CA1182776A

CA1182776A - Method of treating radioactive waste water

Info

Publication number: CA1182776A
Application number: CA000398642A
Authority: CA
Inventors: Rainer Ambros; Jurgen Fischer; Norbert Brenner; Gottfried Paffrath; Heinrich J. Schroeder
Original assignee: Rheinisch Westfaelisches Elektrizitaetswerk AG
Current assignee: RWE AG
Priority date: 1981-03-18
Filing date: 1982-03-17
Publication date: 1985-02-19
Also published as: JPS57161699A; US4476048A; DE3110491A1; DE3110491C2; JPS6367878B2; BR8201373A

Abstract

ABSTRACT OF THE DISCLOSURE
Radioactive waste water containing boric acid is concentrated in an evaporator to which an alkali is added to maintain a substantially neutral pH. The boric acid is sedimented out by cooling the solution and an acid is added to the latter to increase the solids recovery by lowering the pH. The solid sediment is vacuum distilled to yield a moist powder.

Description

77~i MRTHOD OF TREATING RADIOACTIVE WASTE WATER

Our present :invention relates to a method of -treating radioacti.ve waste water and, more particularly, boric-acid-containing waste water produced by nuclear power plants. The invention is especially directed to improvements in the concentration of solids from such waste waters.
Nuclear electricity-generating plants from time to time must dispose of radioactive waste water which contains dissolved solids and particularly boric acid. It is known, as described in DE-OS 29 11 272 to reduce the quantity of radioactive material which must be stored, by concent~ating the waste water, i.e. evaporating H20 therefrom and thereb~
increasing the boric acid concentration to a level which is ~reater than that before the ~`

7~

~~)nc~ntr;ltion ~ste~ ~so IJnclertaken ~nd qreater than the saturation concentration clt room t~mperature~, the 11quid then being cooled to preclpi~ate out the boric ac~Ld. The precipitated .sollds can be separa~ed from the superna~nt liquid by d~can tation and the liquid eæn be recycled to the evaporator in whlch the concentra~ion st~p i~ effected.
The evaporation i~ theroby carrled ou~ to a bor ic acid concentration which i9 les~ than the storage concen-tration at the evaporation tcmpsratur2, but more than the ~torage conc~ntration at room tempera~ur~, ther~by ensurin9 a .~igni:~icant degree of sollcl~ prec.ipltation and ~edlme~ta-t.ion on ~ooling to room temperature~
The apparatus for carryiil~ out thls process thu~
comprises an evaporator whose input side receiveq the waste wa ter fed from the nuclear reactor or recycled wa~te water ~rom the decantation 5 tept and a ~torage ve~sel connect~d to the output of the evaporator and in which th~ cooling can be carried oul to s~dlment o~t the sollcls, namely the boric acid. Th~ outlet t~: this stor~e v~39~1 communlcate~
with th~ inl~t to the ~vaporator for recyclinq the t~ec~ntate and a soll~ or slurry outlet can be provided at the bottom of this ve~sel ~o that the solids can be discharg2d.

The wa~te water which i~ used in the earlier syst~m and Por l~hich the pre~ent method ha~ been ound t~ be particularly ePfective, can contain up to about 4,000 parts per milllon Qf boron, which i~ equivalent ~o 2.5 weiqht percent o~ boric acid as sollds.

7~6 In this earlier system, the solids were generally embedded in a ma-trix of concrete, bitumen or synthet:ic resin directly upon wi-thdrawal from -the s-torage vessel or after storage.
Because of the large quan-tities of the sedimented material which must be stored in many cases, the storage capacity of the plant may be readily exhausted. This, of course, poses a major problem.
It is the principal object of the present disclosure to provide an improved method of treating boric-acid-containing radioactive waste water whereby disadvantages of earlier techniques are avoided.
Another ohject is to provide an improved method of treating such waste water whereby the storage volume which must be available for the boric acid solids can be substantially reduced and the economics of the process improved significantly by comparison with earlier systems. Yet another object is to provide an improved method of concentrating boric acid waste water from nuclear power plants so that the solids are obtained in a condition which allows compact storage and practicall.y any desired subsequent conditioning including the packagi.ng or treatments described in the aforementioned copending application.

~8Z776 These objects are attained, in a method of concen-trating the solids of a boric-acid-containlng waste water of a nuclear power plant wherein the water is evaporated to form a solution which is more concentrated in the boric acid solids and especially has a concentration of boric acid which is less than the saturation concentration at the evaporation tempera-ture but more than the saturation concentration at room temperature, the solution is cooled to precipitate out the boric acid which is sedimented from the solution and the super-natant liquid is separated from the sediment sludge as descri-bed, for example, in the above-identified DE-OS 29 11 ~72.
According to this present disclosure, however, the pH
of the solution during concentration is held at approximately a neutral value and, after separation of the supernatant from the sediment sludge solution, the latter is transformed into a damp powder by vacuum distillation after the pH has been dropped by the addition of an acidic substance to precipitate out additional boric acid.
We have found that, by maintaining the pH substantially at neutral during evaporation and thereafter decreasing the pH, we can markedly increase the quantity of boric acid which can be driven out of the supernatant liquid. The sediment which results, in addition, can be surprisingly effectively reduced in bulk by the vacuum distillation setup which leaves a moist powder. In this case, the apparatus ~.

l~Z77~

described in the aforementioned D~-OS can be ~odified to ~onnect the solution ou~let o~ the sedim~nt wlth a vacuum distillation unlt and to provide mean~ for adding an acld lowering the pH to the ~edlmentatlon vess~l.
Preferably, thi~ acid is oxalio acid.
Th~ pH whlch might tend to be acid originally can be brought to the neu~ral level and maintained at neutral during avaporation by the addition of an alkali ~uch as ~odium hydroxlde.
~ An important advanta~e of thl.~ disclosure is tha~
th~ concentratiorl o~ the boric acld in the ~olut~on befor~
sedimQntatlon 1~ lnltiated can be lncrcased about t~o-fold or ~ore vithout danger that preclpltat~on wlll occur ln the evaporator and, moreover, the quantity of boric acid which ls driven out of the solutlon can be incroased and the precipltate or sedlment sludge recovered in a form whlch allows lt to be transformed into the moist po~der by vacuum dl~tlllatlon.
The moist powder can b~ stored in drum~ and sub~ec-ted to any klnd o* conditivning or packaging, lncludin~ ~h~
storage ~echniqueq described ln the aforementionQd DE-OS
t~ith even greater advantage because, for example, the molst powder is more firmly bound in concrete and like binders. The moist powder which is obtain~d ha~ a signifi-cantly smaller volume than the product previou~ly ob~alned starting from the same waste water, thereby con~erving con-~iderabla space in the plant.

More particularly in accordance with the invention there is provided, a method of treating radioactive waste water containing boric acid, comprising the steps of:
A me-thod of treating radioactive waste water containing boric acid, comprising the steps of:
(a) evaporating said waste water to a boric acid concentration above the saturation level at room temperature but below the saturation level at the evaporation temperature to form a concentrate solution of boric acid;
~b) maintainingthe p~ of said solution during the evapora-tion in step (a) at substantially neutral;
(c) co~ling the solution after the evaporation in step (a) to precipitate boric acid from said solution and forma sediment sludge of boric acid;
(d) separating said sludge from a li~uid phase in which the pH is reduced to increase the precipitation and settling of boric acid therefrom; and (e) vacuum distilling water from said sediment sludge to transform the same into a moist powder.

Specific embodiments of the invention will now be described, reference being made to the accompanying drawing, the sole FIGURE in which is a flow diagram illustrating an apparatus for carrying out the new method here disclosed.
While the apparatus shown in the drawing has been represented in diagrammatic form, it will be apparent that the evaporator and settling vessel of this apparatus can be identical to the units described in the aforementioned DE-OS.
The apparatus comprises an evaporator 1 and a storage vessel 2 which also form a settling tank. A vacuum distillation unit is connected to the vessel 2.

t77~

Boric acid containing waste water from a nuclear power plant is fed as represented by line 4 into the eva-porator 1 from which H20 vapor is discharged at 9, thereby concentrating the solution. A pH ~etector 11 monitors the pH of the solution in the evaporator 1 and controls a dispen-ser 12 which feeds a base, e.g. sodium hydroxide, via line 13 to the evaporator so that the pH is maintained approxi-mately neutral.

.

- 6a -7~

Th~ concentrate is delivered via line ~ to the vessel 2 and lts pH is detected at 14, the pH sensor 1~
controlling a disp~nser 15 ~hich feed3 an acid into the ve~el

2 to decrease the pH. The acid whlch i9 fed by a line 16 to tht? vessel 2 i~ pre~erably oxalic acid.
Ag th~ ~olution cool~, the boric acid precipitates ~nd the supernatAnt liquid is removed a3 shown by llne 5 and recycled to th~ evaporator. L~ne S co~municates ~ith V~9 ~el 2 at the upper hal~ thereo~.
The sedlment ~ludge ls deliv~red via lin~ 7 to the vacuum dlstlller 3 which can al~o be provided with a p~wder dispen~ser equivalent to the unlt 15 for delivering a pulveru-lent acld to the vacuum dl~tiller if any supernatant i9 carried over and befor~ it i9 decanted.
This di~penser ha~ be~n repre~ented at 17 and i~
controlled in rQspon~e to the pH d~tected by the unit 1~.
A radioactive waste water havin~ a borlc acld content up to 2.4X ky weight 18 conc~n~rated in ~vaporator 1 while the pH value i~ maintained b~tween 6.8 and 7~2, pr~ferably 7.0 by ~he addition of 30dium hydroxid~. Tne .qodium hydroxide can be addQd dir~ctly to th~ llne 4.

In the evaporator 1 the water is concentrated above the storage c~ncentration at room temp~rature but below the ~aturstion level at the evaporatlng temperatur2.
Supply of sodl~m hydroxide or a solutlon thereof i5 maintained as long as evaporation i9 carried out to maintain thi.~ pH level.

-7_ Concen~ratlon3 o~ ~0,000 to 90,000 par~3 p~r mllli~n oP boron, corra~pondlrlsl to 2~ to 54 % by weiqht, can be achiev~7d without visible precipitation.
Th~ concentra~ lq than lntroduced into the ves~el 2 ~Ln which i~ is cool~d and the bor~c acld precipi~ated. T~e ~ups~rnat~ t liquid c~n bo r~cyclad a~ descril:~ed . Whlle r~cycl lng c:f the superna~a.nt is ~ar~i~d out, th~s acid i~ not add~d or the ~upernat~nt i~ treated a~ de~crib~d below~ Generally, how~ver, ~h~2 sup~rnata~t in ve~sel 9t i~ tre~t~d with acid to drop the pH.
If, for exampl~, the oxalic acid i~ added to ths V09~ 2 2trld i~ cntrained ln ~he r~cycled d~cantat~, it ls decompo~d with perm~n~nat~ added at lg, i.e. be~or~ the decarltate enter~ the ~vaporator.
The sediment ~lud~?e ~om the ve~;sol 2, corlsisting o~
50 to 70X by w~ight borlc acid, i9 heated ln th~ ve~3~t?1 ~ to ~ t~mporatur~ o~ Sûto 60~C and 1~ th~n introduc~d lnto the vacuum distillatlon unit 3 in which it: i~ conv~rt~d to a mo~ 3t powder which 1~ ~ed by a powd~r m~t~rinq devlc~e 8 ~ nto drum~ .

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE PROPERTY OR PRIVILEGE
IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A method of treating radioactive waste water containing boric acid, comprising the steps of:
(a) evaporating said waste water to a boric acid concentration above the saturation level at room temperature but below the saturation level at the evaporation temperature to form a concentrate solution of boric acid;
(b) maintaining the pH of said solution during the evaporation in step (a) at substantially neutral;
(c) cooling the solution after the evaporation in step (a) to precipitate boric acid from said solution and form a sediment sludge of boric acid;
(d) separating said sludge from a liquid phase in which the pH is reduced to increase the precipitation and settling of boric acid therefrom;
and (e) vacuum distilling water from said sediment sludge to transform the same into a moist powder.

2. The method defined in claim 1 wherein the pH during evaporation is held between 6.8 and 7.2.

3. The method defined in claim 2 wherein the pH during evaporation is maintained by the addition of sodium hydroxide.

4. The method defined in claim 1 wherein the evaporation and the sedimentation of boric acid from said solution are carried out in spaced apart locations and said solution is transferred at a temperature above room tempera-ture from the evaporation location to the sedimenting location.

5. The method defined in claim 1 wherein precipitation of boric acid from said solution is carried out by successively lowering the temperature of the solution and adding an acid thereto to reduce the pH.

6. The method defined in claim 5 wherein the two precipitations of boric acid from said solution in step (c) and step (d) are carried out separately with respective decantations of supernatant liquid.

7. The method defined in claim 5 wherein said acid is oxalic acid.

8. The method defined in claim 7 wherein supernatant liquid containing oxalic acid is recycled to the evaporation step (a), and further comprising treating the recycled supernatant with potassium permanganate.

9. The method defined in claim 1 wherein the boric acid sedimentation and the vacuum distillation are carried out at spaced apart locations and the sludge is heated and is in a flowable state for transfer to the location at which vacuum distillation is carried out.

10. The method defined in claim 9 wherein the solution is heated to a temperature of 50 to 60°C.

11. An apparatus for carrying out the method of claim 1 comprising an evaporator for concentrating boric acid in radioactive waste water, means for monitoring and maintaining pH in said evaporator at substantially neutral, a sedimenting vessel connected to said evaporator for precipitating boric from from a concentrate sludge formed in said evaporator, thereby sedimenting a boric acid sludge in said vessel, and a vacuum distiller connected to said vessel for receiving said sludge therefrom to transform said sludge into a moist powder.

12. The apparatus defined in claim 11 wherein said vacuum distiller is provided with a powder dispenser for filling said powder into a container therefor.

13. The apparatus defined in claim 11, further comprising means for dispensing an acidic suspension into said vessel for reducing the pH of the solution thereof.

14. The apparatus of claim 13, further comprising means for dispensing an alkaline substance into said apparatus for maintaining a substantially neutral pH therein.