Lecture_6.pdf

NUCE 402: Introduction to Nuclear System and Operation Chapter 3.2 (Heat Removal from Nuclear Reactors – Boiling) Dr. Ahmed Alkaabi Previous Lectures  Newton’s law of cooling q  h(Tc  Tb )  Total thermal resistance...

NUCE 402: Introduction to Nuclear System and Operation Chapter 3.2 (Heat Removal from Nuclear Reactors – Boiling) Dr. Ahmed Alkaabi Previous Lectures  Newton’s law of cooling q  h(Tc  Tb )  Total thermal resistance Tb a b 1 R   2k f A kc A hA Tm  Tb q 1 ln( 1  b / a) 1 R R   4k f H 2kc H hA  Temp. along the channel z  V f   V f cos( Tc  Tb  Rh qmax ) qmax z  H Tb  Tb 0  1  sin( ) c p  H  z  V f cos( ) Tm  Tb  Rqmax H  Heat transfer coefficients  Dimensionless numbers  High Re value for turbulent flow k h  0.023( ) Re 0.8 Pr 0.4 D  hDe cp De Re  e Nu  Pr   k k Dittus-Boelter Eq. 2 Boiling  Advantage of boiling  Lower coolant pressure  More heat transfer to coolant  Lower cladding temperature Tb  BWR (boiling water reactor)  Allows extensive boiling  Direct steam cycle  PWR (pressurized water reactor)  Allows limited boiling to enhance heat transfer  Bulk water is subcooled => Indirect steam cycle 3 Boiling Regimes  Boiling patterns Conduction and radiation heat transfer  Experimentally measured => large reduction in  Change surface T of heated rods heat transfer  Measure heat flux  Observe boiling patterns Departure from nucleate Bubbles form on the surface imperfection => boiling (DNB) to film boiling carried away to the bulk coolant => bubbles condensed (Tb < Tsat) Flow velocity More effective heat transfer Bubbles form on the surface imperfection => Convection carried away to the bulk coolant => bubbles dominate persist (Tb = Tsat) 4 Boiling Crisis  Departure from Nucleate Boiling (DNB)  Nucleate boiling => film boiling  Heated rods are exposed to steam vapor  Lower heat transfer  Rapid increase in fuel temp.  Critical heat flux (CHF)  Correlations for CHF  Subcooled boiling  Jens and Lottes  Bulk boiling  GE 5 Boiling Patterns  Boiling patterns  Experimentally measured  Change surface T of vertical heated pipes  Observe changes in flow patterns 6 Boiling Channels  BWR  Saturated boiling  P = 1035psia, Tsat = 550oF A/B 7 Thermal Design of Reactor  Design Philosophy  Fission product should remain within the fuel  Cladding integrity is the pre-requisite  Fuel must not melt  Important temperatures  Melting temp.  UO2: ~5000oF (2760oC) vs. Tm ~ 4500oF (2480oC)  UCN: ~6500oF (3600oC)  Metal U: ~2070oF (1132oC)  Temp. for significant fission gas release: ~750oF for met-U  DNB ratio (DNBR)  Safety margin to prevent DNB  Min. 1.9 for BWR  Min. 1.3 for PWR qc DNBR   qactual  Hot channel factor (HCF) q F  max F  FN  FE  qav 8 Thermal Design of Reactor qc  qmax DNBR  F F  FN  FE  qactual  qav FN    Reactor Power   A P  qav   Fqav qmax  qc  qc Min. DNBR =  q max   qav qmax  F F  (min. DNBR) qc  A   A  P  qav F  (min. DNBR)  To achieve maximum power  Increase critical heat flux  Decrease hot channel factor  Flatten the flux shape  Design proper enrichment distribution => fuel loading pattern  Lower min. DNBR  Need greater confidence with extensive technical justification 9 Summary  Heat generation in reactor  Heat removal  Conduction & Convection T T Tb q m b  Thermal resistance R R a  b  1 1 ln( 1  b / a) 1 R   2k f A kc A hA 4k f H 2kc H hA k  Heat transfer coefficients h  0.023( ) Re 0.8 Pr 0.4 Dittus-Boelter Eq. De  Boiling regime  Flow patterns  DNBR & HCF qc  qmax DNBR  F  qactual  qav qc  A   A  P  qav F  (min. DNBR) 10