September
1951
Hysteria over gross contamination of public waterways by radioactive waste appears unwarranted
B
ui-oui·: the atomic bomb arrived, few persons, if any, realized t h a t difficult radioactive waste disposal problems were on the horizon. T o d a y we face such prol>lems. There are two major sources of radioactive! wastes according to the United States Atomic Energy Commission. T h e first one originates from t h e national atomic energy program and involves reprocessing the residues from isotope separation plants and nuclear reactors. T h e second source arises in the m a n y laboratories and hospitals where radioactive materials are used in medical treatments. Any future explosion of atomic, bombs in populated centers of the United States could release radioactive wastes in sufficient quantities to create temporary if not continuing problems in radioactive waste disposal. As we advance in applying nuclear energy to industrial applications, other radioactive waste problems could be created. Chemists and engineers should keep abreast of such problems. Tin- present source of raw material for nuclear energy production is uraniumbearing ores. T h e earth's crust contains considerable amounts of such ores b u t so far few concentrated deposits have l>een located. Before uranium can be utilized in nuclear reactions, it must be separated :ts pure uranium oxide from the other constituents of the ore and be converted to t h e metal. Uranium ores arc not dangerously radioactive. T h e process of winning uranium from the ore does not therefore require unusual precautions in handling t h e waste residues. Natural uranium is a mixture of three isotopes. T h e U 238 variety is nonfissionsthlc and a m o u n t s to 9 9 . 3 % of the total wight. Fissionable isotopes U 234 and I ' " * are present in much smaller amounts 0.00(1 and 0.7%, respectively. When these fissionable isotopes arc bombarded with neutrons, the a t o m splits forming two fission fragments of lower atomic weight (radioisotopes), and two or three new neut r o n s are liberated with emission of considerable energy. T h e U m isotope is nonfissionable so cannot split into radioisotopes, but when present with U 235 it capt u r e s one of t h e neutrons liberated by V 2 3 S and forms U 239 which decays b y b e t a r a y emission into plutonium Pu 8 3 9 which is fissionable and under favora.ble conditions produces a chain reaction induced by further liberation of neutrons. In this September 1951
way, U 23S becomes usable and increases the potential stockpile of fissionable elements by 140 times as compared to conditions which existed-when only U 235 and the cyclotron were the sole means for bombarding atoms with neutrons. In order to carry out the nuclear reaction, it is necessary to design and build reactors which can breed neutron bombardment. Such studies are carried out under law solely by t h e U. S. Atomic Energy Commission. Eventually when a highly satisfactory breeding reactor is developed, the difficulties in producing nuclear energy efficiently will not be fully overcome until the; chemical engineer develops an efficient chemical process for ( 1 ) recovering plutonium from t h e exposed fuel elements, (2) regenerating t h e residual U23S and U 238 in t h e exposed fuel element for renewed use, (3) separating the fission-produced radioisotopes for medical and industrial applications, and (4) safely disposing of the unusable radioactive residues. At Oak Ridge National Laboratory the chemical technology division is responsible (1) for the control of wastes a t their source, and (2) for the t r e a t m e n t of wastes resulting from the chemical processing as well as t h e separation of reactor-produced radioactive isotopes. It, therefore, devotes its efforts primarily to the t r e a t m e n t of relatively small volumes of concentrated and highly radioactive materials. The radioactive waste disposal research and development section is responsible for development of techniques for treating the residual radioactive wastes. T h e health physics division is designated t o establish t h e limits of radioactive concentration before wastes are discharged into waters outside t h e controlled area of operation. For example, the maximum permissible tolerance concentration of mixed fission products in water is 10 ~7 microcuries per cc. These operating groups a t Oak Ridge have adopted the sanitary engineering approach in developing techniques for processing residual radioactive wastes. Processing of t h e exposed fuel from a nuclear reactor is accomplished a t present by chemical methods. After separation of the more valuable fission products for isotopic use in medicine and industry, the remaining substances are stored in underground tanks to permit dissipation of the short-lived fission products.
I N D U S T R I A L AND E N G I N E E R I N G
Control of wastes at their source is just as important in the treatment and disposal of radioactive wastes as in the treatment and disposal of domestic and industrial wastes. Little information is available concerning the adsorption of radioactive substances by specific muds, soils, plankton, and other flora and fauna which compose t h e life cycle in a receiving stream. Although considerable d a t a are available which indicate t h a t muds, clays, activated carbons, and activated sewage sludge have affinity for adsorbing certain radioactive materials, it is unwise to do other t h a n process radioactive wastes a t the point of origin. Some methods suggested b y Conrad Straub, sanitation engineer assigned t o the Oak Ridge National Laboratory [Sewage and Industrial Wastes, 23, 188 (1951)] are helpful. Evaporation. Although expensive, large volumes of waste waters containing small amounts of radioactive materials may be concentrated b y evaporation. Practically all the activity will be removed from t h e distillate. T h e radioactive constituents will be concentrated in the solid residue or slurry. A decontamination factor of a t least 100,000 is possible b y this method. This method reduces substantially t h e a m o u n t of space required for storage of radioactive materials. Where large volumes of water are required for cooling the nuclear reactor as a t Hanford, Wash., the disposal of the radioactive water is a problem. T o minimize t h e a m o u n t of induced radioactivity, t h e cooling waters are pretreated for the removal of such elements as iron, chlorine, calcium, and sulfur. Coagulation. For removal of arsenic, antimony, molybdenum, selenium, tellurium, and cerium, t h e method of coagulation with a carrier has been used with varying success. Removal of ruthenium and plutonium by this method is very satisfactory. In many instances, coagulation is most effective a t high p H values. Using routine alum and lime coagulation Placak and Lyle of the Oak Ridge N a tional Laboratory removed specific isotopes from water to which radioactive chemicals (Continued on page 114 A)
CHEMISTRY
113 A
Industrial Wastes
Industrial Wastes
available on the removal of radio active substances by sand filtration, Straub states that unpublished communi cations indicate t h a t many of them may be removed by adsorption on sand grains. As shown in Table I I , P 3 2 is removed sub stantially by sand filtration, but I 131 is adsorbed only partially. Ion Exchange. One of the more, sug gestive methods for removing radioactive compounds from liquids is the principle of ion exchange resins. High capacity cationic and anionic synthetic resins have been found to remove radioactive substances, from large volumes of dilute wastes. Γ η published information, according to Straub, indicates t h a t sodium, barium, and lanthanum are removed in cationic resin columns and t h a t tellurium and molybdenum are removed in anionic resin columns. Under suitable conditions stron tium, cesium, cerium, and t h e rare earths may be removed b y cation exchangers. Natural soils and clays in very large amounts have been shown t o remove 80 to 8 5 % of radioactive elements in dilute con centrations. The exchange resins may be recovered effectively by acid and alkali «ash waters. It is quite likely t h a t these resins may prove very useful in cleaning up filtrate waters after removal of gross contamination by coagulation methods. Biological Methods. By a single-stage treatment, Ruchhoft [Sewage Works ./., 21, ">, 89'.) (1940)] demonstrated a 0 6 % reduction in t h e alpha activity of pluto nium by extraction and concentration on aerated activated sewage sludge. Straub removed I 131 in a similar manner and Reid at Johns Hopkins University studied the accumulation of P 3 2 and I 1 3 1 on slimes found in sinks, traps, and drains. If longlived radioactive substances were retained in such sludges, their accumulation could create serious hazards. Crystallization. Preliminary studies in freezing out large fractions of the water t o leave the radioactive materials in a more concentrated solution have indicated definite possibilities for such technique. Conclusion. Hysteria over gross con tamination of public waterways due to radioactive waste appears to be unwar ranted provided adequate care and cont mis are applied a t the source where radio active elements are produced or utilized. Coagulation, adsorption, ion exchange, and other known methods for concentrat ing the radioactive substances into small volumes and then storage of the concen trated radioactive sludges or disposal of the sludge in underground retention pits can meet any reasonable increase in t h e production and use of radioactive com pounds. Correspondence concerning this column forwarded promptly if addressed to the ΐ Kditor. Industrial and Engineering istry, 1155— 16th" St., N . W., Washington
Γ
114 A
will be author, I Chem 6, D.C.
I N D U S T R I A L AND ENGINEERING
had been added. The data on alum co agulation is shown in Table I as com piled by Straub.
T A B L E I. Element
P3S
Sr" ysi
Ce'"
Y90
Sr>»
ALUM COAGULATION OP R A D I O ACTIVE SUBSTANCES Removal, % Remarks Up to 10 Addition of smalt amounts of Cu, Ag, or C increased removals to 75% 98 plus 10 Increasing turbidity from 50 to 1000 p.p.m. increased removal effi ciencies to 50% 45 Using NaOH as alkali Using NaaCOa as alkali; turbidity influencing factor also; for tur bidity ranges of 5Θ to 1000 p.p.m., removals ranged from 17 to 8 2 % 98 Turbidity also influ encing factor; for tur bidity ranges of 50 to 1000 p.p.m., removals ranged from 70 to 8 6 % On standing, more of the Y daughter formed from Sr 10
Lauderdale, also of Oak Ridge, em ployed a modified phosphate coagulation technique. T h e studies were made a t p H 11.5 using up to 200 p.p.m. of sodium (NajPOO or potassium (KH 8 PO.i) phos phates and lime. Element Zn6S was re moved 99 + % ; Sr*9, 9 8 % ; Y " , 9 9 + % ; Sb 124 , 6 7 % ; Ce 1 », 99 + % ; and W 185 , only 10%.
TABLE Element P32
I' 3 1
II.
PILOT PLANT ALUM
TESTS
Treatment Coagulation with alum plus sedimentation Coagulation with alum plus sedimentation plus filtration Coagulation with alum plus sedimentation (sodium silicate also used) Coagulation, sedimenta tion plus filtration Coagulation (alum, so dium silicate, alkali) plus carbon (5 p.p.m.) Coagulation (alum, so dium silicate, alkali) plus carbon plus fil tration
WITH
Removal, %
96 to 98
