600
Days]
PIN 3L 074, (pellet)
-4--__
f5
’
400
(pellet)
/
//
520
0
E d
510
I
s
i-i_-__-
I 500’
I 0
Fig. 8. Diagram of the pin loading plan for the AC-3 fuel bundle. The pins selected for examination are underlined.
cold worked D9 hexagonal duct. These FFTF assemblies are 3.6 m long and are fitted with an orifice plate to regulate the sodium coolant flow to give the design clad operating temperatures with a bulk coolant inlet temperature of 360°C. The loading plan for the assembly is shown in fig. 8.
Table 2 Reactor and peak pin parameters, Cycle
9A 9B 9c 10A 10B
EFPD per cycle
137.7 106.4 97.7 151.8 126.7
a For peak pin 3M24.
I 100
1 300
200 [Effective
’
’ 400
Full Power
’
I 500
Days]
Fig. 9. Calculated pin peak power (a) and coolant outlet temperature (b) histories for the AC-3 test in FFTF for the pins 3L 074 (pellet) and 3L 115 (sphere pat), based on measured in-reactor data, corrected for fuel density.
Table 1 shows the principle characteristics of the fuel in those pins selected later for post irradiation examination.
per cycle Cumulative EFPD
137.7 244.1 341.8 493.6 620.3
1 600
Fluence
Burn up (at%) a Average
Peak
Total average
1.65 2.85 3.92 5.46 6.67
2.08 3.60 4.95 6.91 8.28
4.35 7.53 10.60 14.98 18.56
(X lo** n/cm’)
’
Fast (E > 0.1 MeV) Peak
Average
3.48 6.03 8.48 12.05 14.73
2.74 4.74 6.68 9.44 11.63
R. W. Stratton et al. / Joint US-Swiss mixed carbide test in FFTF
6. Summary of irradiation
conditions
The AC-3 assembly was loaded into FFIYF at the beginning of cycle 9 in August 1986 in a row 4 position. The choice of position was due to restrictions in the overall loading plan affected by the higher priority “Core Demonstration Experiment” assemblies and gave linear powers and burn ups somewhat lower than targeted. Maximum peak linear power (BOL) was estimated to be 84 kW/m (pin 3L 101) Peak cladding temperatures were calculated to be around 550°C at BOL. A summary of each cycle’s operating conditions is given in table 2. Fig. 9 shows the calculated power and coolant temperature history for two selected pins. The experiment operated without incident until it was unloaded in October 1988. After cooling at the reactor the assembly was transferred to the FFI’F Intermediate Examination and Maintenance (IEM) cell for inspection and dismantling. The assembly and individual fuel pins were shown to be in excellent condition with duct distortion within the limits for D9 material under these conditions and with all pins showing only moderate levels of growth and little bow or swelling as shown by the ease with which the duct could be removed. Details of the complete PIE results are given in another paper by Hoth et al. 161.
49
on carbides or nitrides should seek to explore these small differences at higher burn ups and ratings where the potential or the dif~culties are perhaps more evident. On the other hand comparisons could be made at the lower fuel smeared densities which the sphere pat route also offers (single fraction fuel), to take full advantage of a still simpler fabrication alternative. This experiment has shown that the choice of sphere-pat or pellet fuel for reactor application is dependant on preferred differences in fabrication (e.g economics and environmental factors) and not (at these burn ups and ratings) on differences in irradiation behaviour.
It is a particular wish of the authors to give thanks and recognition to the large number of people, too numerous to name, on both sides of the Atlantic, who have taken part in, supported and guided this programme over more than fifteen years of effort. The success of this collaboration is entirely due to their skiils and a determination to see the experiment through to its end. Working with you all was a pleasure.
7. Conclusions References
Fabrication of mixed carbide fuel by the dry, powder, pellet route and by the wet gelation, sphere pat route demonstrated that both are capable of giving a stable homogeneous product meeting all requirements of stoichiometry, density and chemical composition. The successful irradiation of full size fuel pins of both types in the prototypical FFTF conditions of bum up and fluence showed trouble free behaviour up to at least 8% fima with a large potential for higher burn ups and ratings. As shown in the later paper, only minor differences in behaviour were seen between the two fuels which can be attributed mainly to small differences in smeared density. Future work, whether
[I] R.B. Matthews and R.J. Herbst, Nucl. Tech. 63 (1983) 9. [2] C. Prunier, D. Warin, R.W. Stratton and G. Lederberber, presented at Int. Syrnp. on Fuels for Liquid Metal Reactors, Chicago, IL, 1992. [3] R.W. Stratton, Trans. Am. Nucl. Sot. 39 (1981) 421. [4] G. Ledergerber, H.P. Alder, F. Ingold and R.W. Stratton. ENC-86, Geneva (1986). [S] G. Ledergerber, R.J. Herbst, H.U. Zwicky, H. Kutter and P. Fischer, J. Nucl. Mater. 153 (3988) 189. 161 C.W. Hoth, R.E. Mason, B.J. Makenas, R. Wisner, F. Botta, R.W. Stratton, G. Ledergerber, F. Ingold, H.P. Alder, presented at Symp. on Fuels for Liquid Metal Reactors, Chicago, IL, 1992.