Effect of CASMO-5 Cross-Section Data and Doppler ... - Studsvik

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Gerardo Grandi et.al.4. DESCRIPTION OF THE ROD EJECTION ACCIDENT SCENARIOThe problem consists in analyzing a rod ejection in a PWR MOX/UO 2 core at Hot Zero Power conditions:10 -4 rated power, core inlet temperature 560 K, core pressure 15.5 MPa. The model was constructed basedon the OECD “PWR MOX/UO2 Core Transient Benchmark [12].” It is important to remark that: Cross sections were generated using CASMO-4 / ENDF/B IV and CASMO-5 / ENDF/B VII.0. The known state of the core (history data) was assumed to be the same regardless of the crosssectionset used in the transient calculations. The 3D fuel temperature distribution is evaluated by solving the one-dimensional, radial heatconduction equation for the average pin of each node. For safety evaluations, all the pins for eachnode may be modeled [2]. The closed channel thermal hydraulics module provides thetemperature of the coolant surrounding the pin which serves as the boundary condition for thefuel temperature calculation. Fuel thermal conductivity for UO 2 and MOX are computed using the Nuclear Fuel Industriescorrelations as reported in reference [14]. The gap conductance model is taken from the INTERPIN-4 code [15] and it is functionalizedversus exposure and temperature. The intra-pellet power profiles are a function of the fuel type (MOX/UO 2 ) and burnup. Valueswere taken from INTERPIN-4. Fig. 2 illustrates those profiles for 0.0 GWd/T, 20 GWd/T and 40GWd/T.All the control rod banks are fully inserted and all the shutdown banks are fully withdrawn. The initialboron concentration is determined to make the reactor critical in steady state. Note that in the OECDbenchmark; only one of the four rods in position E5 is ejected. However, in the present work all four rodsare ejected. The rod ejection starts at time 0 seconds and the rods are ejected in 0.1 s. The transient isfollowed for 5 s. The chosen spatial discretization is: radial mesh of 2x2 nodes per assembly (x=10.7cm) and 24 axial nodes (z=15.24 cm). The fuel pellet is divided in nine equal volume rings, and thecladding in two rings. Following the recommendation from Grandi [16], solutions were obtained with asmall time step (0.1 ms) to avoid any spurious effect from the numerical integration.5. RESULTSResults for two different exercises will be discussed in what follows. The objective of the first exercise isto assess the effect of the CASMO-5 nuclear data on the rod ejection accident key parameters, namely:peak power, time of peak power, peak power width, and peak fuel enthalpy. The objective of the secondexercise is to assess the influence of the Doppler temperature definition on the previously mentionedparameters.5.1. Effect of the CASMO-5 Cross-Section DataIn this subsection, all the calculations are performed using the ‘BE1’ effective Doppler temperaturedefined by Eq. (1). For simplicity, the S3K results obtained using CASMO-4 nuclear data will be referredas ‘CASMO-4’ case or solution; the results computed using CASMO-5 data will be labeled ‘CASMO-5’;and finally, the results computed using CASMO-5 data with the asymptotic scattering kernel (i.e. withoutthe CASMO-5 resonance elastic scattering model active) will be labeled ‘CASMO-5 Asymptotic’. TableIII presents relevant static -fraction, the static rod worthPHYSOR 2010 – Advances in Reactor Physics to Power the Nuclear RenaissancePittsburgh, Pennsylvania, USA, May 9-14, 20106/13
Effect of CASMO-5 Cross-Section Data and Doppler Temperature Definitions on LWR Reactivity Initiated Accidents(SRW)-re average Doppler. The main observations derived from Table III are: Relative differences in rod worth between ‘CASMO-5’ and ‘CASMO-4’ are less than 1%. Nosignificant differences are observed in the core- * . A difference of ~10% is observed in the Doppler coefficient. This result is consistent with thelattice results presented in Fig. 1. Differences in Doppler coefficient between ‘CASMO-5’ and ‘CASMO-4’ due to differences inthe nuclear data (i.e., ENDF/B VII.0 vs. ENDF/B IV) are less than 1.5%. The difference inDoppler coefficient is mainly due to the 238 U resonance elastic scattering treatment. It is expected, that the rod ejection accident computed with CASMO-5 cross-sections will bemilder. The total energy released during the power burst is proportional to the ratio of the promptreactivity and the Doppler coefficient. This ratio is ~11% smaller if CASMO-5 data is usedinstead of CASMO-4 data.Table III: Steady state REA parameters. Effect of CASMO-5 cross-section data.ParameterCASMO-4CASMO-5CASMO-5Exact kernelAsymptotic kernelValue Diff (%) Value Diff (%)Critical Boron (ppm) 1737 1738 0.1 1740 0.2SRW (pcm) 870 863 -0.8 864 -0.7 (pcm) 544 544 0.0 545 0.2 (1.E-05 s) 1.46 1.48 1.4 1.48 1.4 ($) 1.60 1.59 -0.8 1.59 -0.9 (pcm) 326 319 -2.1 319 -2.1 (pcm/K) -2.93 -3.24 10.6 -2.97 1.4Fig. 3 compares the power evolution for the ‘CASMO-4’ and ‘CASMO-5’ cases. Note that the ‘CASMO-5’ solution has a lower power peak, and a smaller peak width, and the energy release during the powerexcursion is smaller.Relative Power (% Rated Power)1.E+041.E+031.E+021.E+011.E+001.E-011.E-021.E-031.E-040.00 0.50 1.00 1.50 2.00Time (s)CASMO-4CASMO-5(a) Period 0.0 s to 2.0 s. Logarithmic scale.(b) Period 0.1 s to 0.2 seconds. Linear scaleFigure 3. Power evolution. Effect of CASMO-5 cross-section data.Relative Power (% Rated Power)60005000400030002000100000.10 0.12 0.14 0.16 0.18 0.20Time (s)CASMO-4CASMO-5* CASMO-5 delayed neutron fractions are not based on ENDF/B VII.0 data, but on the ENDF/B IV data used by CASMO-4.PHYSOR 2010 – Advances in Reactor Physics to Power the Nuclear RenaissancePittsburgh, Pennsylvania, USA, May 9-14, 20107/13
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