Method and tunnel furnace for sintering blanks

Method for sintering blanks into fuel pellets, in which the blanks are moved through the muffle of a tunnel furnace by pushing the blanks through the muffle on a guiding device which goes through the muffle and protrudes therefrom at least on the input side, in the form of a single-layer column of abutting blanks.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention relates to a method for sintering oxidic nuclear fuel blanks 
into oxidic nuclear fuel pellets, in which the blanks are moved through 
the muffle of a tunnel furnace. 
2. Description of the Prior Art 
Transport boats have been used heretofore, on which a large number of 
blanks were stacked side by side and on top of each other, for 
transporting the blanks through the muffle of the tunnel furnace. Because 
of this stacking of the blanks, the temperature and the action of gases in 
the furnace are of necessity not identical for each blank, with the result 
that the deviation of the characteristics from the standard particularly 
of the dimensions and the density, of the sintered fuel pellets is quite 
large such that the pellets must be measured after the sintering and in 
part reworked by grinding. 
The known method employing transport boats further requires a large cross 
section of the muffle, which means a correspondingly large material and 
power requirement; the latter is additionally increased because the 
transport boats must also be heated. 
A further disadvantage of the known method involves the necessity of 
changing the process parameters, especially the residence time of the 
blanks, the temperature profile and the partial oxygen pressure to obtain 
pellet products of desired characteristics. This can be determined only 
after a transport boat has been unloaded and the fuel pellets have been 
measured; in the meantime, however, the incorrectly adjusted process 
continues to run and produce rejects. 
Also, the latitude for process parameters is small with the known method 
since it is limited by the material properties of the transport boats 
which are subjected to the same process parameters as the blanks. 
Thus, for instance, the temperature in the low-temperature short-time 
sintering process disclosed in German DE-C 28 55 166 and corresponding to 
U.S. Pat. No. 4,348,339, had to be limited because of its effect on the 
material for the transport boats in an oxidizing and reducing atmosphere. 
Also, in oxidatively reducing sintering according to German DE-C 29 39 415, 
corresponding to allowed U.S. application Ser. No. 190,981 filed Sept. 26, 
1980, now U.S. Pat. No. 4,438,050, the transport boats would have to 
withstand temperatures up to 1500.degree. C. with different oxygen 
activities with a reduction in economy as a result thereof in large scale 
operation. 
SUMMARY OF THE INVENTION 
It is therefore an object of the invention to provide a method and a tunnel 
furnace for sintering blanks in which the listed disadvantages do not 
appear; in particular, the deviation of the characteristics of the 
sintered fuel pellets from the standard as well as the material and power 
requirements are reduced substantially. Means for optimizing the process 
parameters are provided. 
With the foregoing and other objects in view, there is provided in 
accordance with the invention a method for sintering oxidic nuclear fuel 
blanks into oxidic nuclear fuel pellets in which the blanks are moved 
through a muffle of a tunnel furnace wherein the blanks are sintered, the 
improvement comprising pushing the blanks in the form of a single layer 
column of abutting blanks through the muffle on a guiding device which 
goes through the muffle and protrudes therefrom at least on the input 
side. 
In accordance with the invention there is provided a tunnel furnace with a 
muffle for sintering oxidic fuel blanks into oxidic nuclear fuel pellets 
in which the blanks in the form of a single layer column of abutting 
blanks are pushed through the muffle, a guiding device for the column of 
blanks which goes through the muffle and protrudes therefrom at least on 
the input side; two partitions with one passage opening each are arranged 
in the muffle at a distance from each other; said guiding device composed 
of prismatic support rods which are placed parallel and side by side on 
the lower edge of the passage opening of the partitions, said guiding 
device going through the muffle having n+1 prismatic support rods for n 
columns of blanks; and wherein the dimensions of the passage openings are 
designed such that the smallest distance between a blank supported between 
two support rods and adjacent edges of the passage opening is smaller than 
one- to two-times the diameter of the blank. 
Other features which are considered as characteristic for the invention are 
set forth in the appended claims. 
Although the invention is illustrated and described herein as embodied in a 
method and tunnel furnace for sintering blanks, it is nevertheless not 
intended to be limited to the details shown, since various modifications 
may be made therein without departing from the spirit of the invention and 
within the scope and range of equivalents of the claims.

DETAILED DESCRIPTION OF THE INVENTION 
According to the invention, the solution of overcoming or at least 
minimizing this problem of listed disadvantages is characterized in a 
method of the type mentioned at the outset by pushing the blanks through 
the muffle on a guiding device in the form of a single-layer column of 
abutting blanks which goes through the muffle and protrudes therefrom at 
least on the input side. 
The invention therefore, works without transport boats. As a result, each 
individual blank traverses, under conditions of constant process 
parameters, the same temperature profile and the same atmosphere. As a 
consequence, deviation of the characteristics of the fuel pellets from the 
standard is more than halved as compared to the method used to date. 
In conjunction with the continuous passage of the blanks, the process 
parameters can be controlled to produce the fully sintered fuel pellets 
with characteristics which lie in a narrow desired-value range such that 
the pellets can be processed further without rework, i.e., in particular, 
without grinding. To this end, the length of the blanks is preferably 
determined at a sufficient distance ahead of the exit opening of the 
muffle, for instance, by an optical method known per se. Dependent on the 
length as determined by the optical method, one of the process parameters 
(running speed, temperature profile, partial oxygen pressure, etc.) is 
changed. The extent of this change is governed by an empirically 
determined characteristic which gives the value of the process parameter 
for each length. Such a curve is, of course, valid only for blanks with 
the same input characteristics (composition, density, dimensions). 
Since the invention works without transport boats, not only is the cost of 
expensive material otherwise required therefor eliminated, but also 
eliminated is the loading and unloading of the boats as well as heating 
and cooling them. 
The elimination of the transport boats and the single-layer arrangement of 
the blanks permits the use of a muffle with a substantially smaller 
overall height, which means correspondingly lower material and power 
costs. As a result of the small height of the muffle, the clear cross 
section between the individual blanks and the confining walls of the 
muffle is small, which lends itself to accurate adjustment of the gas 
atmosphere in the muffle and maintaining the gas atmosphere constant. 
Control of the gas atmosphere in the muffle can be accomplished 
sectionwise. To this end, at least part of the support rods which support 
the blanks in the muffle are designed as hollow support tubes have a 
passage opening at different points along their length. In this manner the 
gas composition can be measured at different points in the muffle and/or 
be changed by injecting a gas in the muffle through these spaced openings 
in the hollow support tubes. Furthermore, temperature sensors can be 
arranged movably in the support tubes to measure in a simple manner a 
temperature profile in the muffle. 
The invention is particularly well suited for low-temperature short-time 
sintering according to DE-C 28 55 166 (U.S. Pat. No. 4,348,399), and the 
method in accordance with the present invention operates effectively and 
efficiently with a sintering temperature of 1300.degree. C. The invention 
employs a particularly simple and effective design of a gas lock for 
separating the sintering zone from the reducing zone. Conventionally, 
blanks of oxidic nuclear fuel bodies are subjected in a muffle furnace to 
sintering in a sintering zone in an oxidizing atmosphere and to reduction 
in a reducing zone in a reducing atmosphere as more fully described for 
example in U.S. Pat. No. 4,348,339 and allowed U.S. application Ser. No. 
190,981, filed Sept. 26, 1980. The optimum length ratio of these zones 
relative to each other for a given length of the muffle can be adjusted 
simply by moving the gas lock. 
Thus, heretofore the blanks were sintered by loading them on transport 
boats and running them through the tunnel furnace. Each transport boat had 
a large number of blanks stacked side by side and on top of each other. 
This, of necessity, resulted in the characteristics, particularly the 
dimensions and the density of the finished fuel pellets having a 
relatively large spread since the temperature at and the gas access to 
each individual blank are different, even if only slightly. The large 
muffle cross section required here further necessitates large expenditures 
for material and energy. By virtue of the present invention, this 
expenditure is substantially reduced. Of even greater significance, the 
deviation of the characteristics of the fuel pellets is reduced 
substantially from the standard, i.e. the usual number of percent of 
pellets which deviate sufficiently to require reworking. In order to 
achieve this, the blanks are pushed through the muffle on guide rods in 
the form of single-layer columns of abutting blanks. For this purpose, the 
muffle has only a very small overall height. The invention makes possible 
control of the process parameters such that the characteristics of the 
finished fuel pellets lie in so narrow a reference range that the fuel 
pellets can be used further without rework (grinding). 
An embodiment example of the invention will be explained in greater detail, 
making reference to the drawings. 
Muffle 10 of Al.sub.2 O.sub.3 of substantially rectangular cross section 
goes through the tunnel furnace and protrudes from the tunnel furnace on 
both sides. The protruding parts of the muffle 10 are covered by two 
terminating pieces 112 and 113. Spaced partitions 11 of 
temperature-resistant material (Al.sub.2 O.sub.3) are disposed in the 
muffle 10 within the tunnel furnace, and partitions 11.1 and 11.2 are 
disposed in the corresponding terminating pieces 112 and 113. These 
partitions have an approximately rectangular passage opening 110 (FIGS. 3, 
4), on the lower edge of which straight support tubes 12 of Al.sub.2 
O.sub.3 are placed side by side and parallel. Straight support tubes 12 
protrude from both ends of the muffle 20 and its terminating sections 112 
and 113, thereby facilitating making connections to charging or cooling 
devices. 
Between every two support tubes 12, a single-layer column of abutting 
blanks 3 is placed. These columns are continuously pushed through the 
muffle simultaneously in the direction of the arrow R in FIG. 1 by a 
charging device 4. 
The charging device 4 includes a flat plate 41 which, on its plane top 
side, has parallel grooves 411 for taking the columns of abutting blanks 
3. On plate 41, the columns are spaced at the same distance from one 
another as on the straight support tubes 12 in muffle 10. The top side of 
plate 41 is arranged parallel to the longitudinal axes of the support 
tubes 12 and at a height placing blanks 3 on plate 41 and in muffle 10 at 
the same level. 
A pushing rake 42 guided in two slots 421 and 422 parallel to the grooves 
411 is provided with one tooth 423 for each groove 411. An endless drive 
chain 424 operated by an electric motor 428 via a cogwheel 425, a coupling 
426 and a gear 427 moves the pushing rake 42 in the guide slots 421 and 
422 in the direction of muffle 10. 
Because of the smooth surfaces of the blanks and the support rails, the 
pressure necessary for each column is rather small, e.g. 1.5 kg in a 
furnace 3 m long. In furnaces of greater length, the required pressure can 
be reduced by an appropriate inclination of the muffle and the support 
rails. 
In any case, the pressure applied to move the blanks through the muffle can 
be kept sufficiently low that the blanks which touch each other at the end 
faces only in a circular area neither bake together nor crumble. 
The passage opening 110 (FIGS. 3, 4) in the partitions 11 is shaped and 
designed so that the smallest distance between a blank placed on the 
support rods and the respective adjacent edges 111 of the passage opening 
is smaller than one- to two-times the diameter of the blank 3. Therefore, 
the unobstructed or impeded, i.e. "free" or "clear" cross section of the 
muffle available for a gas flow is extremely small. 
A gas lock 2, shown enlarged in FIG. 4, is disposed in the muffle 10 and 
divides the muffle into a sintering zone 103 and a reducing zone 105. To 
this end it has two partitions 11 which are placed a short distance from 
each other and enclose between them a decoupling zone 104, through which a 
flushing gas, for instance, nitrogen, flows from the top down. For this 
purpose, a distributor tube 24 with injection openings 241 is arranged 
transversely to the muffle axis in its upper region and connected to a 
feed tube 242. Two exhaust gas collectors disposed transversely to the 
muffle are arranged with exhaust gas openings 261 under the support tubes 
12, and connected to an exhaust tube 262. The flushing gas injected into 
the decoupling zone 104 through the distributor tube 24 is thus discharged 
into the exhaust gas collectors 26, as indicated by the flow arrows. In 
the process, gas which may enter from the reducing zone 105 or from the 
oxidation zone 103 into the decoupling zone 104 is discharged along with 
the gas from zone 104, thereby effectively decoupling the sintering and 
the reducing zones. At least some of the hollow support tubes 12 may have 
a passage opening 121 for feeding in or exhausting gas. The passage 
opening in the individual support tubes are spaced different distances 
from the input opening of the muffle. 
On the outside of the partition 11 of the gas lock facing the entrance 
openings 101 of the muffle 10, distributor tubes 23, 23.1 with injection 
openings 231 are arranged above and below the support tubes 12 which are 
connected to feed tubes 232,232.1, via which latter an oxidizing gas, for 
instance, CO.sub.2 is fed-in. This gas is injected into the muffle through 
the injection openings 231 at an angle toward the input opening 101 of the 
muffle and exhausted via the terminating section 112. The gas therefore 
flows against the feeding direction R of the blanks through the passage 
openings 110 of the partitions 11, preventing undesired air components 
from advancing through the input opening 101 to the sintering zone proper 
ahead of the gas lock 2. Optionally, many partitions can be arranged in 
the muffle at smaller spacings from each other. 
Distributor tubes 25 and 25.1 with injection openings 251 are arranged 
above and below the support tubes 12 on the side of the partition 11 of 
the gas lock 2, facing the exit opening 102 of the muffle. Distributor 
tubes 25 and 25.1 are connected to feed tubes 252 and 252.1, through which 
a gas mixture with reducing action is fed, for instance, 20% hydrogen and 
80% nitrogen. This gas is injected, as shown by the flow arrows, toward 
the exit opening 102 into the muffle and exhausted via the terminating 
section 113. 
The feed tubes for the various gases and the exhaust tube 262 are held in 
the partitions 11 and brought within the muffle up to the terminating 
section 113. In this manner, the gases are heated while being fed-in in 
the muffle 10. 
The two partitions 11 of the gas lock are connected to each other 
mechanically by feed and exhaust tubes and are arranged movably in the 
muffle 10. The position of the gas lock 2 within the muffle and thereby, 
the ratio of the lengths of the sintering zone 103 to the reduction zone 
105 can be changed simply in this manner and can be adapted to the 
respective process parameters. 
LIST OF REFERENCE SYMBOLS 
1: Tunnel furnace 
10: Muffle 
101: Input opening 
102: Exit opening 
103: Sintering zone 
104: Decoupling zone 
105: Reduction zone 
11, 11.1, 11.2: Partitions 
110: Passage opening 
111: Edge 
112, 113: Terminating section 
12: Guiding device, support rod, support tube 
121: Passage opening 
2: Gas lock 
23, 23.1: Distributor tubes 
24: Distributor tube 
25, 25.1: Distributor tubes 
26: Gas collector 
231: Injection openings 
241: Injection openings 
251: Injection openings 
261: Exhaust openings 
231, 231.1: Feed tubes 
242: Feed tube 
252, 252.1: Feed tubes 
262: Exhaust tube 
3: Blank 
4: Charging device 
41: Plate 
411: Grooves 
42: Pushing rake 
421: Slot 
422: Slot 
423: Tooth 
424: Drive chain 
425: Cogwheel 
426: Coupling 
427: Gear 
428: Electric motor