WO1998038647A1

WO1998038647A1 - Dissolution of nuclear fuel rods

Info

Publication number: WO1998038647A1
Application number: PCT/GB1998/000632
Authority: WO
Inventors: Timothy Peter Tinsley
Original assignee: British Nuclear Fuels PLC
Current assignee: Sellafield Ltd
Priority date: 1997-02-28
Filing date: 1998-02-27
Publication date: 1998-09-03
Anticipated expiration: 1999-08-28
Also published as: GB9704256D0

Abstract

A process for the dissolution of a fuel rod comprises maintaining a flow of fuel rod dissolving liquid along an upward flow path in which the rate of flow decreases upwardly along the path. The fuel rods, divided into substantially equal length parts are introduced to the lower end of the flow path as a result of which the fuel rod parts move upwardly along the flow path as the spent fuel is dissolved. When the fuel parts reach the upper end of the flow path they are separated from the liquid. Apparatus for carrying out the process is also disclosed.

Description

DISSOLUTION OF NUCLEAR FUEL RODS

FIELD OF THE INVENTION

This invention relates to processes for the dissolution of fuel rods which have been used in nuclear power stations, for instance, in LWR/BWR reactors.

BACKGROUND OF THE INVENTION

It is known to treat spent fuel rods by dissolving the active fuel contents. In one known method, the spent fuel rods are chopped into lengths of approximately the same size and loaded into a basket. The basket is then lowered into a suitable dissolution medium such as aqueous nitric acid. The basket must be left in the dissolution medium until such time as will be necessary to allow the contents of the slowest reacting chopped length to be dissolved. Furthermore, the equipment for carrying out such a process involves moving parts to lower and raise the basket whereas it is preferable with apparatus handling radioactive materials to have as few moving parts as possible.

STATEMENTS OF THE INVENTION

According to the present invention, there is provided a process for the dissolution of a fuel rod which comprises dividing the fuel rod into substantially equal length parts, maintaining a flow of fuel rod dissolving liquid along an upward flow path, the rate of flow decreasing upwardly along said flow path, introducing said fuel rod parts to the lower end of said flow path, whereby the fuel rod parts move upwardly along said flow path as the spent fuel is dissolved until the fuel parts reach the upper end of the flow path, and separating the fuel rod parts from said liquid.

Accordingly, the present invention is based on the principle of elutriation in which a piece of solid material will sink within a liquid unless the liquid is moving at a rate sufficient to overcome this sinking tendency. If the liquid is capable of dissolving the solid material, then the material will flow with the liquid once it has been dissolved sufficiently to reduce its mass to surface area ratio to a level where it becomes buoyant within the flowing liquid.

The present invention also provides apparatus for the dissolution of a fuel rod which comprises means for dividing the fuel rod into substantially equal length parts, means for maintaining a flow of fuel rod dissolving liquid along an upward flow path, the rate of flow decreasing upwardly along said flow path, means for introducing said fuel rod parts to the lower end of said flow path, and means for separating from said liquid the fuel rod parts which reach the upper end of the flow path.

Preferably, the flow maintaining means comprises an elongate vessel arranged with its longitudinal axis substantially vertical and having a cross-sectional area which increases in an upward direction over at least a part of the length of the vessel.

The liquid may be any suitable liquid for dissolving spent fuel. An example is nitric acid which will dissolve uranium dioxide, the resultant liquid then being suitable for use with the well-known Purex process for treating dissolved spent fuel.

DETAILED DESCRIPTION OF THE INVENTION Reference will be made to the accompanying drawings, in which:-

Figure 1 is a graph of free flowing terminal velocity against % uranium dioxide present in a "standard" hull;

Figure 2 shows a vessel suitable for use in a process of the present invention;

Figure 3 is a graph of vessel internal diameter against uranium dioxide present in the vessel of Figure 2; and

Figure 4 shows apparatus of the present invention.

The terminal falling velocity of a non-spherical particle in a liquid is related to its drag and relative density to the liquor by the following formula: - u_t = K_{l g} D_{s p} -//>/18μ where K, = 0.843 log (φ / 0.065) u, = Terminal or free settling velocity

D_s = 'Spherical' diameter (diameter of a sphere of equal volume)

P - Fluid density

Pp = Particle density μ = Fluid Viscosity φ = Sphericity (surface area of a sphere having the same volume as the particle divided by the surface area of the particle) If the particle is a fuel hull in nitric acid, then, as the uranium dioxide dissolves, the buoyancy of the hull is increased. If φ, g, D_s, P and u are constant, as they will be if all the hulls are the same size and shape, then u, is proportional to Pp, with Pp and hence u, reducing as the hull dissolves. Figure 1 of the accompanying drawings shows an example plot of V against percentage uranium dioxide for a "standard" hull in water. If a different liquid is used, for example, uranyl nitrate solution the u_t will change.

If the dissolver vessel is profiled such that the velocity of the fluid decreases as it passes up the dissolver, then the corresponding u, will also decrease. The effect of this fluid velocity profile will be that different density hulls will be located at different points in the dissolver. Accordingly, as a hull dissolves, it will progress up the dissolving vessel. Figure 2 shows an example dissolver vessel with a feed diameter of 150mm, the corresponding maximum flow rate for water, using the example hull, being 38m³/hr. The terminal velocities and percentages of uranium dioxide at different levels in the vessel for a standard hull in water are indicated in Figure 2. Figure 3 illustrates graphically the relationship between vessel internal diameter and uranium dioxide present in the hulls, again using a standard hull in water.

The fluid velocity at the feed point should be greater than the u, of any hull. The slowest fluid velocity should preferably be slightly greater than the u, of a fully leached hull. Referring to Figure 4, there is shown a schematic diagram of apparatus 1 for dissolving the fuel contents of spent fuel rods. Apparatus 1 includes a vessel 3 which extends upwardly from a relatively narrow base 5 to a relatively wide section 7 about two-thirds of the way up the vessel. Beyond said relatively wide section 7, the width of vessel 3 decreases relatively rapidly until, at the top of vessel 3, the width is about the same as that of the base 5.

Vessel 3 forms part of a liquid flow circuit, liquid being fed via pipe 9 extending from the top of vessel 3 to the base 5 of the vessel. Close to base 5 of vessel 3, there is located an entry point 11 for fuel feed. At point 11 fuel feed, in the form of chopped lengths of spent fuel rods, is fed into pipe 9.

Adjacent entry point 11 there is located a pump 13 which maintains a flow of liquid around the liquid circuit as indicated by the arrows in pipe 9 and vessel 3. Adjacent pump 13 there is located an outlet 15 for liquid to be extracted from the flow circuit and sent to plant for chemical separation processes to be carried out.

Between outlet 15 and vessel 3 there is located a hulls separator 17. At separator 17 hulls are caused to be separated from the circulating liquid. The separating hulls can be sent to waste disposal or to a further dissolver unit which is arranged in series with the unit shown in the drawing.

A dissolver unit, such as that shown in Figure 4, may be made of any suitable material, such as stainless steel. Typically, the fuel feed consists of regular chopped fuel rods (including cladding material) of approximately 50 mm length. This fuel feed is desirably free of any additional fuel assembly parts, that is to say, end appendages, wraps, springs and so on. A fuel dismantling stage is therefore preferably provided for dismantling the fuel assemblies and for single pin or bundle shearing of the fuel rods. In order to operate the dissolver unit shown in Figure 4, fuel feed is fed via inlet 11 to pipe 9 and hence to the base 5 of vessel 3. The fuel dissolves in the liquid in vessel 3 and, as the buoyancy increases, the fuel hulls move upwardly within vessel 3 until they have passed the point where the vessel width is increasing (the beginning of section 7), at which point they are carried out of vessel 3 and towards the hulls separator 17.

The liquid velocity, and hence the buoyancy of a hull in the liquid, are determined by the size and shape of the vessel 3 as well as the liquid flow rate. These factors are, in practice, determined by the overall arrangement of the processing plant, that is to say, the number of dissolver units in series.

Claims

1. A process for the dissolution of a fuel rod which comprises dividing the fuel rod into substantially equal length parts, maintaining a flow of fuel rod dissolving liquid along an upward flow path, the rate of flow decreasing upwardly along said flow path, introducing said fuel rod parts to the lower end of said flow path, whereby the fuel rod parts move upwardly along said flow path as the spent fuel is dissolved until the fuel parts reach the upper end of the flow path, and separating the fuel rod parts from said liquid.

2. A process according to Claim 1 wherein the liquid is nitric acid.

3. Apparatus for the dissolution of a fuel rod which comprises means for dividing the fuel rod into substantially equal length parts, means for maintaining a flow of fuel rod dissolving liquid along an upward flow path, the rate of flow decreasing upwardly along said flow path, means for introducing said fuel rod parts to the lower end of said flow path, and means for separating from said liquid the fuel rod parts which reach the upper end of the flow path.

4. Apparatus according to Claim 3 wherein said flow maintaining means comprises an elongate vessel arranged with its longitudinal axis substantially vertical and having a cross-sectional area which increases in an upward direction over at least a part of the length of the vessel.