30 Separation of Actinides from Purex-Type High Active Waste Raffinates D e v e l o p m e n t o f E x p e r i m e n t a l Studies a t J R C - I S P R A Establishment
L. CECILLE, H. DWORSCHAK, F. GIRARDI, B. A. HUNT, F. MANNONE, and F. MOUSTY JRC-ISPRA Establishment, 21020 ISPRA (Varese), Italy The reduction of the potential long term hazard of radioactive wastes generated by the nuclear fuel cycle has been the objective of European and USA
R&D program-
mes. Several conceptual and experimental studies on the feasibility of the actinide separation from radioactive wastes (waste partitioning) have been developed at various national (USA, Sweden) laboratories (1—4). Experimental investigations in this field were also carried out until 1977 in France, at the CEA laboratories (5). Various separation techniques currently applied for actinide recovery processes ( precipitation,
solvent extraction and ion exchange) were regarded as potentially pro-
mising for partitioning purposes. The aim of the waste partitioning is however quite different from that of existing actinide recovery processes, which mainly serve to purify actinides from fission products (FP). Consequently,
existing actinide recovery
processes need to be suitably adapted in order to meet the new requirements of waste partitioning. If the separation of actinides from HAW is demonstrated to be feasible, a number of alternative waste management options could open up, due to large differences in the radiation properties of the two waste fractions.
HAW Partitioning Studies Partitioning studies at the Joint Research Centre - Ispra Establishment of the Commission of the European Communities were oriented towards highly radioactive wastes generated by the 1st TBP extraction cycle of the Purex process (HAW raffinâtes) and towards actinide separation techniques operating under low acidity conditions such as solvent extraction by TBP or HDEHP and oxalate precipitation (6-11). The HAW partitioning was deemed to be thefirststage of an advanced waste management strategy based on the continuous recycling of separate actinide wastes within the fuel cycle. Other alternative waste management options based on separate storage and disposal of the two waste fractions, will be studied at a later stage. It was, in fact, considered that, while a decision on waste partitioning or not must be reached within a few years, a final decision on a separated actinide management can be deferred for much longer times. 0-8412-0527-2/80/47-117-427$05.00/0 © 1980 American Chemical Society
428
ACTINIDE SEPARATIONS
The main objectives of HAW partitioning studies are: (1 ) to demonstrate on a laboratory scale the technical feasibility of the actinide separation from HAW raffinâtes. Laboratory experiments at tracer and full activity level are performed in order to obtain fundamental data and other information needed to develop on a laboratory scale complete HAW partitioning flow-sheets; (2) to evaluate the engineering implications of the partitioning flow-sheets developed on laboratory scale. The engineering assessment is performed assuming that only established technologies and equipment will be utilized; (3) to prepare reference conceptual designs for preliminary cost evaluations.
Experimental To integrate the HAW partitioning into a waste management scheme including concentration, intermediate storage and conditioning steps, two options can be envisaged. The HAW may in fact be partitioned: (1 ) either immediately after being generated by the 1st Purex extraction cycle (DIRECT partitioning); (2) or after a suitable period of intermediate storage in concentrated form (DELAYED partitioning). Experimental studies were therefore directed to investigate the removal of actinides from both diluted (5000 l/t) and concentrated (about 500 l/t) HAW solutions. Three alternative processes have been selected for this purpose. They all rely upon actinide separation at low acidity conditions requiring a preliminary denitration step. Two of them (TBP and HDEHP processes) are based on solvent extraction techniques using as extractants a neutral (TBP) and an acidic (HDEHP) organophosphorus compound respectively. The third process (OXAL) applies as the first step the precipitation of actinides and lanthanides FP as oxalates. It is assumed that: (1 ) the HAW raffinate to be partitioned is generated by reprocessing of LWR spent fuels irradiated at 33,000 MWd/t, and cooled for 150 days before reprocessing; (2) 99.5% of U and Pu originally present in the spent fuel is removed by the Purex reprocessing operations; (3) the separation of Np is attained either by co-extraction with U and Pu during the Purex process or, without affecting conventional reprocessing operations, during an additional extraction cycle of acidic HAW aimed at removing residual Pu before the acidity reduction step. Even though the degree of the actinide removal still needs to be more precisely defined, actinide decontamination factors (DF) of 10 for Pu, Am and Cm are taken as target values to be attained (12) by a specific partitioning process on HAW raffinâtes. A DF of about 10 for Np appears to be sufficient; higher values would indeed yield only an insignificant advantage (12). 3
The TBP extraction process. The extraction of trivalent actinides by TBP may succeed, as well known, at low acidity conditions, provided a sufficient nitrate salt content is reached in the feed solution (13,14). However, the large amount of salting agents to be added to the HAW (tons of Na and Al nitrate per ton of spent
30. C E C i L L E E T A L .
Separation of Actinides at
JRC-ISPRA
429
fuel) would lead to an unacceptable volume increase of the vitrified waste. On the contrary, if about ten-fold concentrated HAW is processed, the amounts of Na and Al nitrate to be added per ton of spent fuel will remain comparable with the proportions of the additives required for the glass making process. This means on the other side that the TBP process can be applied only for a D E L A Y E D partitioning, i.e. after an adequate additional cooling period of the HAW. After 5-years cooling, the activity level will decrease by a factor of 10 approximately with respect to that at 150days cooling. A still higher benefit may, moreover, come from the virtually total decay of Z r a n d Nb, the most detrimental isotopes to solvent integrity. 9 5
9 5
The concentration of HAW raffinâtes and the addition of suitable amounts of nitrate salts are therefore the key steps on which the applicability of the TBP process for partitioning purposes is based. The main steps of this process are illustrated in Fig. 1. The HAW is assumed to be concentrated at the reprocessing plant to about 500 l/ton and will have at the end of the 5-years storage period a nitric acid concentration of about 4 M. Concentrated and stored HAW solutions are expected to contain a considerable amount of precipitates that may have been formed freshly during the foregoing steps and may include also solids from reprocessing operations. After separation, these precipitates are expected to need further treatment for actinide removal. However, laboratory results obtained with simulated HAW solutions showed that maintaining the acidity above 4 M during the HAW concentration, the fraction of Pu adsorbed on the precipitates may be significantly reduced. Therefore, provided the acidity may be kept at the same level also during the storage period, the treatment of these precipitates could eventually be limited to a simple washing. Two options are proposed for this process. In the case op option 1, after removal of precipitates (clarification), an additional extraction cycle of acidic HAW is introduced before the denitration step. The purpose of this cycle is to get acidic HAW virtually free from extractable Pu (and also from Np and U) so that, during the successive HAW denitration, the formation of Pu-bearing precipitates should be substantially minimized. A conventional Purex type co-decontamination cycle is proposed for the selective removal of Pu, U (and Np, if the latter has not been previously co-extracted during the Purex reprocessing operations). According to option 2, the denitration of concentrated HAW is carried out without any previous Pu extraction. During denitration, as the acidity of the HAW solution decreases, hydrolysis and polymerisation of extractable Pu is expected, together with precipitation of hydrolysable FP still remaining in the solution. It appears, however, at least from results obtained on simulated HAW solutions that by carrying out the denitration under suitably selected process conditions, it should be possible to reduce the retention of Pu on the precipitate to less than 1% of its initial amount. A detailed study on the denitration process and reaction mechanisms involved is presently under way. The TBP countercurrent extraction step will provide for the separation of Am, Cm and Lanthanides from other FP. From the latter, the most part of extractable Mo and Zr could have been already eliminated in the foregoing steps (Pu, Np, U extraction and precipitation at low acidity) whereas the Ru remaining from the foregoing precipitation (about 50%) should distribute nearly equally between the organic and aqueous phases.
EXTR.
HAW VITRIFICATION
ALPHA-SOLID T R E A T M E N T
EXHAUSTIVE (U, Pu, Np)
INTERIM
EXTRACTION
A C T J N IDE W A S T E
MANAGEMENT
RE/ACT. SEPARATION ( T A L S P E K Process)
RE + A C T . (DTPA)
-|RE + A C T I N I D E
CLARIFICATION
DENITRATION
O.1 -O.2MHNO3
•
STORAGE
HNO3)
CONCENTRATION (10-FOLD, 4M
33
TBP
3
ΑΚΝ0) NaN0
HCOOH
Figure 1. Process scheme for the delayed HAW partitioning based on the actinide extraction by TBP (TBP Process)
FROM PUREX PROCESS
TBP
y
CLARIFICATION
H A W 5000 l/t -
I
!
>
ο
CO
30.
CECiLLE E T AL.
Separation of Actinides at
JRC-ISPRA
431
Actinides and RE will be back-extracted from TBP by a complexing O.1 M pentasodium diethylenetriaminepentaacetate (Na DTPA) solution in 1 M glycolic acid at ρ H = 3. The aqueous strip solution will form directly the feed solution of the Am, Cm/RE countercurrent separation step, to be performed according to the operating conditions of the T A L S P E A K process (15). The presence of a complexing agent like DTPA will prevent the extraction of Am and Cm whereas the RE will be more easily extracted by di(2-ethylhexyl)phosphoric acid (HDEHP) in n-dodecane (pH = 3). By a separate scrubbing of the loaded solvent with a fresh aqueous DTPA solu tion, the Am-Cm fraction lost to the organic phase should be completely removed. The two DTPA aqueous phases will constitute the starting solution for final actinide conditioning processes. 5
The HDEHP extraction process. The proposed HDEHP flow-sheet is illus trated in Fig. 2. In this flow-sheet the only extractant used through the whole pro cess is HDEHP. It is a well known acidic organophosphorus compound with extrac tion capacities for metal-ions highly dependent on the hydrogen-ion concentration. This extractant has not been used in fuel reprocessing plants but in many actinides separation processes such as DAPEX (16), PUBEX and C L E A N E X (17). Its pro perties (extractive capacity, radiation stability, etc..) are therefore reasonably well known (17,18). The use of this extractant has created some problems deriving from the reduced loading capacity of the solvent as well as from its strong complexing properties, causing difficulties for the back-extraction of some elements (Zr, Mo) and for its clean-up. The solutions so far experimented to solve these problems, namely: 1 ) the use of large volumes of solvent to balance its reduced loading capacity, 2) the use of suitable chemicals and operating conditions in order to improve the back-extraction efficiency, 3) the addition of TBP to the HDEHP/n-dodecane mixture in order to avoid the third phase formation during the alkaline washing, appear to be effective from the chemical and hydraulic points of view but unsatisfac tory from that of the overall waste balance, due to the generation of non-recyclable waste streams in which the absence of alpha-contaminants cannot be guaranteed. Also for the HDEHP process, two options are proposed that are quite similar to those already proposed for the TBP flow-sheet, i.e. an additional U, Pu and Np ex traction cycle to be performed before denitration (option 1) or alternatively a deni tration step without any previous HAW treatment (option 2). The operating condi tions of the denitration process are similar to those indicated for the TBP flow-sheet. Results obtained using simulated HAW solution showed also in this case the possibi lity of minimizing the Pu adsorption on precipitates, if suitable process conditions are applied in carrying out the denitration step (HCOOH/HN0 =2; pH = 2). After the denitration, the process steps sequence of option 2 is similar to the "Reverse Talspeak Process" already developed on a cold laboratory scale at the KfK laboratories of Karlsruhe (19) for the recovery of actinides from HAW. It involves the coextraction of actinides with RE by HDEHP/TBP in n-dodecane at about pH2, the selective stripping of actinides by a Na DTPA-glycolic acid solution and the stripping of RE by 4 Μ H N 0 . In the case of option 2, the stripping of co-extracted 3
5
3
432
ACTINIDE
Θ
CLARIFICATION
EXHAUSTIVE (U, Pu, Np)
SEPARATIONS
- H A W (5000 l/t)
EXTR.
HCOOH
DENITRATION pH = 2
ALPHA-SOLID TREATMENT
CLARIFICATION
RE + ACTIN. E X T R A C T I O N (Reverse T A L S P E A K )
FROM PUREX PROCESS
Am, Cm (DTPA)
Am, Cm STRIPPING ^
HDEHP
RE STRIPPING
Pu, N p S T R I P P I N G
HAW VITRIFICATION
ACTINIDE WASTE MANAGEMENT
TO PUREX PROCESS
Figure 2. Process scheme for the direct HAW partitioning based on the actinide extraction by HDEHP (HDEHP Process) H A W (5000 l/t)
i DENITRATION pH ~
+ PRECIPITATION O.2
HCOOH ~ ~ H C 0
"
2
2
4
CLARIFICATION
PPT ( R E + A C T . )
OXALATE
DISSOLUTION
|^ H N o
3
Pu, N p S E P A R A T I O N ( E X T R . or P A R C H R O M . )
NaOH (pH ~ 2) RE/ACT. SEPARATION ( P A R C H R O M or T A L S P E A K
ACT.
HAW VITRIFICATION
ACTINIDE WASTE MANAGEMENT
Figure 3. Process scheme for the direct HAW partitioning based on actinide oxalate precipitation (OXAh Process)
30.
CECILLE
ET AL.
Separation of Actinides at
Pu and Np is carried out by a O.8 M H C 0 2
2
4
433
JRC-ISPRA
solution.
The O X A L process. The flow-sheet of the Oxal process is shown in Fig. 3. The denitration is carried out by slow addition of the waste solution to the boiling mixture of formic and oxalic acid. The presence of the oxalic acid during the denitration prevents the polymerisation and precipitation of hydrolysable ions such as Zr and Mo ions, and assures the precipitation of the RE and actinide oxalates from homogeneous solutions in a we 11-crystallized form. After clarification, the supernatant is sent to vitrification and the oxalates are dissolved and destroyed by nitric acid so that a final solution (3M H N 0 ) is obtained. The final separation of Am and Cm from RE can be done by applying the solvent extraction techniques developed in the HDEHP and TBP flow-sheets. As an alternative, partitioning techniques based on column extraction chromatography (LEVEXTREL-TBP and LEVEXTREL-HDEHP) are being experimentally studied on fully active HAW solutions. A final decision on the techniques for the actinide/RE separation will be reached at a later stage of the programme. 3
Results and Discussion A synthetic waste solution simulating chemically a 150-day decayed HAW raffinate (Table I) was used to carry out tracer laboratory experiments on TBP, HDEHP and O X A L processes. Fully active laboratory scale experiments were started using firstly a Windscale HAW solution (5000 l/t) generated by the reprocessing of Magnox fuel elements with a burn-up value of 3500 MWd/t. The overall decay time was about 10 months and as the composition was not known, only relative activity measurements were performed. Other fully active HAW solutions were subsequently prepared in Ispra hot cells by dissolving U 0 samples irradiated at 26 - 36,000 MWd/t and cooled for about 4 years. Successive TBP batch-extraction steps were carried out under the 1st extraction cycle conditions of the Purex process to remove the bulk of U and Pu. 2
Denitration experiments. Results reported in Table II are relating to the denitration tests performed on the unconcentrated HAW solutions (5000 l/t) as proposed in the HDEHP process scheme (option 2). By operating the denitration of simulated HAW with a HCOOH/HN0 molar ratio of about 2, it seems possible to maintain almost quantitatively the Pu in the extractable form. The following mechanism is tentatively proposed to explain the Pu behaviour during denitration: the lower retention of Pu into denitration precipitates at pH2 is probably due to the reduction of Pu (IV) to Pu (III), even if Pu is initially present under polymeric form. This reduction is probably promoted by the formic acid decomposition in the presence of the Pd and Rh black acting as catalysts (20,21). Preliminary results of fully active runs (Table II) are quite comparable although some Pu was lost to the precipitate. The higher Pu fraction adsorbed on the precipitate separated in the active run nr. 3 was probably due to the incomplete dissolution of Pu formate by the 4M H N 0 wash. 3
3
434
ACTINIDE
TABLE I :
SEPARATIONS
Composition of simulated HAW solution calculated from ref. 22 & 23 assuming a value of 5000 l/ton U, (burn-up = 33,000 MWD/ton; initial U enrichment = 3.3%; decay time = 150 days) 2 3 5
Element
g/i
Ge(IV) As(lll) Se(IV) Br(V) Rb(l) Sr(ll) Y(IM) Zr(IV) Mo(VI) RuNO(lll) Rh(lll) Pd(ll) Ag(l) Cd(ll) In(lll) Sn(IV) Sb(lll) Te(IV) KV) Cs(l) Ba(l I)
7.10 1.75-10 O.01 3-10" O.066 O.18 O.094 O.73 O.69 O.45 O.08 O.26 1.2-10" O.017 2.4- 10" O.011 3.5- 10" O.113 O.054 O.544 O.278 s
s
3
2
4
Element
g/i
La(lll) Ce(lll) Pr(lll) Nd(lll) Pm(lll) Sm(lll) Eu(lll) Gd(lll)
O.254 O.576 O.24 O.78 O.021 O.16 O.036 O.021
Al(lll) Cr(lll) Ni(ll) Cu(ll) Zn(ll)
O.0021 O.096 O.047 O.02 O.024
Na(l) Fe(lll)
1.61 1.88
U(VI) Np Pu Am Cm
3
>
O.95 15.2-10" 9.08-10" 30.6-10 7.06-10"
3
3
-3
3
T A B L E II : Behaviour of Pu during denitration of unconcentrated HAW solutions by formic acid Run nr.
1
2
3
4
HAW solution (5000 l/t)
simulated
simulated
fully active
fully active
ionic
polymeric*
probably ionic
probably ionic
HCOOH/HNO3 molar ratio
2
2
-2
2.2
% Pu leftinthe supernatant solution
86
61
70
56
% Pu recovered by formic and nitric washings
14
39
28
43.4
% Pu leftinthe precipitate
99.9 80 92 99.5
4M
HN0
3
O.3 M HDEHP O.2 M TBP pH = 2.11 in n-dodecane
after a single extraction stage cumulative, after 4 extraction stages
3
4
>99 >98 -O.5 >98 >99 >98 >99
% Extr. yield 1
1
>99
4
4M 2
HNO3
3
>99.5 3
pH = 2.76
~ 1.0 >99.5 >99.5 >99.5 >99.9
not measured solvent: O.3M HDEHP - O.2M TBP in n-dodecane
Pu, Am and Cm separation yields appear to be sufficiently high considering that the indicated values correspond to a single extraction stage. According to the preliminary results of a single back-extraction test performed on fully active laboratory scale, after three successive back-extraction stages, the overall Pu stripped from the loaded HDEHP/TBP solvent originated by the exhaustive extraction step (option 1 ) was nearly quantitative. As far as the back-extraction of loaded HDEHP/TBP solvent is concerned (option 2), only simulated HAW solutions have been so far tested. Results reported elsewhere (10) indicated that Pu, Mo, Zr and U may be almost quantitatively removed from the loaded HDEHP/TBP solvent by an oxalate solution at pH 2.5 and confirmed that this organic mixture, employed according to the "Reverse Talspeak
I
436
ACTINIDE SEPARATIONS
Process", is able to attain a well satisfactory separation of trivalent actinides from RE. Hot cell extraction tests are in progress. Also in the case of the TBP process, only simulated HAW solutions have been until now used to study the process feasibility. Results have already been reported (9), which indicated that separation yields of about 99.9% can be obtained for Am after three successive extraction stages operating with a concentrated HAW solution adjustedto O.1 — O.2 Μ H N 0 , O.65 M A I ( N 0 ) and 1.3 M N a N 0 . The back-extraction of all the actinides and RE from loaded TBP (30% in n-dodecane) was carried out by means of Na DTPA-glycolic acid solution which can be then used directly to separate Am and Cm from RE according to the Talspeak process 3
3
3
3
5
(15). Oxalate precipitation experiments. Simulated and fully active HAW solutions have been utilized to carry out experimental tests on the separation of actinides by oxalates precipitation and on the actinide/RE separation steps. The Oxal process was initially tested by carrying out separately the HAW de nitration and the oxalate precipitation. Results obtained from simulated and Windscale HAW solutions are in good agreement. The best DF for Am and Cm (~ 2 χ 10 ) were obtained, however, on the Windscale solution, operating at about pH 2. To prevent during the denitration step the formation of precipitates on which Pu and Am were partially and irreversibly adsorbed, denitration and oxalate precipi tation were carried out in a single step by addition of the waste solution to the for mic and oxalic acid mixture, the latter acid acting as a metal complexant during the denitration step. By experimental tests performed on simulated HAW according to this modified process scheme, separation yields of about 99.5% for Pu and 99.8% for Am were measured. A further reduction of the actinide content was reached by flowing the clarified HAW solution through a Dowex 50 resin column. The oxalate precipitation experiments on fully active HAW solutions have practically been com pleted. The results obtained from five runs (Table IV) confirmed the previous results obtained on simulated solutions. 3
T A B L E IV :
Percent distribution of actinides and RE measured after denitration-oxalate precipitation tests on fully active HAW solutions
"^£lejrients Streams - ^ ^ ^
% Distribution Am, Cm
Pu
RE
100
100
100
Supernatant after denitration + O.1 - O . 8 oxalate precipitation
