FR2509900A1 - METHOD FOR VITRIFICATION OF HIGHLY RADIO-ACTIVE WASTE USING GLASS SPHERULES AS VITRIFICATION ADDITIVES - Google Patents

Abstract

HIGHLY RADIOACTIVE WASTE VITRIFICATION PROCESS USING GLASS SPHERULES AS VITRIFICATION ADDITIVES. IN THE PROCESS FOR VITRIFICATION OF HIGHLY RADIO-ACTIVE WASTE ACCORDING TO THE INVENTION, INTRODUCED THROUGH DUCT 3 IN CRUCER 1 HEATED BY OVEN 2, THE GLASS SPHERULES USED AS VITRIFICATION ADDITIVES HAVE A DIAMETERING ADDITIVES INCLUDED BETWEEN 1 AND 4MM, AND PREFERREDLY EQUAL TO 1MM, ARE INTRODUCED INTO THE VITRIFICATION CRUCIBLE THROUGH THE INDEPENDENT DUCT 5 IN CONTINUOUS AND IN DOSED QUANTITY USING THE MEASURING DEVICE 6. THE GASES EMERGING AT THE COURSE OF TRANSFORMATIONS TAKING PLACE WITHIN THE CRUCIBLE, EXIT THROUGH TUBE 7.

Description

Process of vitrification of highly radioactive waste

  using glass spherules as vitreous additives

  fication. The present invention relates to a method of vitrification of highly radioactive waste which uses

  glass spherules as vitrification additives.

  These additives are generally composed of mixtures which contain

  If O 2 (50% -60%), B 203 (15% -20%), Na 20 (10% -15%) and small

quantities of other oxides.

  The prior art exploited solutions or suspensions

  aqueous solutions of vitrification agents dissolved and / or suspended

  or solid materials in the form of

  regulars that are sent directly to the crucible.

  The present invention more particularly relates to a vitrification process which uses glass spherules of a suitable size, the diameter of which is between 1 and 4 mm, 2 mm being the preferred diameter, which have an aptitude

  at high flow and high bulk density.

  Such spherules, added to nitric solutions of radioactive waste make it possible, quite surprisingly, to obtain a glass block while reducing the formation of foams and / or a liquid phase in the crucible vitrification In addition, they allow the liquid phase materials of the waste to be evaporated continuously and continuously, and considerably reduce the duration of the complete vitrification operation compared to the prior art: by

  therefore, an increase in the efficiency of the system is obtained.

  It is known that vitrification is currently the most widely adopted method on an industrial scale to solidify liquid waste with high radioactivity,

  which come from the treatment of nuclear fuels.

  According to the known method, the nitric solutions which contain the fission products undergo numerous state transitions due to the increase of the temperature, so that they are first evaporated to dryness and then converted into a solid state. calcined powder due to the decomposition of nitrates into oxides and finally are enclosed in a vitreous body The last step is made possible by the addition of a vitrification material which, by melting at high temperature, retains the entire mass for forming a block having a glassy structure It is possible to arrange for all these transformations to take place in a single reactor which is generally a cylindrical crucible, which is thus fed with the aqueous phase and the additives and in which the molten glass is obtained ; alternatively, it is also possible to use two separate reactors: in the latter case, the heating of the liquid phase takes place in the first reactor and in the second

  reactor, calcination of the heated solids and the

  clusion into the vitreous body by the addition of vitriol

fication.

  The separation in space of the two stages, heating

  and vitrification, makes it possible to carry out both

  calcination and transfer of calcined oxides to the reactor

  vitrification If the latter is emptied or replaced by

  mittence, one can obtain a continuous production operation.

  In the batch process which uses a single reaction

  the calcined solid is separated below the aqueous phase, then melted with the vitrification additives in the same crucible, by raising the temperature. In this case, the must be interrupted when the maximum effective level has been reached in the crucible, so that the transformation

  of the liquid phase, which has been accumulated in the upper layers

can be done.

  The rate of evaporation, the latter being the trans-

  training which governs the speed of treatment, must now be

  at a high level, but it must be regular in order to prevent massive

  in which the fumes are treated.

  The feed speed, in turn, must not exceed

  the evaporation capacity of the crucible so as to pocket that the volume of liquid can increase inside the

  crucible, because the liquid level, under the conditions of equi-

  free, must be kept constant and to a minimum.

  The control of the evaporation operation is thus a critical factor in the vitrification process when not in use.

  only one reactor and defines the limits of volume and pos-

  applicability to industrial installations.

  In the existing pilot plants that adopt this process, the following facts have been established: The evaporation capacity of the cylindrical crucible is a practically linear and increasing function of its diameter, so that the efficiency of the plant can be increased, all the other parameters being the mimes, adopting

  cylindrical containers of larger diameter.

  As the diameter of the crucible increases, it is observed that

  the volume of liquid inside the crucible

  above the mass which has already been transformed into a calcined mass or molten glass, increases more than simply proportionally to a fraction

  the total effective volume of the crucible.

  liquid is the result of poorer conditions of heat exchange between the walls of the crucible and the interior of the crucible due to the presence of calcined spongy mass on the walls of the crucible. This effect, which also occurs in crucibles of medium size (25 cm), is even more important when increasing the diameter of the crucible, since the insulating layer is

thicker.

  Due to the accumulation of the liquid phase inside

  of the crucible, the feeding speed must be reduced

  gressively, often at values that are equal to half or even a third of the initial speed This fact results in a significant prolongation of the duration of the vitrification cycle

  and thus a decrease in the efficiency of the system.

  In addition, the accumulation of a considerable volume of liquid gives rise to a significant volume reduction when the entire mass is brought to its temperature of

  melting, so that the actual volume of the crucible is insufficiently

  exploited and this fact is another factor that reduces

the potential performance of the system.

  In relation to the evaporation zone, we notice the tendency to the formation of foams which can be attributed to the presence of compounds which modify the surface tension and also to the simultaneous release of gaseous products from the liquid mass This phenomenon can become important because of the increase in volume and / or the rate of irregular evaporation The calcined solids could thus be separated in a spongy form having a low bulk density: these solids, besides the fact that they cause an increase in volume and thus a premature filling of the crucible, can enclose the aqueous phase or prevent a complete release of gases and vapors, so that these

  could be released suddenly at a

ture higher.

  The presence of foam at the evaporation interface and / or

  sudden release of gases and vapors

  massive amounts of vitrification material, in particular boron trioxide or silica dust, and fine powder droplets containing radioactive isotopes, in the flue gas treatment section. In the first case,

  there may be severe problems with the pipelines

  cause a danger of fouling; in the case of radioactive particles, the pollution of the gaseous effluent increases so that the stage of treatment of fumes

  subsequent becomes more difficult to achieve.

  In accordance with the process of the present invention, it is possible to overcome such drawbacks of the vitrification process carried out in a single reactor by using vitrification additives in the form of spherules, the diameter of which can be chosen between 1 and 4 mm, the size preferred being 2 mm, these spherules being introduced continuously

  in the reaction crucible in a metered quantity.

  The adoption of vitrification additives in the form of

  spherules in accordance with the present invention makes it possible

hold:

  Regular and continuous nutrition of vitreo-

  with a satisfactory control of the quantity introduced

  duity per unit of time; Constant and constant evaporation of the liquid supply and the maintenance of a very small volume of liquid in the evaporation zone; Maintaining a constant flow rate of the radioactive waste solution throughout the duration of the feed cycle; A duration of the whole vitrification operation which is remarkably reduced compared to that of the processes used hitherto, the result being an improved potential yield of the whole system; A considerable reduction in dust entrainment

  of vitrification material in the treatment pipes.

  The use of spherules according to the present invention can be extended to a few variants of the process:

  it is quite possible to feed a driving line

  with the liquid waste solution mixed with the spherules; it is possible to load the spherules in the crucible before starting the supply of liquid, which allows the spherules to exert in all cases their control action on the evaporation operation Finally, the vitrification agents in form spherules can be introduced into the crucible when the calcination operation is complete

(Figure 2).

  The present invention will be better described with reference to FIGS. 1 and 2 of the attached drawing, which are given by way of

illustrative and not limiting.

  Figure 1 shows the ideal diagram of vitrifi-

cation.

  In the usual case, well known in the art,

  shown in FIG. 1A, the crucible 1 in which vitrification

  cation is executed, is heated by induction furnace 2

  at medium frequency or by an electrical resistance.

  The solution containing the fission products (highly radioactive waste) is introduced and metered using line 3 shown schematically in FIG. 1A. Line 4 indicates the feed line for vitrification additives which can be mixed with prior to the liquid waste solution (line 3) or the additives can be introduced directly into the crucible.

  The solution already contains, in suspension, vitrification additives

  cation (Si O 2, B 203, Na 2 and minor components) or alternatively, these are added as a separate solution or suspension to be combined with 3 before entering the crucible, be introduced directly into the

crucible itself.

  Another variant of the usual operation provides for the

  troduction of additives directly into the crucible and under a

  solid form, which is composed of flakes or iridescent blocks

  However, this variant does not allow a food

  regular and continuous as in the case of spherules.

  The typical case of which the process of the invention is an improvement can be considered as that in which the additives are introduced into an aqueous medium represented by the solution of the radioactive waste themselves, or under

  form of an independent liquid stream.

  Line 7 represents the gas outlet tube that emerges during the transformations taking place at

inside the crucible.

  In accordance with the process of the present invention, the vitrification additives are introduced directly via the independent line 5 of FIG. 1B and dosed continuously using the dosing device 6. The solution containing the radioactive waste assets is introduced at the same time via line 3, both

power supplies being simultaneous.

  In the case of spherules, therefore, water is not introduced in addition to that of the waste solution as carrier for the vitrification agents, and a material is used which, because of its properties of high flowability and high bulk density,

  can be measured and dosed as if it were fluid.

  FIG. 2 represents the possible variants with regard to the use of the spherules in accordance with the

present invention.

  Figure 2 C shows the case of the diet of the

  liquid solution of waste (1) and spherules (2) simul-

temporarily and separately.

  In FIG. 2D, the spherules 2, although introduced via a separate pipe, are introduced into the crucible by the same pipe as that which feeds the liquid solution (1). Indeed, the two streams ( solid spherules and liquid waste solution) are mixed before

  to enter the crucible This case is very close to the pre-

  with the exception that only one input pipe is

  Lisée. The other two variants (FIGS. 2E and 2F) provide for a non-simultaneous supply of the liquid solution of

  radioactive waste and spherules.

  In the variant shown in FIG. 2 E, the spherules which are necessary for the transformation into glass are introduced into the crucible 2 beforehand, after which the liquid solution of radioactive waste 1 is introduced and evaporated to dryness, and finally the whole mass

  is brought to its melting point.

  In the case of the variant of Figure 2 F, the first part of the cycle is used to completely introduce the liquid waste solution and simultaneously, the latter is converted into calcined powder 1; at this point, the spherules of vitrification material 2 are added-s, which allows when the temperature has reached about 1100 'C, the totality

  of the mass to be transformed into vitreous agglomerate.

  In FIGS. 2E and 2F, reference numeral 2 indicates the mass of spherules which are in any case always introduced via line 5 of FIG. 1B. In FIG. 2F, the supply lines are the same as in FIG. 1B, that is, mark 3 indicates the power supply

  of liquid solution, and the reference 5 the supply of spherules.

Claims (5)

  1 Process for the vitrification of liquid radioactive waste arising from the reprocessing of nuclear fuel, characterized in that vitrification additives are used in the form of glass spherules whose diameter is   chosen between 1 and 4 mm, and is preferably equal to 2 mm.   Process for the vitrification of liquid radioactive waste according to Claim 1, characterized in that the vitrification additives in the form of spherules are introduced in a quantity dosed continuously or discontinuously into the vitrification crucible, together with the charge. liquid from the radioactive waste solution, and in the form of of a separate current.   3.Process vitrification of liquid radioactive waste according to claim 1, characterized in that the additives in the form of spherules are premixed with the liquid solution of radioactive waste and introduced as a single stream in the vitrification crucible. 4 vitrification process of liquid radioactive waste according to claim 1, characterized in that the addifives in the form of spherules are introduced into the vitrification reactor before the   liquid solution of radioactive waste to vitrify.   5 vitrification method of liquid radioactive waste according to claim 1, characterized in that the additives in the form of spherules are introduced into the reaction crucible after the operation   evaporation and when the liquid charge has been interrupted.