Core Thermal Limits Flashcards
Total Peaking Factor Fq(T)
Max local linear power density in the core divided by average linear power density in core
Fq(T)=(Max Kw/ft)/(Ave Kw/ft)
How to calculate Max linear power density in the core?
Given the Total Heat Flux Hot Channel Factor (or Total Core Peaking Factor, Fq(T)).
Calculate Ave Linear Power Density.
Max Kw/ft=Fq(T)•Ave Power Density.
Calculate Average Linear Power Density.
Ave Kw/ft=(MW•1000kW/MW)/(#fuel assemblies•#rods/assembly•ft/rod)
Can substitute total rods for #assemblies•#rods/assembly if already known.
Thermal limit for Cladding and why.
2200F to prevent EXCESSIVE cladding oxidation from zirconium water reaction.
1800F is the point where the oxidation reaction starts.
How is cladding thermal limit protected?
Limit on Nuclear Enthalpy Rise Hot Channel Factor
Protects against DNBR.
What is Nuclear Enthalpy Rise Hot Channel Factor?
FdeltaH(N)
=(Total integrated kW for Rod with greatest kW/ft)/(Total kW for Ave fuel rod)
DNBR concern
Melting point for PWR UO2 fuel
5200F
Limit for core temp to prevent melting
4700F
Typical temp doesn’t exceed 4000F during normal operations.
What does the High Power Trip protect against?
Fuel Pellet melting
What does the Low Pressure RCS trip protect against?
DNBR
What does the RCS Low Flow Trip protect against?
Fuel Pellet melting and DNBR
Why are slow transients a potential concern on the plant?
Slow depressurization of the RCS could be a DNBR concern.
Calculate DNBR
DNBR=CHF/AHF
Higher is mo betta!
When is AFD not applicable per tech specs?
Below 50% power.
Explain Most Limiting Channel concerns with respect to DNB
Unrodded fuel assemblies have an area with 4 fuel rods.
Assemblies with rods have thimble spaces for the rod and 3 fuel assemblies.
Thimble tube is wide and reduces flow thru that assembly. Power production is less with 3 rods making coolant colder in that assembly which results in higher flux. Net result is higher than Ave power density in Thimble assemblies and higher Enthalpy rise.
Net result is thimble assemblies are more likely to reach DNB.
What happens to the Pool Boiling Curve if RCS pressure is lowered?
The curve shifts to the left which puts the actual RCS condition (based on heat flux) further to the right on the curve (closer to DNB).
What is Fuel Densification?
First 200 hours of fuel cycle.
Elimination of small pores in the UO2 pellets.
Helium gap gets bigger and thermal conductivity gets worse.
Fuel centerline temp increases during this period.
Also increases linear power density due to same kW with smaller area.
How to calculate QPTR
Ratio of max upper excore to ave upper excore.
OR
Ratio of max lower excore to ave lower excore.
Whichever of these is greater is the QPTR
What does DNBR ensure?
Provides 95% probability that 95% of the fuel is not experiencing DNB.
In other words, a small percentage of fuel is allowed to exceed CHF by design.
Thermal Hydraulics question:
Head loss is proportional to what?
H(L) is directly proportional to the square of fluid velocity
And
Inversely proportional to channel cross sectional area (Diameter).
Thermal hydraulics:
Calculate core bypass flow.
Q-dot(Rx vessel)=Q-dot(core)+Q-dot(bypass)
m-dot(Rx vessel)=m-dot(core)+m-dot(bypass).
Thermal hydraulics:
Relationship of power change to change in mass flow rate for natural circ.
Cube root of power change is proportional to the change in mass flow rate.
[Power(f)\Power(o)]^(1/3)=[m-dot(f)/m-dot(o)]
Thermal hydraulics:
Ratio of change in core Delta T to change in mass flow rate for natural circ
Square root of change in Core Delta T is proportional to the change in mass flow rate.
[DeltaT(f)/DeltaT(o)]^(1/2)=m-dot(f)/m-dot(o).
Thermal hydraulics:
Ratio of power change to change in Core a delta T for natural circ
Cube root of power change is proportional to square root of Core Delta T change.
[power(f)/power(o)]^(1/3)=[DeltaT(f)/DeltaT(o)]^(1/2). OR
[power(f)/power(o)]^(2/3)=[DeltaT(f)/DeltaT(o)].
