Modelling of decay chain transport in groundwater from uranium tailings ponds

Transcription

1 Modelling of decay chain transport in groundwater from uranium tailings ponds Nair, R.N., Sunny, F., Manikandan, S.T. Student : 曹立德 Advisor : 陳瑞昇老師 Date : 2014/12/04

2 Outline Introduction Model Result and discussion Conclusions

3 Introduction Uranium tailings ponds The principal radiation risk

4 Introduction Percentage of inventory of the world Total volume : m 3 33% 23% 13% 14% 17% Kazakhstam Germany Ukraine USA Others

5 Introduction

6 Nuclide Half-life U Introduction years U years Uranium Th-230 ore deposit years Ra years Th years *Rn-222(g) years Pb years Bi years Po years Pa-234m 1.17mins Po mins Pb mins Bi mins Po ms

7 Introduction Limition Number of species Retardation factors Homogeneous and isotropic medium Dimensionality of model The source depletes only by radioactive decay

8 Introduction Development Bauer developed a Laplace domain solution using a recursive form with distinct retardation factors.(2001) Their recursive formula can be used to build a more complex multi-species transport solution.

9 Model

10 Model Parent radionuclide for an inhomogeneous and anisotropic aquifer, the equation : N N N N N N N R (D ) (D ) (D ) u v w R N t x x y y z z x y z x y z Symbol Definition Dimension N 1 the concentration of the parent in groundwater atoml 3 or mol D x, D y, D z hydrodynamic dispersion coefficient L 2 T -1 x,y,z longitudinal, lateral, vertical distance L u,v,w groundwater seepage velocity LT -1 λ 1 radioactive decay constant T -1

11 Model Decay chain equation for ith member N N N N N N N R (D ) (D ) (D ) u v w R N R N t x x y y z z x y z i i i i i i i i x y z i i i i 1 i 1 i 1 i >1 to M, M : number of the total nuclide Last term : ingrowths of the progenies from the preceding parent radionuclides.

12 Model Retardation factor of the radionuclide R i 1 ( K di b ) K di : the distribution coefficient of nuclide i ρ b : the bulk density of the aquifer material(μl 3 ) θ: the porosity

13 Model Hydrodynamic dispersion coefficients: Symbol Definition D D D u v w D V V V x L L L m α L the longitudinal dispersivity L α T the lateral dispersivity L v u w D V V V α V the vertical dispersivity L y L L L m V the velocity vector LT w v u D V V V D m the molecular diffusivity L 2 T -1 z L L L m τ tortuosity - Dimension

14 Model

15 Model The release rate of radionuclides from the tailing ponds into the groundwater : ( t) N ( t) K exp[ ( K ) t] i i li i li Symbol Definition Dimension Φ i (t) the release rate of the ith nuclide atoms T -1 N i (t) K li the inventory of the ith nuclide(atoms) at time t the leach late/ fractional release rate of the ith nuclide t time T atoms T -1

16 Model The leach rate from tailing ponds can be calculated as shown below: K li S vs VR Symbol Definition Dimension v the infiltration rate of the water from LT -1 the tailing pond s the surface area of the tailing pond L 2 θ s the porosity of the tailings - V the volume of the tailing pond L 3 R is the retardation factor of nuclide i in the tailings pond is -

17 Model The concentration of the ith species at the source area: N () t N () t exp[ ( K )] t i 0i i li S VRis

18 Model The number of atoms of the parent radionuclide at the source area can be calculated using following equation: dn dt 1 ( 1 Kl1 ) N1

19 Model The number of atoms ith daughter radionuclide can be calculated using the following equation : dn dt i N K N i 1 i 1 ( i li ) i

20 Result and discussion Parameter: 316.2m x 316.2m x 20m The groundwater seepage velocity : 0.03m/day Porosity(θ) : 0.3 Bulk density : 1.5g/mL

21 Result and discussion Bauer-1D transient model t=300days λ=(7.0,5.0,4.5,3.8) R=(5.3,1.9,1.2,1.3) 10-4 day -1 u=1m/day C 0 =(100,0,0,0)mmol θ=0.15

22 Result and discussion Bauer-1D steady state model λ=(7.0,5.0,4.5,3.8) R=(5.3,1.9,1.2,1.3) 10-4 day -1 u=1m/day C 0 =(100,0,0,0)mmol θ=0.15

23 Result and discussion Bauer-3D transient model λ=(7.0,5.0,4.5,3.8) R=(5.3,1.9,1.2,1.3) 10-4 day -1 u=1m/day C 0 =(100,0,0,0)mmol θ=0.15

24 z x 0.03 Θ=0.3 ρ=1.5g/ml y

25 Result and discussion

26 Result and discussion Time history of U 238 and its progenies n 222 >Po 210 >Bi 210 =Pb 210 >Ra 226 >U 238 =U 234 >Th 234 >Th 230

27 Result and discussion Lower C ( high K d value) Higher C ( low K d value and ingrowth Rn-222 : dissolve in groundwater and does not escape from groundwater.

28 Result and discussion U-238 as distance from tailing ponds y=0, z=0, t=100years

29 Result and discussion With & without decay chain

30 Result and discussion Percentage contribution 99.75% Rn 222,Po 210,Pb 210,Ra %

31 Conclusions Low K d & ingrowth High C When distance, C C progenies > C parent The effective dose with decay chain transport are 100 times than without decay chain transport. Rn 222,Po 210,Pb 210,Ra 226 : 99.75%, and other radionuclide : 0.25%

32 Thank you for listening!