EXTERNAL REPORT SCK CEN-ER-86 09/EWe/P-20 Update of the near field temperature evolution calculations for disposal of UNE-55, MOX-50 and vitrified HLW in a supercontainer- based geological repository Eef Weetjens ONDRAF/NIRAS contract CCHO-2004-2470/00/00 February, 2009 SCK CEN Boeretang 200 BE-2400 Mol Belgium IPA-PAS
EXTERNAL REPORT OF THE BELGIAN NUCLEAR RESEARCH CENTRE SCK CEN-ER-86 09/EWe/P-20 Update of the near field temperature evolution calculations for disposal of UNE-55, MOX-50 and vitrified HLW in a supercontainer- based geological repository Eef Weetjens ONDRAF/NIRAS contract CCHO-2004-2470/00/00 February, 2009 Status: Unclassified ISSN 1782-2335 SCK CEN Boeretang 200 BE-2400 Mol Belgium
SCK CEN Studiecentrum voor Kernenergie Centre d étude de l énergie Nucléaire Boeretang 200 BE-2400 Mol Belgium Phone +32 14 33 21 11 Fax +32 14 31 50 21 http://www.sckcen.be Contact: Knowledge Centre library@sckcen.be RESTRICTED All property rights and copyright are reserved. Any communication or reproduction of this document, and any communication or use of its content without explicit authorization is prohibited. Any infringement to this rule is illegal and entitles to claim damages from the infringer, without prejudice to any other right in case of granting a patent or registration in the field of intellectual property. SCK CEN, Studiecentrum voor Kernenergie/Centre d'etude de l'energie Nucléaire Stichting van Openbaar Nut Fondation d'utilité Publique - Foundation of Public Utility Registered Office: Avenue Herrmann Debroux 40 BE-1160 BRUSSEL Operational Office: Boeretang 200 BE-2400 MOL
Table of contents 5 1 Introduction...7 2 Methodology...7 2.1 Source term...7 2.2 Material data...8 2.3 Geometry...9 3 Results...10 3.1 Spent fuel UNE-55...10 3.2 Spent fuel MOX-50...11 3.3 Vitrified HLW...12 4 Discussion...13 5 References...14
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7 1 Introduction In 2004-2005, right after the decision of ONDRAF/NIRAS to opt for the supercontainer for accommodating category C waste in a geological repository (ONDRAF/NIRAS, 2004), heat transport calculations have been performed in the context of the GTA (Repository Conceptual Design) working group. These calculations, reported in SCK CEN report R-4277 (Weetjens and Sillen, 2005) aimed at calculating the evolution of the near field temperatures in a backfilled repository in order to evaluate whether these were acceptable (according to a rather arbitrary temperature criterion limiting the temperature in the supercontainer buffer to 100 C). The applied thermal properties of the EBS materials were at that time fairly unknown, since detailed material characterisation studies were only in an embryonic phase. Moreover, there seemed to be quite some uncertainty on the value of the thermal conductivity of the Boom Clay as well. In fact, a sensitivity study (Weetjens and Sillen, 2005) showed that the uncertainties on the thermal properties of the Boom Clay had more impact on the calculated near field temperatures than these of the EBS. As recently new thermal data were reported for both Boom Clay (results of the ATLAS III experiment, to be published) and the concrete buffer (Craeye et al., 2009), the near field heat transport calculations are updated, taking new geometry developments (supercontainer dimensions) into account as well. Apart from an update for vitrified HLW and UOX spent fuel (UNE-55), the analysis is complemented with thermal calculations for MOX spent fuel (MOX-50), previously not yet reported. 2 Methodology The same general methodology and governing equations are used as in R-4277 (Weetjens and Sillen, 2005). When different assumptions, input data, etc. are applied, they are explicitly mentioned in this note. 2.1 Source term The evolution of the heat generation is represented through a sum of exponentials: = λi t Q A e (Q in W/tHM) [1] i i where the coefficients A i and λ i (defined in Table 1) are fit to external data. These fits are shown below for vitrified HLW (Figure 1) and UNE and MOX spent fuel with a burn-up of 55 GWd/tHM and 50 GWd/tHM respectively (Figure 2). The heat output for VHLW, shown in W/tHM, can be converted into W/m knowing that reprocessing of 1.333 thm results in 1 canister and there are 2 canisters packed in a supercontainer of 4.2 m length. Likewise, the thermal output for spent fuel can be converted into W/m for application in our model considering that the initial mass of heavy metal is 0.459 thm/assembly for UNE and 0.457 thm/assembly for MOX-50. A supercontainer for UNE-55 spent fuel is 6.177 m long and contains 4 assemblies, whereas for MOX-50, the supercontainer (of 6.1 m length) contains only 1 assembly. Table 1: Coefficients for equation [1] for the waste types considered in this study. A1 A2 A3 A4 A5 λ1 λ2 λ3 λ4 λ5 VHLW 5021 1205 27.04 0.7576 0.1 3.89E-01 2.46E-02 1.63E-03 6.55E-05 0 UNE-55 1813 231.1 140.5 23.46-2.24E-02 3.81E-03 9.95E-04 2.70E-05 - MOX-50 3782 1545 326.4 100.6-2.27E-02 2.84E-03 3.74E-04 2.86E-05 -
8 10000 Thermal output (W/tHM) 1000 100 10 Vitrified HLW (Put) Vitrified HLW (ORIGEN) NAGRA Vitrified HLW 1 JNC-H12 Vitrified HLW XSi/2003/201 1 10 100 1000 10000 Time after waste production (a) Figure 1: Heat generation in vitrified HLW. Fit using [1] by Put (Put and Henrion, 1992) to independent calculations (ORIGEN code) and measurements of NAGRA and JNC (H-12 report, JNC, 2000). 1.0E+05 Thermal output (W/tHM) 1.0E+04 1.0E+03 1.0E+02 UNE-45 BN data UNE-55 BN data URE-55 BN data MOX-50 BN data UNE-45 fit UNE-55 fit URE-55 fit MOX-50 fit 1.0E+01 1.0E+00 1 10 100 1000 10000 100000 Time after spent fuel unloading (a) Figure 2: Heat generation in several spent fuel types. Best fit using formula [1] to data obtained from Belgonucléaire. In these calculations, a cooling time of 60 years was assumed. This covers the time period from unloading from the reactor (SF) or waste vitrification (VHLW) to backfilling of the disposal galleries. The initial temperature throughout the model domain was assumed to be 16 C. 2.2 Material data The thermal properties of the EBS components and of Boom Clay are listed in Table 2. The data for waste and overpack, filler (second phase concrete) and backfill remained unchanged compared to R-4277. In Craeye et al. (2009) two variants for the supercontainer buffer are considered: self-compacting concrete (SCC) and traditional vibrated concrete (TVC). The thermal data for these concrete compositions are the ones reported in Craeye et al. (2009). Here, we will only consider SCC since this concrete has the most penalising, i.e. lower value
9 for the thermal conductivity. The thermal conductivity value for the concrete gallery lining is provided by ONDRAF/NIRAS. The most drastic change compared to previous thermal calculations is the applied value for the thermal conductivity of the Boom Clay. Results of the ATLAS III experiment revealed that there is substantial anisotropy in the thermal conductivity, namely 1.7 W/(m K) in the horizontal and 1.25 W/(m K) in the vertical direction (ATLAS III, results to be published). As this anisotropy is impossible to implement in the 2D axisymmetric calculations performed here, we used a conservatively rounded mean value of 1.45 W/(m K). Table 2: Thermal properties of the EBS materials and host formation (α h =ρ C p ). λ [W/(mK)] ρ [kg/m 3 ] C p [J/(kgK)] α h [J/(m 3 K)] waste and overpack 40 7850 500 second phase concrete 1 2400 880 SSC buffer (self-compacting concrete) 1.89 2.420E+06 TVC buffer (traditional vibrated concrete) 2.02 2.440E+06 cementitious backfill 1 2400 880.00 concrete lining 1.5 2400 880.00 Boom Clay 1.45 2.900E+06 The buffer will probably be surrounded by a stainless steel liner. This liner is not explicitly modelled since its limited thickness and high thermal conductivity make that it has very little influence on the temperature field. 2.3 Geometry The heat transport simulations are done for a 2D axisymmetric model of half a supercontainer (see Figure 3, dimensions not to scale). The simulations are run with the FE calculation tool COMSOL Multiphysics 3.2 (COMSOL, 2005). The dimensions used in the model are based on a table presented by H. Van Humbeeck at the 13 th EURIDICE exchange meeting (2009), complemented with other data provided by ONDRAF/NIRAS (see Table 3). Table 3: Supercontainer dimensions used in the calculations (based on Van Humbeeck, 2009) UNE-55 MOX-50 VHLW overpack diameter (m) 0.8430 0.3650 0.5160 thickness 2nd phase concrete (m) 0.0500 0.0500 0.0500 supercontainer diameter (m) 2.1040 1.6000 2.0280 radial thickness buffer (m) 0.5805 0.5675 0.7060 supercontainer length (m) 6.1770 6.1000 4.2000 overpack length (m) 5.0920 5.0920 2.8000 top/bottom thickness buffer (m) 0.4925 0.4540 0.6500 inner gallery diameter (m) 3.0000 3.0000 3.0000 gallery lining thickness (m) 0.3000 0.3000 0.3000
10 r 0 Boom Clay gallery lining cementitious backfill OPC concrete buffer filler o o o t s s a i t o r waste and overpack θ Figure 3: Model geometry: only half a supercontainer needs to be modelled for symmetry reasons (example for disposal of vitrified HLW, dimensions not to scale). 3 Results 3.1 Spent fuel UNE-55 The results in terms of thermal profiles through the middle of the supercontainer are depicted in Figure 4. The coloured lines represent the temperature at different times after waste emplacement. The interface between the 2 nd phase concrete filler and the buffer concrete is shown as well. This is the location where the NF temperature limit (currently set at 100 C; Bel and Bernier, 2005) is evaluated. As can be seen from Figure 4, this temperature limit is exceeded for some time. temperature ( C) 120 110 100 90 80 70 60 50 40 30 20 filler/buffer interface NF temperature criterion initial temperature 1 year 2 years 5 years 10 years 20 years 50 years 100 years 10 UNE-55 0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 distance from gallery axis (m) Figure 4: Near field temperature profiles for disposal of UOX spent fuel (UNE-55) in a supercontainerbased repository (cooling time: 60 years)
11 Figure 5 shows the evolution of the temperature at different locations. The curve in light-blue represents the temperature at the interface between the concrete filler and the buffer. It is clear that the temperature exceeds the temperature limit during about 20 years. 120 110 100 90 2nd phase concrete filler SCC concrete/backfill interface lining/clay interface filler/scc buffer interface backfill/gallery lining interface 1 m deep in Boom Clay NF temperature criterion temperature ( C) 80 70 60 50 40 30 20 initial temperature 10 UNE-55 0 0.0 10.0 20.0 30.0 40.0 50.0 60.0 70.0 80.0 90.0 100.0 Time (a) Figure 5: Temperature evolution at different locations in the near field of a disposal gallery filled with UOX spent fuel (type UNE-55; cooling time 60 years) 3.2 Spent fuel MOX-50 Similar results were obtained for the temperature evolution around a disposal gallery filled with MOX spent fuel. Temperature profiles and temperature evolution at specific locations are shown in Figure 6 and Figure 7 respectively. Please note that also the MOX-50 was allowed to cool down for 60 years prior to disposal. The temperature at the interface with the concrete buffer reaches 112 C and exceeds the near field temperature criterion during a period of about 50 years. The reason why the problem is even more pronounced for MOX spent fuel is because the overpack diameter is smaller since it contains only 1 assembly than for UOX spent fuel and hence, the temperature criterion is evaluated close to a more compact source.
12 temperature ( C) 130 120 110 100 90 80 70 60 50 40 30 20 10 filler/buffer interface NF temperature criterion initial temperature MOX-50 0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 distance from gallery axis (m) 1 year 2 years 5 years 10 years 20 years 50 years 100 years Figure 6: Near field temperature profiles for disposal of MOX spent fuel (MOX-50) in a supercontainerbased repository (cooling time: 60 years) 120 110 100 90 2nd phase concrete filler SCC concrete/backfill interface lining/clay interface filler/scc buffer interface backfill/gallery lining interface 1 m deep in Boom Clay NF temperature criterion temperature ( C) 80 70 60 50 40 30 20 initial temperature 10 MOX-50 0 0.0 10.0 20.0 30.0 40.0 50.0 60.0 70.0 80.0 90.0 100.0 Time (a) Figure 7: Temperature evolution at different locations in the near field of a disposal gallery filled with MOX spent fuel (type MOX-50; cooling time 60 years) 3.3 Vitrified HLW Results for disposal of vitrified HLW are shown in Figure 8 in terms of temperature profiles and in Figure 9 in terms of temperature evolution at the EBS interfaces. The temperature criterion is not exceeded in this case.
13 120 110 100 90 80 NF temperature criterion 1 year 2 years 5 years 10 years 20 years 50 years 100 years temperature ( C) 70 60 50 40 30 20 filler/buffer interface initial temperature 10 VHLW 0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 distance from gallery axis (m) Figure 8: Near field temperature profiles for disposal of vitrified HLW in a supercontainer-based repository (cooling time: 60 years) 120 110 100 90 2nd phase concrete filler SCC concrete/backfill interface lining/clay interface filler/scc buffer interface backfill/gallery lining interface 1 m deep in Boom Clay NF temperature criterion temperature ( C) 80 70 60 50 40 30 20 initial temperature 10 VHLW 0 0.0 10.0 20.0 30.0 40.0 50.0 60.0 70.0 80.0 90.0 100.0 Time (a) Figure 9: Temperature evolution at different locations in the near field of a disposal gallery filled with vitrified HLW (cooling time 60 years) 4 Discussion It is clear that, due to the smaller thermal conductivity of the Boom Clay compared to the previous thermal analysis, the heat generated by the waste is less efficiently dissipated into the host rock resulting in increased near field temperatures, and in case of the two considered
14 spent fuel types, an exceedance of the (arbitrary) temperature limit. There are several ways to tackle this problem: 1. increase cooling time: is the easiest solution, but might be "overkill" 2. insert a thermal "diluter" or "heat spreading ring" around the waste package: from Figure 4 and Figure 6 it can be seen that the source zone needs to be expanded to a radius of 0.7 m for UOX and 0.5 m for MOX. This thermal "diluter" is preferably a material with high thermal conductivity, although the thermal conductivity will only affect temperatures towards the inside and the duration of the thermal transient period. However, inserting such a thermal diluter might be problematic as well (in terms of construction feasibility, shielding properties, and long term behaviour: potential gas generation/geochemical evolution). 3. review the currently arbitrary near field temperature limit and improve its argumentation base. When there is sufficient experimental support that a slightly higher temperature criterion will not jeopardize the buffer integrity or cause secondary effects influencing long-term behaviour, it might be considered to increase the temperature limit. Shifting the temperature criterion (and its substantiation) to the Boom Clay could also be a solution. 5 References Bel, J. and Bernier, F. "Temperature criterion related to the engineered barriers in the framework of a geological repository of heat producing radioactive waste (vitrified waste or spent fuel)" ONDRAF/NIRAS, Brussels (2005) COMSOL, COMSOL Multiphysics 3.2, Earth Science Module Model Library, COMSOL AB, Stockholm, Sweden (2005). Craeye, B., De Schutter, G., Van Humbeeck, H. and Van Cotthem, A., Early age behaviour of concrete supercontainers for radioactive waste disposal. Nuclear engineering and design Vol. 239, p.23-35 (2009). EURIDICE ESV, The role of cementitious materials for deep disposal of high-level waste in Boom Clay. 13 th EURIDICE exchange meeting (2009). JNC. "H12: Project to establish the Scientific and Technical Basis for HLW Disposal in Japan." Project overview report TN1410, JNC, Japan (2000). ONDRAF/NIRAS, Multi-criteria Analysis on the Selection of a Reference EBS Design for Vitrified High Level Waste. ONDRAF/NIRAS, Brussels (2004). Put, M. and P. Henrion, Modelling of radionuclide migration and heat transport from an HLW-repository in Boom Clay, EC, Nuclear Science and Technology, Luxembourg, EUR 14156 (1992). Weetjens, E. and Sillen, X, Thermal analysis of the Supercontainer concept. 2D axisymmetric heat transport calculations. SCK CEN report R-4277 (2005).