LRS Seminar February 2006

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Evaluation of the PWR Rod Ejection Pulse Width for Realistic UO2/MOX Cores Nuclear Energy and Safety Research Department Laboratory for Reactor Physics and Systems Behaviour Nuclear Energy and Safety Research Department Laboratory for Reactor Physics and Systems Behaviour H. Ferroukhi, M.A. Zimmermann Laboratory for Reactor Physics and Systems Behaviour Paul Scherrer Institut, Switzerland




Reactivity-Initiated-Accident (RIA) refer to transients characterized by a positive reactivity insertion into a critical core The consequent power excursion can lead to fuel damage Classified as Design-Basis-Transients Consequences of such transient must be assessed as part of the safety evaluation during reactor licensing process For PWRs, the Rod-Ejection-Accident (REA) serves as RIA Design Basis Rapid ejection of Control Rod (~0.1s) Very fast power excursion due to the inserted reactivity Power excursion reversed by (mainly) Doppler feedback (~0.1 to 0.2 s) Because so fast, almost adiabatic heat-up of fuel pins Deposited energy can lead to fuel failure and eventually dispersal of molten fuel


RIA Safety Acceptance Criteria defined as upper limits on Maximum Fuel (Delta) Enthalpy Core Coolability Limit Fuel Rod Failure Limit Pure Core Dynamical Transient  Core Analysis Methods Required Typical REA evaluation performed in Three Step Process assuming HZP Initial Conditions: Step 1 Steady-State Identification of CR with highest reactivity worth Step 2 Transient analysis with ejection of identified “worst” Rod Step 3 Assessment of maximum reached enthalpies against Acceptance Criteria Limits


Analytically, it can be shown that for a given and step reactivity insertion the Power Maximum is : The total energy release is: To quantify how fast the energy is released, the PULSE WIDTH is usually used and defined as Using this definition and the analytical model above, it can be shown that the Pulse width is which implies that it is inverse proportional to the energy release :


For RIAs, burnup-independent acceptance criteria have historically been used, based on old fuel design and based on database limited to burnup levels below 40 GWD/MT However, due to the trend towards higher burnups and increasingly more complex fuel design, the reliability of the existing criteria become uncertain and requires better insight into high-burnup transient fuel behavior Recent experimental programs revealed fuel rod failure for high burnup fuels at lower enthalpy depositions, indicating that revised criteria taking into account burnup dependency should be established EPRI Robust Fuel Program launched to develop new criteria Based on a combination of experimental data and advanced analytical fuel rod analysis, revised burnup dependant criteria were developed and proposed for UO2 LWR Cores with rod average burnup < 75 GWD/MT HZP BWR RDA and HPZ/HFP PWR REA


Revised criteria proposed also for Switzerland From that perspective, PSI Performed HSK On-Call 21 aimed a Plant Specific Verification of New curves (KKM and KKB1) Additionally, similar limits developed for MOX fuel due to operation of MOX cores in Switzerland However, methodology based on Pulse width magnitude of 40 ms Applicability verified for range 20 – 40 ms Verification of these assumption on pulse width magnitude required As follow-up to OC21, PSI performed a study aimed at the quantification of the PWR REA Pulse Width in MOX v.s. UO2 Cores


PSI Core Management System 3-D Full Core Model 2-Group ANM Nodal Diffusion (ARROTTA) 2-Fluid 6 Eq T/H (VIPRE-02) CORETRAN 3-D Kinetic Analysis Reactor Power Node and Pin Power Densities Fuel Temp. and Node-Enthalpy CASMO-4 SIMULATE-3 2-D Lattice Transport 3-D Core 2-G Static Diffusion Core follow analyses Verification vs. Plant Measurements Plant_1 Plant_k Plant_K Cycle 1 Cycle 2 Cycle n Cycle N


CORETRAN 3-D Model Full Core Neutronic + Thermal-Hydraulic Lower-to-Upper Plenum Model Analytical Nodal Method with 1x1 Radial Assembly Mesh and 1ms uniform time-step Dynamical Rod Worth Model added MATPRO Fuel Thermal Properties Uniform Gap Conductance Analysis Assumptions Hot-Zero-Power Initial Conditions All Rods in + SCRAM Out Ejection of Highest-Worth Rod within 0.1 s Analyses performed at both BOC and EOC for all Cycles


BOC EOC Rod Worth ($) larger at EOC for all Cycles and particularly for MOX Cores Realistic Rod Worths for both UO2 and MOX (up to 4% Pu): 1.4 – 2.0 $ (300-500 Excess Reactivity Above Prompt-Criticality)


Both for UO2 and MOX, pulse width smaller at EOC MOX pulse width smaller than UO2 (and particularly at EOC) Pulse width clearly smallest for highest NRW (High RE + Low Beta ) => 3-D Confirms analytical functionality Typical Rod Worths [1.5 $ - 2.0 $] [300 – 500] pcm excess reactivity  Pulse Width around 15-30 ms Study Relationship between Pulse Width and RE SRW NRW RE




C3 – BOC Conditions


Rod Worths in Realistic MOX Cores 1.5 – 2.1 $   300 to 500 pcm Excess Reactivity For such Rod Worths, MOX Pulse width around 5-10 ms lower than UO2 Similar Trends with increased Cycle Exposure (BOC vs. EOC) Typical MOX pulse Width in range 10-20 ms Nodal Enthalpy doubled (e.g vs. 20-40 ms range) but below 100 cal/g Further Analysis Indicate For very high rod worths, pulse width dominated by global core average kinetic parameters (Total MOX Fraction) For Realistic Rod Worths, non-negligible effect (5 to 10 ms) of local supercell characteristics Particularly , Increased MOX in Supercell Lower rod worth Local Kinetic Effect (Beta-eff)  Lower Pulse Widths Stronger Enthalpy response in adjacent UO2 assemblies


Further Studies Sensitivity upon Spatial Kinetics Method Effects of Neutron Data Libraries Applicability of XS-Model Case Matrix Kinetic Analyses with Representative Fuel Rod Model Properties from Fuel Performance Code Core Loading (Global vs Local Effects of MOX on REA Behaviour)

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