Core Thermal Limits

Question 1

Total Peaking Factor Fq(T)

Answer 1

Max local linear power density in the core divided by average linear power density in core

Fq(T)=(Max Kw/ft)/(Ave Kw/ft)

Question 2

How to calculate Max linear power density in the core?

Answer 2

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.

Question 3

Calculate Average Linear Power Density.

Answer 3

Ave Kw/ft=(MW•1000kW/MW)/(#fuel assemblies•#rods/assembly•ft/rod)

Can substitute total rods for #assemblies•#rods/assembly if already known.

Question 4

Thermal limit for Cladding and why.

Answer 4

2200F to prevent EXCESSIVE cladding oxidation from zirconium water reaction.

1800F is the point where the oxidation reaction starts.

Question 5

How is cladding thermal limit protected?

Answer 5

Limit on Nuclear Enthalpy Rise Hot Channel Factor
Protects against DNBR.

Question 6

What is Nuclear Enthalpy Rise Hot Channel Factor?

Answer 6

FdeltaH(N)
=(Total integrated kW for Rod with greatest kW/ft)/(Total kW for Ave fuel rod)

DNBR concern

Question 7

Melting point for PWR UO2 fuel

Answer 7

5200F

Question 8

Limit for core temp to prevent melting

Answer 8

4700F

Typical temp doesn’t exceed 4000F during normal operations.

Question 9

What does the High Power Trip protect against?

Answer 9

Fuel Pellet melting

Question 10

What does the Low Pressure RCS trip protect against?

Answer 10

DNBR

Question 11

What does the RCS Low Flow Trip protect against?

Answer 11

Fuel Pellet melting and DNBR

Question 12

Why are slow transients a potential concern on the plant?

Answer 12

Slow depressurization of the RCS could be a DNBR concern.

Question 13

Calculate DNBR

Answer 13

DNBR=CHF/AHF

Higher is mo betta!

Question 14

When is AFD not applicable per tech specs?

Answer 14

Below 50% power.

Question 15

Explain Most Limiting Channel concerns with respect to DNB

Answer 15

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.

Question 16

What happens to the Pool Boiling Curve if RCS pressure is lowered?

Answer 16

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).

Question 17

What is Fuel Densification?

Answer 17

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.

Question 18

How to calculate QPTR

Answer 18

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

Question 19

What does DNBR ensure?

Answer 19

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.

Question 20

Thermal Hydraulics question:
Head loss is proportional to what?

Answer 20

H(L) is directly proportional to the square of fluid velocity
And
Inversely proportional to channel cross sectional area (Diameter).

Question 21

Thermal hydraulics:
Calculate core bypass flow.

Answer 21

Q-dot(Rx vessel)=Q-dot(core)+Q-dot(bypass)

m-dot(Rx vessel)=m-dot(core)+m-dot(bypass).

Question 22

Thermal hydraulics:
Relationship of power change to change in mass flow rate for natural circ.

Answer 22

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)]

Question 23

Thermal hydraulics:
Ratio of change in core Delta T to change in mass flow rate for natural circ

Answer 23

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).

Question 24

Thermal hydraulics:
Ratio of power change to change in Core a delta T for natural circ

Answer 24

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)].