Modeling of an Optimized Multilayer Cover Design for a Uranium Mill Tailings Disposal

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1 ABSTRACT Modeling of n Optimized Multilyer Cover Design for Urnium Mill Tilings Disposl Mri de Lurdes Dinis, António Fiúz Porto University, Engineering Fculty (U.P. FEUP) Geo-Environment nd Resources Reserch Center (CIGAR), Ru Dr. Roberto Fris, Porto, Portugl, mldinis@fe.up.pt; fiuz@fe.up.pt This pper presents computer bsed lgorithm to evlute the performnce of n engineered cover design for urnium tilings disposl. The developed reserch ws pplied to specific cse study, the Urgeiriç urnium mining site locted in the centrl north region of Portugl. A multi-lyer cover design ws proposed in the remedition progrm for this site to be plced bove the tilings disposl. The tilings composition is highly heterogeneous, in prticulr for rdium concentrtion which vries between 3,000 nd 67,000 Bq/kg. A vlue of 7.1 Bq.m -.s -1 ws clculted for rdon exhltion through the tilings, considering n verge rdium concentrtion of 1,900 Bq/kg. The cover ws designed to mitigte the releses of rdon gs to the surfce nd to minimize wter infiltrtion to the mill tilings. An lgorithm ws developed to simulte the rdon flux ttenution given by n rbitrry cover providing s n output the rdon flux exhltion through the cover system. In ddition, it is lso possible to estimte the thickness of the cover tht llows rdon flux inferior to the cceptble one, to ccomplish legl limits. The lgorithm is bsed on the diffusion theory describing the long-term rdon trnsport through the tilings nd the cover s mteril nd it ws implemented in Mtlb. The lgorithm ws simulted for the multi-lyer cover design proposed in the remedition progrm, for n optimized multilyer cover design, nd nother simultion ws done to ccomplish defined threshold. The cover proposed in the remedition progrm, with 5.15 meters thickness, eliminted lmost ll the rdon exhltion (99.8%). The optimized multilyer cover with 3 or meters could chieve similr results: the rdon exhltion decreses, respectively 99.55% or 96.39%. To ccomplish the legl limits cover system with 1 meter would be enough to reduce the verge rdon exhltion to the defined threshold. INTRODUCTION Usully the environmentl remedition of urnium mining site includes, long with mny other ctivities, the design nd implementtion of n engineered cover to contin rdioctive nd nonrdioctive mterils from the tilings disposl. The cover system must lso provide long-term stbiliztion nd control of the tilings in order to minimize or eliminte rdition helth hzrds to the public. In prctice, the mjor concern is to reduce rdon exhltion to ner-bckground levels lso preventing geochemicl rections to occur within the rdionuclides. This pper presents computer bsed lgorithm to evlute the long term efficiency of n engineered cover design for the remedition of urnium tilings disposl. The developed reserch ws pplied to 1

2 specific cse study, the Urgeiriç urnium mining site locted in the centrl north region of Portugl. Approximtely 13 h of urnium mill tilings with n estimted volume of m 3, were left s prt of the legcy of the urnium mining nd milling in this region. Also, two other contmintion focuses were prt of the legcy: deposit of wste rock contining some low-grde ore nd few tons of highgrde urnium ore tht were not milled [1]. The remedition project for this site included trnsferring the deposits of ore nd wste rock into the tilings pile. It lso included the geotechnicl stbiliztion of the tilings re, confined in-situ by concrete support structure with deep dringe system, nd seled off with multi-lyer cover composed by geologicl nd synthetic mterils over the surfce of the tiling deposit [1]. The cover ws designed to mitigte the releses of rdon gs to the surfce nd minimize wter infiltrtion to the mill tilings. The cover lso hd the purpose of stopping the infiltrtion of gseous oxygen preventing the formtion of cid mine dringe. These were the relevnt impcts needed to be mitigted. SITE DESCRIPTION The Urgeiriç mine is locted ner Cns de Senhorim, Nels in centrl Portugl (Figure1). The Urgeiriç mine ws ctive from 1913 to 1991 nd until 1944 the ore ws mined for rdium extrction. After World Wr, in order to fce the interntionl demnd for urnium, the purpose of this mine chnged recovering only urnium s rdioctive substnce [1]. The Urgeiriç ore processing plnt ws built in 1951 nd it ws used to process not only the ore from this mine but lso from 6 others urnium mines exploited in Portugl t the time. The solid wstes produced in this fcility were disposed in three min loctions: tilings pile with 1.46 million m³ on 13.3 h, known s Old Dm ; deposit of wste rock contining some low-grde ore, extrcted from the Urgeiriç mine, with bout 91 thousnd m³ on 1.5 h nd iii) few tons of unprocessed high-grde urnium ore deposited ner the plnt (45 thousnd m³). The Old Dm tilings composition is highly heterogeneous due to former ore tretment nd disposl prctices. In prticulr, rdium content vries between 3,000 nd 67,000 Bq/kg. Site studies nd pst investigtions [1] showed the presence of cid wter resulting from groundwter percolting into the interior of the old mine nd the nerby mill tilings. This wter is minly sulfuric cid diluted nd is lible to trnsfer significnt mounts of dissolved metls nd rdionuclides into the environment (cid mine dringe) in prticulr into the "Ribeir d Pntnh" strem, tributry to the Mondego river.

3 Fig. 1: Urgeiriç site loction (ntionl, regionl nd locl), extrcted from []. METHODOLOGY Conceptul Approch Rdon emission occurs due to the rdioctive decy of elements present within the tilings; in prticulr Rdium-6 (R-6) nd Thorium-30 (Th-30). Rdon- (Rn-) genertion continues for mny yers due to the reltively long hlf-lives of these two rdionuclides, both present in the urnium mill tilings. Rn- emntes from the rdium present in bre tilings into the soil pore spce followed by diffuse nd dvective trnsport in both liquid nd gs phses into the tmosphere. Rn- ttenution to ner bckground levels cn be chieved by clculting the thickness of the cover mterils tht refrins the difusionl trnsport of the rdon so tht it decys into solid dughter before it reches the tmospheric surfce, becoming trpped in the cover mterils [4]. A multi-lyer cover design ws proposed in the remedition progrm for the Urgeiriç site to be plced bove the Old Dm urnium tilings. The tilings overlie uncontminted bedrock of ltered grnite nd re composed by crushed fine grined mterils of metssedimentry grnitic nd some bslt composition. The surfce is highly rdioctive; n verge vlue of.39 μgy/h ws mesured for externl gmm rdition bove the tilings [3]. A computer bsed lgorithm ws developed to evlute the performnce of the engineered cover design proposed. The efficiency ws investigted minly in wht concerns to Rn- exhltion to the tmosphere. The conceptul model dopted includes contminnt source nd relese informtion, description of trnsport mechnisms nd pthwys. The model ws simulted for the multilyer cover proposed in the remedition progrm, for n lterntive optimized multilyer cover, nd nother simultion ws done to ccomplish defined threshold. 3

4 Mthemticl Approch The developed lgorithm is bsed on the diffusion eqution pplied to the Rn- diffusion in the ir spce nd in the wter spce of multiphse system s proposed by Rogers nd Nielsen [4], [5]. Rn- diffusion through two-phse medium in the pore spces of erthen mterils cn be chrcterized by the following diffusion equtions [4], [5]: D C x C RE Tw 0 (Eq. 1) D w C x w C w RE w Tw 0 (Eq. ) The necessry prmeters for solving these equtions re: Rn- diffusion coefficient D (m.s -1 ), Rn- decy constnt λ (s -1 ), rdium concentrtion in the pores spce C (Bq.m -3 ), rdium concentrtion in the tilings R (Bq.kg -1 ), bulk density of the dry mteril ρ (kg.m -3 ), Rn- emntion coefficient E (dimensionless), totl porosity (dimensionless), moisture (dimensionless) nd Rn- trnsfer rte from wter to ir T w (Bq.m -3.s -1 ). The subscript refers to ir-filled pore spce nd w refers to wter filled pore spce. Rn- concentrtion in ir filled pore spce (C ) nd wter filled pore spce (C w ) re relted by this expression [4], [5]: C ( ) C C w (Eq. 3) nd combining equtions 1, nd 3 yields: C RE C 0 x D (Eq. 4) where D is the Rn- diffusion coefficient in the totl pore spce nd E is the totl emntion coefficient (E + E w ) of the mteril. The generic solution of the diffusion eqution (Eq. 4) gives the Rn- flux relesed considering either no cover system, J t, either the Rn- flux through designed cover system, J ci [6]. The following eqution ws used to clculte the Rn- flux, J t (Bq.m -.s -1 ) from bre tilings with thickness of x t (m) [4], [5]: J t R E Dt tgh x t (Eq. 5) Dt For thick tilings sources inferior to m, the hyperbolic tngent term (tgh) is equl to unity nd my be ignored [4], [5]. The surfce Rn- flux exhltion through homogeneous cover mteril (J c, Bq m - s -1 ) plced over the tilings pile, ws clculted with the following eqution [4], [5]: bcxc te J J c (x c ) (Eq. 6) t bcxc t 1 tgh b t x t 1 tghb t x t e c c where b i, i nd m my be clculted with equtions 7, 8 nd 9, respectively [4], [5]: 4

5 i 1 i c ou t, m bi (Eq. 7) D i i i km, m s D (Eq. 8) m 10 M (Eq. 9) nd x c (m) is the cover thickness; M (dry weight percent) is the moisture content; m (dimensionless) is the frction of sturtion nd k represents 0,6 Bq/m 3 in wter per Bq/m 3 in ir. The frction of sturtion cn be estimted by the empiricl correltion proposed by Rogers nd Nielsen [4]: mm m D 7 x10 e (Eq. 10) t The cover thickness required to ccomplish Rn- flux inferior to specified vlue, cn be deduced from eqution 6, resulting in [4], [5]: Dc J t / J c x c ln (Eq. 11) t 1 tgh b t t x t 1 tgh b t x t Jc c c Jt To clculte the Rn- flux for multilyer cover system it ws used sequentil ppliction of eqution 6 nd eqution 11 to ech lyer. The process trets ech lyer s single cover system with n updted source considering the tilings nd ll the cover lyers beneth the lyer in question [4], [5]. To perform these clcultions, n lgorithm ws developed nd implemented in Mtlb code. It simultes the Rn- flux exhltion from the tilings nd from the cover, using the mthemticl formultion given by equtions 5 nd 6 nd then optimizes the cover thickness (Eq. 11) to stisfy given flux constrint. The code simultes not only Rn- flux but lso the Rn- concentrtion t 1 m bove the ground nd the dispersion with the wind in severl directions nd in the dominnt wind direction [7], [8]. Cover Systems Studied In wht concerns the geotechnicl stbiliztion, three different cover systems were studied to quntify the Rn- exhltion: i) multilyer cover proposed in the remedition progrm for Urgeiriç site composed by six different lyers, ii) n optimized multilyer cover considering the regionl nturl bckground levels, nd iii) multilyer cover designed to ccomplish defined threshold. The multilyer cover system proposed in the remedition pln to be plced over the surfce of the tilings, from bottom to top, ws composed by [] [3]: A compcted lyer of geologicl mterils from other mine tilings (-5 m); A cly lyer (0.60 m); An HDPE liner in conjunction with geo-textile membrne ( mm); A lyer of well sorted grvel (0.30 m); A fine snd lyer (0.5 m); And lyer of composite soil (0.5 m). The re nd the thickness of the tilings were estimted in 133,000 m nd 14 m, respectively [3]. Different 6 R concentrtions were considered bsed on smples collected in drill-holes perforted in the 5

6 tilings [9]. The totl re ws individulized into seven smller res ccording to the rdium concentrtion. These res nd the respective R-6 concentrtion re presented in Tble I. Tble I: R-6 Concentrtion for Ech Considered Are. Are A (m ) R-6 (Bg.kg -1 ) A 1 110,575 4,898 B 1 1,575 9,764 B 1,575 9,764 B 3,750 8,75 B 4 6,85 1,500 B 5 4,075 7,00 B 6 5,65 63,313 The model prmeters for the tilings mteril re the Rn- emntion coefficient (0.4), the totl porosity (0.37) nd the bulk density (1.67 g/cm 3 ). For the totl cover, n verge thickness of 5.15 m ws used, s proposed in the site rehbilittion project [3]. Considering the cover composition of cly, snd nd grvel, n verge vlue of 1.67 g/cm 3 nd 0.30 were dopted for the cover bulk density nd totl porosity, respectively. As the flux of Rn- through the tilings nd cover mteril depends on Rn- diffusion coefficient, the following verge vlues were clculted for the tilings nd for the cover: 5 x 10-7 m /s nd 9. x 10-7 m /s, respectively [3]. The cover thickness ws optimized in second stge, to reduce Rn- flux to ner nturl bckground levels. For the Urgeiriç region this vlue vries between 0 nd 30 Bq/m 3. This my be generted by n verge Rn- flux of 1.36 Bq.m -.s -1, considering previous studies in this site [6], [7], [8]. Sme mterils with the sme chrcteristics but with different thickness ( nd 3 meters for totl thickness) were investigted, by djusting the bottom lyer composed of geologicl mterils from other mine tilings. In the third stge, the US EPA (1983) vlue for the stndrd Rn- flux exhltion from the surfce of n inctive urnium mill tilings pile ws set t the mximum permissible limit to ccomplish; 0 pci.m -.s -1 or 0.74 Bq.m -.s -1 [10]. As strting point for the optimiztion of the cover thickness, cly lyer of 0.5 m ws considered in the simultion. In ddition, n overburden lyer with unknown thickness ws lso considered. RESULTS AND DISCUSSION A simultion ws performed to clculte the Rn- flux through the bred tilings; initilly it ws used single verge vlue for R-6 concentrtion in the tilings (1,900 Bq/kg). The resulting Rn- flux ws 7.19 Bq.m -.s -1. In n lterntive pproch, simultion ws done considering the individulized res ccording to rdium content in the tilings. The resulting verge flux, weighted by the respective re, ws 5.31 Bq.m -.s -1. The Rn- flux exhltion through the multilyer cover system defined in the remedition project ws lso simulted for ech one of these res. The results re presented in Tble II. 6

7 Tble II: Rn- Flux from the Tilings (J t ) nd from Ech One of the Lyers (J C ) Bq.m -.s -1. Thickness x c = 5.15 m. Are J t J C1 J C J C3 J C4 J C5 J C6 A x x x x x x 10-5 B x x x x x x 10-5 B x x x x x x 10-5 B x x x x x x 10-4 B x x x x x x 10-4 B x x x x x x 10-4 B x x x x x x 10-4 The verge Rn- exhltion rte through the tilings (J t ) is 5.31 Bq.m -.s -1. On the top of the sixth cover lyer, the verge Rn- exhltion is pproximtely equl to 9.64 x 10-5 Bq.m -.s -1. The multilyer cover design with the thickness of 5.15 m reduced lmost ll the Rn- exhltion; 99.8% from 5.31 to n verge vlue of 9.64 x 10-4 Bq.m -.s -1 (Rn- flux from J C6 verged with the respective re). This mens tht the Rn- exhltion is much lower thn the regionl nturl bckground level. Figure shows the Rn- exhltion decrese through the different cover lyers for ech one of the 7 individulized res. Fig. : Rn- exhltion rte through the multilyer cover system proposed to be plced over the Old Dm. The re identified s B 6 hs the highest Rn- exhltion rte s expected, due to the higher R-6 concentrtion present. It is lso cler tht the vrition of Rn- exhltion rte between the seven res is much less significnt in the upper lyer (J C6 ) nd it seems to be more or less homogenous long its thickness. In the second stge, the purpose of the simultion ws to optimize the cover thickness in order to reduce Rn- flux to ner the nturl bckground level. The sme mterils with the sme chrcteristics were investigted; t first it ws considered totl cover thickness of 3 meters nd fter, vlue of meters. This ws done by djusting the bottom lyer composed of mining wstes from other mines with different geologicl chrcteristics. The results re presented in Tble III nd Tble IV, respectively. 7

8 Tble III: Rn- Flux from the Tilings (J t ) nd from Ech One of the Lyers (J C ) Bq.m -.s -1. Thickness x c = 3 m. Are J t J C1 J C J C3 J C4 J C5 J C6 A x x x x x x 10-4 B x x x x x x 10-4 B x x x x x x 10-4 B x x x x x x 10-4 B x x x x x x 10-4 B x x x x x x 10-4 B x x x x x x 10-3 The results showed tht the multilyer cover design with 3 m reduced the verged Rn- exhltion in 99.55%; it decresed from 5.31 Bq.m -.s -1 to.36 x 10-4 Bq.m -.s -1 (Rn- flux from J C6 verged with the respective re). The results re similr with those obtined in the previous simultion. These results re presented in Fig. 3. Fig. 3: Rn- exhltion rte through the lterntive multilyer cover system, x c = 3m. As the results for Rn- exhltion re of the sme mgnitude in both simultions, nother reduction in the cover thickness my be optimized. For this lterntive the efficiency of the multilyer cover with the two meters thickness ws tested. Tble IV: Rn- Flux from the Tilings (J t ) nd from Ech One of the Lyers (J C ) Bq.m -.s -1. Thickness x c = m. Are J t J C1 J C J C3 J C4 J C5 J C6 A x x x x x x 10-4 B x x x x x x 10-3 B x x x x x x 10-3 B x x x x x x 10-3 B x x x x x x 10-3 B x x x x x x 10-3 B x x x x x x 10-8

9 The multilyer cover design with m reduced the Rn- exhltion rte in 96.39%; the vlue decresed from 5.31 to 1.9 x 10-3 Bq.m -.s -1 (Rn- flux from J C6 verged with the respective re). The results re presented in Fig. 4. Fig. 4: Rn- exhltion rte through the lterntive multilyer cover system, x c = m. For the third stge the purpose ws limiting the surfce Rn- exhltion to 0.74 Bq.m -.s -1 (0 pci.m -.s - 1 ). As first pproch, it ws considered lyer of 0.5 m of cly plus lyer of overburden to chieve n Rn- exhltion rte inferior to this limit. The diffusion coefficient for the cly cover lyer ws the sme vlue used in the previous simultions. For the overburden lyer diffusion coefficient of. x 10-6 m.s -1 ws clculted nd totl porosity of 0.37 ws used. The diffusion coefficient for the new source term (tilings plus cly lyer) ws clculted in the lgorithm (7.17 x 10-7 m.s -1 ) nd used to estimte the second lyer cover thickness. Considering single verge vlue for rdium content in the tilings (1,900 Bq/kg), the vlue obtined for the second lyer is x c = 0.57 m which gives totl cover thickness of 1.07 m. On the other hnd, considering the heterogeneous R-6 concentrtion in the tilings, we stipulted tht the limit should be chieved for ll the individulized res nd in these conditions; the second cover lyer thickness ws optimized to 1.49 m, which gives 1.99 m for totl cover thickness. This ssures tht for ny individulized re the Rn- flux is inferior to the EPA limit. The obtined verge Rn- exhltion rte weighted by the respective re ws 0.3 Bq.m - s. -1.The Rn- exhltion rte ws reduced in 95.67%; from 5.31 to 0.3 Bq.m - s. -1. CONCLUSIONS A computer bsed lgorithm ws implemented in Mtlb code to evlute the efficiency of n engineered finl cover for the remedition of urnium tilings disposl. The developed studies were pplied to specific site, the Urgeiriç urnium mining site. 9

10 Three different situtions were simulted in the lgorithm bsed on the multilyer cover system proposed in the rehbilittion project for this site. Different criterions were estblished nd subsequently different resulting thicknesses were obtined for finl covers; 5.15 m, 3 m nd m. The multilyer cover design with 5.15 m proposed in the remedition progrm reduced the Rn- flux in 99.8%. The lterntive optimized cover with 3 meters reduced the Rn- exhltion in 99.55%. When the desired performnce criterion is to chieve the Rn- stndrd emission rte from the surfce of inctive urnium mill tilings piles, cover with thinner thickness could perfectly be used. In the simultion done, the two lyers cover with thickness of meters llowed to reduce the Rn- exhltion to 0.3 Bq.m -.s -1 (decresed in 95.67%) which is inferior to the defined threshold. The cover proposed in the remedition project leds to complete blck out of Rn- exhltion which mens tht the Old Dm tilings pile exhltion is much lower thn the nturl bckground. In this context, or 3 meters for the totl cover system could be dopted. The results showed tht for ll the three simulted situtions the cover system llows reducing the Rn- exhltion rtes to negligible vlues, once the regionl nturl bckground is inferior to 30 Bq.m -3. The min difference between different simultions refers to the thickness of the bottom compcted lyer composed by geologicl mterils from other mine tilings. This my suggest tht the very high efficiency of the cover system is predominntly ssured by upper lyers, in prticulr, the HDPE liner in conjunction with the geo-textile membrne liner, isolte the hydrologic situtions in the dringe lyer bove from the cly lyer. Nevertheless, other importnt prmeters could nd should hve been vried in the simultions developed, in prticulr porosity nd sturtion degree, s the Rn- diffusion coefficient is function of both. Considering the costs ssocited with this type of rehbilittion works, not only due to vriety of mterils involved but lso due to their movement nd trnsporttion, the simultion of the ttenution of the rdon flux ttenution throughout severl lterntive multilyer covers llows to minimize the costs ssuring, simultneously, the environmentl fesibility of the remedition. REFERENCES 1. A. JANSSENS, B.A. ORDAX, V.D. STRICHT, nd P. VALLET, Verifictions Under the Terms of Article 35 of the Eurtom Trety, Europen Commission, Directorte-Generl for Energy nd Trnsport, Art. 35 Technicl Report - PT-06/07 (006).. EXMIN, Recuperção mbientl d Áre Mineir d Urgeiriç. Plno de Intervenção e Projecto de Selgem e Recuperção Ambientl d Brrgem Velh, Vol. I Resumo Não Técnico (004). 3. A.J.S.C. PEREIRA, J.M.M. DIAS, L.J.P.F. NEVES, nd J.M.G. NERO, Modelling of the long term efficiency of rehbilittion pln for urnium mill tiling deposit (Urgeiriç Centrl Portugl), XI Interntionl Congress of the Interntionl Rdition Protection Assocition (IRPA) (004). 4. V.C. ROGERS, nd K. NIELSON, Rdon Attenution Hndbook for Urnium Mill Tilings Cover Design. U.S. Nucler Regultory Commission, NUREG/CR-3533 (1984). 5. NCR, Clcultion of the Rdon Flux Attenution by Erthen Urnium Mill Tilings Covers, Regultory Guide 3.64, U.S. Nucler Regultory Commission (1989). 6. M.L. DINIS, nd A. FIÚZA, Simultion of the Rdon Flux Attenution in Urnium Tilings Piles, in: Methods nd Techniques for Clening-up Contminted Sites. pgs NATO Science for Pece nd Security Series, Environmentl Security, Annble, M. D.; Teodorescu, M.; Hlvinek, P.; Diels, L. (Eds), V, 80 p., ISBN: (008). 10

11 7. M.L. DINIS, nd A. FIÚZA, Simultion of Libertion nd Dispersion of Rdon from Urnium Mill Tillings, in: Environmentl Contmintion from Urnium Fcilities nd Remedition Mesures, pgs Proceedings of the Interntionl Workshop on Environmentl Contmintion from Urnium Fcilities nd Remedition Mesures, 6 p., IAEA Scientific & Technicl Publictions, Vienn, Austri, STI/PUB/18, ISBN (005). 8. M.L. DINIS, nd A. FIÚZA, Integrted Methodology for the Environmentl Risk Assessment of n Abndoned Urnium Mining Site, in: Urnium, Mining nd Hydrogeology, pgs Merkel, Broder J.; Hsche-Berger, Andre (Eds.) XXII, 958 p. 38 illus., ISBN: (008). 9. EXMIN, Estudo Director de Áres de Minérios Rdioctivos.ª fse, Compnhi de Indústri e Serviços Mineiros e Ambientis, SA., (003). 10. EPA, Environmentl Protection Agency, Helth nd Environmentl Protection Stndrds for Urnium nd Thorium Mill Tilings, CFR 40 Prt 19 subprt A, United Sttes (1995). 11