ESTIMATION OF THE GENERATION OF 13C AND 14C IN THE REACTOR GRAPHITE USING MCNP6 MODELLING, ISOTOPE RATIO MASS SPECTROMETRY AND 14C MEASUREMENT TECHNIQUE

Characterization of irradiated graphite in terms of C activity is crucial for the optimization of treatment technology: geological disposal, landfill storage, recycling, etc. The main contributor to C generation in the RBMK reactor graphite is N(n, p)C reaction. The generation of carbon isotopes C and C in the virgin RBMK graphite samples irradiated at the LVR-15 research reactor (Research Centre Řež, Ltd.) were investigated in order to obtain the impurity concentration level of N. Afterwards the modeling of graphite activation in the RBMK-1500 reactor was performed by computer code MCNP6 using obtained N impurity concentrations and new nuclear data libraries. The irradiation parameters – neutron fluence have been checked by method based on coupling of stable isotope ratio mass spectrometry and computer modelling. The activity of C in the different constructions of irradiated graphite of the RBMK-1500 reactor has been measured by the β spectrometry technique (LSC) and has been compared with the simulated one. Obtained results have indicated the importance of C production from N in the RBMK-1500 reactor and in the LVR-15 neutron spectrum. Measured C specific activity values in the samples varied from 130700 kBq/g in the RBMK-1500 irradiated samples and from 3-12.5 Bq/g in the LVR-15 irradiated graphite samples. This corresponds to 15±4 80±10 ppm impurity of N in various graphite samples of RBMK reactor.


INTRODUCTION
C is one of 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 [1]. Two RBMK-1500 reactors operated in Ignalina NPP (Lithuania): Unit 1 in 1984-2004, Unit 2 -in 1987Unit 2 -in -2009. The graphite used as a moderator and reflector in both reactors representing some 3800 tones. According to the existing radiological classification the graphite waste is attributed to the long-lived low and intermediate activity waste [2]. The timescale for geological disposal of spent nuclear fuel and operational radioactive waste of INPP is previewed by 2066 in Lithuania [3]. 14 C is generated in the graphite moderator-reactor due to the neutron capture by 13 C in the graphite matrix or by the neutron activation of 14 N and 17 O impurities. The main contributor to 14 C generation in the RBMK reactor is 14 N(n, p) 14 C reaction due to high neutron reaction cross section. 14 N concentration in the graphite is determined by initial impurity concentration and because of 14 N abundance in the helium-nitrogen mixture used to flush the graphite stack in the core. There is an urgent need of 14 C activity determination for the optimization of spent graphite treatment technology (e.g. geological disposal, landfill storage, recycling, etc.). The determination of concentrations of radioactive impurities in irradiated graphite has a primary importance to its management strategy and may lead to considerable savings of decommissioning funds. In this research importance of both 14 C generation channels via 13 C and 14 N was investigated with virgin and with irradiated graphite. Firstly the 14 N impurity concentration level was estimated in the virgin RBMK graphite samples irradiated at the LVR-15 research reactor. Afterwards the modeling of graphite activation in the RBMK-1500 reactor was performed by computer code MCNP6 using obtained 14 N impurity concentrations and new nuclear data libraries. The irradiation parameters -neutron fluence have been checked by method based on coupling of stable isotope ratio mass spectrometry and computer modelling.

ESTIMATION OF 14 N IMPURITY CONCENTRATIONS IN THE VIRGIN RBMK-1500 REACTOR GRAPHITE
For the impurity concentration of 14 N in the virgin RBMK graphite determination the samples irradiated in LVR-15 research reactor (Research Centre Řež, Ltd.) for INAA experiment -have been latter on investigated by the β spectrometry technique (LSC) at CPST (Lithuania). The formation of carbon isotopes 13 C and 14 C in INAA RBMK graphite specimens was obtained after irradiation in the LVR-15 research reactor at the thermal neutron flux of 4.9u10 13 cm -2 s -1 [4]. Neutron flux distribution is presented in Table 1. Table I. Parameters of the neutron spectrum at LVR-15 H8 irradiation channel.

E (MeV)
) (cm -2 s -1 ) ) (%) 0,0 -5,0u10 - 8 3,0u10 13 61.2% 5,0u10 -8 -0, 1 1,0u10 13 20.4% 0,1 -20 9,0u10 12 18.4% Modelling of sample activation in LVR-15 neutron flux was performed using MCNP6 [5]. 14 N(n, p) 14 C, 13 C(n, J) 14 C and other reaction rates have been obtained in the samples irradiated at H8 irradiation channel. Calculated reaction rates are as folows: were, RRi -rate at which reactions are occurring in the nuclide i (reactions/s); N -number of target atoms in the sample; V(E)energy-dependent microscopic cross-section; I(E) -energy-dependent neutron flux in the sample (n/cm 2 s). The impurity concentrations have been obtained according to measured activities. The experimental reactions rates are calculated taking into account irradiated samples activity and irradiation parameters [6]: were At -activity of the sample at the end of irradiation (Bq), Φ -neutron flux (n/cm 2 s), σ -microscopic neutron cross section (cm 2 ), ta -time of activation (s), ߣ -decay constant (s -1 ). The real concentration of impurity is determined by comparing calculated and experimentally obtained activity values. The 14N reaction rate was checked by 60 Co activity and corresponding impurity concentration (see Table II).
The obtained results show, that 14 C production from 13

RBMK-1500 REACTOR MODELING AND ESTIMATION OF THE NEUTRON FLUENCE IN THE REACTOR GRAPHITE BY ISOTOPE RATIO MASS SPECTROMETRY
The activation of 14 C in the irradiated graphite of the RBMK-1500 reactor has been modeled previously using different simulation codes [9,10,11]. In this research the full scale 3D RBMK-1500 reactor model (MCNP6) with detail materials composition, power history of the reactor, operation time, the most recent graphite impurity concentrations [4] were used for MCNP6 model upgrade. The new ENDF-VIII nuclear library [12], which includes carbon isotopes, was used for detail 14 C production pathways estimation. The irradiation parameters -neutron fluence have been checked by estimating stable carbon isotope ratio (δ 13 C) [as in 8]. The modeling of (δ 13 C) in the RBMK-1500 spent graphite construction is presented in Table III.   The full 3D of the RBMK-1500 reactor, graphite mass (%) share and calculated 14 C activities in the different graphite constructions are presented Fig. 1. The simulated activity of 14 C was compared with the measured 14 C activity using β spectrometry technique (liquid scintillation counter Quantulus-1220 (PerkinElmer, USA) and also with the rapid method of specific activity determination [13]. Results have indicated that production of 14 C from 14 N in the RBMK-1500 reactor is considerable (60% to 90% depending on 14 N initial concentration) and has to be taken into account in order to make proper evaluation of 14 C activity in the model. Measured 14 C specific activity values in the samples varied from 130-700 kBq/g in the RBMK-1500 irradiated samples. This corresponds to 15±4 -80±10 ppm impurity of 14 N in various graphite samples of RBMK reactor.
The estimation of the real neutron flux in different parts of RBMK-1500 reactor graphite was performed by measurement of the stable carbon isotope ratio (δ 13 C) in the irradiated graphite samples. The measurements of RBMK spent graphite samples were performed by using the elemental analyzer FlashEA 1112 connected to the stable isotope ratio mass spectrometer ThermoFinnigan Delta Plus advantage. The measurements procedure is presented in [8]. The experimentally determined δ 13 C values are in range from -20.0 to -25.6 in graphite core samples, while in raw graphite sample δ 13 C value is about -30.7 (the similar result is obtained for low neutron fluence regions as side cooling reflector top and bottom parts). In the sample from the central zone of the reactor δ 13 C value of -24.5 is obtained, while in graphite samples from the plateau zone of the reactor it differ from -20.0 to -24.4. In the sample form peripheral zone the δ 13 C value of -24.1 was found. In summary, the results indicate clear difference between irradiated and non-irradiated raw graphite samples. However, the dependence of δ 13 C value on the irradiated graphite sample location in reactor core is not found. This indicates that the neutron flux is quite uniform in the RBMK-1500 reactor core. The measured ' 13 C/ 12 C values in graphite at different positions of the reactor core indicates that according to modelling results the neutron flux in the samples varies from 910 13 n/cm 2 s -to 1.510 14 n/cm 2 s (see Table III for details).

CONCLUSIONS
The research on importance of 14 C generation channels via 13 C and 14 N in the virgin and irradiated RBMK-1500 graphite was performed. Obtained results have indicated that production of 14 C from 14 N in the RBMK-1500 reactor graphite is about ~63% in case of RBMK-1500 neutron flux, and 70-90% in case of LVR-15 neutron flux. We should note what irradiation in the LVR-15 environment (irradiation in the air) corresponds to surface 14 N concentration of the spent graphite. Additional experiments are needed for investigation of samples irradiated in the RBMK-1500 reactor environment (with N+He mixture flushing gas) to confirm the 14 N concentrations in the bulk of graphite. Measured 14 C specific activity values in the different irradiated graphite samples varied from 130-700 kBq/g in the RBMK-1500 irradiated samples and from 3-12.5 Bq/g in the LVR-15 irradiated graphite samples. This corresponds to 25±4 -90±10 ppm impurity of 14 N in various graphite samples of RBMK reactor. Without additional proof if the high 14 C activity is imposed by thin surface layer, the RBMK-1500 spent graphite activation seems to be too high for additional treatment or reconsidering for lower category of waste.