Gas-feed nozzle for a pyrolytic particle coating apparatus

An inner tube through which the coating gas is supplied terminates short of he end of the outer tube which forms an annular channel around the inner tube for supply of the carrier gas, so that a chamber is formed between the ends of these channels and the connection of the outer tube to the bottom of the heated reaction container. A constriction is provided where the outer tube joins the reaction container with its aperture aligned with the common axis of the two gas supply tubes. The position of the end of the inner tube is adjustable to suit the particular gas feed rate. The inner tube never reaches the temperature at which the coating gas decomposes. The upper portion of the outer tube, particularly at the constriction, is at or above that temperature, but the flow of carrier gas along the hot surfaces prevents the deposit of a coating on these surfaces. A porous barrier in the annular channel assures a uniform velocity profile of the carrier gas, so that in the nozzle and even in the lowermost part of the reactor it will sheathe the stream of coating gas.

This invention concerns a gas feed nozzle for supplying a decomposable 
coating-producing gas and also a carrier gas to an apparatus for the 
coating of fuel kernels for a nuclear reactor, and more particularly, to 
gas feed nozzles of that type provided at the bottom of the reaction 
container of a fluidized-bed coating apparatus. 
The coating-producing gas and the carrier gas that is generally necessary 
to be used with the coating-producing gas in the coating of the kernel 
form fuel and breeder elements of a nuclear reactor are commonly provided 
in coaxial piping, in which a central channel serves to lead the 
decomposable gas that provides the coating and an annular channel coaxial 
with the central channel serves to lead the carrier gas to the coating 
apparatus. 
Fuel and/or breeder material kernels of a diameter of a few hundred microns 
made for use in a nuclear reactor are coated with a suitable material in 
order to prevent or mitigate the giving off of fission products. According 
to a known coating process, the kernels are treated in the turbulent layer 
of a fluidized bed with one or more thermally decomposable gas or gases, 
such as methane, acetylene, propane, propylene, chlormethylsilane, 
molybdenum-V-chloride (MoCI.sub.5) or the like, as well as a carrier gas 
that is suitable because under the reaction conditions it is inert, for 
example, argon, helium, hydrogen, nitrogen or carbon monoxide. The 
reaction temperature is between 1000.degree. and 2200.degree. C. The 
fluidized bed reactor consists of a double-walled water-cooled cylindrical 
vessel preferably arranged for heating electrically by resistance heating 
or inductive heating, with a reaction tube or fluidized bed tube provided 
within the vessel. The turbulent layer is formed in the fluidized bed tube 
which is made of graphite because of the temperature at which the 
pyrolysis is intended to take place. The gases required for the pyrolysis, 
including any necessary carrier gas for dilution of the pyrolytic gas, are 
supplied to the fluidized bed through a nozzle by which the supplied gas 
must be so distributed that the kernels of fuel or breeder material are 
held in a circulating movement, so that all kernels of fuel or of breeder 
material are evenly coated. 
Various constructions of feed nozzles are known. For example, one is known 
in the shape of a capillary which leads into a generally conical fluidized 
bed tube (see Melvin F. Browning, Dale A. Vaughan, Joseph F. Dettore, John 
M. Blocher, Jr.: Characterization of Pyrolytic-Carbon Fuel Particles 
Coating prepared with Acetylene, Battelle Memorial Institute, Rep.-No. 
BMI-1735, 1965, A1, A2). It is also known to use for the nozzle a tube 
that is connected to the conical end of the fluidized bed tube after the 
fashion of a ball and socket joint. Nozzles with a single opening have the 
disadvantage, however, that at higher coating temperatures deposits are 
formed on the conical walls of the fluidized bed tube, as the result of 
which the flow characteristics are seriously impaired and occasionally 
even stopping up of the nozzle is caused. 
In order to avoid these disadvantages, nozzles have been developed in which 
several openings are provided. According to one known form, the nozzle in 
this case consists of a molybdenum tube with a tapered down end, that 
serves for supply of carrier gas and coating gas, while bores are provided 
concentrically in the conical part of the fluidized bed tube through which 
an inert gas--conveniently the same gas that also is used as the carrier 
gas--is supplied into the fluidized bed tube, in order to mitigate the 
deposit of solid materials (see H. Beutler, G. B. Redding, J. R. G. Gough: 
Development of Coated Particles, Fuel Element Symposium, 1963, Rep. 151). 
Further types of known nozzles exist for which in the supply of coating and 
carrier gas a central opening is provided, while for additional quantities 
of carrier gas, bores arranged coaxially with the central opening are also 
provided (R. L. Pilloton, J. A. Carpenter: Motion of Particles in 
Fluidized Beds and Implications for the Preparation of Coated Nuclear Fuel 
Particles, Oak Ridge National Laboratory TM-1170-1965), or in which a 
porous plate is used that can be regarded as an arrangement of countless 
small nozzles arranged next to each other (H. Bildstein, P. Koss: Coated 
Particles, Beschichtete Teilchen, Jahresbericht 1965, Reaktorzentrum 
Seibersdorf, Osterreichische Studiengesellschaft fur Atomenergie mbH). In 
the case of nozzles with several openings, it has been found, however, 
that small bores arranged alongside the main opening get stopped up after 
a relatively short period of use in spite of intensive cooling. 
Other forms of known nozzles consist of providing a ring gap encircling a 
central opening that serves for supply of the coating gas, the carrier gas 
being blown into the fluidized bed through the ring gap (see R. L. 
Bickerdicke et al.: Studies on Coated Particle Fuel Involving Coating, 
Consolidation and Evaluation, D. P. Report 139, 1963 and R. L. R. Lefevre 
et al.: The choice of Pyrocarbon Deposition Agent for Nuclear Fuel 
Particles, D. P. Report 800, 1972). Furthermore, German patent No. 
1,808,550 discloses a nozzle for the supply of gases that is made of 
graphite in which a central channel is provided for the decomposable gases 
and a surrounding annular channel coaxial to the central channel serves 
for supply of the carrier gases. These approaches do not overcome the 
problem, however, that parts of the nozzle structure that are exposed to 
the decomposition temperature for the coating gases come into contact with 
the coating gases. Layer growths and even stoppages are not to be avoided 
even with these last-mentioned nozzles. 
It is an object of the present invention to supply a feed nozzle apparatus 
for the supply of gases in which the formation of deposits on parts of the 
nozzle structure, and hence a stopping up of the nozzle, is prevented. 
SUMMARY OF THE INVENTION 
Briefly, the outer tube of the annular channel is extended beyond the month 
of the inner tube that supplies the decomposable coating gas and a 
constriction is provided located at least in part within the end of the 
outer tube that connects to the gas feed orifice of the coating chamber, 
so that the adjacent ends of the separate central and annular channels are 
spaced from the constriction. The constriction constitutes a nozzle for 
the end of the outer tube and has a portion of minimum aperture which 
provides a central opening, for exit of gas from the outer tube, through 
which the coating-producing gas flows within a similarly moving sheath of 
carrier gas that flushes the surface of the nozzle construction. 
The orifice of the central channel from which the coating gas streams out 
lies well inside the nozzle structure, since it is spaced away from the 
constriction at the entrance to the fluidized bed as the result of the 
outer tube extending axially beyond the mouth of the inner tube is 
therefore at a lower temperature than that which is necessary for 
decomposition of the coating gas. A formation of a deposit at the orifice 
of the central channel and on the parts forming that orifice is therefore 
not possible. Furthermore, the laminar stream of coating gas issuing from 
the central channel towards the middle of the constriction is laterally 
surrounded by the similarly directed laminar stream of carrier gas stream 
coming out of the annular channel and is therefore kept away by the 
carrier gas stream from the nozzle configuration of the walls that lead 
towards the place of minimum aperture of the constriction, so that the 
formation of deposits on the remaining portions of the nozzle is also 
prevented. This holds in particular for the portions of the nozzle forming 
the constriction, which are actually at a temperature high enough for the 
decomposition of the coating gas. In this connection, it has been found 
desirable to provide a sharply inwardly-running edge surface for the 
upstream side of the constriction, culminating, for example, in an annular 
inward cusp. Preferably the direction of gas flow is vertically upward, so 
that the lower or under side of the constriction, which preferably has a 
concavely curved transition surface leading to the cusp, is the upstream 
side of the constriction The upper (i.e., downstream) surface of the 
constriction preferably has a downstream upwardly widening flare above the 
place of minimum aperture, which widens out at a rate substantially 
greater than that at which the portion of the reaction container 
immediately thereabove gradually widens. 
An advantageous further development of the nozzle according to the 
invention provides an inner tube for the central channel which is at least 
in part axially shiftable, so that the axial position of at least an end 
portion of the tube, and hence also of the end of this tube, can be 
adjusted. In this way, it is possible to set a spacing between the 
constriction and the downstream ends (i.e., the upper ends in the usual 
case) of the separate central channel and of its surrounding annular 
channel, which is optimal for the particular conditions that may be 
produced by operations at a particular rate of gas delivery into the 
fluidized bed, which is of course a process variable. 
A further advantageous development of the nozzle apparatus according to the 
invention consists of insertion of means for providing an evenly 
distributed velocity profile of gas leaving the annular channel, such 
means being preferably provided in the form of a porous barrier in the 
annular channel. Thus, a surrounding layer of carrier gas of uniform 
velocity profile is provided around the central stream of coating gas. The 
porous barrier in this case may be regarded as defining the upstream 
boundary (i.e., usually the lower boundary) of an end portion of the 
annular channel in which end portion a laminar flow of carrier gas is 
established.

As shown in the drawing, the nozzle apparatus in each case has a central 
channel 1 inside an inner tube 6 and an annular channel 2 surrounding the 
central tube and bounded on the outside by an outer tube 3. The outer tube 
extends beyond the mouth of the inner tube 6 and ends at 8 in a 
constriction 4 where it fits in an orifice in the lower end of the 
fluidized bed container 5 that is only partly shown in the drawing. The 
minimum aperture of the opening 11 through the constriction 4 is so chosen 
that no fuel kernel or moderator material kernel can drop down into the 
inside of the nozzle apparatus at the lowest gas delivery rate to be used, 
which is easily provided, for example, with an aperture of 6 mm diameter, 
when kernels of a diameter of a few hundred microns are in the apparatus. 
The constriction 4 constitutes a nozzle for the end 8 of the outer tube and 
has a portion 10 of minimum aperture which provides a central opening 11 
for exit of gas from the outer tube 3. 
As is further evident from the drawing, by virtue of the extension of the 
tube 3 beyond the mouth of the inner tube 6, the upper end 9 of the 
central channel 1 inside the tube 6 and the adjacent upper end 9a of the 
annular channel 2 between the tubes 6 and 3 are spaced up-stream (i.e., 
downwards) from the constriction 4. In other words, the illustrated end of 
the inner tube 6 terminates short of the corresponding end of the outer 
tube 3. The tube 6 that forms the central channel 1 on this account is not 
at a temperature high enough for producing the decomposition of the 
coating gas. The cross-section of the central channel 1 within the portion 
of the inner tube 6 shown in each of the figures of the drawing, namely 
the end portion of the tube, is substantially constant. 
The stream of coating gas issuing out of the central channel 1 and also the 
stream of carrier gas issuing out of the annular channel 2 proceed by 
laminar flow in separate stream paths, although in contact with each 
other, to the constriction 4, so that the material of the constriction 
structure is flushed by a gas consisting only of the inert carrier gas. In 
order that the gas velocity of the carrier gas issuing from the annular 
channel 2 may be evenly distributed (i.e. equalized), an annular plate 7 
of porous graphite or sintered metal is provided as a grid in the annular 
channel, at the same time serving as a positioning member for centering 
the central tube 6 that is shiftable in the axial direction. Means for 
shifting either the entire tube 6 or an end portion thereof are shown at 
12 in FIG. 3 in a symbolic manner. A wide variety of such means can be 
provided, not only mechanically, but also magnetically with permanent 
magnets, or electromagnetically, for instance. 
Since the carrier gas is forced to flow through the porous layer 7, a 
highly uniform velocity profile in the carrier gas stream is obtained. 
The portion of the outer tube 3 between the position of the porous plate 7 
and the throat 11 of the constriction 4 can have various shapes 
internally. FIG. 1 shows a structure in which the end of the tube 3 is 
cylindrical right up to a substantially flat bottom upstream surface 4a of 
the constriction 4. FIG. 2, on the other hand, shows a structure in which 
this end portion of the outer tube 3 gradually narrows as shown at 3a from 
the porous plate 7, in conical form, up to the narrowest part of the 
constriction 4, thus providing a conical lower upstream portion for the 
constriction 4 that extends to a place on the inner wall of the outer tube 
3 that is upstream of the mouth of the inner tube 6. Its lower end, 
adjacent to the porous plate 7, is well upstream of the end of the tube 6. 
In the form shown in FIG. 3, the inside of the tube 3 remains cylindrical 
for some distance past the mouth 9 of the tube 6 and then concavely 
narrows 3b to a cusp 10a at the narrowest part of the constriction 4. The 
downstream (upper) side of the constriction in all of these illustrated 
embodiments widens (flares) quite sharply (i.e. it has a high axial rate 
of flare) until the cross-sectional area is about the same as that of the 
outer tube 3 upstream of (below) the beginning of the constriction. In 
other words, the portion of the container 5 immediately above the nozzle 
constriction has an internal cross-sectional area not substantially 
smaller than that of the outer tube 6. Above the downstream flare of the 
nozzle constriction the cross-sectional area, now relating to the 
construction of the fluidized bed container, widens upward much more 
gradually. 
The aperture of the constriction preferably has an area which ranges from 1 
to 2.5 times the cross-sectional area of the central channel 1. 
The inside diameter of the outer tube 3 preferably has a diameter 5 to 10 
times the diameter of the central channel 1. However, the inner diameter 
of the outer tube 3 has to be such that for a needed flow rate of carrier 
gas the flow of the carrier gas in the annular channel 2 is laminar. 
The spacing between the end of the tube (6) and the place of smallest 
cross-sectional area of the constriction 4 as shown in FIG. 1, is 
preferably 5 to 40 times the diameter of the central channel 1. In case 
tubes of non-circular cross-section are used for the tube 6, these 
relations would be referred to the minimum inside width of the tube. 
For materials for the tube 3 and likewise for the inner tube 6, or at least 
their end portions shown in the present drawing, firm graphite material is 
preferable. For the inner tube 6 also metals of high thermal stability 
such as temperature resistant steel of the American designations AISI 309 
or AISI 810 may be used. 
Although the invention has been described by reference to particular 
illustrative embodiments, modifications or variations are possible within 
the inventive concept.