Temperature measurement on Zircaloy-clad fuel pins during high-temperature excursions Page: 1 of 2
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TEMPERATURE MEASUREMENT ON ZIRCALOY CLAD FUEL PINS
DURING HIGH TEMPERATURE EXCURSIONS
R.H. MESERVEY
Special thermocouples and attachment techniques have been developed
for use in measuring surface temperatures of Zircaloy-clad fuel pins during
high temperature excursions. Although thermocouples have historically
been used as the mechanism for measuring temperatures of nuclear reactor
fuel pins, their use has not extended to the measurement of surface temp-
eratures of Zircaloy-clad pins under accident conditions. This is basically
a result of the incompatibility of common thermocouple materials with
Zircaloy at temperatures above about 17000F. To overcome these problems,
a thermocouple was developed which is metallurgically compatible with Zir-
caloy, and with the reactor coolant environment. This latter condition
requires that the sheath material not only be compatible with 6500F,
2500 psi water, but that it survive for short periods at 28000F in steam.
Thus, the thermocouple must be metallurgically bonded to Zircaloy and both
the thermocouple and attachment must operate for extended periods of time
under normal pressurized water reactor coolant conditions and for short
periods under accident conditions.
i'..anium was selected as the sheath material and a laser welding process
.sing a titanium filler was selected as the method of attachment to the
fuel pins. Thermocouples were designed for operation over two separate
temperature ranges. The first was to cover temperatures to 23000F and these
ise Chromel/Alumel thermoelements. A second range to 28000F is required
to cover core hot spot measurements and these thermocouples utilize plati-
num/10% rhodium-platinum thermoelements. Magnesium oxide is used as the
insulation Mattr41l for these thermocouples.
The cables are swaged to a final diameter of 0.046 in. and have a
thicker wall than is normally used for cables of this diameter. To allow
for deeper penetration of the laser attachment welds a wall thickness
of 0.009 in. was specified.
A spade of "duck bill" grounded type measuring junction is used to
improve the thermocouple response time. This, too, presents come special
problems since i.he thermoelement materials are not compatible with the
titamium sheath. This problem has been solved by placing a small barrier
of tantalum between the thermoelements and the sheath during fabrication
of the grounded-type junction.
Attachment of the thermocouples to fuel pins is accomplished using an
automated laser welding system. Fillet welds are made at regular intervals
along each side of the thermocouple cable using a titanium filler wire. The
automated tooling was developed to automatically position the thermocouple,
fuel pin, and filler wire during welding. A commercial laser welder is used
in conjunction with the tooling.VISTFIUTION QE SAIS 00CUM7NT 19 UNLMITWE 'A
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Meservey, R.H. Temperature measurement on Zircaloy-clad fuel pins during high-temperature excursions, article, January 1, 1975; Idaho Falls, Idaho. (https://digital.library.unt.edu/ark:/67531/metadc867322/m1/1/: accessed April 19, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.