US3492217A - Electrolytic dissolver - Google Patents

Description

Jan. 27, 1970 Filed July 14, 1967 D. P. KEELER T AL ELECTROLYTICYIDISS'OLVER 3 Sheets-Sheet l Inventors Donald P keelef Mail 6am 13. Kerr lllt'llawfrf Rounds 1970 D. P. KEELER ET AL ELECTROLYTIC? DISSOLVER 3 Sheets-Sheet 2 Filed July 14, 1967 3. Keri Razz/2d Jan. 27, 1970 o. P. KEELER ETAL 3,492,217

I ELECTROLYTIO'DISSOLVER Filed Jul 14, 19s? 5 sheets-Sheet s I a- S IO 7 I il I w/Q Inventors:

[ I 5 p mzd J? ffeeer media/1r E. Kerr mane/ Row/{d5 United States Patent Ofice 3,492,217 Patented Jan. 27, 1970 3,492,217 ELECTROLYTIC DISSOLVER Donald P. Keeler, Bartlesville, kla., and William B.

Kerr and William Rounds, Idaho Falls, Idaho, assignors to the United States of America as represented by the United States Atomic Energy Commission Filed July 14, 1967, Ser. No. 654,036

Int. Cl. B01k 3/00 U.S. Cl. 204-272 7 Claims ABSTRACT OF THE DISCLOSURE An electrolytic dissolver wherein dissolution occurs both by metal contact and by solution contact including a cathodic shell containing an acid electrolyte, an anodic niobium, tantalum, niobium-tungsten or tantalum-tungsten basket having perforations over a portion of the circumference disposed in the shell, a metal anode fastened to the inside of the shell, an impact cone having metal anode strips therein at the base of the dissolver, and means for passing a direct current through the dissolver. Dissolution is by metal contact when the article being dissolved is in contact with the niobium basket and by solution contact when it is not.

Contractural origin of the invention The invention described herein was made in the course of, or under, a contract with the United States Atomic Energy Commission.

Background of the invention This invention relates to an electrolytic dissolver. In more detail, the invention relates to an electrolytic dissolver for dissolving nuclear-reactor fuel elements wherein dissolution occurs by both solution contact and metal contact.

In a solution-contact electrolytic dissolver, the article to be dissolved is submerged in an acid electrolyte in an electrolytically neutral perforated niobium, tantalum or titanium basket between a platinum anode and a stainless steel or titanium cathode. Electrical current flowing through the electrolyte between the electrodes intercepts the article being dissolved. When the article intercepts all or part of the current, the article is cathodic with respect to the anode but anodic with respect to the cathode. This anodic condition satisfies the requirements for dissolution as well as if the metal were in direct contact with the anode.

In a metal-contact electrolytic dissolver, the article to be dissolved is submerged in an acid electrolyte in a perforated cylinder disposed in a stainless steel cathode shell. The perforated cylinder is made from a metal which exhibits the electrolytic valve effect. For electrolytic dissolutions in nitric acid the most important valve metals" are niobium, tantalum and alloys of these metals with tungsten. When such a metal is made anodic, an insulating oxide film forms on the surface, which, once formed, causes a high resistance to the flow of anodic current from the metal to the solution in which it is immersed. This allows the flow of current from the dissolving fuel to pass through the holes in the basket to the cathode. The film on the valve metal anode offers a reasonably low resistance to electrical flow to the articles being dissolved in contact with it and hence completes the electrical circuit. In addition to conduction through the film, local penetration of the film to make positive contact may occur.

Summary of the invention The electrolytic dissolver according to the present invention combines the advantages of a solution-contact dissolver and a metal-contact dissolver. It incorporates a valve metal basket which contacts the article being dissolved to give metal-contact dissolution and it also incorporates a platinum anode, electrically connected to the valve metal basket, which gives solution-type dissolution if metal-to-rnetal contact between the basket and the article being dissolved is broken. To accomplish this result, the valve metal basket is perforate over half of its circumference and imperforate over the other half of its circumference, the platinum anode being attached to the imperforate portion of the basket opposite the perforate portion of the basket. The dissolver thus eliminates the primary disadvantage of the known electrolytic dissolvers.

While acceptable results can be attained with known electrolytic dissolvers, a solution-type dissolver is of low efficiency due to the fact that much of the current bypasses the article being dissolved, taking a path through the electrolyte between anode and cathode. While efficiency of a metal-contact dissolver is much higher so long as the basket and article being dissolved are in contact, it has been found difficult to maintain this contact and electrolytic dissolution essentially stops when contact is broken. According to the present invention, the advantages of metal-contact dissolution are enjoyed so long as contact is established; once contact is lost, however, solution-type dissolution occurs until contact is reestablished.

Another feature of importance of the electrolytic dissolver is an impact cone disposed in the base of the dissolver which contains platinum anodes inset into the cone. This arrangement ensures maximum probability of dissolution of the heel of the article being disposed.

Brief description of the drawing The invention will next be described in connection with the accompanying drawing wherein:

FIG. 1 is a vertical viewpartly in elevation and partly in se'ctionof an electrolytic dissolver constructed in accordance with the present invention,

FIG. 2 is a vertical sectional view of a part of the lower portion of the dissolver showing the impact cone.

FIG. 3 is a sectional view taken on the line 33 in FIG. 2, and

FIG. 4 is a horizontal sectional view, greatly enlarged, taken on the line 4-4 in FIG. 1 and rotated degrees.

Referring first to FIG. 1, the dissolver comprises an elongated, cylindrical, stainless steel vessel 10 incorporating an inlet 11 for electrolyte and a drain 12 near the bottom of the vessel and a vapor outlet 13 and a. liquid outlet 14 near the top of the vessel. An anode basket assembly 15-to be described in more detail hereinafter is disposed concentrically in vessel 10 and electrically insulated therefrom by insulators 15a. Vessel 10 is made cathodic with respect to anode basket assembly 15 by leads 16 contacting vessel 10 by connections 17. Basket assembly 15 is made anodic by leads 18 contacting the basket assembly by means of flexible connections 19 which are insulated from vessel 10. At the base of the vessel 10, an impact cone 20 takes the shock of fuel elements dropped into the dissolver. Impact cone 20 will be described in more detail hereinafter.

As shown in FIGS. 2 and 4., anode basket assembly 15 includes a cylindrical niobium basket 21 (or other valve metal) having a perforate portion 22 at the bottom of the basket extending the entire circumference of the basket, 2.

perforate portion 22a extending over about half of the circumference of the cylinder and opposite to anode connection 19, a solid portion 23 extending over the remainder of the circumference.

A platinum-% indium plate 24 is fastened to the interior of the imperforate portion 23 of anode basket 21 and an anode guard 25 consisting of a perforated niobium sheet covers platinum anode 24, being fastened to basket 21 at its ends. Also shown is anode bus 26 which extends through flexible connector 19 to make electrical contact with anode basket assembly 15.

As shown in FIGS. 2 and 3, impact cone 20 comprises a cone-shaped niobium anode 27 having undercut portions 28 therein extending the length of the cone. Elongated platinum-10% indium strips 29 are inlaid into undercut portions 28 and a cone-shaped perforated niobium guard 30 covers anode 27 to protect the platinum strips 29. Also provided is an impact cover plate 31 which extends across the top of the cone while flow openings 32 are disposed around the circumference of the base of the cone to allow the dissolvent to flow through the dissolver and permit sludge to drop out of the dissolver for removal through drain 12.

Impact cone 20 is provided with a base 33 which supports anode basket assembly and places impact cone and anode basket assembly 15" in electrical contact. Just below base 33 of impact cone 20' is an nonconducting impact plate 34 having fiow ports 35 therein aligned with flow openings 32 and below plate 34 is a bearing plate 36, having flow ports 37, which is supported from vessel 10 by support 38.

In operation, nuclear-reactor fuel elementsor other articles to be dissolved-are dropped into the dissolver from above. The electrolyteessentially a metal nitratenitric acid solutionis circulated continuously upwardly through the dissolver to remove heat generated by electrical power losses in the dissolver. Solution overflowing the dissolver is passed through a cooler and then returned. Make-up nitric acid is added continuously to the circulating solution. The rate of acid addition is adjusted to give the desired metal concentration in the circulating product s lution. A constant liquid inventory in the circulating system is maintained by continuously removing a side stream of solution to a recovery system for recovery of the uranium. Also off gas from the dissolver escapes through vapor outlet 13, condensible gases being condensed and returned to the circulating system.

A direct electrical current is passed through the dissolver during operation. So long as the article being dissolved is in contact with anode basket 21, electrical current flows directly from anode basket 21, through the article and then through the electrolyte to and through the perforations in perforate portion 22 of the anode basket 21 to cathode shell 10. Current does not flow directly from the anode to the cathode due to the electrolytic valve effect, as explained hereinbefore. Most of the current is expected to pass directly from the anode basket to the article being dissolved by direct contact; this would result in a minimum cell resistance. If, however, the article being dissolved is not in contact with the anode basket, the dissolver continues to operate by solution contact. According to this manner of operation, current flows from platinum plate 24, through the electrolyte, to the article being dissolved, through the article being dissolved, through the electrolyte to and through the perforations in anode basket 21 and to cathode shell 10.

Simultaneously current is flowing from platinum strips 29 in impact cone 20 to and through the perforations in the lower portion of anode basket 21 to vessel 10. As the metal collapses, it falls to the bottom of the dissolver vessel .where the solution path between anode and cathode is shortest, thus concentrating the current. Flow openings 32 in impact cone 20 permit any remaining sludge to 4 drop out of the bottom of the dissolver through drain Specific embodiment of the invention According to a preferred embodiment of the present invention, the electrolytic dissolver is to be used to dissolve fuel elements from existing reactors. For example, it may be used to dissolve fuel elements incorporating stainless steel and urania employing nitric acid as the acid electrolyte. For this purpose, dissolver vessel 10 is 24 feet 6 inches long; the upper 23 feet 1 inch has an inner diameter of 5% inches and /2-inch thick walls; the lower 1 foot 5 inch portion is a heavy forging 4 /2 inches in internal diameter with 2 -inch-thick walls. Anode basket 21 is 15 feet long by 5% inches in outside diameter and has a thickness of /s inch. The perforated portion thereof contains /s-inch diameter holes to provide afree area of approximately 30%. Platinum-indium plate 24 is 6 inches wide by 96 inches long and is O.l0-inch thick.

The electrical current used may, for example, be 6000 amperes at 20 volts. At 6000 amperes a dissolvent recirculation rate of 72 liters per minute is required.

The current utilization on simulated Army Package Power Reactor (APPR) fuel elements was found to be about 0.6 gram of metal dissolved per ampere-hour. At 6000 amperes the dissolution rate would be 3600 grams of stainless steel per hour. For a product concentration of 50 grams of dissolved metal per liter, the acid feed rate of 8 molar nitric acid is 72 liters per hour.

Finally, the electrolyte solution is introduced into the dissolver at 100 F. and is heated therein to F. to maintain an average temperature of approximately 125 F.

It will be understood that the invention is not to be limited to the details given herein but that it may be modified within the scope of the appended claims.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. An electrolytic dissolver wherein dissolution occurs both by metal contact and by solution contact comprising a metal vessel for containing an acid electrolyte, means for connecting said vessel to the negative terminal of a power source, a valve metal basket having an imperforate portion and a perforate portion disposed in the vessel for containing metal to be dissolved, means for connecting said basket to the positive terminal of said power source, an anode plate formed of a metal that is not a valve metal fastened to the interior of the imperforate portion of the basket, and means for passing a direct current through the dissolver.

2. An electrolytic dissolver according to claim 1 wherein the imperforate portion of the backet covers about of the circumference of the basket.

3. An electrolytic dissolver according to claim 2 wherein the vessel is stainless steel, the basket is niobium, the anode plate is a platinum-10% indium alloy, and the acid is nitric acid.

4. An electrolytic dissolver according to claim'3 and including a perforated niobium anode guard covering the platinum-indium plate.

5. An electrolytic dissolver according to claim 4 and including an inlet for electrolyte at the bottom, an outlet for electrolyte at the top, a drain at the bottom, and a vapor outlet at the top.

6. An electrolytic dissolver according to claim 1 and including an impact cone at the base of the dissolver in electrical contact with said basket, the entire circumference of the basket being perforated immediately surrounding the impact cone.

7. An electrolytic dissolver according to claim 6 wherein the impact cone includes a cone-shaped niobium anode having undercut portions therein, platinum-10% indium strips inlaid in the undercut portions, a cone-shaped perforated niobium guard covering the niobium anode, an impact cover plate covering the top of the impact cone,

and flow openings disposed around the circumference of 3,278,410 10/1966 Nelson 204-285 the base of the cone.

JOHN H. MACK, Primary Examiner D. R. JORDAN, Assistant Examiner References Cited UNITED STATES PATENTS 5 2,865,832 12/1958 Pitzer. U.S. Cl. X.R. 2,955,999 10/ 1960 Tirrell. 204-278, 290, 285

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