Coolability of UO2 debris beds in pressurized water pools: DCC-1 and DCC-2 experiment results

Nuclear Engineering and Design 97 (1986) 81-88 North-Holland, Amsterdam

81

COOLABILITY OF U O 2 D E B R I S B E D S IN P R E S S U R I Z E D W A T E R POOLS: DCC-I A N D DCC-2 E X P E R I M E N T R E S U L T S * A.W. REED, E.D. BERGERON,

K.R. BOLDT and T.R. SCHMIDT

**

Reactor Safety Theoretical Physics, Sandia National Laboratories, Albuquerque, N M 87185, USA Received 9 October 1985

The DCC-1 and DCC-2 experiments were designed to examine post-accident heat removal from reactor fuel debris using prototypic materials over a pressure range of 1 to 170 atmospheres. The purpose of these experiments is to provide dryout data for comparison with current predictive models. The experiments displayed two unexpected features. In DCC-1, the pressure dependence of the dryout flux was less than anticipated. In DCC-2, localized thermally stable dryouts were observed.

1. Introduction DCC-1 and DCC-2 are the first two experiments in the United States Nuclear Regulatory Commission's ( U S N R C ) Degraded Core Coolabifity (DCC) Program being performed at Sandia National Laboratories. The D C C program is part of the N R C ' s Severe Fuel D a m a g e ( S F D ) program established after the accident at the Three Mile Island-2 reactor. The main objective of the D C C program is to verify, with a limited number of in-pile experiments, the accuracy of existing debris coolability models in previously untested S F D parameter regimes. The primary test matrix consists of three experiments, the first two of which are reported here. Both of these addressed the coolability of a deep U O 2 rubble bed in a pressurized water pool, with pressure varying from I to 170 atmospheres. The beds were fission-heated in the Annular Core Research Reactor ( A C R R ) [1] to simulate the internal heat generation from decay heat in an S F D accident. DCC-1 and DCC-2 differed primarily in their particle size distributions for the debris. DCC-1 contained a broad distribution of small particles (table 1), simulat* This work was part of the SFD program supported by the US Nuclear Regulatory Commission and its foreign partners and performed at Sandia National Laboratories which is operated for the US Department of Energy under contract number DE-AC04-76DP00789. ** The authors are members of the technical staff in the Reactor Safety Research Department at Sandia National Laboratories, Albuquerque, NM, USA.

Table 1 Particle size distributions Particle size (ram) 0.075-0.106 0.106-0.125 0.125-0.150 0.150-0.180 0.180-0.250 0.250-0.355 0.355-0.500 0.500-0.850 0.850-1.00 1.00 -1.18 1.18 -1.40 1.40 -1.70 1.70 -2.00 2.00 -2.36 2.36 -2.80 2.80 -3.35 3.35 -4.00 4.00 -4.75 4.74 -5.60 5.60 -6.30 6.30 -6.70 6.70-8.00 8.00 -9.50 9.50 -11.2

Weight percent DCC-1

DCC-2

2.55 1.87 2.69 3.43 8.22 11.56 13.29 21.21 5.91 5.49 5.01 4.83 3.33 2.76 2.24 1.78 1.29 0.88 0.60 0.31 0.13

0.82 0.19 0.19 0.17 0.34 0.34 0.92 1.38 1.30 1.60 1.70 5.50 14.83 21.47 21.66 16.02 7.47 2.52 0.68 0.45 0.45 0.00

0.28 0.17

0.00

0.17

0.00

100.00

100.00

Note: The effective diameter includes a factor of 0.78 to account for the non-spherical shape of the particles [3,12].

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82

A. W. Reed et al. / Coolability of UO2 debris beds

ing the expected debris from an energetic melt-water reaction. The mean particle diameter was 0.75 mm with an effective (Fair-Hatch [2] weighted) diameter of 0.31 mm. The measured porosity was 0.345. DCC-2 (table 1) has a narrow distribution of large particles, simulating the rubble that might be expected from the quench of solid fuel. The average and effective diameters were 2.43 mm and 1.42 mm respectively. The measured porosity was 0.41. In addition to being representative of different accident scenarios, the DCC particle distributions were selected to span the range of coolability regimes. Fig. 1 shows the predicted dryout heat flux at one atmosphere as a function of particle diameter and bed porosity using the Lipinski zero-dimensional coolability model [2]. The graph is subdivided into the three general coolability regimes corresponding to the coolant flow regimes: laminar, transition and turbulent. Mapping the DCC-1 and DCC-2 effective particle diameters and porosities onto this graph illustrates that the experiments are predominantly in the laminar and transition regimes, respectively.

2. Experiment description The DCC-1 and DCC-2 experiment packages (fig. 2) were nearly identical in design, assembly and operation. The debris bed consisted of UO 2 particulate which was seeded with Gd203. The gadolinium was added to absorb some of the neutrons moderated by the water within the debris bed. This reduced the sensitivity of the bed power of the water content of the bed (bed saturation). Each bed had a diameter of 100 mm and a height of about 500 mm, and was contained in a double-wall, insulated crucible. The crucibles provided a nearly adiabatic boundary condition on the bed bottom and walls.

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A.W. Reed et al.

/ Coolabifity of UO2 debris beds

The debris, crucible, and water bath were enclosed in the primary containment vessel (fig. 2). To prevent fission product release in the event of any primary boundary failure, a secondary containment vessel completely surrounded the primary vessel. A concentric-flow heat exchanger was attached to the secondary vessel. The manifold on the top of the heat exchanger routed cold helium gas down the outside of the secondary containment vessel and received the return flow from the outer annulus of the package. The main diagnostic instruments for determining bed conditions were 1.59 mm diameter sheathed thermocouples located at various radial and axial positions in the debris bed (fig. 3). To minimize their effect on the debris bed, the thermocouples were shaped like an " L " with the vertical shaft pressed against the vessel wall and the end penetrating the bed at an angle 90 ° to the wall. This array of thermocouples was used to interpret

83

subcooled, boiling and dryout conditions in the bed. Pressure in the primary vessel was measured using pressure transducers and verified by the saturation temperature. The DCC auxiliary systems and their interconnection to the experiment package are shown in fig. 4. The DCC debris bed was fission-heated in the central irradiation cavity of the ACRR. Cooling lines connected to a mobile helium cooling loop located outside of the reactor room. The flow from the loop to the package could be diverted to a heat exchanger in the liquid nitrogen tank to provide additional cooling capacity. Diagnostic instrumentation from the package was monitored by an HP-9845/HP-1000 computer-based data acquisition system. A separate HP-9845 computer-based data acquisition system. A separate HP-9845 computer-based data acquisition system was used to monitor cooling loop parameters for diagnostic and control functions.

3. Results

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The major problem involved in data reduction is determining the power coupling between the ACRR and the bed power. This coupling was measured directly by correlating step changes in reactor power to temperature increases in the particle bed. Because a boiling bed is isothermal, this procedure was used only with a subcooled bed or a bed with an extensive dry region. With this technique, it was found that the coupling with a completely saturated bed at 20oC was about 1.8 times greater than the coupling with a completely dry bed. This is because the interstitial water moderated some of the fast neutrons, and the gadolinium absorbed only part of the resulting thermal neutrons. In a boiling bed, the saturation varies spatially between zero and unity, and the coupling factor lies somewhere between that for the fully wet and fully dry bed. Because the saturation was not measured directly in the experiment, the Lipinski 1-D model was used to calculate the saturation profile at the time of dryout. This calculation accounted for both the spatial and saturation dependence of the local power generation. The effect of decreasing water density with increasing saturation temperature was included in the calculation of moderation. This procedure is described in detail in ref. [3]. The information on saturation was then used with the measured coupling factors to determine the power coupling at the time of dryout. The major uncertainty in the data lies in the

84

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determination of saturation at dryout. An estimate of this uncertainty was obtained by assuming that the bed average dryout saturation lay between 0.2 and 0.8. Coupling factors for each of these saturations were computed using the measured calibration data. These extremes in coupling factor were applied to the lower and upper bounds on reactor power to determine the uncertainty bounds in dryout flux.

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The DCC-1 dryout heat flux data is shown in fig. 5. Near one atmosphere, the bed dried out at about 41 W / c m 2, corresponding to a decay heat of 12 W / k g . This increases slowly with saturation pressure to a maximum of about 78 W / c m 2 (23 W/kg). Note that the span in uncertainty of the data decreases at higher pressures. This is due to the decreased amount of neutron moderation caused by decreased water density. Also the position of the data point moves higher within the bands with increasing pressure. This is because the predicted average saturation at dryout increases with increasing pressure. The pressure dependence of the dryout data contrasts sharply with that predicted by most of the models. At low pressures, the dryout heat fluxes are best predicted by the Lipinski [12] and the Henry [10] mod-

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A. W.. Reed et al. / Coolabifity of UO2 debris beds

els. At higher pressures, the Lipinski, Gabor [8,9], Dhir-Catton [5] and the Jones [6,7] models come closest. The dependence on pressure of the dryout fluxes is overpredicted by all of these models. While the Theofanous-Saito [4] model comes closer to predicting the pressure dependence, it overpredicts the dryout fluxes by an order of magnitude. This is because it is based on flooding data, and is therefore inappropriate for small particle beds. The Dhir-Barleon [11] model is also based on flooding and is meant only for large particles. The probable cause of the disparity between the predictions and the measurements is the breadth of the particle size distribution coupled with the depth of the bed. Most of the world data are based upon beds composed of narrow particle size distributions. It is therefore consistent to expect predictive models to work better for narrow distributions than for broad distributions. To gain some insight into the problem, the capillary pressures of the DCC-1 bed and several other UO2/water beds were measured [13]. While the pressures of narrow distribution beds agreed very closely with the data of Leverett [14], pressures for broad distribution beds deviated significantly from Leverett's correlation. Burdine [15] and Brooks and Corey [16,17] demonstrated that the relative permeabilities were related to the form of the capillary pressure curve. Unfortunately, the form of the measured DCC-1 capillary pressure curve does not lie within the data base covered by these relative permeability correlations, and the predictions based upon these new relative permeabilities still do not predict the observed pressure dependence adequately [3]. The quench behavior in DCC-1 resembled those described in subsequent out-of-pile experiments on small particles [18]. After the dry region was formed and the power reduced, the liquid front moved downward in a uniform manner. No liquid fingers were observed. Quench times were several hours long.

meability. Water could flow through this zone of higher permeability to the bottom of the bed (fig. 6). Vapor generated in the boiling zones convected through the dry zone, stabilizing the particle temperature at some level above the saturation temperature. In global dryouts, the dry zone extends across the width of the bed, thus preventing any liquid from reaching the bottom. Local and global dryout fluxes measured in DCC-2 are shown in fig. 7. Unlike DCC-1, several models predict the global dryout data. Of the models shown, the Theofanous-Saito [4] and Lipinski [12] models predict the dryout levels and the pressure dependence well. The Henry [10] and Gabor [8,9] models predict the dryout levels adequately, but do not track the pressure dependence. The Dhir-Catton [5] and Jones [6,7] models are meant for small particles and are probably inappropriate for DCC-2. The predictions of the Lipin-

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In DCC-2, "local" dryouts were observed in addition to the anticipated "global" dryouts. Local dryout is characterized by a thermally stable local dry zone surrounded by a boiling zone. This behavior is believed to have been caused by a region of low permeability formed by the relocation and concentration of small particles at the time of bed construction. Reactor powers capable of drying this region were unable to dry the horizontally adjacent zone which had a lower concentration of fines, and a correspondingly higher per-

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86

A. IV. Reed et aL

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ski 1-D model are extremely close to those of the 0-D model, and have been omitted from the figure for the sake of legibility. The quenches in DCC-2 took only minutes to complete. The temperature field in the dry region prior to the initiation of these quenches was radially asymmetric with the low permeability zone having the higher temperatures. Upon initiation, the quench front moved down the high permeability side, reached the bottom, and then moved upward into the low permeability zone. This strongly resembles the fingering behavior observed in out-of-pile experiments with large particles [19,20], but the inhomogeneities in the temperature and permeability field probably modified the process.

4. C o m p a r i s o n w i t h o t h e r d a t a

Fig. 8 is a plot which appeared in Muller and Schulenberg's review of post-accident l~eat removal research [21]. It shows the dryout heat flux as a function of particle diameter over a broad range of diameters. In addition to the DCC-1 and DCC-2 data which have been added, the plot contains experimental data from

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four different laboratories. Predictions of the Dhir-Catton model for small particles, the Sowa flooding model for large particles (basically the same as Theofanous model), and the Lipinski model for bed heights of 10 and 50 cm are included on the plot, along with the flat plate critical heat flux predicted by the Zuber model. The DCC-1 dryout level at 0.1 MPa appears slightly below the Lipinski 1-D model whereas it is slightly above the predictions in fig. 5. The difference is the porosity assumed in the predictions. The prediction shown in fig. 8 is based on a porosity of 0.4 compared to the measured DCC-1 porosity of 0.345. The DCC-1 data point fits in well with the data of Barleon et al. [201. The DCC-2 global dryout level at 0.37 MPa appears somewhat above the dryout data at 0.1 MPa, while the local dryout point at 3.5 MPa is located in the center of the data. However, these points should be adjusted for saturation pressure. Fig. 7 shows that, according to the Lipinski model, the dryout power at 0.1 MPa should be about a factor of 1.8 lower than that at 0.37 MPa. This would place the pressure*adjusted DCC-2 global dryout point in the mainstream of the displayed data.

A.W. Reed et al. / Coolabifity of

5. Summary DCC-1 and DCC-2 have provided the first data on dryout and quench behavior of internally heated UO2 debris in water. DCC-1, designed to test the laminar flow region of dryout, displayed unexpected results. While the dryout fluxes at one atmosphere are approximately predicted by the Lipinski and Henry models, none of these predicts the pressure dependence of the dryout data. This is believed to be due to a combination of the broad particle distribution and the deep bed, but corroboratory data on such beds are necessary to confirm this hypothesis. The quench behavior of DCC-1 displays none of the "fingering" observed in out-of-pile large-particle experiments, and the quench times extend for hours. The DCC-2 dryout data conform in magnitude and pressure dependence with the Theofanous-Saito and Lipinski models. In addition to "global" dryouts, in Which the dry zone extends across the width of the bed, "local" dryouts were observed. These are zones of lower than average permeability which dry out and achieve stable temperatures without eliminating coolant flow to the bottom of the bed. The quench times in DCC-2 are only a few minutes long. Fingering, modified by the radially varying temperature of the dry particles, was observed.

References [1] K.R. Boldt et al., Sandia annular core research reactor (ACRR) safety analysis report, Sandia National Laboratories, SAND77-0208 (November 1981). [2] R.J. Lipinsld, A Model for boiling and dryout in particle beds, Sandia National Laboratories, NUREG/CR-2646, SAND82-0765 (June 1982). [3] A.W. Reed, K.R. Boldt, E. Gorham-Bergeron, R.J. Lipinski and T.R. Schmidt, DCC-1/DCC-2 analysis report, Sandia National Laboratories, Albuquerque, New Mexico, NUREG/CR-4390, SAND85-1967 (October 1985). [4] T.G. Theofanous and M. Saito, An Assessment of Class-9 (Core-mel0 accidents for PWR dry-containment systems, Nucl. Engrg. Des. 66 (1981) 301-332. [5] V.K. Dhir and I. Catton, Prediction of dryout heat fluxes in bed of volumetrically heated particles, in: Proc. Int. Meeting on Fast Reactor Safety and Related Physics, Chicago, IL, October 5-8, 1976, CONF-761001, p. 2026-2035 (October 1976). [6] J.D. Gabor, M. Epstein, S.W. Jones and J.C. Cassulo, Status report on limiting heat fluxes in debris beds, NS/RAS80-21, Argonne National Laboratory, Argonne, IL (September 1980). [7] S.W. Jones, M. Epstein, J.D. Gabor and S.G. Bankoff,

UO 2

debris beds

87

Investigation of Limiting Boiling Heat Fluxes from Debris Beds, Trans. Am. Nucl. Soc. 35 (1980) 361-363. [8] J.D. Gabor and J.C. Cassulo, Induction heated simulant materials, reported by J.D. Gabo et al., Status report on debris accommodation technology for LBRs, ANL/RAS 81-19, Argonne National Laboratory, pp. 34-59 (May 1981). [9] J.D. Gabor, J.C. Cassulo, S.W. Jones and D.R. Pedersen, Studies on Heat Removal from Fuel Debris, Trans. Am. Nucl. Soc. 38 (1981) 642-643. [10] R.E. Henry and H.K. Fauske, Core melt progression and the attainment of a permanently coolable state, Meeting on Light Water Reactor Safety, Sun Valley, ID, August 1981. [11] V.K. Dhir and L. Barleon, Dryout heat flux in a bottomheated porous layer, Trans. Am. Nucl. Soc. 38 (1981) 385-386. [12] R.J. Lipinski, A coolability model for postaccident nuclear reactor debris, Nucl. Technol. 65 (1984). [13] A.W. Reed, H. Meister and D.J. Sasmor, Measurements of capillary pressure in urania debris beds, Sandia National Laboratories, Albuquerque, New Mexico, SAND85-2217J (to be published). [14] M.C. Leverett, Capillary Behavior in Porous Solids, Petroleum Trans. AIME 142 (1941) 152-169. [15] N.T. Burdine, Relative permeability calculations from pore size distribution data, Petroleum Trans. AIME 198 (1953) 71-78. [16] R.H. Brooks and A.T. Corey, Hydraulic properties of porous media, Hydrology Papers, Colorado State University, No. 3 (March 1964). [17] R.H. Brooks and A.T. Corey, Properties of porous media affecting fluid flow, J. Irrigation and Drainage Division, Proc. ASCE, IR2, pp. 61-68 (June 1966). [18] L. Barleon, K. Thomauske and H. Werle, Extended dryout and rewetting of small-particle core debris, Proc. Sixth Information Exchange Meeting on Debris Coolability, November 7-9, 1984, University of California at Los Angeles. [19] D.H. Cho, D.R. Armstrong, L. Bova, S.H. Chan and G.R. Thomas, Experiments on quenching of a hot debris bed, presented at the Information Exchange Meeting on Post Accident Debris Cooling, Karlsruhe, July 1982. [20] T. Ginsberg, J. Klein, J. Klanges and C.E. Schwarz, Phenomenology of transient debris bed heat removal, presented at the Information Exchange Meeting on Post Accident Debris Cooling, Karlsruhe, July 1982. [21] U. Muller and T. Schulenberg, Post-accident heat removal research: A state of the art review, KfK 3601, Kernforschungszentrum, Karlsruhe, FRG (November 1983). [22] E.S. Sowa, C. Hesson, R.H. Gebner and G.T. Goldfuss, Heat transfer experiments through beds of UO2 in boiling sodium, Trans. Am. Nucl. Soc. 14 (1971) 725. [23] V.K. Dhir and I. Catton, Dryout heat fluxes for inductivity heated particulate, beds, J. Heat Transf. 99 (1977) 250-256.

88

A.W. Reed et al. / Coolability of U02 debris beds

[24] L. Barleon, K. Thomauske and H. Werle, Cooling of debris beds, Nucl. Technol. 65 (1984). [25] I. Catton, V.K. Dhir, C.W. Somerton and D. Squarer, An experimental study of debris bed coolability under pool boiling conditions, EPRI, NP-3094 (May 1983). [26] J.D. Gabor, M. Epstein, S.W. Jones and J.C. Cassulo,

Status report on limiting heat fluxes in debris beds, ANL/RAS 80-21 (Sept. 1980). [27] R.E. Trenberth and G.F. Stevens, An experimental study of boiling heat transfer and dryout in heated particulate beds, AEEW-R 1342 (July 1980).