Materials of the Heat Insulating Package

Researchers of RIPRA “Luch” have invested considerable efforts into the develop­ment of various alternative manufacturing techniques for processing heat insulating materials (HIP) of the nuclear fuel assemblies (NFA) of Nuclear Rocket Engines (NRE), shows two alternative packaging arrangements of heat insulation packages (HIP) for NFA.

Five-layer HIPs based on three ZrC + C casings Fig.2.9b are designed for the thermal flow values of 2 x 106 W-m-2 [30]. The HIP protects the NFA housing from the heat impact of the working medium. This design features a multilayer sectional package structure that minimizes the possibility of cracks penetrating into the housing and offers the possibility to vary HIP material composition over its length and across its thickness.

The pyrolytic graphite-based outer casings of the HIP, along with good thermal insulation, provide for a ‘soft’ contact with the housing, thereby facilitating the structure assembling process and minimizing abrasion of the metal case by thermal insulation. The inner casings are thin-walled carbide-graphite cylinders. For the low — temperature section these casings are made of zirconium carbide + graphite (ZCG) and for the hot section the niobium carbide + graphite (NCG) casings are used. These casings serve as a supporting frame for the HRA preventing the penetration of fragments of heat insulation elements into the hot sections’ duct. The casings ensure the assembly of HSs and their mounting into a heat-insulating package and reduce the effects of erosion and aggressive impact of the working medium on the heat insulation. Casings made of low-density pyrographite and porous ZCG and NCG and typically placed between metal housings. Casings made of low-density pyro-graphite are located in the low-temperature section (T = 1,500 — 2,000 K). For higher temperatures, the first designs contained casings based on so-called “laminate” consisting of carbide layers in a graphite matrix. The next-generation designs had these replaced by casings made of porous Zr and Nb carbides.

The HIP internal casings are made of ZCG or NCG containing 5 wt% of the free carbon. Use of ZCG is limited by the eutectic melt temperature of compositions. The NCG layer serves as a rigid carcass preventing the possible penetration of HIP frag­ments into the hot section duct. The low-density pyrolytic graphite layers (deposited on the surface of one or two casings) are used in the zones characterized by the max­imum power flux. The length of this zone is limited by the temperature of reaction with the environment (hydrogen).

Preforms of ZCG-based on laminate pyrolytic compositions (ZrC + Pq) were processed by depositing carbon and zirconium particles from the vapor-phase mix­ture of ZrCl4 CH4 + H2 + Ar on the surface of a thin-walled graphite substrate at 1,700-1,800K [31].

Pyrolytic graphite (PGV) is a polycrystalline form of graphite deposited from a vapor-phase carbon source at pressures of 25-150 mm Hg on a heated graphite substrate [32, 33]. The PGV casing is used in the relatively cold zone of HIP. PGV casing provides reliable heat-shielding of the duct housing and prevents metal hous­ing exposure to hydrogen. PGV has a highly desirable feature for the given operating conditions: its thermal conductivity slightly decreases with temperature rise.

Pyrographite properties depend primarily on the substrate temperature, pressure and speed of the flowing vapor and on the composition of a vapor mix. By rising the deposition temperature from 1,800 to 2,500K it is possible to obtain pyro-graphites with the density varying from 1.2 to 2.25g/cm3.

Such density variations lead to significant changes in the physical properties of the pyrographite; thus the density is considered as the basic parameter influencing the main properties, including thermal conductivity. Distinctive feature of pyrographites is their significant anisotropy of properties. In particular, perpendicularly to the depo­sition surface (c axis) thermal conductivity maybe 50-100 times lower than that along deposition surface (a axis); depending on density; the thermal conductivity may vary from 1.5 to 7 W/m-K [33]. Due to the lower thermal conductivity along c-axis pyro — graphites may effectively be used in heat shields for various power plants with high operation temperatures.

The results of analysis of the pyrographites thermal conductivity values show considerable scatter both in absolute values, and in their change with temperature along c axis [31-33]. This further complicates thermal calculations and the prediction of performance of specific thermally stressed parts of HGA. In order to obtain the

Fig. 4.25 Temperature dependence of heat con­ductivity of casings from low density LPGV with various density 1.35-1.60g/sm3 and PGV

reliable data on thermal conductivity of pyrographites, measurements were made on samples processed by standard techniques routinely used for production of standard NFA parts. PGV thermal conductivity does not change monotonously. As temperature rises within the certain range, the thermal conductivity decreases reaching a certain minimum and then rises again (see Fig. 4.24).

The maximum difference between absolute values of heat conductivity for the samples performed at temperatures ~ 1,300 K does not exceed 20 %, while for those made at ~2,400 K it does not exceed 13%, which is quite acceptable for the pyrolytic graphite processed by the same technique. The measurement error X for pyrolytic graphite in all cases does not exceed 8-10 %.

Scatter of thermal conductivity values for standard high-density pyrographite as well as the one of the literature values is attributable primarily to multiple defects, as

Fig. 4.26 )f pyrolitic graphites PGV (a) and low density PGV (b) (magnification x70)

well as to micro — and macrostructural differences in PGV parts (see Fig. 4.24) caused by differences in processing history (deposition temperature, heat treatment, gas pressure and flow), sample geometry and other factors. PGN thermal conductivity is typically higher than that of PGV deposited at 2,300-2,500 K (Fig. 4.25). Such change in thermal conductivity with density is due to the structural changes in the material, this assertion being confirmed by the X-ray structural analysis and microstructural characterization.

PGV structure is typically characterized by presence of a significant amount of microcracks (see Fig. 4.26). The reasons for microcracking being (1) stresses gener­ated by temperature gradient across the layer during cooling, and (2) anisotropy of thermal expansion along c and a axes. The thicker the PGV layer, the greater are the stresses and consequently, the higher the propensity to microcracking.

The standard PGV products with wall thickness S of 0.75 mm may feature microc­racks of considerable length that typically pass through multiple growth cones. Upon reaching the cone the crack stops, and the new one is generated in the adjoining lay­ers. Virtually defect-free structure is highly unusual in standard PGV products. As the microcrack plane runs nearly parallel to the deposition surface such microcracks act as large thermal resistances during heat transfer by thermal conductivity in the direction perpendicular to the deposition surface. The number of microcracks has a considerable effect on the PGV thermal conductivity value and on its behavior with temperature change.

Powder metallurgy technique of processing the heat insulating materials has a considerable effect on their structure and properties [10, 27]. Porosity increase in ZrC-based samples from 5 to 70 % leads to thermal conductivity drop by an order of magnitude depending on the structural features, particularly, on skeleton structure factor and closed or open porosity.

For example, porous fibrous ZrC (see Fig.4.27d) processed by thermal diffusion impregnation of carbonized fiber by zirconium show lower thermal conductivity than carbides with continuous skeleton matrix, despite their equal density. Natural structural variation in a porous composition inevitably leads to the scatter in heat conductivity values between samples from the same batch. It is to be noted that the temperature coefficient a thermal conductivity in highly porous carbide materials is similar to the one for the dense carbides, with some influence of heat radiation in pores noticeable only at temperatures above 1,800 °C.

Fig. 4.27 A microscopic structure of porous ZrC carbides (a, b), foam ZrC carbide (c). fibrous ZrC carbide (d) all with porosity (P ^ 75%) layered carbide-graphite (ZrC + Pq. P = 25%) (e), (a, b, c x200), (d x70)

ZrC-based fibers with nearly stoichiometric composition had diameters of 15­20 ^m. Bulk density of carbide, depending on a carbonization regime, varied from 70 to 90 % with thermal conductivity ~2 W/m-K at porosity ~70 %. Compressive strength values of these samples are rather low only 1.2-1.5 times that of tensile strength [33]. At equal porosity levels, porous carbides are stronger than fibrous ZrC, though demonstrating somewhat lower thermal insulating capacity than the latter. Greater shrinkage of fibrous ZrC at temperatures >2,300 K even under moder­ate compressive loads makes porous carbide the material of choice for use in HIPs. Greater structural stability of porous carbide, attributable to its structure, is influ­enced by the choice of processing technique [34]. Zirconium carbide and potassium bromide powders in ratios 1:10 to 1:300 were mixed with binder (natural rubber). This premix was pressed and pre-sintered in vacuum for de-bonding at temperatures above carbide sintering point, with subsequent final sintering to achieve consolidated porous parts. Pre-sintering was effected with heating ramp up to 2,300 K during 11 h, with subsequent isothermal dwell time of 0.5h, whereas final sintering was carried out at heating ramp up to 2,800K within 5h with subsequent dwell time of 0.5h at maximum temperature. This sintering technique yielded products with porosity of 50-85 %, depending on the introduced filler amount. Pore stability at high tempera­tures was achieved through the presence of rather large cellular pores corresponding to the pore-forming agent (filler) particle size.

Formation of special structure of a heterophase composition from alternating carbide-graphite layers (ZrC+Pq) with porosity of 30 % (Fig. 4.27e) deposited from a gas phase get to gain stronger material with thermal insulating properties (Fig. 4.28), which are not conceding to properties of high porous carbides.

It should be note that the temperature coefficient of thermal conductivity of high porosity carbide materials is analogous to that for the dense carbides (Fig. 4.29), some influence of heat radiation in pores are detected only at temperatures above 1,800 °C.

Fig. 4.28 Heat conduc­tivity of compositions ZrC + Pq from alternating carbide-graphite layers:

ZrC 35-40 weight.%, PGV 60-65 weight%, (1) and ZrC 55-60 weight%. PGV 40­45 weight% with a thickness of 0.25-0.5 mm every layer (2)

Fig. 4.29 Heat conductivity of ZrC, П = 6% (1), ZrC +5%C (2), ZrC +5%UC (3) pyrolitic graphite (4), fibrous ZrC carbide p = 70% (5), foam carbide ZrC П = 65 % (6)

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