GB2110868A

GB2110868A - Dissolution of solids such as solid nuclear reactor fuels

Info

Publication number: GB2110868A
Application number: GB08233210A
Authority: GB
Inventors: Friedrich-Wilhelm Ledebrink; Dieter Schafer
Original assignee: Alkem GmbH
Current assignee: Alkem GmbH
Priority date: 1981-11-27
Filing date: 1982-11-22
Publication date: 1983-06-22
Also published as: FR2517218A1; DE3147199A1; GB2110868B; BE895151A; DE3147199C2; JPS58100793A; JPH0374356B2; FR2517218B1

Abstract

In order to dissolve a solid such as a solid nuclear reactor fuel in a liquid such as nitric acid, there is used a rectifier apparatus (2) having a sieve plate column (3) and a sump (4) at the lower end of the sieve plate column (3). The solid and the liquid are introduced into the sieve plate column (3) and conducted in co-current flow through the sieve plate column (3) to the sump (4), while gas is passed in counter-flow to the solid and the liquid from the sump (4) through the sieve plate column (3), and the liquid containing the dissolved solid is drawn off from the sump (4). <IMAGE>

Description

SPECIFICATION Dissolution of solids such as solid nuclear reactor fuels This invention relates to a method of dissolving solids, more particularly solid nuclear reactor fuels, in a liquid, and to an apparatus for use in this method.

It is usual for nuclear reactor fuels, such as uranium dioxide (UO2), plutonium dioxide (PuO2), thorium dioxide (ThO2) and mixed oxides such as (U/Pu)O2, which are relatively difficult to dissolve, to be dissolved in nitric acid in so-called "flat tanks", which have a long narrow base of a relatively small area and a pair of sides of a particularly large area, and which thus have a slit-like inner chamber as a reaction chamber. As a result of this shape, these tanks do not have any critical requirements.

These so-called flat tanks are operated discontinuously, i.e. the flat tank is first filled with nitric acid and heated, after which the nuclear fuel is added in batches. The nitric acid and the nuclear fuel can be mixed by means of a flow of air passed through the tank.

The charging and emptying of the tank, and the heating and cooling of the reaction mixture or the product which it contains, leads to a long idle time and thus to a relatively low flow rate through the tank.

In order to achieve an adequate mechanical stabilitythese flat tanks may only be made of steel. It is therefore impossible to maintain visual control of the progress of the reaction in these tanks, and therefore the only reliable way of avoiding the reaction mixture frothing over in the tanks is to add the charge of nuclear reactor fuel to be dissolved in batches to the nitric acid in the tanks. In order safely to avoid the reaction mixture frothing over, the chosen acid concentration and the reaction temperature as well as the level of filling of the tanks are several times lower than those actually permissible, which reduces still further the flow rate through these tanks.

According to the present invention, there is provided a method of dissolving a solid in a liquid, in a rectifier apparatus comprising a sieve plate column having a sump at the lower end thereof, wherein the solid and the liquid are introduced into the sieve plate column and are passed in co-current flow through the sieve plate column to the sump, wherein a gas is passed through the sieve plate column from the sump in counter-current flow to the flow of the solid and the liquid, and wherein the liquid, having the solid dissolved therein, is removed from the sump.

The present invention also provides an apparatus for use in the method of the invention, the apparatus comprising a sieve plate column, and a sump, provided with a heating means, at the lower end of the sieve plate column.

In a rectifier apparatus of the invention, the solid can be dissolved in the liquid as the solid and liquid are conducted through the apparatus, so that it is possible for the solid which is to be dissolved and the liquid to be supplied continuously to the apparatus and also for the liquid containing the dissolved solid to be continuously drawn off. Furthermore, the rectifier apparatus may be very slim and made of glass so that its critical safety requirements can be achieved without special expense, and so that it is possible to maintain a visual control of the progress of the reaction in the apparatus.

An apparatus consisting of a sieve plate column and a sump is known as a rectifier apparatus for the fractional distillation of mixtures such as crude oil.

This known apparatus is not, however, used for chemical solution reactions (as in the case of dissolving nuclear reactor fuel in nitiric acid) but for purely physical separation reactions.

The invention will now be described, by way of example, with reference to the drawing.

The drawing shows a rectifier apparatus 2 consisting generally of a vertically arranged glass tube. The apparatus includes a sieve plate column 3. At the lower end of the sieve plate column 3, there is a sump 4, while at the upper end, there is a condenser 5, namely a coil condenser. A series of sieve plates 6 provided with holes are arranged one on top of the other in the column 3, and each plate has an overflow weir 7, consisting of a pipe passing through the respective sieve plate, leading to the next lower sieve plate 6 orto the sump 4.

Above the top sieve plate 6, a supply line 8 for nitric acid, which is the dissolving liquid, opens into the solumn 3. A further supply line 9 for powdered nuclear reactor fuel (e.g. UO2 or PuO2) opens into the column 3, between the top sieve plate 6 and the next lower sieve plate 6.

There is a condensate collecting partition 10 in the rectifier apparatus 2, between the top sieve plate 6 and the coil condenser 5. This partition 10 has a central opening 11.

A condensate outlet line 12 is provided above the condensate collecting partition 10, and is in turn connected both to a bypass line 14, provided with a stop valve 13 and discharging into the sump 4, and also to an extraction line 16 provided with a stop valve 15.

A gas supply line 17 having a stop valve, and a supply line 18 for solid nuclear fuel, provided with slide valves, discharge into the rectifier apparatus 2, between the lowest sieve plate 6 and the sump 4.

The apparatus is also provided with a filling level regulator 19 for the sump 4, between the lowest sieve plate 6 and the sump 4.

The sump has a bypass branch 20 containing an electrical heating device 21. A basket 22, which may consist of, for example, a sieve, is also arranged in the sump 4, between the two discharge points 20a and 20b of the bypass branch 20, which are arranged vertically one above the other. A discharge line 23 provided with a filter 24, leads from the sump 4, below the discharge points 20a and 20b of the bypass branch 20. Below the outlet of the discharge line 23, a compressed air supply line 25 and a suspension discharge line 26 are provided in the sump 4. Above the coil condenser 5, a gas outlet pipe 27 is connected to the uppermost end of the rectifier apparatus 2.

Liquid nitric acid (HNO3) is supplied to the sieve plate column 3 via the supply line 8, and powdered nuclear reactor fuel, e.g. uranium dioxide (U02) and plutonium dioxide (PuO2), is supplied via the supply line 9. The powdered nuclear reactor fuel can be conveyed pneumatically in the supply pipe 9 and advantageously has a particle size of < 300 um. The sump 4 including the bypass branch 20 is filled with nitric acid to the level of the point 20a of the bypass branch 20. This nitric acid is heated electrically by the heating device 21. This not only causes the nitric acid to circulate in the sump 4 but also establishes the desired temperature, i.e. the boiling temperature of the nitric acid used.Furthermore, steam is produced in the sump 4, which steam passes through the holes in the sieve plates 6 of the sieve plate column 3 so that the suspensions on the individual sieve plates are agitated, without the use of mechanically moving parts.

In stationary operation, the nitric acid supplied through the line 8 and the powdered uranium and plutonium dioxides supplied through the line 9 are conducted by the force of gravity in co-cu rrent flow over the individual sieve plates 6 through the overflows 7 connecting these sieve plates, to the sump 4. Steam produced in the sump 4 by means of the heating device 21 passes in counter-flow from the bottom through the column 3 to the uppermost part of the apparatus, i.e. to the coil condenser 5.

During the period of direct contact in this sieve plate cascade, the powdered uranium and plutonium dioxides dissolve in the nitric acid forming NO, until finally a suspension of undissolved residual constituents is produced in the sump 4.

These residual constituents of the uranium and plutonium dioxide powder, which remain insoluble despite a longer period of direct contact in the rectifier apparatus 2, collect in a dead zone directly on the bottom of the rectifier apparatus 2, while clear filtered nitric acid solution containing dissolved nuclear fuel is drawn off continuously via the discharge line 23. The residual constituents, such as highly sintered ThO2 and PuO2 for example, which are insoluble in nitric acid, may be removed during breaks in the dissolving process, after the individual sieve plates 6 have been emptied by allowing the liquid to drain off through the holes of the sieve plates 6 and after the sump 4 has been emptied via the discharge line 23.The insoluble constituents which have collected at the bottom of the rectifier apparatus 2 can then be fluidized in the residual amount of solution left in the sump 4, by blowing in compressed air via the supply line 25, the suspension produced in this way being drawn off through the suspension discharge line 26.

The soiubuility of powdered uranium and plutonium dioxide depends on its preparatory heat treatment. Accordingly, the extent of foam formation during the dissolving of the uranium and plutonium dioxide is different in each case, depending upon the preparatory heat treatment of these dioxides. As the surface area of the dioxide powder (in addition to the other operational variables such as nitric acid concentration, temperature, concentration of the solution, pressure and other transport sizes) directly determines the speed at which the dioxides dissolve, it is advantageous for increasing the production output of solution if the uranium and plutonium dioxide, when in the form of for example pellet waste, is crushed before it is dissolved.Although high concentrations of nitric acid accelerate the dissolving of uranium and plutonium dioxide, dilute nitric acid must be used occasionally in order to prevent too great a formation of foam in the rectifier apparatus 2. As the rectifier apparatus 2 is made of glass, it is very easy to maintain a visual control of the extent of foam formation.

If concentrated nitric acid is used as the solvent in the rectifier apparatus 2, residual water can be condensed as a slightly acidic condensate by means of the coil condenser 5 and can be drawn off via the condensate discharge line 12 from above the conde nsationcollecting partition 10.

If dilute nitric acid has to be used to avoid the formation of foam in the rectifier apparatus 2, condensate from the discharge line 12 can be conducted back from the discharge line 12 via the supply line 14 into the sump 4 again, and the nitric acid concentrations in the sieve plate column 3 can be adjusted so that the greatest reaction takes place on the second from top sieve plate 6, where the proportion of solid uranium and plutonium dioxide is the highest, but where the nitric acid concentration is the lowest, so that the reaction is slowed down. On the next sieve plate 6 both the nitric acid concentration and also the uranium and plutonium dioxide concentration in the solution increase by virtue of the fact that reaction has taken place.

Uranium and plutonium dioxides, which are difficult to dissolve, require a longer period of direct contact in the rectifier apparatus 2. In this connection the liquid flow rate through the sieve plate column 3 can be regulated with the aid of the return flow of condensate through the supply line 14 to the sump 4, by supplying to the sump 4 via the supply line 10 exactly as much liquid per unit of time as evaporates in the sump 4.

Uranium dioxide or plutonium dioxide pellets which have not been crushed can be supplied through the supply line 18 semi-continuously to the sump 4 onto the basket 24, while filtered uranium dioxide or plutonium dioxide solution can be drawn off continuously along the discharge line 23. In this case too, liquid nitric acid is advantageously introduced through the supply line 8 into the sieve plate column 3 so that uranium dioxide and plutonium dioxide pellets added via the supply line 8 can be dissolved in parallel to each other.

According to one embodiment of the method according to the invention, 9 litres of concentrated nitric acid and 2.34 kg of a UO2/PuO2 powder mixture having a particle size of < 300 can be added to the rectifier apparatus 2. At a rate of evaporation of 7 litres of nitric acid per hour in the sump 4, 1 litre of distillate is drawn off every hour from the rectifier apparatus 2 via the discharge line 16, and the remaining distillate, i.e. 6 litres, is returned via the supply line 14 to the sump 4to lengthen the period of direct contact of the uranium dioxide and plutonium dioxide powder (passed into the sieve plate column 3 via the supply line 6) in the sump 4 and possibly also in the sieve plate column 3.In order to aid in effecting the mixing in the sieve plate column 3 and in order to effect NO, oxidation by the use of 0.5 cbm of air per hour, an oxidation-promoting reaction gas may additionally be supplied to the rectifier apparatus 2, between the sieve plate column 3 and the sump 4via the gas supply line 17. Thus, 8 litres of a solution of uranium dioxide and plutonium dioxide in nitric acid, with a concentration of 52 g per litre, are drawn off continuously along the discharge line 23, controlled by the filling level regulator 19. As the mixture of uranium dioxide and plutonium dioxide powder supplied to the rectifier apparatus 2 contains approximately 3% of insoluble constituents, approximately 709 of solid is deposited each hour in the dead zone directly on the bottom of the rectifier apparatus 2, which solid accumulates and can be removed as a suspension via the suspension discharge line when appropriate during a break in the dissolving process.

Claims

1. A method of dissolving a solid in a liquid, in a rectifier apparatus comprising a sieve plate column having a sump at the lower end thereof, wherein the solid and the liquid are introduced into the sieve plate column and are passed in co-current flow through the sieve plate column to the sump, wherein a gas is passed through the sieve plate column from the sump in counter-current flow to the flow of the solid and the liquid, and wherein the liquid, having the solid dissolved therein, is removed from the sump.

2. A method according to claim 1, wherein the liquid is evaporated in the sump.

3. A method according to claim 2, wherein the liquid is caused to circulate in the sump.

4. A method according to claim 3, wherein the liquid is caused to circulate in the sump by heating the liquid.

5. A method according to any of claims 1 to 4, wherein the solid is additionally fed into the liquid in the sump, and wherein the liquid, having the solid dissolved therein, is removed from the sump via a filter.

6. A method according to any of claims 1 to 5, wherein gas issuing from the top of the sieve plate column is condensed, and wherein the condensate is removed.

7. A method according to claim 6, wherein the condensate is returned to the sieve plate column.

8. A method according to claim 6, wherein the condensate is returned to the sump.

9. A method according to any of claims 1 to 8, wherein a reaction gas is added to the gas flowing through the sieve plate column.

10. A method according to any of claims 1 to 9, wherein the solid is solid nuclear reactor fuel.

11. A method according to claim 10, wherein the liquid is nitric acid.

12. A method according to claim 1, substantially as hereinbefore described with reference to the drawing.

13. An apparatus for use in a method according to claim 1, comprising a sieve plate column, and a sump, provided with a heating means, at the lower end of the sieve plate column.

14. An apparatus as claimed in claim 13, wherein the heating means is located in a bypass branch of the sump.

15. An apparatus as claimed in claim 13, substantially as hereinbefore described with reference to, and as shown in, the drawing.