Thermal Test Methods

The thermal properties in different temperature ranges were measured by different methods using different setups. Comparative estimates of the thermal diffusivity a, the heat conduction X, the heat capacity C, and the electric resistance p were made by fast transient methods with a typical accuracy of ±10% in the temperature range from 300 to 1,000 K. The heat conduction of carbide materials at high temperatures (up to 2,800K) was measured with an accuracy of ±15 % by the axial method by passing current through a rod sample or a fuel element. Simultaneously, the electric resistance was easily measured.

Special technique and equipment were developed for thermal conductivity charac­terization of the multilayer heat insulated packets (HIPs). The radial heat flux method based on indirect internally applied heating (cylinder technique, see Fig. 3.3) yielded reliable data on heat conductivity Xeff and thermal resistance (Rn) of the HIPs by the radial thermal stream method with indirect heating.

The bearing capacity of thermally loaded constructions was estimated by methods with parameters closest to the operating conditions of devices. The thermal loading of an HRE was simulated by cooling the heated HRE in water, while the thermal loading of cases in the HIP was simulated by heating in melted tin [1]. The thermal strength of materials at temperatures exceeding 0.3-0.5Tm (where Tm is the melting point), when micro — and macroplasticity can appear, was measured by electron, plasma, and induction heating methods, which require sophisticated equipment. These methods

Fig. 3.3 The circuit design of assemblage for measurement of heat conductivity TIP.

image0241 heater, 2, 3, 4 casings: accordingly, internal, testing and outdoor, 5 pyrometric channels, 6, 8 apertures for potential and potentiometer sounds, 7 a collet system and a crimped lock, 9 the pyrometric channel

were used to vary and control the level and type of temperature fields and the rate and cycle of their variation, thereby permitting quantitative estimates of the ther­mal strength of materials under conditions close to operating conditions at high temperatures.

The interaction of the HRA units with hydrogen at working temperatures and changes in the composition, structure, and physical-chemical and mechanical prop­erties were investigated using electrical-thermal setups in which the required tem­peratures could be produced, up to the melting points of materials directly in HREs made of Zr-UC + C and UC-ZrC-NbC. Hydrogen or hydrogen-methane mixture flows passing through working chambers provided the gas rate near the sample sur­face approximately from 0.1 to 40ms-1 at pressures up to 10MPa. The hydraulic resistance of the HRA channel in the construction was optimized on hydraulic mounts (Fig. 3.4).

A unit for thermal heat resistance testing by radiation heating under nonstationary conditions includes, as a rule, a hot (a) and a cold chamber (b, c) (Fig. 3.5). The specimen in the hot chamber is heated in an inert medium or in a vacuum to 2,000 °C by means of heater 1, which is a hollow graphite cylinder placed inside an inductor. The heater is shielded from the surroundings by water-cooled double quartz tubes. In

Fig. 3.4 Circuit design of the working chamber of installation B-2. 1 the tested sample (Fuel element), 2 a quartz tube in diameter 10mm, 3 bearing pipe, 4, 5 current leads with an air-floating head, 6-9 clutches, the 10 steel case

image025image026Fig. 3.5 Circuit design of the combined installation RAD for definition of ther­mal strength resistance by a heating (cooling) method of the sample by radiation or cooling in liquid medium [1]. 1 Heater, 2 specimen, 3, 5 shield, 4 piston, 6 sound duct

order to create a uniform temperature field along the length of the graphite heater, the heater’s edges were protected by molybdenum or graphite shields. The temperature of the heater is monitored by an optical pyrometer. After the heater has reached the desired temperature, specimen 2 (protected on the side face by shield 3) is transferred pneumatically from the cold chamber to the hot chamber as piston 4 is driven by pressure applied from beneath. After the specimen has been positioned in the middle of the heater, shield 5 is released and allowed to drop by gravity, and the side face of the specimen is exposed to the heater. The instant at which the specimen fails is recorded by a piezoelectric sensor connected to the specimen through a sound
duct 6; the signal from the sensor is transmitted through an amplifier to a recording oscillograph which is switched on automatically after shield release.

The transfer of heat from the heater at temperature Th to the specimen’s surface at temperature Ts can be treated in terms of constant heat flow q = const with an accuracy better than 5 % if the surface temperature is not in excess of 0.475 Th [1]; this makes determination of the thermal stress resistance a much simpler procedure.

The maximum relative error in measuring the temperature does not exceed ±0.7 %; this error is associated with the thermocouple calibration error (±0.3 %), measuring device error (±0.05 %), and delay effect error (±0.3 %) which is caused by the heat resistance at the thermocouple—specimen contact through an In-Ga eutectic at temperature change rates of <100K/s. As the heat flow for the system “heater — specimen” is known, the integral mean temperature drop to failure is determined by merely measuring the heating time to failure rf.

The total relative error in the thermal stress resistance composed of the time — to-failure error, heat flow error, temperature extrapolation error, and thermal para­meter errors does not exceed 10% with a probability of 0.68. As a rule, failure is manifested by complete disintegration of the specimen.

It should be noted that the cylinders and disks used as test specimens are more convenient than hollow cylinders or rings. However, in both cases heat resistance of the material in terms of mean integrated temperature ATm does not exceed 100 °C, and the temperature at failure can be varied only within 300-700 °C.

Test process consists in a definition of the temperature pattern at the body fracture moment and a calculation of fracture mean integrated temperature drop in a body ATm = Tm — T, where Tm—mean integrated temperature on cross-section of a body and T is a point temperature (on a surface or on a body center), where thermal stress are defined

a = aEATm/(1 — i), (3.1)

where E—modulus of elasticity, a—a coefficient of linear expansion, ^—a Poisson’s ratio.

The complex (1 — i)/aE = ATm is accepted to name as the first criterion of thermal strength resistance R [1].

High-temperature thermal strength resistance under electron beam heating was measured on UTU-1 system (see Fig.3.6) developed at RITM, Moscow. The said technique involves both sophisticated equipment and an elaborate measurement procedure, but its capabilities if high-speed thermal loading of the sample (up to 1,000K/s) in a wide temperature range up to 2,800K [2] led to effective simulation of the operating conditions of some HGA blocs in NREs. The cylindrical samples in the UTU-1 system were heated by four electron beam guns placed in a horizontal plane at 90° in a sealed chamber under vacuum (min. 0.01 Pa). Thermal strength behavior at various temperatures was measured by slow heating of the sample to the preset temperature, with subsequent high-speed heating ramp up to the sample fracture. The fracture criterion used was the abrupt slope change of the temperature curve as registered on the oscilloscope tape. Temperature stress calculated using numerical methods based on the first boundary conditions, with surface temperature

being a given function of time. The general measurement error of the thermal strength resistance was estimated at ±15 %.

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