(e)

Nuclear graphite waste management strategy during decommissioning of Ignalina NPP is the pending decision in Lithuania. In the RBMK type reactor graphite is a neutron moderator and

reflector. The total mass of radioactive graphite from the both Ignalina NPP units is up to 3,800 t. 14C is the limiting radionuclide for long-term disposal of irradiated graphite due to half-life of

5730 years and relatively high activity as well as mobility in geological media. Characterization of irradiated graphite in terms of both 14C activity and chemical bonds in the lattice is crucial

for the optimization of treatment technology (e.g. geological disposal, landfill storage, recycling, etc.). For this purpose numerical simulations and experimental analysis are performed.

Elena Lagzdina, Danielius Lingis, Jevgenij Garankin, Rita Plukienė, Andrius Garbaras, Arūnas Gudelis, Laurynas Juodis, Mindaugas

Gaspariūnas, Vitalij Kovalevskij, Ieva Matulaitienė, Gediminas Niaura, Artūras Plukis, Vidmantas Remeikis

Center for Physical Sciences and Technology, Savanorių pr. 231, LT-02300 Vilnius, Lithuania

elena.lagzdina@ftmc.lt

1. Plukiene et al., 2014, Nuc. Eng.

Des. 277, 95-105

2. Remeikis et al., 2009, Nucl.

Eng. Des. 239, 813-818

3. Remeikis et al., 2018, PLOS

ONE, In Press

Conclusions References

Determination of 14C specific activity in irradiated graphite

Experimentally validated numerical 3D model of RBMK-1500 is used for 14C profile determination in different graphite constructions (stack,

sleeve, top, bottom, side reflectors). 14C activity measurements in graphite samples is carried out by using express method or liquid scintillation

counting (LSC) technique.

The evolution of graphitic sp2-related content as well as formation of an amorphous structure serves for understanding of location and stability of 14C in graphite matrix, while the thermal treatment carries information about recrystallization process. Further structural investigations are

currently in progress.

12C+ ion implantation and thermal treatment

14C measurements are usually carried out by using liquid scintillation counting (LSC) technique

after time consuming sample preparation procedure. Recently we proposed an express analysis

method for the specific 14C activity determination in small graphite samples in the range of 1-

100 μg [3]. This method is based on the graphite sample combustion in the commercial

elemental analyzer and determination of 14C specific activity by using the semiconductor

detectors. This method is planned to apply for determination of the graphite homogeneity profile

in terms of 14C activity.

Characterization of 14C in neutron

irradiated RBMK-1500 graphite

For further graphite treatment technology optimization the structural investigations of graphite should be performed. 14C mobility and position in graphite matrix is determined by neutron

irradiation in the reactor at certain operation conditions. In order to understand the processes in the irradiated graphite we observe the propagation of defects induced by 12C+ ion implantation

at energy of 700 keV at varying fluences. The structural changes after implantation and thermal treatment later on are investigated by Raman spectroscopy. The SRIM-2013 code is also used

to estimate the damage profile in the surface of the graphite samples.

RBMK-1500 graphite

Fig. 4. 700 keV 12C+ ion implantation parameters according to SRIM: (a) the projected

range of implanted ions; (b) the damage profile.

Fig. 1. (a) Neutron flux distribution in the RBMK reactor graphite horizontal and (b) vertical cross

sections. (c) Horizontal cross section full scale reactor core; (d) vertical cross section of reactor core

with bottom, top reflectors, metal plates and cooling tubes system on the top. (e) Magnified view of

RBMK-1500 core fragment (3x3) with fuel assemblies, inserted and extracted control rods.

Fuel channel

Control rod channel

Graphite reflector

Top and bottom reflector

Stack

Sleeve

Channel

Fuel

Coolant

B4C absorber

Al tube

Control rod graphite

Experimentally validated numerical 3D model of RBMK-1500 (MCNP6 and SCALE

6.1.) is effective for description of the change of radiological characteristics of different

parts of nuclear reactor during operation and decommissioning periods [1]. Both

experimental measurements and modeling data are used for scaling factor determination

[2], which subsequently could be used for sorting of spent graphite radioactive waste.

0 50 100 150 200 250 300 3500

50

100

150

200

250

300

350

1

2

3

4

5

6

7

8

9

1 sample number

linear aproximation

mT

CD(

g)

mweighing

(g)

0 50 100 150 2000

20

40

60

80

100

120

140

160

sample No.

linear fit

Counts

by

det

ecto

r

14C activity (Bq)

(c)

(d) (b)

(a)

(c) (b)

(a)

Fig. 2. (a) Rapid system for 14C specific activity determination in the sample. (b) The correlation of the graphite

sample mass as determined by two independent methods: weighing and combustion in the elemental analyzer.

(c) Correlation between LSC and semiconductor detector data.

1000 1200 1400 1600 1800

0.0

0.2

0.4

0.6

0.8

1.0

Inte

nsi

ty, a.

u.

Raman shift, cm-1

Virgin graphite

3.5x1015

C+/cm

2

1.18x1016

C+/cm

2

7.2x1015

C+/cm

2

1.15x1015

C+/cm

2

1000 1200 1400 1600 1800

0.0

0.2

0.4

0.6

0.8

1.0 Virgin graphite

1,2x1016

C+/cm

2

1,2x1016

C+/cm

2, 400

oC, 5h

1,2x1016

C+/cm

2, 600

oC, 5h

1,2x1016

C+/cm

2, 800

oC, 5h

Inte

nsi

ty, a.

u.

Raman shift, cm-1

Theoretical

calculations

(SRIM)

Sample

preparation

Ion implantation

(ion accelerator

Tandetron 4110A)

Raman

characterization

Thermal

treatment

Raman

characterization

Fig. 3. SEM images of raw RBMK-1500 stack graphite samples.

(b) (a)

Fig. 5. Raman spectra of raw RBMK-1500 stack graphite samples (a) implanted at a fluence of (b) 1.2×1015 ions/cm2 (c)

3.5×1015 ions/cm2 (d) 7.2×1015 ions/cm2 (e) 1.2×1016 ions/cm2 and subsequently annealed at various temperatures ranging

from 400ºC to 800ºC for 5h.

1000 1200 1400 1600 1800 2000

0.0

0.2

0.4

0.6

0.8

1.0

Inte

nsi

ty, a.

u.

Raman shift, cm-1

7.2x1015

C+/cm

2

7.2x1015

C+/cm

2, 400

oC, 5h

7.2x1015

C+/cm

2, 600

oC, 5h

7.2x1015

C+/cm

2, 800

oC, 5h

Virgin graphite

800 1000 1200 1400 1600 1800 2000

0.0

0.2

0.4

0.6

0.8

1.0

Raman shift, cm-1

Inte

nsi

ty, a.

u.

Virgin graphite

3,5x1015

C+/cm

2

3,5x1015

C+/cm

2, 400

oC, 5h

3,5x1015

C+/cm

2, 600

oC, 5h

3,5x1015

C+/cm

2, 800

oC, 5h

800 1000 1200 1400 1600 1800 2000

0.0

0.2

0.4

0.6

0.8

1.0

Inte

nsi

ty, a.

u.

Raman shift, cm-1

Virgin graphite

1,2x1015

C+/cm

2

1,2x1015

C+/cm

2, 400

oC, 5h

1,2x1015

C+/cm

2, 600

oC, 5h

1,2x1015

C+/cm

2, 800

oC, 5h

(a) (b) (c)

(d) (e)