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Ceramic Materials in Reactor Construction.

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C O N F E R E N C E REPORTS
Ceramic Materials in Reactor Construction
The Confercnce took place o n November 8th and 9th, 1962
a t Baden-Baden (Germany), in conjunction with the Annual
Gcneral Meeting of the Deutsche Keramische Gesellschaft
(German Ceramic Society) as a joint meeting of the AusschuB fur Reaktormetalle (Reactor Metals Committee) of
the Deutsche Gesellschaft fur Metallkunde (German Metallurgical Society) with the German Ceramic Society.
T h e discussions were held under the chairmanship of Prof.
Dr. A . Dietiel of the Max-Planck-lnstitut fur Silikatforschung, Wurzburg (Germany), and Prof. Dr. E. Gebhnrdt, of
the Max-Planck-Institut fur Metallforschung, Stuttgart (Germany).
T h e reports and discussions made it clear that the importance
of ceramic materials is steadily increasing, especially for the
construction of power reactors. T h e aini in plants of this kind
is t o attain temperatures that are as high as possible, in order
t o make the reactors economically competitive i n comparison
with conventional plants for the generation of electric energy.
This necessitates the use of ceramic materials, but some
problenis remain unsolved, especially with regard to fuel
elements: these include radiation damage under reactor
conditions, escape of fission gases, corrosion problems, and
economic production methods [I].
The part of the conference conducted by Prof. Gebhtirdt
dealt mainly with uranium oxides and uranium carbide.
H . Birmm, Karlsruhe (Germany), showed that uranium
dioxide and uranium monocarbide a r e being increasingly
used in place of metallic uranium. Furthermore, the production of metal-ceramic compositions (cermets) and the
combination UO;!/BeO were discussed.
D. Harkort, G. Honcici, and If. G. Kling, Frankfurt/Main
(Germany), studied the physical properties of sintered UO;!
preparations in relation t o B e 0 additions. Specimens with
beryllium oxide contents of 5 , 10, 15, and 20 % by volume
were cold-pressed and sintered at temperatures of up t o
1600°C under hydrogen and attained densities of up to 95 %,
of the theoretical density. It was found that the thermal
conductivity increases with increasing B e 0 content, e.g. t o
exactly double the value for the thermal conductivity of
uranium dioxide for an addition of 20 ”/, B e 0 by volume.
A relationship between the thermal expansion and the
amount of B e 0 added could not be observed, nor does
beryllium oxide appear t o be soluble in uranium dioxide.
T h e question was raised, during the discussion, whether the
conditions of annealing v i i . 1600°C for one hour, are
adequate for establishing equilibrium in this system. Safety
precautions were carried o u t in conformity with the regulations of the AEC, a n d it was seen that the maximum
permissible concentration of 2 pg of beryllium per m3 of air
was not exceeded. The dust filters used were designed for
dusts with particle diameters > 3 y.. It would be interesting
to know whether it is worthwhile increasing the conductivity
of uranium dioxide at low temperatures by additions of BeO,
since heat transfer in high temperature reactors takes place
by radiation.
Studies of particle growth in uranium dioxide preparations
by H. Stehle, Erlangen (Germany), attain special interest
because it is possible t o utilize particle growth for indirect
temperature determinations and, furthermore, because the
escape of fission products from the fuel element may b e
influenced by particle growth. I t was noticed that in sintered
UOz preparations, large particles grow a t t h e expense of
small ones by a thermally activated process; seeding is not
[ I ] The lectures given have be% published in “Rerichtc der
Deutschen Keramischen Gesellschaft” (Reports of the German
Ceramic Society) 40, N o . 2 (1963).
Aigew. Chem. internat. Edit. / Yol. 2 (1963) NO. 6
involved. T h c results may be expressed i n the form:
6: b:,
~
.
h O . L: ~ - 8 7000/RT
.
where D is the mean diameter of the particle i n y. at time t,
and ko is a constant equal t o 0 . 3 1012
~
1 . 2 ~10121~3/h.T h e
experimental results also show that large pores grow at the
expense of small ones.
I t was emphasized during the discussion that sintered
uranium dioxide is free from intrinsic tensions and, therefore,
the observed activation energy of 87 kcal/inole is t o be
ascribed almost exclusively t o the initial step i n the grainboundary shift. It was noticed, too, thal particle growth may
be influenced by radiation in two dii-ections. O n the one
hand, it is accelerated by stimulation of the initial steps,
while, on the other, the fission products limit the particle
sizes and thus may cause delay in pariicle growth. A rough
estimate shows that the total influences of radiation probably
leave the final rcsult unchanged.
Sintering of U 0 2 was discussed by N . Miiller, Wolfgang bei
Hanau (Germany). Uranium dioxide has a higher melting
point (2800°C) than uranium metal. It docs not undergo
phase inversion on heating and is thercfore preferred as fuel.
despite its lower thermal conductivity. Moreover, uranium
dioxide powder has t o be sintered to form pellets. T h e
specific surface area of the original powder and the density of
coinpressed pellets should be as high as possible for high
sintered densities. Finer powders illso have considerably
smaller pore diameters, and it was shown, in accordance
with Stehle’s results (see above), that pores with mean
diameters of --2000 8, disappear 011 sintering. T h e final
density is apparently reached when the diameter of the
remaining pores is t o o large for shrinkage to take place. In
the case of super-stoichiometric sintering, experiments show
that a plot of the sintered density against the sintering
temperature passes through a maximum at about 12OO0C.
If sintering is carried out at higher or lower temperatures,
prepxations of lower density are obtained. This effect was
observed using preparations with O/U ratios of 2.05, 2.12,
and 2.21. Oxygen has higher partial pressures at higher
temperatures and hence probably delays the sintering
process and leads t o lower densitics. With regard t o extraneous admixtures, Ti02 and V:O? cause higher sintered
densities a t low temperatures, Ca(OH)2 causes lower ones.
To a certain extent, sintering temperature and sintering time
are interchangeable variables. All thcsc circumstances have
to be balanced o u t for economic production of uranium
dioxide pellets.
The pellets produced by sintering pass through a n intricate
production process and have t o be adjusted to their final
dimensions by expensive grinding. T h e waste amounts t o
5~-15 A search for alternative processes was made with a
view t o reducing costs. F. Hofmnnn, H. Kroll, B. Liebnzann,
and 0. Neumann, Wolfgang bei Hanau (Germany), examined
es of producing UOzfuel elements by vibrational
compacting and hammering. A promising idea, that is also
particularly suitable for processing mixed uranium/plutonium
oxides, which may only be handlcd with stringent safety
precautions, involves preliminary compacting of the powders
in a tubular envelope by vibration and subsequent compression to the final density by hammering. T h e powders have to
bc carefully prepared for this process by fusing U 0 2 in a
special smelting plant and subsequent milling. A scrutinizing
final check is indispensable after fabrication of the fuel
element rods. T h e thermal conductivity of the preparations
that of sintered
obtained by this process is only 35 -50
pellets. Subsequent sintering does occur, however, owing t o
the intense neutron flux in the reactor and, in this way, the
x.
‘x
329
thermal conductivity is raised virtually to the level of sintered
pellets.
The production and processing of uranium monocarbide for
fuel elements was described by P. Hirnrnelstein, B. Liebrnnnn,
and L. Schafer, Wolfgang bei Hanau (Germany).
Uranium monocarbide has a high uranium density and a
thermal conductivity exceeding that of uranium dioxide by a
factor of 3-6. The preparation was carried out according to
the equation:
UO(Z.,X, t (3 -1-
X)C
=
uc
t (2 + x)CO.
The original powder had an O/U ratio of 2.00 and a specific
surface area of about 0.5 mz/g. The carbon was incorporated
in the form of finely divided natural graphite. The ingredients
were thoroughly mixed i n a Lodige mixer, the mixture pressed
into pellets, and the latter sintered in a vacuum induction
furnace at 1700 "C. The pellets were subsequently fused in an
argon atmosphere in a special arc-type centrifugal casting
plant, uranium monocarbide rods of 13 mm 0 and 100 mm
length being produced by the centrifugal casting process. The
rods showed satisfactory uniformity. It was pointed out, in
particular, that only about the szme number of steps is
required for the production of uranium monocarbide as for
uranium dioxide. The considerably higher price of the
carbide is due to the fact that there is less experience in this
field. It could be lowered appreciably if this fuel were required
in larger quantities for future reactors.
A survey of less-known uranium compounds and their
potential use as nuclear fuels was given by F. Tlziirnrnler,
Stuttgart (Germany). Nitrides, silicides, sulfides, aluminides,
beryllides, borides, phosphides, selenides, and tellurides of
uranium were discussed. The most important compound of
this series is UN, the use of which as a disperse fission
material, in conjunction with niobium, molybdenum, stainless steel, and similar metals, is attaining popularity. Furthermore, compounds like US, U3Si2, UP, and UA12 are of
interest. The selenides and tellurides were studied mainly for
their thermoelectric properties.
Investigations of the cermet system uranium dioxide/molybdenum were carried out by E. Gebhardt and G. Ondracek,
Stuttgart (Germany). The U02/Mo cermet is produced by
mixing, cold-pressing, and sintering of the corresponding
powders. During the sintering operation, the linear shrinkage
was measured; it remains constant with increasing sintering
time in the case of pure molybdenum and uranium dioxide,
but unexpectedly riscs for the cermet samples. It was ascertained by metallographic, X-ray, and electron-microscopic
investigations that the reaction
3 UOz
+ Mo
=
2 MOO,
+3U
most probably does not take place during the sintering
operation. In contrast to uranium dioxide pellets, the cermet
bodies did not show any superficial cracks after irradiation
with lOl(' thermal neutrons per cm2. With molybdenum
additions of between 30 and 50
by weight, the electrical
conductivities of the samples come very close to that of pure
molybdenum, which indicates that, from this concentration
upwards, the molybdenum forms a continuous basic mass,
which can also be seen from the irregular shape of the
density curves in this concentration range. Measurements of
the thermal expansion showed a decrease of the coefficient
of expansion of uranium dioxide with rising molybdenum
content. Shaving experiments on the lathe yielded dust-free
turnings from uranium dioxide preparations containing
additions of molybdenum.
In the discussion, thc experimental fact was mentioned that,
while the density of pellets increases with rising molybdenum
content between 30 and 60 % by weight of molybdenum, the
density of the sintcred preparations decreases. This phenomenon is explained by assuming the formation of a
molybdenum continuum in this concentration range.
[VB 668/83 IE]
SELECTED ABSTRACTS
An acid chloride, containing phosphorus and sulfur, with the
structure either ( I ) or ( 2 ) , has been prepared by M . BeckeGoeliring and J . Hartetwtein by treiting triphosphorus
nitrilochloride at 70 80°C with chlorosulfuric acid. Fission
of the triphosphorus nitrilcchloride ring leads to a colorless
0
R2N-CHZ-Hal
I R -SH -t NR"3 + R~N-CHI-SR' .I- [R"NH] HalE liquid with the formula PNS205C14, which can be vacuum2 RzN-CH2-S -R' I- I ? + 2 R~N-CHZT4-R-S- S-R'
distilled without decomposition (b.p. 55 'CjO.02 mm, or
63 'C/O. I mm Hg). I n the nuclear magnetic resonance specsolvents at room temperature to form dialkyl or diarql ditrum of the phosphorus, there is a line with a chemical shift
sulfides together with the corresponding dialkylaminomethyl
of 4 ppm (relative to orthophosphoric acid), indicating the co[Rd 526/147 IE]
halides. / Chem. Ber. 96, 604 (1963) / -BO.
ordination number of 4 for the phosphorus.
Dialkylaminomethyl thioethers were obtained by H . Bohme
and K. H w t k e by treating dialkylaminomethyl halides with
thiols or thiophenols in the presence of tertiary amines. The
thioethers were resplit by iodine or bromine i n indifferent
The constitution of laurelliptine, a new aporphine alkaloid
from Beilschrnicdia ellipticn White and Francis (Lnuroceoe),
has been elucidated by P. S . Clezy, A . W. Nichol, and E. GelIert. The alkaloid, ( I ) , m.p. 191 " C , [crl'd = 47' (alcohol),
+
CI3P-N -SO2 -0 -SO- -C1
CI 4 0 2 -N=PC12 -0
4 0 2 -Cl
(21
(1)
Results of chemical examination support the symmetrical
formula (2). / Z . anorg. allg. Chem. 320, 27 (1963) / -KO.
[Rd 5321151 IE]
'
forms a dimethyl ether and an O,O,N-triacetyl derivative,
gives N-methyllaurelliptine on hydrogenation with Raney
nickel in the presence of formaldehyde, and shows the characteristic ultraviolet absorption of the aporphine alkaloids.
O,O,N-Trimethyllaurelliptineis identical with glaucine. / Ex[Rd 493/143 IE]
perientia 19, I (1963) / -Ma.
330
Enrichment of 137Cs in organisms has been investigated by
K. Pfleger et al. The ratio Cs~+/K.I
is higher in man than in
food by a factor of three. The transport of K+ and Cs+
through cell membranes was investigated by incubating
rabbit erythrocytes with 4 2 K + and 137Cs+. In accordance with
the literature, it was found that K' is absorbed 4.5 times
Faster than Cs+. However, from measurement of the rates
of diffusion out of the cells (I< '/CsC = 1.5), it must be concluded that, contrary to previous observations, Cs cannot be
concentrated in the cells by exchange between serum and the
[Rd 497/142 IE]
cells. / Experientia 19,25 (1963) / -Re.
Angew. Clieiir. internut. Edit.I Vol. 2 (1963)
/ No. 6
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