GB2056156A

GB2056156A - A Column for Handling Fissile and/or Fertile Materials

Info

Publication number: GB2056156A
Application number: GB8018402A
Authority: GB
Original assignee: Nukem GmbH
Current assignee: Nukem GmbH
Priority date: 1979-06-13
Filing date: 1980-06-05
Publication date: 1981-03-11
Also published as: GB2056156B; DE2923870A1; FR2458879A1; SE8004392L; FR2458879B1; JPH0146839B2; BR8003430A; JPS562598A; DE2923870C2

Abstract

The extraction columns hitherto used for fissile and/or fertile materials have only a limited throughput per column unit on account of the necessary safe geometry. A considerably increased throughput is obtained if one or more rods containing a neutron poison is/are arranged throughout the length of an otherwise conventional extraction column. Safe geometry is maintained by the number or by the shaping of the rods at particular places in the column.

Description

SPECIFICATION A Column for Fissile and/or Fertile Materials This invention relates to a column, more particularly an extraction column, for fissile and/or fertile materials having safe geometry and an increased throughput.

In the nuclear field and, more particularly, in the reprocessing of burnt-up fuel elements, uranium and plutonium are purified by liquid-liquid extraction. For criticality reasons, the columns used should not exceed a certain cylinder diameter, depending on the U-235 and plutonium contents of the solutions. The throughput of the extraction column is limited by this restriction. In the case of large installations, therefore, several extraction lines (one line contains from 3 to 6 columns for example) have to be arranged parallel to one another which involves high capital investment. Accordingly, a desirable increase in or doubling of the column throughput would not only reduce expenditure on further extraction lines, it would also save production space which, in reprocessing plants for example, is particularly expensive on account of the relatively thick concrete shieid.

Accordingly, it has already been proposed to contaminate the column heterogeneously using hafnium metal (Reaktortagung-Reactor Convention-Dusseldorf 1976). In this connection, it was proposed that the sieve plates of the columns should consist of approximately 2.5 mm thick hafnium plate.

However, this proposal is attended by a number of disadvantages. Thus, the geometry of the columns can only be slightly widened so that the throughput cannot be doubled. In addition, it is not possible to prevent stress dorrosion because the column consists of stainless steel and local elements can develop. In addition, the wall thickness of the sieve plates has to be frequency checked because any reduction in the thickness of the metal plate has a considerable effect on absorption and increases criticality.

Accordingly, an object of the present invention is to provide a column, more particularly an extraction column, for fissile and/or fertile materials having safe geometry and an increased throughput which does not have any of the disadvantages referred to above and, more particularly, provides for a considerably increased throughput without any corrosion problems.

According to the invention one or more of the rods containing a neutral poison are arranged in the column over its entire length. One or more rods are with advantage centrally arranged in the column. In addition to extraction columns, other columns, such as rectification columns of washing columns, may also be provided with the rods according to the invention. The columns may also contain other fittings such as, for example, plates or packings. It is preferred to use a single central neutron adsorbing rod.

The accompanying drawing diagrammatically illustrates one example of an embodiment of the column according to the invention.

Arranged in a column (1), which is provided with feed and discharge pipes for the various liquids, for example the extractant and the solution to be extracted, is a rod (2) which has an increased diameter at the base (3) and head (4) of the column (1 ) corresponding to the external form of the column (1). It is even possible to install additional rods in these widened zones of the column. The rod (2), which in this case is centrally arranged, consists of a shell (5) of which the interior (6) is filled with a neutron poison. In addition, the column contains sieve plates (7).

A central hollow rod (2) which is made of the same material as the column itself and which is filled with neutron poisons is advantageously arranged in the middle of the column (1). This central tube which is filled with neutron poison and closed on all sides may be widened as required so that the diameter of the column may also be increased virtually as required and a considerably larger crosssection per unit area and, hence, throughput can be obtained. Column diameters of 1 metre (without a central rod) are already known in the purification of natural uranium and may now also be used (with a central rod) for enriched uranium and plutonium. Stress corrosion is ruled out by using a uniform column material.To forewarn against neutron poison being dissolved out from the rod (2), a leakage indicator is advantageously built into the rod (2), immediately releasing an alarm signal when liquid penetrates into the rod. This eliminates the need for expensive inspection work.

Suitable neutron adsorbing materials are, for example, boron carbide (B4C) in the form of a dense powder or tablets, cadmium metal, even in the form of alloys, or gadolinium oxide (Gd203) and other rare earths (Dy203, Sm203 etc).

It is best to use boron carbide which is inexpensive. It is only in rare cases that an even larger adsorption cross-section of the material for neutrons is required. The tube of the central rod is made of the same material as the column itself with equally thick or thinner walls, is filled for example with B4Cpowder and tightly welded. This central tube is also used for example for holding the sieve plates which are arranged at intervals of approximately 5 cm over the entire column. The widened stabilisation zones at the base and head of the column may be made safe from the criticality point of view in the same way by a correspondingly thicker neutron adsorbing rod. Increases in diameter such as these (stabilisation zones) are required for keeping the phase boundary important to the control of the column free from flocculations.

The invention is illustrated by the following Examples: Example 1 For uranyl nitrate solution with a 4% enrichment of U-235, the safe column diameter is 38.5 cm.

In order to be able to double the throughput of a given column, the throughflow area has to be doubled.

The safe column diameter of 38.5 cm corresponds to a throughflow area of 11 64 cm2. The throughflow area can be doubled to 2328 cm2 by increasing the diameter of the column to approximately 58 cm and by providing the central neutron adsorbing rod with a diameter of 1 9.5 cm and filling it for example with boron carbide powder. The column diameter of 58 cm then has a throughflow area of 2642 cm2 which is reduced by about 300 cm2 by the area of the central absorption rod, leaving a throughflow area of 2342 cm2 so that the throughput may be substantially doubled.

If the column is widened to the usual approximately 3-fold throughflow surface at the phase interfaces at the head or base of the column, the column ends have a diameter of 94.5 cm and the absorption rod a diameter in these zones of 56 cm. The absorption rod may be in one piece because the thicker ends are still smaller in diameter than the interior of the column.

Example 2 For plutonium nitrate solutions, the safe column diameter is 1 5 cm. Doubling the throughput requires twice the throughflow area in the column. A column 1 5 cm in diameter has a throughflow area of 1 77 cm2 so that a throughflow area of 354 cm2 is required for double the throughput.

With a central absorption rod (B4C-filling), a column for double the throughput has the following dimensions: Column diameter=22.5 cm with S=398 cm2 B4C-rod diameter=7.5 cm with S=44 cm2 This gives a free throughflow surface of S=354 cm2.

The widened column ends have a diameter of 37 cm and the ends of the absorption rod a diameter of 22 cm.

Claims

1. A column for fissile and/or fertile materials, having safe geometry and having one or more rods containing a neutron poison arranged in the column over its entire length.

2. A column as claimed in Claim 1, which is an extraction column.

3. A column as claimed in Claim 1 or 2, wherein the rods are centrally arranged in the column.

4. A column as claimed in any of Claims 1 to 3, wherein the rods consist of a shell and are filled in their interior with a neutron poison.

5. A column as claimed in Claim 4, wherein the shell of the rods consists of the same material as the column.

6. A column as claimed in any of Claims 1 to 5, wherein the rods are provided with a leakage indicator.

7. A column as claimed in any of Claims 1 to 6, wherein at both ends the rods are widened in such a way that the criticality limits are not exceeded even in widened stabilisation zones of the column.

8. A column as claimed in any of Claims 1 to 7, wherein the column has only a single centrally arranged rod.

9. A column for fissile and/or fertile materials substantially as described with particular reference to the accompanying drawing.