An overview of high-level waste concentration and storage operations and some of the problems involved in them are given. (LK

Burns, R.E.

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HIGH-LEVEL WASTE CONCENTRATION AND STORAGE - AN OVERVIEW by R. E. Burns - . -- . . -- 'Ibh rapart wu pmpad as an account of work lposDDd by tbe United %Ias O o ~ n l .  Ndthor the UaYDd %tr nor tlm united %ta Eoorg l a a a r c l l . r d ~ ~ ~ n o r l a ~ 0 f  cha ompbysor, nw UIY of ihotr mntnctq Wmmcion, or Ualr ompbyra, mku .nY -. - m Implkd, or mmc~ .ay lrpl b b i s h o r r a P d b w ~ r o r t b . - . ~  dl - d .Y infrmrrion, .pp.nioh podw or  Ikb~od ,  or mpmenis 0111 Ht um would 1101 ~ n h t o h l o r n c d I l d l t L  Battel  le Paci f t c Northwest Laboratories R i  chl and, Washington 99352 T h i s  paper i s  based on work performed under U.S. Energy Research and Development Administration Contract E(45-1) : 1830 DISCLAIMER This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency Thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof. DISCLAIMER Portions of this document may be illegible in electronic image products. Images are produced from the best available original document. 01 ' . & I%, ,"I L- - a 19. . BNWL-SA- 54 7 7 I ' CONCENTRATION AND STORAGE OF HIGH-LEVEL RADIOACTIVE WASTES Chemical reprocessing of irradiated nuclear fuels to recover I ! I and purify uranium and plutonium (and possibly other valuable i constituents such as neptunium) produces waste streams which are highly radioactive, hence, extremely toxic to man. These wastes must be managed and disposed of by techniques which,are safe and economically reasonable. Under current regulations in the United : States, applicable to commercial fuel reprocessors, these wastes may be stored in liquid form for a period up to five years and then they must be converted to solids. Later these solids will -. be transf.erred to a federal repository for storage and/or ultimate disposal. Nearly all nuclear f11e1 reprocessing  plant^ in thc world . . today, and those planned for the near future,,use a version of the Purex solvent extraction process. Large volumes of aqueous . . high-level waste which are moderately concentrated in nitric acid are produced in these processes. As a precursor to solidifying the wastes, they are concentrated and stored for some interim period. Most of the nitric acid present' is recovered for reuse in the plant; s o m e 3  destroyed to reduce the acidity of the This paper is intended to be an overview of high-level waste &.d 4t+# 7-C concentration,operations and some of the problems involved in them. Other papers in this session will discuss some of these problems in greater detail. Papers in other sessions.of this meeting will . discuss waste solidification operations and problems. . . Composition of High-Level Wastes First, we need to define, as well as we can, what we mean by high-level waste, i.e., what is it that is to be concentrated and ... &+ stored. For this paper, I will discusslthe high-level waste produced in the reprocessing of commercial power reactor fuel. However, many of the operations ar~d problems are also common to . ' the. processing of ERDA fuels as we shall see from other papers in this session. As an example of a-commercial power reactor fuel, consider fully irradiated fuel.from the light water reactor, Diablo Canyon. Reference (1) gives the fission product and actinide element content of fuel from this reactor based on 33,000 MWD/MT integrated exposure at 30 MW/MT. If we assume that (1) the fuel is reprocessed at 150 days out of the reactor, (2) all of the xenon and krypton and 50 percent of the iodine present are volatilized from the fuel prior to or during dissolution operations, (4) 0.5 percent of the uranium and plutonium is lost to the waste during the reprocessing and (5) essentially c all of the remaining fuel constituents appear in the high-level waste, then the fission product and actinide element composition will be that shown in Table 1. Also shown are the Curie levels of the various constituents as well as the radioactive decay I energy contributed by them. Thus, we see that the high-level 4 waste will contain about 3.5 x-10 g of fission products and * actinide elements per tonne of4uel processed, that it will 6 contain about 5.4 x 10 Curies of radioactivity and that it will be generating about 2 x lo4 watts per tonne of fuel processed (at 150 days). While the Curis level and decay heat level of the L However, whether waste streams from later cycles are returned to the first cycle will not change greatly the volume and acidity of the first cycle waste since these are determined by the operating conditions for the first gycle. Hence. the volume and acidity of the Hanford Purex Process first cycle waste may be used as an example. The volume and nitric acid concentration of the waste are about 6250 L/MTU and 3 M, - respectively. Since 99%+ 'of the fission products 0.5% of the U and Pu and 99%+ of the remaining actinide elements present in the irradiated fuel should be in this waste stream, it will have d2c te. . the composition shown in Table 3. A Table 3 shows only HN03 and those waste constituents which come from the irradiated fuel. Other chemicals may or will be present depending on the flowsheet and operating conditions # - used. Chemicals added for'plutonium valence adjustment may be present if wastes are backcycled. Corrosion products derived ' -  from processing equipment will be present. Since austenitic stainless steels are generally used for process equipment, the TABLE 1. FISSION PRODUCT AND ACTINIDE ELEMENT CONTENT OF DIABLO CANYON REACTOR FUEL AGED 150 DAYS .9/MT Curie s/MT Watts/MT S e 5.2 x 10 3.93 x 10-I 1.49 x 1 Br 1.5 x 10 --- --- 1 Rb 3.33 x 10 2 1.85 x 10 0 8,69 x 2 5 sr 8.96 x 10 1.73 x 10 4.45 x 10 2 TABLE 1. (continued) i g/MT Curies/MT TOTAL: . 3.493 x 10 4 5.4 x 10 6 3.24 x lo-' 8.88 x 8.11 x 5.16 x 1.33 x 10 0 6.30 x 10 0 6.38 x 10 2 '8 . .  TABLE ELEMENTS CTONRIBUTING 1% OR MORE OF TOTAL MASS. RADIOACTIVITY AND DECAY HEAT Others 4.96 : .major corrosion products present will be iron, chromium' and nickel. Their concentration will depend on process conditions. Some degradation products of the solvent will be present; for TBP these will be di- and monobutyl phosphate and phosphate ion. If a soluble poison such as gadolinium or boron is used in the process, it too will appear in the high-level waste. The Hanford Purex process high-level waste contains also a small amount of sulfate and sodium ion from backcycled waste streams containing Pu and Np valence adjustment chemicals. It also contains some aluminum and fluoride which result from the - fact that zirconium cladding is chemically dissolved in ,the headend operation. Wastes generated in processing commercial power reactor fuels will not have the aluminum and flsoride since a chop-leach headend is used instead of chemical decladding. Waste Concentration Operations Recovery of nitric acid from the dilute high-level waste is desirable for two reasons. The recovered nitric acid, though slightly contaminated with radioactive constituents, can be z TABLE y. BASIC COMPOSITION OF FIRST CYCLE WASTE -Other Fission Products and Actinides H+ - especially subject to attack. Stainless-steel equipment, . sometimes utilizing titanium tube bundles in the evaporator have given good service in ERDA installations. Since the waste from commercial power reactor fuel reprocessing will contain higher concentrations of fission products and actinides and since flow- sheets used may vary from plant to plant, evaluation of the corrosion potential and selection of optimum material for the evaporator will be needed on a plant by plant basis. The possibility of volatilizing waste components during nitric acid recovery must be considered. Such volatilized components could appear in the recovered acid, in the acid fractionator overhead and in the offgas. Ruthenium is perhaps of greatest concern. Significant v~latiliza~tion of ruthenium can occur if the acidity of the . . evaporator bottoms is maintained too high ( > 6 ~ ) .  One measure - to prevent ruthenium volatilization while maximizing the amount * of HN03 which can be recovered is to add water to the system * and recycle it to the evaporator to keep the evaporator bottoms !. acidity low. Another is to add a reducing agent to the evaporator : \ : /  to generate oxides of nitrogen and reduce ruthenium volatiliza- I L' , tion. Sucrose is used for this purpose in the t an ford Purex plant. Iodine, if present at this point, may be volatilized and may distribute among the recovered acid, the fractionator overhead and the offgas. This has not been a significant problem with ERDA fuels due to aging before processing for 1-131 decay and the small amount of 1-129 present. It may be a problem with commercial ' power reactor fuels because of the much larger 1-129 content. Tritium present. in the high-level waste will distribute p among the various streams (concentrated waste, recovered acid, fractionator overhead-and offgas) in relation to the water present in them. <P The concentrated waste will contain some solids. How much solids and their physical characteristics will depend on the .I .: , fuel being process'ed, the chemical flowsheet used (process , additives present) and processing conditions (e.g., potential ! i ; for degradation of solvent). They may be crystalline salts - !  they may be gelatinous materials. Their precise nature and their potential for causing probems should be determined for each waste evaporated. They may simply coat heat transfer ? surfaces and thus reduce evaporator capacity. They have been known to completely fill the spaces between tubes in a vertical - heat transfer tube bundle. In commercial power reactor fuel wastes, the solids may have a high radioactive content, F hence, a high decay heat output. They could create hot spots equipment if allowed to accumulate. C The volume and acidity of the higli-level waste tollowing * nitric acid recovery will; of course, depend on the waste L *5. composition and the operating conditions for any given installation. In one il~ude of operating the Hanford Purex plant system, a volume reduction of about 8 is achieved (to Ca. -. Lr C. 7702/MT fuel) and the residual HN03 concentration is 2.8M. -d is still a relatively large volume and be an I j undesirably high acidity for acid side storage. Further . . concentration and further reduction in acidity is desirable. One used technique for reducing the HN03 is simply to destroy it by adding a reducing agent to produce volatile oxides 3 of nitrogen. ' Formaldehyde (9) and sugar ' are the two reagents most studied and used for this purpose. Both are effective. 'Formaldehyde has an induction period before reaction starts and process conditions must be controlled to avoid a runaway reaction. The sugar reaction is slower and requires no special controls. Oxalate is among the decomposition products of sugar. Some . W fission product and actinide element oxalates are sparingly soluble .. . in the waste and may add to the amount of solids present. While I either reagent can be used to produce an acid deficient waste, the terminal acidity normally sought - 8 ;. Further reduction in volume might be done in an'evaporator ' ! $or by allowing the wastes to self-concentrate by boiling in a I I! j .  :;storage tank. The latter has been done at Hanford after the J 'waste was made alkaline and stored in large (500,000 or 1,000,000 gal) mild steel underground tanks. Problems involved in allowing stored wastes to boil will be discussed under Waste Storage. The problems involved in concentrating the denitrated waste in aG.c" an evaporator are essentially the same as those discussedfiunder - jDsVE nitric acid recovery. The degree to which the high-level waste is concentrated I L prior to or during interim storage will depend on the fuel being processed, the particular flowsheet being used, the interim storage period involved and the design of the interim storage tank. Table IV shows the concentration of fission products and r:- actinide elements which would be present in waste from processing fully irradiated Diablo Canyon Reactor fuel and .concentrating the . . .. .. .. . . . .  . waste to 3782 (100 gal) per tonne of fuel processed. TABLE IV. COMPOSITION OF DIABLO CANYON FUEL WASTE AT 378 R/MT Constituent S r Y Zr Mo Tc Ru Rh Pd Te Cs Ba La Ce Pr Nd Sm u NP Other Fission Products and Actinides <1 The concentrations given in Table I11 are based on the assumption that all constituents are in solution. in fact s ' this will not be the case. High-level waste concentrated to this degree can be expected to contain a significant amount of solids and these solids can be expected to contain a significant fraction of the radioactive constituents present. This is an important consideration in storage of the waste. Note also that the concentrated waste has a radioactivity level of about 14,000 Ci/e which results in a decay heat evolution rate of about 50 W/e (180 BTU/hr - 2). The latter is also of great significance to interim storage waste. Interim High-Level Waste Storage Problem involved in interim storage of high-level waste are due to the decay heat evolved by the waste, the solids , .. present in the waste, the corrosiveness of the waste (particularly . . if stored acidic), radiolysis'effects due to its high radioactivity * and, to a much less extent, to the potential for volatilization of constituents from it. Concentrated and stored high-level radioactive wastes will : boil due to the radioactive decay heat generated in them. This ! i j gives rise to several problems which must be accounted for in I the design and operation of the waste storage system. For one thing, the wastes are more corrosive in both the liquid and the vapor phase if allowed to boil than if maintained below the boiling point. If the management decision is to cool the wastes I to prevent boiling and to reduce the corrosiveness, an adequate cooling system must be incorporated. Backup provisions to maintain cooling are needed in case the primary system fails. The stored waste may be allowed to boil either to provide some in-storage concentration or simply as a means of removing the decay heat. In either case, if the boiling waste is acidic, provisionk must be made for condensing the offgas and either . . returning the condensate to the storage tank or disposing of it. Tritium, if present in the waste, will be present in the condensate. Solids present in the stored wastes have a high decay heat k generation rate. 1f these solids are allowed to settle, they 'may compact, form a barrier to heat transfer by convection, and .6ecome dry. The temperature of the solids could then rise to levels much above the boiling point of the liquid waste, perhaps high enough to threaten the integrity of the tank liner. Means such as air sparging or air lift circulators may be used to keep the solids suspended and prevent the formation of deposited solids which could produce hot spots. A phenomenon called "bumping" was observed in stored - alkaline wastes at Hanford. These wastes were stored in large underground tanks which had no provision for keeping solids suspended. Solids settled to a layer on the bottom of the tanks. Since the solids generated decay heat more rapidly than the liquid, the liquid develqped a temperature gradient. As the buck liquid approached the boiling point, liquid near the solids *. was superheated. Once boiling started, liquid convection was rapid and the superheat was released rapidly as steam. A momentary pressurization, amounting in cases to a few tens of inches of water, occurred. Installation of air lift circulators to keep the liquid agitated and the solids suspended stopped the I bumping action. Radiolysis in the stored waste will generate hydrogen. If the storage tank does not have an adequate flow of air (or other diluting gas) through it, the potential for developing an explosive offgas could exist. Air supplied or air . lift circulators, or simply air inleakage, is adequate to provide the needed dilution. Stored high-level wastes have the capability of evaporating themselves to dryness if all means of maintaining the liquid phase should fail. The dried wastes could then become quite hot. Calcination and perhaps melting could occur. Volatilization of radioactive constituents could occur and the tank integrity could be threatened. Obviously, tank design, operation and backup facilities must be adequate to ensure the tank contents cannot concentrate to dryness. Summary This overview has cited recognized problems in the concentration and storage of high-level radioactive waste. The discussion was intentionally kept general since each fuel reprocessing site will have its own spectrum of fuels to process, its own reprocessing flowsheet and its own sequence of waste concentration and storage. The Hanford Purex plant and associated waste storage system was used as an example since -. the waste concentration and storage operations are 'similar to those in commercial plants except that the waste is made alkaline before - .. storage. Designs and procedures have been developed to cope effectively . ! 8 .  with these problems and to make the operations safe and reliable. 1. J. A. Blomcke, W. C. McKee and J, P. ~ichols, ~rojections of Radioactive Wastes to be Generated by the U. S, Nuclear Power Industry, USAEC Report, ORNL-TM-3695, February 1974. 2. Separations' Process Engineering Staff, Purex Chemical Flowsheet for Processing N-Reactor Fuels, USAEC Report ARH-2362, June 1972: 4 3. R. C. Forsman, G, C. Oberg, Formaldehyde Treatment of Purex Radioactive Wastes, USAEC Report, HW-79622, October 1963. 4. L. A. Bray, Denitration of Purex Waste With Sugar, USAEC .Report HW-76973 Rev., April 1973. 5. R. E. Tomlinson, B. F. Campbell, E. Doud, "Management of High-Level Wastes - Current Practice," HW-SA-2478, February 1962. 

High-level waste concentration and storage: an overview

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High-level waste concentration and storage: an overview

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