Nuclear fuel failure protection method
A method that provides a more direct indication of peak fuel rod centerline temperature and peak fuel rod clad temperature than conventionally inferred from the power distribution by directly and continuously measuring the fuel temperatures of the fuel pellets in one or more of the hottest fuel elements in the core. The peak fuel rod clad temperature is then obtained from the maximum measured peak fuel rod centerline temperature in combination with the maximum coolant core exit temperature and the minimum coolant flow rate.
1. A method of determining a maximum measured internal fuel rod temperature and a peak clad temperature of one or more of fuel rods in an operating nuclear core comprising the steps of:
directly measuring the temperatures with sensors at one or more locations within the one or more of the fuel rods, wherein signal processing elements ae structured to receive the measured temperatures from the sensors;
identifying via the signal processing elements a hottest temperature within the one or more fuel rods as the maximum measured internal fuel rod temperature and a core location where the hottest temperature is measured;
measuring the maximum temperature of coolant exiting the core at the core location where the hottest temperature is measured and the minimum flow rate of the coolant; and
determining via the signal processing elements the peak clad temperature from the maximum measured internal fuel rod temperature, the maximum temperature of the coolant exiting the core at the core location where the hottest temperature is measured and the minimum flow rate of the coolant.
2. The method of claim 1 wherein the directly measuring step measures the temperature along a plurality of axial locations in the one or more fuel rods.
3. The method of claim 2 wherein in the plurality of axial locations include the tops and bottoms of the one or more fuel rods.
4. The method of claim 1 further comprising:
determining the bulk coolant temperature from the maximum temperature of the coolant exiting the core at the core location where the hottest temperature is measured and the minimum flow rate of the coolant.
5. The method of claim 4 , wherein determining the peak clad temperature is based on knowledge of heat transfer and geometric characteristics of fuel pellets in the one or more fuel rods and cladding of the one or more fuel rods.
6. The method of claim 5 , wherein determining the peak clad temperature includes determining the peak clad temperature using the following equation:
T
C
(
t
)
=
T
F
(
t
)
+
T
M
(
t
)
-
T
F
(
t
)
φ
(
t
)
Where
:
φ
(
t
)
=
(
r
h
2
k
f
(
T
F
(
t
)
)
A
r
+
c
A
r
+
c
h
k
c
A
r
+
c
A
m
+
1
)
And
:
A
m
=
2
π
c
L
ln
(
r
+
c
r
)
Where
:
r
=
fuel
pellet
diameter
c
=
cladding
thickness
L
=
fuel
rod
length
k
f
=
fuel
pellet
thermal
conductivity
k
c
=
cladding
thermal
conductivity
A
r
=
cross
sectional
area
of
fuel
pellet
A
r
+
c
=
cross
sectional
area
of
fuel
pellet
and
padding
h
=
cladding
heat
transfer
coefficient
by
convection
.
7. The method of claim 1 wherein the step of directly measuring the temperature at one or more locations on the fuel is performed substantially continuously.
8. The method of claim 1 , further comprising:
comparing the peak clad temperature and/or the maximum measured internal fuel rod temperature with one or more threshold values; and
implementing protective measures in response to determining that the peak clad temperature and/or the maximum measured internal fuel rod temperature with one or more threshold values.
9. The method of claim 8 , wherein the protective measured include a turbine runback or a reactor trip.
10. A method for use with an operating nuclear core, the method comprising:
directly measuring temperatures with sensors at one or more locations within one or more of the fuel rods, wherein signal processing elements are structured to receive signals indicative of the temperatures;
identifying via the signal processing elements a hottest temperature within the one or more fuel rods as a maximum measured internal fuel rod temperature and a core location where the hottest temperature is measured;
measuring a maximum temperature of a coolant exiting the core at the core location where the hottest temperature is measured and a minimum flow rate of the coolant; and
determining via the signal processing elements a peak clad temperature from the maximum measured internal fuel rod temperature, the maximum temperature of the coolant exiting the core at the core location where the hottest temperature is measured, and the minimum flow rate of the coolant.
11. The method of claim 10 , wherein the one or more locations comprise axial locations in the one or more fuel rods.
12. The method of claim 11 , wherein the axial locations include top and bottom locations in the one or more fuel rods.
13. The method of claim 10 , further comprising determining a bulk coolant temperature from the maximum temperature of the coolant exiting the core at the core location where the hottest temperature is measured and the minimum flow rate of the coolant.
14. The method of claim 13 , wherein determining the peak clad temperature is based on heat transfer and geometric characteristics of fuel pellets in the one or more fuel rods and cladding of the one or more fuel rods.
15. The method of claim 14 , wherein determining the peak clad temperature includes determining the peak clad temperature using the following equation:
T
C
(
t
)
=
T
F
(
t
)
+
T
M
(
t
)
-
T
F
(
t
)
φ
(
t
)
Where
:
φ
(
t
)
=
(
r
h
2
k
f
(
T
F
(
t
)
)
A
r
+
c
A
r
+
c
h
k
c
A
r
+
c
A
m
+
1
)
And
:
A
m
=
2
π
c
L
ln
(
r
+
c
r
)
Where
:
r
=
fuel
pellet
diameter
c
=
cladding
thickness
L
=
fuel
rod
length
k
f
=
fuel
pellet
thermal
conductivity
k
c
=
cladding
thermal
conductivity
A
r
=
cross
sectional
area
of
fuel
pellet
A
r
+
c
=
cross
sectional
area
of
fuel
pellet
and
padding
h
=
cladding
heat
transfer
coefficient
by
convection
.
16. The method of claim 10 , further comprising comparing the peak clad temperature and/or the maximum measured internal fuel rod temperature with one or more threshold values.
17. The method of claim 16 , further comprising implementing protective measures based on results of the comparing of the peak clad temperature and/or the maximum measured internal fuel rod temperature with the one or more threshold values.
18. A method for use with an operating nuclear core, the method comprising:
receiving signals indicative of temperatures detected by sensors at one or more locations within one or more fuel rods of the operating nuclear core;
based on the signals, identifying a hottest temperature within the one or more fuel rods as a maximum measured internal fuel rod temperature and a core location where the hottest temperature is identified;
determining a maximum temperature of a coolant exiting the core at the core location where the hottest temperature is measured and a minimum flow rate of the coolant; and
determining via signal processing elements a peak clad temperature based on the maximum measured internal fuel rod temperature, the maximum temperature of the coolant exiting the core at the core location where the hottest temperature is identified, and the minimum flow rate of the coolant.
19. The method of claim 18 , wherein the one or more locations comprise axial locations in the one or more fuel rods.
20. The method of claim 19 , wherein the axial locations include top and bottom locations in the one or more fuel rods.
21. The method of claim 18 , further comprising determining a bulk coolant temperature from the maximum temperature of the coolant exiting the core at the core location where the hottest temperature is identified and from the minimum flow rate of the coolant.
22. The method of claim 21 , wherein determining the peak clad temperature is based on heat transfer and geometric characteristics of fuel pellets in the one or more fuel rods and cladding of the one or more fuel rods.
23. The method of claim 22 , wherein determining the peak clad temperature includes determining the peak clad temperature using the following equation:
T
C
(
t
)
=
T
F
(
t
)
+
T
M
(
t
)
-
T
F
(
t
)
φ
(
t
)
Where
:
φ
(
t
)
=
(
r
h
2
k
f
(
T
F
(
t
)
)
A
r
+
c
A
r
+
c
h
k
c
A
r
+
c
A
m
+
1
)
And
:
A
m
=
2
π
c
L
ln
(
r
+
c
r
)
Where
:
r
=
fuel
pellet
diameter
c
=
cladding
thickness
L
=
fuel
rod
length
k
f
=
fuel
pellet
thermal
conductivity
k
c
=
cladding
thermal
conductivity
A
r
=
cross
sectional
area
of
fuel
pellet
A
r
+
c
=
cross
sectional
area
of
fuel
pellet
and
padding
h
=
cladding
heat
transfer
coefficient
by
convection
.
24. The method of claim 18 , wherein the temperatures are continuously detected.
25. The method of claim 18 , further comprising comparing the peak clad temperature and/or the maximum measured internal fuel rod temperature with one or more threshold values.
26. The method of claim 25 , further comprising implementing protective measures based on results of the comparing of the peak clad temperature and/or the maximum measured internal fuel rod temperature with the one or more threshold values.