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

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Gerardo Grandi et.al.Fig.6 shows the evolution of the Doppler reactivity and the Doppler temperature during the transient.During the initial rapid power increase the problem is almost adiabatic, and the pin fuel temperature peaksnear the pellet surface due to the intra-pellet power shape (see Fig. 2). As the transient progresses, the heatconduction in the pin gradually reduces the surface temperature with respect to the centerline temperature.This behavior is illustrated in Fig. 7 for a pin in a fresh MOX assembly close to the ejected rod, and a pinin the UO 2 assembly where the rod is ejected.Doppler Reactivity ($)0.200.00-0.20-0.40-0.60-0.80NEABE1BE2GDTLDoppler Reactivity ($)-0.80-0.85-0.90-0.95-1.000.0 1.0 2.0 3.0 4.0 5.0Time (s)-1.000.0 0.5 1.0 1.5 2.0 2.5 3.0Time (s)Doppler Fuel Temperature (K)7006506005505000.0 0.5 1.0 1.5 2.0 2.5 3.0Time (s)NEABE1BE2GDTLFigure 6. Doppler reactivity and Doppler temperature. Effect of Doppler temperature definitions.MOX Fuel - Burnup 0.15 GWd/TUO2 Fuel - Burnup 37.5 GWd/TTemperature (K)1200110010009008007006005000 0.2 0.4 0.6 0.8 1Relative radiusTime (s)0.0000.1400.1450.1500.1600.2040.5041.0042.0045.000Temperature (K)1200110010009008007006005000 0.2 0.4 0.6 0.8 1Relative radiusTime (s)0.0000.1400.1450.1500.1600.2040.5041.0042.0045.000Figure 7. Evolution of the intra-pellet fuel temperature for 2 representative fuel pins.Table V summarizes the rod ejection accident results, in terms of power and fuel enthalpy, for the fourdifferent effective Doppler temperature cases. Differences with respect to the ‘BE1’ solution are alsoprovided. The main observations derived from Table V and Figs. 5-7 are: The ‘NEA’ effective Doppler temperature may predict a lower power peak depending on the corelife. However, the power generated in the after-burst computed using by the ‘NEA’ Dopplertemperature will always be the highest because the heat conduction in the pin gradually increasesthe centerline temperature with respect to the surface temperature.PHYSOR 2010 – Advances in Reactor Physics to Power the Nuclear RenaissancePittsburgh, Pennsylvania, USA, May 9-14, 201010/13
Effect of CASMO-5 Cross-Section Data and Doppler Temperature Definitions on LWR Reactivity Initiated AccidentsThe ‘NEA’ definition produces larger pulse widths as noted by other researchers [4]. This effectpartially compensates the lower power peak, and the energy generated during the powerexcursion differs only 1% with respect to the ‘BE1’ case.The prompt enthalpy rise is barely affected by the choice of the Doppler temperature definition.However, the maximum fuel enthalpy could be affected. The under-prediction of the Dopplerreactivity by the ‘NEA’ temperature results in a conservative estimate of the peak fuel enthalpy.Table V: Transient parameters: effect of the Doppler temperature definitionParameterBE1NEABE2GDTLValue Diff (%) Value Diff (%) Value Diff (%)Peak power (MW) 168700 162500 -3.7 167600 -0.7 170200 0.9Peak power time (s) 0.146 0.146 0.0 0.146 0.0 0.146 0.0Pulse width (ms) 18.2 18.7 2.7 18.2 0.0 18.1 -0.5Pulse energy release (MJ) 3070 3039 -1.0 3050 -0.7 3081 0.3Peak power part time (s) 0.1642 0.1647 0.3 0.1642 0.0 0.1641 -0.1Prompt fuel enthalpy (cal/g) 41.81 41.63 -0.4 41.73 -0.2 41.97 0.4Prompt enthalpy rise (cal/g) 24.71 24.53 -0.7 24.63 -0.3 24.87 0.6Peak fuel enthalpy (cal/g) 44.39 51.32 15.6 45.48 2.5 43.72 -1.5Peak enthalpy rise (cal/g) 27.29 34.22 25.4 28.38 4.0 26.62 -2.56. CONCLUSIONSThe effect of the cross-section data generated by CASMO-5, as well as the impact of different effectiveDoppler temperature definitions on LWR RIA was investigated. Key parameters of a PWR rod ejectionaccident scenario for a MOX/UO 2 core assuming the known core state have been compared.The main conclusions are: The LWR Doppler coefficient predicted using CASMO-5 cross-section data is ~10% morenegative at typical HZP conditions. The proper treatment of the 238 U resonance elastic scattering is the main contributor to the morenegative Doppler coefficient. The use of the volume-averaged Doppler temperature, Eq. (1), may overestimate the Dopplerreactivity. The ‘NEA’, Eq. (2), effective Doppler temperature definition leads to very conservativeresults because the Doppler worth is underestimated ~30% for quadratic profiles. Temperature distribution effects for Doppler could be accurately accounted by the empiricalweighting scheme described by Eq. (4). LWR RIA calculated using CASMO-5 cross-section data should be milder. This means that thefuel safety parameters computed with CASMO-5 core-section data show more margin than thesame parameters computed using CASMO-4 data for PWR rod ejection accidents and BWR roddrop accidents. The prompt enthalpy rise is barely affected by the choice of the Doppler temperature definition.The under-prediction of the Doppler reactivity by the ‘NEA’ Doppler temperature results in aconservative estimate of the peak fuel enthalpy. Results computed with the ‘BE1’, ‘BE2’, and‘GDTL’ effective Doppler temperatures, differ only a few percent.PHYSOR 2010 – Advances in Reactor Physics to Power the Nuclear RenaissancePittsburgh, Pennsylvania, USA, May 9-14, 201011/13
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