(1) SCK CEN, Boeretang 200, B-2400 Mol, Belgium (2) Belgonucléaire, Av. Arianelaan 4, B-1200 Brussels, Belgium

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1 The REBUS Experimental Programme for Burn-up Credit Peter Baeten (1)*, Pierre D'hondt (1), Leo Sannen (1), Daniel Marloye (2), Benoit Lance (2), Alfred Renard (2), Jacques Basselier (2) (1) SCK CEN, Boeretang 200, B-2400 Mol, Belgium (2) Belgonucléaire, Av. Arianelaan 4, B-1200 Brussels, Belgium An international programme called REBUS for the investigation of the burn-up credit has been initiated by the Belgian Nuclear Research Centre SCK CEN and Belgonucléaire with the support of EdF and IRSN from France and VGB, representing German nuclear utilities and NUPEC, representing the Japanese industry. Recently also ORNL from the U.S. joined the programme. The programme aims to establish a neutronic benchmark for reactor physics codes in order to qualify the codes for calculations of the burn-up credit. The benchmark exercise investigates the following fuel types with associated burn-up: reference fresh 3.3% enriched fuel, fresh commercial PWR fuel and irradiated commercial PWR fuel (54 GWd/tM), fresh PWR MOX fuel and irradiated PWR MOX fuel (20 GWd/tM). The experiments on the three configurations with fresh fuel have been completed. The experiments show a good agreement between calculation and experiments for the different measured parameters: critical water level, reactivity effect of the water level and fission-rate and flux distributions. In 2003 the irradiated BR3 MOX fuel bundle was loaded into the VENUS reactor and the associated experimental programme was carried out. The reactivity measurements in this configuration with irradiated fuel show a good agreement between experimental and preliminary calculated values. KEYWORDS: Burn-up Credit, VENUS, Criticality Experiment 1. Introduction Present criticality safety calculations of irradiated fuel often have to model the fuel as fresh fuel, since no precise experimental confirmation exists of the decrease of reactivity due to accumulated burn-up. In other occasions only actinide depletion is allowed to be taken into account and the influence of fission products has to be disregarded. The fact that this so-called "burn-up credit" cannot (completely) be taken into account has serious economical implications for transport, storage and reprocessing of irradiated fuel. For long-term geological storage it is almost imperative to apply burn-up credit. 2. General description of the REBUS programme The aim of the REBUS programme is to establish an experimental benchmark for validation of reactor physics codes for the calculation of the loss of reactivity due to burn-up, both for and MOX fuel bundles. The reactivity effect of PWR spent fuel will be measured in the VENUS critical facility on a bundle of commercial spent fuel with a burn-up of 54 GWd/tM. The same will be done on a MOX bundle with a burn-up of 20 GWd/tM. Other fuel types can be investigated in future extensions of the programme, like BWR fuel. At the same time we will measure fission rate and flux distributions in the different configurations. Since the commercial PWR spent fuel rods have a length of 4 meter, these rods have to be refabricated into 1 meter rodlets. Afterwards the rodlets will be cleaned thoroughly, since the contamination level of the VENUS reactor has to remain very low. Also the BR3 spent fuel rods have to be cleaned, but no refabrication is needed. After cleaning the rodlets will be assembled in the experimental 7x7 bundle. This will be executed in the SCK CEN hot cell laboratory. Together with a precise measurement of the reactivity effect, it is also indispensable to have a good characterization of the fuel. The characterization of the spent fuel is performed in two steps. The first step is non-destructive and is performed before the reactivity measurements. All spent fuel rods will be measured by gross gamma-scanning in order to determine the distribution of total gamma activity in the fuel rods. One specific rod will be investigated by gamma-spectrometry, together with Corresponding author: Tel , Fax , pbaeten@sckcen.be

2 a well-qualified calibration source, to determine the 137 Cs content and in this way the burn-up of the rod. Via the gross gamma-scans this will give a good picture of the burn-up of all rods. This first step is necessary to verify that the selected rods have a similar burn-up and is necessary to account for the burnup distribution of the rodlets for an accurate simulation with the computer codes. The second step is destructive and is consequently performed after the reactivity measurements. It aims at determining both the actinides content, some burn-up indicators (Cs, Nd) and the 19 most important fission products with respect to neutron absorption (representing >80% of the neutron absorption in the spent fuel). The sample for this destructive, radiochemical assay is taken from the same rod, on which gamma-spectrometry has been performed. PWR MOX fuel, originating from the BR3, an experimental Belgian PWR Irradiated PWR MOX fuel (20 GWd/tM), also from the BR3 reactor All test bundles will be loaded as a 7x7 fuel assembly in the centre of a 23x23 3.3% enriched fuel driver zone surrounded with two outer rows of 4.0% enriched fuel. The 7x7 assembly is chosen because calculations show that such an assembly will result in a reactivity effect that is large enough for benchmark purposes (~2000 pcm) and for practical reasons because VENUS has removable grids where this assembly fits in. The reference 3.3% enriched fuel bundle (figure 2a) consists of the same 3.3% enriched fuel as there is in the driver zone. Its purpose is to validate a k eff calculation. The fresh fuel content has been well-documented during fabrication. 3. Short description of the VENUS reactor The VENUS critical facility 1) is a water-moderated zero-power reactor. It consists of an open (nonpressurized) stainless-steel cylindrical vessel including a set of grids which maintain fuel rods in a vertical position. After a fuel configuration has been loaded, criticality is reached by raising the water level in the vessel. Fig. 2a: Reference fuel configuration design The fresh commercial fuel bundle (figure 2b) consists of a 5x5 fuel assembly, surrounded with 3.3% enriched fuel. The central investigated fuel is 3.8% enriched fuel, fabricated at Framatome ANP (formerly SIEMENS), Germany. It is the original composition of the irradiated commercial fuel. Fig. 1: Vertical cross-section of VENUS 4. Measurements performed at VENUS in the framework of REBUS Five fuel bundles or assemblies will be investigated in the framework of the REBUS programme. These five bundles are: Reference fresh 3.3% enriched fuel commercial PWR fuel, constructed by Framatome ANP (formerly SIEMENS) Irradiated commercial PWR fuel (54 GWd/tM), originating from Neckarwestheim NPP, Germany The irradiated commercial fuel bundle has the same configuration as the fresh bundle, as is obvious for experimental reasons (clean comparison of the fresh and irradiated bundle). The central investigated irradiated fuel is provided by GKN, the operator of the Neckarwestheim NPP in Germany. Fig. 2b: Central 7x7 bundle

3 The fresh BR3 MOX fuel bundle (figure 2c) consists of 24 fuel rods (a 7x7 overmoderated assembly). In this way the highest reactivity effect could be obtained with the available rods. The fuel is 6.9% enriched fissile MOX fuel, fabricated at Belgonucléaire, Belgium. It is the original composition of the irradiated BR3 fuel. The irradiated PWR MOX fuel of intermediate burn-up originates from the BR3 reactor, Belgium. The fuel is provided by SCK CEN. The bundle design of the irradiated bundle is the same as for the fresh bundle (figure 2c). Fig. 3: Measurement positions for horizontal fission rate distribution Fig. 2c: Central 7x7 MOX bundle The reactivity effect will be measured by loading the different bundles in the centre of the driver zone and measuring each time the critical water level and the reactivity effect of a change of the water level. From these measurements the reactivity effect can be estimated. However, a more direct way of validating the reactor codes is simply calculating the k eff for the different configurations at the measured critical water levels. This is a more direct comparison and more accurate, since the determination of the reactivity effect from the measured parameters is subject to some approximations. In addition of the reactivity measurements the fission rate distribution and the flux distribution at the main axes will be measured (see figure 3). Due to the impossibility of measuring this parameter in the spent fuel assembly, wire activation measurements will be performed in this assembly to measure the thermal and epithermal neutron flux. Table 1: Overview of the measurements in the different configurations. Config Critical dρ/dh Fission Flux level rate Ref. X X X X X X Irr. X X In driver X MOX X X X X Irr. MOX X X in driver X 5. Results 5.1 Characterization of spent fuel rods The fresh fuel rods originating from the VENUS and BR3 (driver zone, reference bundle and MOX fuel) have already been used in previous benchmark programmes and their characteristics are welldocumented. Additionally, the VENUS rods have been elongated from 50 cm to 1 m and during this elongation we took the opportunity to perform some extra measurements with respect to the cladding inner and outer diameter and the position of the lower end plug. The irradiated MOX fuel from the BR3 has been examined in the hot cell laboratory with respect to gross-gamma scanning, burn-up by gammaspectrometry, profilometry and fuel column length GROSS GAMMA MEASUREMENT OF ROD F7654 (date : ) 0,00 100,00 200,00 300,00 400,00 500,00 600,00 700,00 800,00 900, , , ,00 Axial corrected position (in mm. from bottom end.) Fig. 3: Measurement of the gross-gamma scan of an irradiated MOX PWR rod The gross-gamma scan (see figure 3) results show for certain MOX rods a rather high degree of Cs migration, indicating a high linear power during irradiation.

4 5.2 Experiments in the critical facility VENUS: fresh configurations In 2002, the three configurations with only fresh fuel, namely the reference configuration, the fresh GKN configuration and the fresh BR3 MOX configuration were loaded in the VENUS critical facility. For all three configurations, the predefined experimental programme as laid out in paragraph 4 was executed. Table 2 summarizes the results for the determination of the critical height and makes a comparison with calculations which were made for the pre-determination of the cores. Two different calculation schemes, a 3-D Monte Carlo (WIMS KENO) scheme and a 3-D deterministic (WIMS- THREEDANT) scheme, were used to calculate the critical water level. In both calculation schemes the geometrical model was the same. Table 2: Comparison of calculated and experimental critical heights (C-E) (pcm) Ref. MOX WIMS KENO (3D) WIMS THREEDANT (3D) Table 2 shows that there is not only a good agreement between the two different calculation schemes, but also between calculation and experiment. A small systematic bias of about -300 to -400 pcm is observed. For the comparison between measured and calculated water level reactivity variations, the 2-D deterministic WIMS scheme and the 3-D deterministic WIMS THREEDANT scheme was used (see table 3). The Monte Carlo KENO was not suited for this purpose due to the statistical uncertainty. Table 3: Comparison of calculated and experimental water level reactivity variation (C-E)/E (%) Ref. MOX WIMS (2D) WIMS THREEDANT (3D) Table 3 shows that for the 3-D code calculation scheme the calculated and measured values correspond very well, but for the 2-D calculation scheme a systematic bias of about - 10% occurs. For the fresh BR3 MOX configuration and the fresh GKN configuration, also the horizontal fission-rate distribution was measured along two axes, the X-axis and the X-Y diagonal. Figure 4 depicts these measured and calculated fission-rate distributions, which were normalized at fuel pin location 7. Relative fission-rate Fuel pin location X (M) X (C ) XY (M) XY (C ) Fig. 4: Comparison of calculated (C) and measured (M) fission-rate distributions for the fresh MOX bundle along the X-axis and the X-Y diagonal Figure 4 shows that there is a good agreement between the measured and calculated fission-rate distributions along the two axes. In figure 5, the measured and calculated fission-rate distributions for the fresh GKN configuration are illustrated. Relative fission-rate Fuel pin location X axis (M) X axis (C ) XY diag (M) XY diag (C ) Fig. 5: Comparison between calculated (C) and measured (M) fission-rate distributions for the fresh bundle along the X-axis and X-Y diagonal Figure 5 points out that there is relative good agreement between calculations and experiment, but that one notices a discrepancy of about 7 % (C- E)/E in the central zone with GKN fuel. 5.3 Loading of irradiated fuel into the VENUS reactor As for every zero power reactor, the loading of irradiated fuel coming from a nuclear power plant is not a standard operation, since in such types of reactors only the manual loading of fresh fuel pins is foreseen. In order to achieve this goal, a specific loading procedure and necessary equipment was developed and constructed. For the transport from the hot-cell laboratories to the VENUS facility, the REBUS container with the

5 irradiated REBUS bundle is put in an additional transport container to reduce the radiation level below 2 msv/h in contact. In the reactor hall the covering plates of the transport container are removed and the REBUS container is lifted out of its transport container and put in a vertical position (Figure 6). distributions outside the bundle. Also Co activation wires were inserted at several positions in the core and in the bundle around core midplane. The analysis of the measurement data is ongoing, but already the critical height and the reactivity effect were determined. The calculation results are based preliminary raw calculations corrected by axial burnup profile, anisotropic cross sections effect P0/P1 and calculation bias. By comparing the calculated value with the experimental one, we see that for the critical water level an underestimation of 3% is present and for the reactivity coefficient the calculated value was overestimated with 16%. Figure 6: Lifting of the REBUS container out of the transport container Afterwards, the REBUS container is transferred into the bunker by means of the tackle and positioned in a unique location on top of the reactor (Figure 7). When the cable of the telescopic arm is connected to the top of the irradiated bundle in the REBUS container, the shielding doors of the bunker are closed and the bundle is lowered remotely into the reactor from the control room. The reactivity effect of replacing the fresh MOX bundle by the irradiated bundle yields a reactivity effect between ~ 1900 and 2200 pcm depending on the way that the reactivity effect is inferred from the critical height and the reactivity coefficients. Such a reactivity effect is anyway larger than the reactivity effect foreseen in the early phase of the programme (~1600 pcm). 6. Conclusions The REBUS programme provides an experimental benchmark for burn-up credit, taking into account both fissile isotopes depletion and the production of neutron absorbing fission products. With the results of the REBUS programme a validation of reactor physics codes with respect to burn-up credit of PWR fuel with intermediate and high burn-up is possible. Based on the results of this validation, and the quoted uncertainties of the experiment, safety margins can be determined that have to be implemented for burn-up credit calculations. The REBUS programme is being executed completely at SCK CEN in its hot cell laboratory, radiochemistry lab and the VENUS reactor. Figure 7: Positioning of the REBUS container on the supporting structure over VENUS 5.4 Reactivity measurements of the irradiated BR3 MOX configuration The experimental programme for the configuration with the irradiated BR3 MOX bundle was completed in June The critical water level and the reactivity effect of the water level were measured together with axial and radial fission-rate Gamma scans, profilometry, length determination and gamma-spectrometry measurements on the MOX fuel have been performed. The experiments for the three fresh configurations show a good agreement between calculation and experiments for the different measured parameters: critical water level, reactivity effect of the water level and fission-rate distributions. In 2003 the irradiated BR3 MOX bundle was loaded into the VENUS reactor. The reactivity effect of replacing a fresh MOX bundle by the same bundle after irradiation in a power reactor was determined experimentally.