ML19345G483
| ML19345G483 | |
| Person / Time | |
|---|---|
| Issue date: | 03/30/1981 |
| From: | Picklesimer M NRC OFFICE OF NUCLEAR REGULATORY RESEARCH (RES) |
| To: | NRC OFFICE OF NUCLEAR REGULATORY RESEARCH (RES) |
| Shared Package | |
| ML19345G484 | List: |
| References | |
| NUDOCS 8104070353 | |
| Download: ML19345G483 (13) | |
Text
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M. L. Picklesimer 4
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Fuel Behavior Research Branch
SUBJECT:
RELOCATION OF FUEL FRAGMENTS IN BALLO 0!iED FUEL RODS Neutrographs have shown that pellet fragments are observed to be relocated from other, undefonned sections of the fuel rod to fill up the ballooned section of Zircaloy clad LWR fuel rods burst after in-pile LOCA simulation tests. This has been observed and reported by Karb (Reference 1) in his studies in the FR2 at Kernforschungzentrisn Karlsruhe, FRG, and by Yackle (Reference 2) in the LOC-3 and LOC-5 tests in PBF at INEL. Questions have been raised as to when the relocation occurs, to the effects of the fuel fragment relocation on the centerline temperature of the fuel, to the peak surface temperature of the cladding, to the progression of the ballooning of the cladding, and to what account of this observation should be made in LOCA analyses for licensing purposes. This memorandum has been prepared to assemble infonnation pertinent to the provision of an analysis of the problem and answers to the questions raised.
Ex-pile tests of ballooning rods by Wiehr (Reference 3), using internal i
electrical cartridge heaters in fuel rod simulators and x-ray photography, and Chung, et.al., (Reference 4) using pellet-constrained Joule-heated cladding specimens have shown that once a local plastic instability has formed during ballooning, the remaining defonnation required to form the balloon and burst occurs in fractions of a second to a second. Circumferential strains, averaged l
axially over the ballooned specimens away from the innediate neighborhood of the burst, seldom are higher than 20-25% and should represent the approximate strain present at the time the plastic instability developed. Pertinent figures from their reports are shown in Figures 1 and 2.
Karb (Reference 5) has shown in a special in-pile ballooning test that:
(1) the total time of ballooning to large strains over an extended length (approx-imately 60% strain over 20 cm of length) required no more than 1-2 seconds; (2) fuel relocation must have occurred after the ballooning started; and (3) ao significant h:mperature increases were measured on the cladding, though no thermocouple was located precisely on the ballooned section. Other tests by Karb have shown that the fuel fragmentation was present in the fuel before ballooning and was not caused by the LOCA testing (a companion irradiated, but unballooned, fuel n>d had the same particle size distribution as ballooned and burst rods). An excerpt of the data report is presented in Appendix A.
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All of.the observations of fuel fragment relocation in ballooned sections of I
fuel rods have been made on specimens used in experimental in-pile tests i.
examining the behavior of fuel rods in simulated LOCA.
In all cases, the ruptured specinens have been subjected to vibration during reflood, removal I
of the test train from the in-pile tube test apparatus, removal of the specimens from the test trains, transport to the neutrographic facility, and instal-lation in the apparatus for neutrography. Thus, it is not certain when the fuel relocation occurred - during the ballooning, subsequent to the balloon-ing during reflood, or during handling and transport after the tests were I
complete. Since the latter cases pose no problem or hazard, and the first could cause increased ballooning strain it is assumed, for the purposes of this memorandum, that the fuel relocation takes place during ballooning.
i.e.. the worst case.
l' i
Since the fragment sizes are not changed by the LOCA ballooning (see Figure A-l 3 of Appendix A for companion rods ballooned and unballooned), the relocation i
is in effect, a change in the bulk density of the fuel pellets. Though cracked and relocated sufficiently to fill the as-built cladding to pellet gap, the fuel pellet in the unballooced rod section is still at an effective or bulk density nearly identical to its original as-sintered density (nom-inally 93-95% theoretical). When displaced into the volume of the balloon.
the bulk density is decreased 'significantly, just as is crushed rock formed from bedded layers in a quarry. Handbook values of bulk density for rock, j1 rubble, and gravel fomed by crushing large solid bodies range from 0.56 to l
0.69 of the original density. Measurements of the ratios of radioisotopic readings on ballooned and unballooned sections of fuel rods from the LOC-3 and LOC-5 PBF tests (Reference 6) give values of about 0.65, which should be proportional to the ratio of the bulk to theoretical densities for the fuel fragments.
[.
Using simple assumptions, a set of calculations have been made to estimate the amount of new fuel entering an axial node of a balloon as a function of I '
the amount of circumferential strain present and the bulk density of the fuel
(
fragments. The values are presented in Table 1.
These show, for example.
that if the bulk density of the fuel fragments is 0.65 theonttical density.
l then a circumferential cladding strain of more than 24% and a volume increase l
of more than 54% must be present at a node if any new fuel is to enter that node from other regions. A ballooning strain of 75% is required for the amount of fuel at an axial node of a balloon to be doubled, again for a bulk density of 0.65 theoretical.
Yackle (Reference 6) has performed a set of calculations of centerline and cladding surface temperatures on ballooned fuel rods during LOCA for three levels of cladding circumferential strain, two gases (helium and oxygen), and three levels of fuel fragmentation. The essence of the results are presented in Figure 3 and the complete results in Appendix B.
For a circumferential I
l strain of 44%. the cladding temperature rises only about 25 K above that for
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MAR 3 01981 Files cladding with no strain, while the. centerline temperature for the fuel frag-i ments in a ruptured rod increases about 450 K above that for an unstrained a"
rod with no fuel fragmentation.
If the circumferential strain is increased to 89% the cladding temperature is increased about 225 K. while the fuel i,
l fragment centerline is increased about 1270 K, both above the conditions for I
an undefomed rod. To put these strains into perspective, in present day PWR's the cladding of neighboring rods will touch if both balloon coaxially with about 33% circumferential strain, while about 66% strain is required for a ballooning md to touch an unballooned neighbor. The maximum circumferential strains observed in single rod burst tests with slow heating rates and heated shroud (near adiabatic conditions) range from about 75% to about 90%.
t l
If it can be accepted that the bulk density of fragmented fuel is approximately 1
0.65 theoretical density, and that no new fuel can enter a ballooning region until space has been made for it to enter at that bulk density, then the circumferential strain in the balloon must exceed 24% before new fuel enters l
the balloon. Enamination of the internal rod pressum curve (P74) in Figure A-2 in the time period between 52 and 58 seconds shows that the major volume increases in the ballooned region of specimen rod E-5 occurred in the 1-1.5 seconds between about 56.5 and 58 seconds elapsed time. It is thought that the ballooned section developed a pin-hole rupture just before 58 seconds which was then covered over by the shroud as the balloon continued to develop (PIE of the specimen is underway). With the termination of the nuclear l
power, the ballooned section cooled off enough to allow the cladding at the rupture to pull away from the shroud and allow the slow depressurization to proceed. When the system pressum was increased at about 75 seconds (P60),
the pressure inside the ballooned rod also increased (P74). It can be argued then that the new fuel enters the ballooned region after the balloon-ing has occurred and, thus, ca'n not affact the ballooning strains (there isn't time and the strain must be greater than 24% for any new material to enter the local region).
It can be concluded that: (1) the fuel fragmentation reported existed in the fuel rod prior to the LOCA ballooning (2) the fragmentation was not increased by the LOCA ballooning or thermal shock of quenching, (3) the filling of the balloon at each elevation in the cladding by fuel fragments occurred after theballoontherewasformed,(4)thetemperatureincreasestobeexpectedon the cladding due to the insertion of more fuel fragments into the balloon is at most 100 to 200 K even for very large balloons, and (5) the temperature increase to be expected at the center 11rc in the fragmented fuel is not sufficie.
to produce melting of the fuel fragments in very large balloons filled with steam, unless the ' surrounding steam temparature becomes higher than about 1100 K.
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It can also be concluded that the fuel fragment relocation does not produce a problem in the fuel rod during or following the ballooning and bursting.
It i.
may, however, constitute a problem of " washout" through the rupture opening during reflood and subsecuent flow of coolant to depcsit fuel fragments elsewhere in the primary system.
6 M. L. Picklesimer Fuel Behavior Resse.ith Branch Division of Reactor Safety Research
Enclosures:
As stated-DISTRIBUTION SUBJ CIRC CHRON BRANCH RF MLPicklesimer RF j
MLPicklesimer LHSullivan l
GPMarino j
bec w/ encl.
PDR i
W. V. Johnston, NRR/CPB l
D. A. Powers, NRR/CPC R. O. Meyers, NRR/CPB I
R. Van Houten RES/FBRB D. A. Hoatson, RES/FBRB D. Bessette, ACRS l
I RECORD NOTE: Drafted and submitted to W. V. Johnston, NRR/CPB on October 8, 1980, acceptance confimed by D. A. Powers, NRR/CPb on March 23, 1981 by telecon.
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.T REFERENCES 1.
E. Karb, M. Prussmann, L. Sepold, "In-Pile-Experimente zum Brennstabverhalten beim KUhlmittelverluststorfall, Bericht 'u*ber die Versuchsserie F," KfK i-2956, Mai 1980 (Kernforschungzentrum Karlsruhe, FRG).
2.
T. R. Yackle, et.al., "An Evaluation of the Thermal-Hydraulic Response and Fuel Rod Thermal and Mechanical Deformation Behavior During PBF Test LOC-3," Proceedings of the ANS Topical Meeting on Thermal Reactor Safety, April 6-9, 1980, Knoxville, Tennessee, USA, Conference-800403, Vol. I, pp. 387-394.
3.
K. Wiehr, He. Schmidt, "Out-of-Pile-Versuche zum Aufbli*hvorgang von Zircaloy-Hullen Ergebnisse aus Vorversuchen mit verku'rzten Brennstabsimulatoren,"
KfK 2345, October 1977, Gesellschaft fur Kernforschung M.B.H., Karlsruhe, FRG.
4.
H. M. Chung and T. F. Kassner, " Deformation Characteristics of Zircaloy Cladding in Vacuum and Steam Under Transient-Heating Conditions: Summary Report," NUREG/CR-0344 (ANL-77-31), July 1978, pp. 30-32.
5.
E. Karb, "In-Pile Experiments in the FR2 DK-Loop on Fuel Rod Behavior During a LOCA," Presentation to the Workshop on Fuel Behavior, June 1980 Karlsruhe (PNS/NRC/JAERI Annual Information Exchange on Cladding and Codes, Kernforschungzentrum Karlsruhe, Karlsruhe, FRG, June 1980).
C.
T. R. Yackle, " Steady State Fuel Rubble Thermal Analysis," HJZ-317-80 correspondence from H. J. leile, EG&G, Idaho, to R. E. Tiller, DOE /ID, Idaho Falls, September 29, 1980.
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Axial Node (1)
(2)
(3)
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AVrod c/co c/co=.60 o/co=.65 o/co=.694
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.440 1.074 35.0 46.3 56.2
.500 1.250
.600 1.560 53.6 66.4 77.7
.75 2.063 83.8 99.1 112.6
.890 2.570 114.2 132.1 147.8 (1) Circumferential strain = C-Co, g.
Co (2) AV = Y~Yo = c +2c 2
Yo (3) Ratio of bulk density of fuel fragment to theoretical density for filling of
- ballooned volume by only the fuel at the axial node of the calculation (4),(5),&(6) Per cent new fuel at the axial node required to fill volume of balloon at that node for bulk density ratios of 0.60, 0.65, 0.694 NOTE:
2 2,
g, c-co V = nr L. c = 2nr wr AV = V-Vo, av = c +2c 2
Vo l'
% new fuel = 100 (AV od - IVp/p,)
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NUREG/CR-0344 NUREG/dR-0344 ANL-77-31 ANL-77-31 i
DEFORMATION CHARACTERISTICS OF ZlRCALOY CLADDING IN VACUUM AND STEAM UNDER TRANSIENT-HEATING CONDITIONS:
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REPORT FIGURE 2 by 1
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1 PROJEKT PNS 4237 - VERa. NR.
- E5 VOM 04.03.80 ZEIT'llH03M24S 950 NITTL. THERN. LEISTUNG NTH =39,8 V/CM 50
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Fuel Particle Size Distribution, Sieve Analyses Fig.
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