SEVERE CORE DAMAGE ACCIDENT PROGRESSION WITHIN A CANDU CALANDRIA VESSEL

P. Mani Mathew AECL Chalk River Laboratories,

Fuel and Fuel Channel Safety Branch Chalk River, Ontario, K0J 1J0

Abstract This paper focuses on the current understanding of the progression of a Severe Core Damage Accident within a CANDU calandria vessel, the supporting research and the analysis conducted at AECL. In general, the progression of a Severe Core Damage accident in a CANDU reactor is slow because (a) the fuel is surrounded by a large quantity of light and heavy water, which acts as a heat sink to remove the decay heat, and (b) the mechanical deformation mechanism leading to disassembly of the core is creep, which is a slow process. To understand the progression of a severe core damage accident in a CANDU reactor, single and multi-channel tests are conducted in a scaled-down facility and models are developed to explain the test findings. Tests in an inert atmosphere have been completed and tests in an oxidizing atmosphere are being performed. The progression of a severe core damage accident in a CANDU plant is analyzed using the MAAP4 CANDU code, which contains CANDU-specific models, such as horizontal fuel channels within the core, calandria vessel and calandria vault. This paper presents the findings of the single and multi-channel tests, the models developed to explain the findings and the calandria vessel response to a Station Blackout accident sequence resulting in severe core damage.

Introduction

The core of the CANada Deuterium Uranium (CANDU®) power reactor is comprised of several hundred horizontal fuel channels in a large cylindrical calandria vessel. The calandria vessel is surrounded by a calandria vault, which contains a large volume of light water. Each fuel channel consists of an internal pressure tube, containing the fuel and the hot, pressurized heavy or light water primary coolant, and an external calandria tube, separated from the pressure tube by an insulating gas-filled annulus. The calandria vessel contains cool low-pressure heavy-water moderator that surrounds each fuel channel. The primary coolant is distributed amongst the fuel channels by common headers and individual feeder pipes. The fuel can be natural or slightly enriched UO2 fuel in bundles about 0.5 m long. The fuel bundles are replaced on-power by fuelling machines at both ends of the core. Figure 1 shows a schematic of the arrangement of fuel bundles and fuel channels inside the calandria vessel. The calandria vault is not shown in the figure.

Each safety function in a CANDU reactor is performed by at least two independent means. For any design basis or severe-accident, a CANDU reactor can be shutdown by two independent shutdown systems, in addition to the control system. If the reactor coolant pressure boundary remains intact, the decay heat is removed by the steam generator heat sink or a dedicated decay heat removal system [1]. In loss-of-coolant accidents, the decay heat is removed by the Emergency Core Cooling System. In postulated “limited core damage accidents” such as a loss of coolant accident, plus unavailability of the Emergency Core Cooling System, the separately cooled moderator provides an effective heat sink. The Moderator Cooling System has sufficient heat removal capacity to remove the decay heat and thus prevents gross melting of the fuel and maintains fuel channel integrity. In addition to the adequate heat removal capability of the moderator, the local sub-cooling should be sufficient to prevent overheating and failure of the calandria tube on pressure-tube contact [2]. Prevention of prolonged film boiling on the calandria tube surface is adequate to avoid channel failure.

Under a very low probability accident scenario, such as in a Station Blackout scenario (SBO) with loss of all AC power, including both sets (Group 1 and Group 2) of redundant emergency diesel generators, the moderator circulation/cooling system itself could also be unavailable. Then, the fuel channels are uncovered as the moderator boils off. The channels will gradually sag during heat-up and break-up forming debris. Even in such accidents, called “Severe Core Damage Accidents”, the debris will be contained within the calandria vessel as long as it remains cooled on the outside by the shield-tank water [3,4,5]. The slow boil-off of the shield tank water will delay the failure of the calandria vessel for about a day [3], which will give the operator sufficient time to implement Severe Accident Management measures to arrest the accident progression.

This paper describes small-scale CANDU core disassembly tests conducted with single and multiple channels to understand the behaviour of CANDU channels during moderator boil-off, when the channels are expected to sag and to break up to form debris [6]. A sagging channel eventually contacts the lower channel and when the lower channel is also uncovered it is expected to sag under its own weight as well as that of the supported channel. This process would continue, as more channels are uncovered. As sagging increases, it is expected that the channel segments separate near the bundle junctions. A suspended debris bed is thus formed, which moves downward with the falling moderator level. The submerged channels will be able to support a finite number of channels after which they are also expected to fail. The loading on the submerged channels increases with the accumulation of debris from top channels, thereby leading

to progressive failure of the lower channels and ultimately resulting in the collapse of the core into the moderator pool in the bottom of the calandria vessel.

The progression of a severe core damage accident in a CANDU reactor is analyzed using the MAAP4

CANDU code, which is the CANDU-version of the MAAP code. AECL and Ontario Power Generation Inc., in co-operation with Fauske and Associates Inc., have developed the MAAP4 CANDU code for severe core damage accident analysis in a CANDU reactor. In addition to the core disassembly experiments described above, this paper also discusses the expected behaviour of CANDU corium inside the calandria vessel, which is based on the results of a Station Blackout (SBO) analysis using the MAAP4 CANDU code. As expected, the analysis results show that long times are available for administering Severe Accident Management measures to arrest the progression of the accident. Small-Scale Core Disassembly Tests

In a first step to understand the core disassembly phenomenon, single-row and two-row channel disassembly tests, using small-scale channels, scaled-down to one-fifth the scale of a CANDU 6 channel, were conducted in an inert atmosphere. The tests investigated channel deformation, failure and disassembly mechanisms. A numerical model was developed to explain the observed mechanical deformation behaviour of the single channel test. Scaling Calculations

The fuel channel geometry considered in the experiments is a pressure tube-calandria tube composite, with the pressure tube ballooned into circumferential contact with the calandria tube. The pressure tube material, Zirconium-2.5 wt% Niobium (Zr-2.5Nb), was chosen as the channel material. The creep and tensile properties of the CANDU pressure tube material were achieved in the small-scale channel by appropriate heat treatment processes. The tensile stress distribution, temperature distribution and end-restraints were replicated in the experiments. The remaining factors, including geometry, flexural stiffness and support by lower channels, were addressed through scaling. The tests reported here were conducted in an inert atmosphere to understand the deformation and failure mechanisms of the channels under simplified conditions; the effect of oxidation will be addressed in separate tests.

A constant scaling ratio of the significant dimensions of the full-size channel could be maintained in the scaled-down geometry so that geometric similarity between the full size and the small-scale channel was achieved. The stress level of the full-size channel was maintained in the small-scale channel by reducing the channel wall thickness, so that the deformation and failure mechanisms of the channel were scale-independent. Scaling calculations showed that the simulated fuel bundles (heaters) inside the small-scale experimental channel required a high-density material to maintain the high stress levels of the full-size channel. Therefore, the heaters were made of high-density tungsten, which could also withstand the high temperatures during a test. The scaled heaters were 99 mm long with an O.D. of 20.8 mm. Each channel had twelve heaters to represent the twelve fuel bundles in a CANDU channel. The scaled dimensions of a test channel were as follows: distance between the end-supports was 1.2 m long with a channel I.D. of 24.1 mm and a wall thickness of 0.4 mm. The vertical separation between the channels for the two-row test was 32 mm.

Test Facility

A facility called the “Core Disassembly Test Facility (CDF)” was designed and built to study the disassembly behaviour of a CANDU core using small-scale channels. Figure 2 shows the test chamber made of stainless steel with a front window, which allows visual monitoring and videotaping of the experiment. The facility can test up to four vertically-stacked channels. The top plate of the chamber has gas-tight openings, which allows laser measurement of channel sag; the back plate of the chamber contains electrical fittings that provide power to the heaters.

To conduct a test, the test channels are instrumented with thermocouples, spot-welded to the sheath along the top, bottom and sides of the channel at different locations to measure the channel temperatures. The channel end restraints are designed to allow the same degrees of freedom as in the reactor: one end of the channel is completely fixed against any movement, and the other end, the “floating end”, is allowed to move horizontally, but not rotate. To measure the horizontal movement of the channel during heat-up and cooling, a linear variable displacement transducer is positioned at the floating end. A restraining plate is installed at the floating end to stop the channel from inward movement beyond the original position during high temperature sag. The typical maximum channel temperature during a test is about 14000C. Various test data such as temperature, sag at channel’s mid-point, channel horizontal displacement and channel power can be monitored using the LabVIEWTM software. In a typical test, the two end heaters are not powered to minimize the heat load on the end supports. Test Procedure

In a typical test, the channel is held in the range of reactor operating temperature at ~3000C until thermal equilibrium is reached. The power is then increased so that the channel reaches a maximum temperature in the range 1300 to 14000C. The heat-up rate up to the maximum test temperature is varied in the range 0.1 to 1.20C/s in different tests to cover the expected heat-up rate of fuel channels when uncovered by the moderator. The channel is held at the maximum temperature for holding times in the range 600 to 5500 s in the single channel tests. In the two-row test, after the top channel temperature increases from its equilibrium temperature of ~3000C, the channel below is powered from its equilibrium temperature of ~3000C after a time lag of ~2000 s. The time lag corresponds to the approximate time required to uncover the next channel below in a CANDU core undergoing a severe core damage accident.

In addition to monitoring the various test data and videotaping the test, post-test examination of the channel is conducted in which the axial sag profile, any changes to the original diameter and wall-thickness along the top and bottom at various locations are measured. The channel is also radio-graphed to determine the post-test location of the heaters. Model Development

A finite element model using the ABAQUS code [7,8] was developed to explain the single channel test findings. The ABAQUS finite element program was used for the modelling because it allows the solution of thermal-mechanical problems involving highly non-linear deformations such as the significant creep sag observed in the experiments.

The full length of the channel was modeled with 93 beam elements and 94 nodes. An element, 1 mm

long, was assigned to the gap between the heaters. As in the test channel, one end was restrained against displacements and rotations in all directions. The other end was similarly restrained in all directions except for the axial direction, where it was allowed to move freely but was restrained to move inward beyond the original location.

Post-test destructive channel examinations revealed significant wall thinning at the bundle junctions, suggesting accelerated creep and high stress concentration in the gap region between the heaters. Therefore, the debris could consist of small channel segments. A constant factor to magnify the effect of stress concentration in the gap region was applied to the creep rate equation for the gap element, which was used to model the deformation in the gap region (localized strain model). This stress intensity factor was obtained by matching the measured transient sag of one test with the calculated results. The same factor was applied to a different test and obtained good agreement between the model and the test results. The modeling results demonstrated that such a simple approach adequately models the complicated creep deformation mechanism and mechanical interaction between the heaters and the test channel. Test Results

Although six single channel tests and two multi-channel tests, one with two-row and another with three-row channels, were conducted in an inert atmosphere within the experimental program, results of a single channel test and a two-row test are described in this paper to illustrate the general findings. Single Channel Results

A test (CD-9) with an almost linear heatup rate from 8400C to 13900C was chosen as the base case for modelling. The model using the same stress concentration factor was applied to another test (CD-7) to demonstrate the adequacy of the simple approach to model the experiments. The top and bottom measured temperatures as a function of time were divided into linear segments and used as input in the ABAQUS model. At first, calculations were performed without considering accelerated creep at the bundle junctions for test CD-9. The calculated sag curve is plotted as a function of time in Figure 3, which shows that, while the pattern of the curve agrees with the experimental plot, the magnitude of the sag is lower than the measured sag by about 25% at the end of heatup at 16,650 s. Good agreement was obtained between the calculated curve and the experimental results, when accelerated creep at the bundle junctions was accounted for by applying a constant factor of 4.75 on the stress in the creep rate equation and applying it only at the 1.0 mm gap at the heater junctions (localized strain model) The factor was arrived at on a trial and error basis. This case is plotted in Figure 3, which shows that the sag curve is only about 2% lower than the experimental result at 16,650 s. The overall characteristics of the sag curve agree reasonably well with the experimental results, especially in predicting the rapid increase of sag at 5000 s when the temperatures exceed about 8000C.

The localized strain model was applied to simulate another single channel test CD-7 after implementing the corresponding linearized temperature history. Figure 4 shows that the overall characteristics of the calculated sag curve agree reasonably well with the experimental results. Good agreement was obtained between the model and the test for permanent sag for test CD-7 (Figure 5), when

the same stress intensity factor was used. The pattern of concentrated creep strain in the gap regions between the heaters along the bottom side of the channel was also correctly predicted by the model. The good agreement between the model and the experiment shows that the deformation behaviour of the channel during core disassembly experiments is dominated by creep and localized effects. The experimentally observed wall thinning in the gap suggests that debris formed by this mechanism will likely be small channel segments as a result of the creep-sag deformation and localized effects. Two-Row Channel Test Results

Results of the two-channel test (CD-8) are described here. Figure 6 shows the horizontal displacements of the Row 1 and Row 2 channel free-ends. As both channels heat up to the range 300 to 400°C, the thermal expansion causes a horizontal displacement of about 2 mm. At ~3000 s the heat up of Row 1 channel is increased, which results in an increase of the horizontal displacement. At the maximum horizontal displacement of ~3.0 mm the channel free-end reverses direction because the continuing thermal expansion is fully compensated and then exceeded by the increasing sag rate. At point “A” in Figure 6, the Row 1 channel bottom temperature exceeds 800°C, which results in a rapid decrease of the horizontal displacement since the sag rapidly increases. From A to A1 time period, the channel free-end moves back and at point A1 the Row 1 channel contacts the Row 2 channel (temporarily halting the movement of its floating end) and transfers heat to the Row 2 channel. The Row 2 channel heats up and reaches 800°C at B, when it begins to sag rapidly. During the time period from L1 to L2, the Row 1 channel is supported by the Row 2 channel, which is reflected in the horizontal plateau L1 to L2 in Figure 6. At 5400 s, the Row 1 channel moves to its original location, where it is stopped from moving inward at point C by the fuel channel design. The horizontal displacement of the Row 2 channel remains a constant from L1 to L3 because the sag is compensated for by the thermal expansion during that time period. At L3 the channel heat up essentially stops; but the sag continues, which results in a slow decrease in the observed horizontal displacement. At 8600 s the Row 2 channel moves back to its original location, where it is stopped from moving inward by the fuel channel design (location D in Figure 6). Post-test examination showed cracking of the bottom side of the Row 1 channel at both sides, four bundle lengths away from the channel mid-point. This cracking had occurred at high temperatures as shown in a photograph reproduced in Figure 7. The channel assembly was sectioned along the axis and the wall thickness was measured along the top and bottom of the channel at different locations. The Row 1 bottom and Row 2 top walls of the channel had bonded in the mid-region over a length of ~308 mm while at high temperature. These tests demonstrated, that the support provided by the lower channels would result in a slow down of the creep rate of the top channel and in subsequent failure of the top channel at locations near the end of the channel. Thus the debris formed by this process is long, unlike the single channel test observations, where smaller bundle segments were observed. Tests in an oxidizing atmosphere are underway to study the effect of oxidation on channel disassembly behaviour. MAAP CANDU Analysis Results of CANDU Corium Behaviour in the Calandria Vessel

The experimental results described above showed that fuel channel segments separate near the bundle junctions as a result of high temperature sag. As discussed in the introduction, a suspended debris bed will then be formed inside the calandria vessel, which will be supported on submerged channels until the

loading on the submerged channels increases with the accumulation of debris from top channels. This process will lead to progressive failure of the lower channels and ultimately resulting in the collapse of the core into the moderator pool in the bottom of the calandria vessel. The core debris behaviour within the calandria vessel was analysed with the MAAP4 CANDU code (version 4.0.4A+) for a CANDU 6 Station Blackout scenario with the loss of off-site AC (Class IV) power and subsequent loss of all on-site standby and emergency electric power supplies. For reference, the CANDU 6 calandria vessel has an I.D. of ~7600 mm and a wall thickness of ~29 mm. The CANDU 6 core has an inventory of ~98.8 Mg uranium dioxide and ~38.7 Mg Zircaloy. The timing of some of the events calculated by the code is summarized in Table 1. Details of the accident progression following a SBO accident sequence is described elsewhere [9]. Only the core debris behaviour calculated by the code is discussed here.

In the SBO accident scenario analyzed, the reactor is shut down; the core decay heat is transferred to the steam generators by natural convection. As a result, the steam generator secondary side boils off discharging steam from the secondary side to outside of the containment through the main steam safety valves. The steam generators dry off at ~2.5 h and the heat transport system (HTS) pressure increases until it reaches the liquid relief valve set point and oscillates at the relief valve set point, which finally results in fuel channel dryout. Subsequent heatup of the pressure and calandria tubes at ~10 MPa HTS pressure results in ballooning and rupture of one channel/loop causing a rapid blowdown of the HTS coolant into the calandria vessel at ~4.4 h. In parallel, the moderator level in the calandria vessel decreases as a result of moderator expulsion and boiloff, which eventually causes the uncovery of some of the top fuel channels.

As fuel channel axial segments heat up and sag, they break up and form debris. In the MAAP CANDU model, the fuel channel fragments relocate to “holding bins”, where they are held temporarily as a “suspended debris bed”. The suspended debris bed heats up further from the decay heat and from the Zr/steam exothermic reaction resulting in partial melting. Some of the molten material relocates from the suspended debris bed to the bottom of the calandria vessel and is quenched in the water. When the suspended debris bed cannot be supported by the intact channels covered by water, the suspended debris bed and most of the intact channels relocate to the bottom of the calandria vessel by core collapse, calculated to occur at ~8.3 h. A significant amount of the heat is transferred to the calandria vessel top by convection and radiation from the debris. The heat flux from the debris to the water surrounding the calandria vessel bottom is less than 50% of the critical heat flux.

Following core collapse, the moderator in the calandria vessel is depleted at ~8.9 h. The water in the calandria vault cools the external calandria vessel wall. Steam generated in the calandria vault is released into the containment. The calandria vault water reaches saturation temperature at ~14.5 h. The containment pressure increases gradually and reaches the containment failure pressure of 500 kPa (a), at ~27.1 h. When water level in the calandria vault reaches near the calandria vessel bottom, the vessel bottom heats up rapidly from the heat from the core debris and the calandria vessel fails due to creep at ~42.4 h. The debris relocates into the concrete calandria vault floor and is cooled by the water, which prevents significant corium/concrete interactions until the calandria vault dries out at ~ 46 h. At ~104.3 h the calandria vault floor fails as a result of corium/concrete interactions and the mixture relocates into the basement, where it interacts with the sump water.

Figure 8 shows the inside surface temperature of calandria vessel wall for the various nodes from the bottom of the calandria vessel. Initially, the temperatures of all those nodes at different elevations are about the same because the calandria vessel shell is immersed in the water in the calandria vault. The

spike at ~11.1 h for the two bottom nodal temperatures is the result of collapsed debris melting inside the calandria vessel. At ~14.5 h the water in the calandria vault starts to boil off and at ~27.5 h the top node of calandria vessel is no longer immersed in water. The temperature of the top node (7.56 m) and the node 5.34 m from calandria vessel bottom, therefore, sharply increase to the range ~1000 to 1070 K. The top calandria vessel wall temperatures are relatively low because heat is transferred from the top wall nodes by several mechanisms: i) to the bottom nodes by circumferential conduction and eventually to the water in calandria vault and ii) by convection/radiation to the calandria vault atmosphere and concrete walls. When the calandria vault water level decreases to ~2.5 m at ~42.4 h, the calandria vessel bottom heats up rapidly (see nodes from 0.04 to 0.95 m from calandria vessel bottom in Figure 8). The vessel bottom then fails due to creep at ~42.4 h, when the debris relocates into the calandria vault, where it is cooled by the calandria vault water. Following the relocation of the debris into the calandria vault, the calandria vessel wall temperatures drop as shown in Figure 8. At ~104 h, the calandria vessel wall temperatures drop again because the reactor vault floor fails and the corium relocates into the basement.

Figure 9 shows the results of MAAP4 CANDU analysis for total corium mass (including particulates

(solid debris), corium crust, and molten corium), the mass of the corium crusts (bottom, side and top surfaces) and the mass of particulates in the calandria vessel. A crust is formed on the calandria vessel wall; the crust thickness on the calandria vessel walls is in the range 5 to 10 cm. Similar corium crust thickness (7 to 8 cm) was predicted for similar conditions by independent studies [4]. After the water in the calandria vessel is depleted, the core debris in the calandria vessel begins to heat up and eventually the solid debris melts and joins the molten debris pool at ~14 h. The transient molten pool temperature and the average corium temperature are shown in Figure 10. When the calandria vessel fails, all corium falls into the reactor vault at ~42.4 h, which is shown in Figure 9 as a sudden drop of the total debris mass in the calandria vessel. Conclusions The following general conclusions are drawn from the study reported here:

• Experiments showed that significant sag of the CANDU fuel channel driven by creep occurs above 8000C. Since creep deformation is a slow process, the core damage progression in a CANDU core is expected to be a slow process.

• A simple ABAQUS beam element model confirms that the deformation behaviour of the channels during the core disassembly experiments is dominated by creep and localized effects.

• Significant strain localization observed at the bundle junctions along the bottom side of the channel suggests that the core debris are formed by accelerated creep in the gap region between the bundles.

• Single channel tests showed that channel debris during fuel channel heat up would likely consist of small channel segments, whereas the two-row channel test showed that the support provided by the bottom channel to the deforming top channel would produce long channel debris segments. The experiments also showed that the channel break up temperature is less than 14000C.

• The MAAP4 CANDU analysis for a severe Station Blackout sequence demonstrated that significant time is available for operator action during the accident to arrest the accident progression. For example it takes ~4.4 h for the moderator and ~14.5 h for the calandria vault water to reach the saturation temperature.

References [1] Snell, V.G., Bonechi, M. and Kupferschmidt, W.C.H., “Advances in Nuclear Safety”, Proceedings of Pacific Basin Nuclear Conference, Seoul, Korea, October 29-November 2, 2000. [2] Gillespie, G.E., Moyer, R.G., Hadaller, G.I. and Hildebrandt, J.G., “An experimental investigation into the development of pressure tube/calandria tube contact and associated heat transfer under LOCA conditions”, Proc. of the 6th Annual Canadian Nuclear Society Conference, Ottawa, ON, 2.24-2.30, 1985. [3] Meneley, D.A., Blahnik, C., Rogers, J.T., Snell, V.G. and Nijhawan, S., “Coolability of Severely Degraded CANDU Cores”, International Seminar on Mass and Heat Transfer in Severe Reactor Accidents, Cesme, Turkey, May 22-26, 1995. [4] Rogers, J.T. and Lamari, M.L., “Transient Melting and Re-Solidification of CANDU Core Debris in Severe Accidents”, Proceedings of 20th Canadian Nuclear Society Simulation Symposium, Niagara-on-the-Lake, Ontario, Canada, September 7-9, 1997. [5] Muzumdar, A.P., Mathew, P.M., Rogers, J.T. and Lamari, M.L., ”Core Melt Retention Capability of CANDU Reactors”, Proceedings of Pacific Basin Nuclear Conference, Banff, Canada, May 3-7, 1998. [6] Mathew, P.M., Kupferschmidt, W.C.H., Snell, V.G. and Bonechi, M., “CANDU-Specific Severe Core Damage Accident Experiments in Support of Level 2 PSA”, Proceedings of 16th International Conference on Structural Mechanics in Reactor Technology, SMiRT 16, Washington DC, August 12-17, 2001. [7] ABAQUS Program, HKS Inc., Version 5.7.3, 1998. [8] Mathew, P.M., White, A.J., Snell, V.G. and Bonechi, M., “ Severe Core Damage Accident Analyses and Experiments for CANDU Applications”, Proceedings of 17th International Conference on Structural Mechanics in Reactor Technology, SMiRT 17, Prague, August 17-22, 2003. [9] Mathew, P.M., Kupferschmidt, W.C.H., and Bonechi, M., “Application of PSA to CANDU Design and Licensing”, Proceedings of Pacific Basin Nuclear Conference, Shenzhen, China, October 21-25, 2002.

Table 1. Sequence of Some Events for a CANDU 6 Station Blackout Scenario

Time (h) Comments

0 Loss of AC and all Backup and Emergency Power supplies 2.5 Steam Generator secondary side is dry 4.4 Pressure and calandria tubes are ruptured 4.4 Moderator in calandria vessel reaches saturation temperature 4.8 Beginning of core disassembly 6.3 Beginning of core debris relocation onto the CV bottom 8.3 Collapse of entire core onto the CV bottom 8.9 Calandria vessel water is depleted

11.1 Beginning of collapsed debris melting 14.5 Water in calandria vault reaches saturation temperature 27.1 Containment failed 42.4 Calandria vessel failed due to creep 46.0 Calandria Vault Water depleted

104.3 Calandria Vault failed as a result of concrete erosion

Fig. 1 Schematic of a CANDU Reactor Core showing the Fuel Channel Concept

Fig. 2 A photograph of the core disassembly test facility showing the test chamber

Fig. 3 Comparison of Measured Sag at Channel Mid-point with Model Results with and without localized Strain Model for Test CD-9

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Fig. 5 Comparison of Measured Post-Test Permanent Sag with Model Results using localized Strain Model for Test CD-7

Fig. 6 Row 1 and Row 2 Horizontal Displacements (CD-8)

Fig. 7 View of One End of the Row 1 Channel, showing Hot-Tear at Two Heaters away from Channel End, sitting on top of Row 2 Channel at Temperature (CD-8)

Break up of Row 1

Intact Row 2

Fig. 9 MAAP4 CANDU Results for Corium Mass in Calandria Vessel (SBO). (Total mass includes molten, crust and particulate components)

Fig. 8 MAAP4 CANDU Results for Calandria Vessel Wall Temperature for various nodes from Calandria Vessel Bottom (SBO)

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Fig. 10 MAAP4 CANDU Results for Core Debris Temperaturein the Calandria Vessel (SBO) (*Since the code does not calculate the debris and molten pool temperatures until those phases are formed, MAAP4 CANDU assigns a value of 0K to those temperatures.)

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