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LWR NUCLEAR CHARACTERISTICS

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LWR NUCLEAR CHARACTERISTICS

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    1. LWR NUCLEAR CHARACTERISTICS NUEN 609 Nuclear Safety September 22, 2005 William E. Burchill Department Head & HTRI Professor Department of Nuclear Engineering Texas A&M University

    13. REACTIVITY COEFFICIENTS

    14. Reactivity Factors Geometry Fuel composition fissile isotopes fission products burnable poisons Moderator/coolant composition voids soluble boron coolant additives, e.g., mat’l. degradation inhibitors Power distribution Control rods positions

    16. Fuel Temperature (Doppler) Coefficient As TF increases, epithermal capture cross section resonances broaden – hence, - ?? mostly U-238 at BOL add Pu-240 at EOL As TF increases, decreased thermalization due to reduction of fuel scattering cross sections – about 10% of - ?? effect As TM increases, absolute magnitude of - ?? decreases As void fraction increases, absolute magnitude of - ?? increases

    18. Moderator Temperature Coefficient Increased TM reduces ?M which reduces thermalization – hence, - ?? However, in PWR with soluble boron, increased TM reduces boron concentration giving reduced neutron absorption – hence, + ?? Thus, soluble boron concentration must be limited to keep MTC negative (limit on boron concentration produces need for burnable poisons in fuel) MTC changes over fuel cycle due primarily to boron concentration with small secondary influence of fuel isotopic composition and neutron spectrum

    19. Moderator Temperature Coefficient Increased TM also reduces the hydrogen scattering cross section which hardens the neutron spectrum – impact depends on fuel isotopic composition U-235 has 1/v fission cross section, so increased TM reduces fission rate - hence, - ?? Pu-239 has a fission cross section resonance near top of thermal energy range (~ 0.65 ev), so increased TM increases fission rate - hence, +?? These “spectral” effects are much smaller than those due to moderator density changes

    22. Isothermal Moderator Density Coefficient As ?M increases, thermalization increases – hence, + ?? In PWR with soluble boron, increased ?M increases boron concentration giving increased neutron absorption – hence, - ?? Magnitude of thermalization effect is usually much larger than magnitude of boron concentration effect – hence, magnitude of + ?? decreases with increased boron concentration At very high boron concentration (BOC), coefficient can be slightly negative

    24. Reactivity as f(moderation)

    27. Reactivity Control Design Basis The reactivity control system must be able to make the core subcritical from any power operating condition with the highest worth control rod stuck out. (See 10CFR50 App. A GDC 27)

    34. Reactivity Temperature Defect Temperature defect = reactivity change from room temperature to hot standby About 4% ˜ $6 (ß = .007) or $10 (ß = .004)

    38. LWR Nuclear Characteristics LCOs Power distribution Core reactivity Boron concentration (PWR) MTC (PWR) Void coefficient (BWR) Power coefficient Single control rod (bank) worth Allowable control rod (bank) insertion f(power level) Allowable control rod insertion rates SCRAM time Shutdown worth

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