Speaker
Description
Today’s searches of effective technology to deactivate irradiated reactor graphite are very acute due to the large volumes of accumulated irradiated graphite in the world (about 100 thousand tons) and the challenging problem of uranium-graphite reactors decommissioning period. It is also well known that 14C isotope makes the greatest contribution to the irradiated graphite residual activity from RBMK reactors which are now on the way of out-of-operation stage. The problem is that 14C isotope suffers beta-decay with long lifetime ~ 5700 years and is dangerous when entering into biological food chains. It is also known that 14C isotope is mainly produced during operation of RBMK reactors as a result of neutron absorption by nitrogen atoms from a helium-nitrogen gas cooling mixture. In this way 14C atoms are precipitated from the gas and accumulated on the surface of reactor graphite pile. Another channel of 14C formation and accumulation is due to nitrogen intercalation and migration in between graphene-graphene layers of graphite, where nitrogen atoms are activated by neutrons. Predominantly surface localization of 14C in the irradiated reactor graphite was confirmed in [1-2].
We are developing ion-plasma deactivation technology [3] of irradiated reactor graphite, which provides sputtering of the reactor graphite surface of a given thickness by micro-plasma discharge in inert gas (argon) at high pressure up to atmospheric one. The deposition of the sputtered layer, including radioactive atoms, is carried out on the cooled collector plate with control of the discharge operating parameters (current, voltage, pressure, etc.). So, graphite surface structure data are of great importance for the development of effective technology for the irradiated graphite deactivation.
We experimentally studied the surface of the pure reactor graphite (GR-280) non-irradiated samples by SEM with X-ray microanalysis and BET-porosimetry. It was obtained that the values of the specific surface area of GR-280 reactor graphite are near to 2.1 m2 / g; pore volume ~ 0.004 cm3 / g, pore radius ~19.4 A. Based on these data, the surface layer depth of reactor graphite enriched by 14C isotope was estimated around 420 nm, this result is also consistent with the data from [1,2]. A number of another atomic species were observed by SEM X-ray microanalysis spectra in contrary to declared bulk atomic content of GR-280. It is important for a choice of graphite surface temperature and exposure duration under plasma treatment, as this temperature may vary in the range of 600-1800 K depending on the processing conditions and the input power level [4].
Plasma technology described [1] can also be used to decontaminate the surfaces of metal structures of nuclear power plants, is patented in our collaboration with Concern Rosenergoatom JSC and Rosatom and also is suitable for Fukushima NPPs accident dismantling efforts.
Аcknowledgements
This work is supported by The Foundation for Assistance to Small Innovative Enterprises (FASIE) – contract №3832ГС1/63219.
[1] M.L. Dunzik-Gougar, T.E. Smith, Journal of Nuclear Materials 451, 328 (2014)
[2] D. Vulpius, et al., Journal of Nuclear Materials 438, 163 (2013)
[3] A.S. Petrovskaya, A.B. Tsyganov, M.R. Stakhiv "Method for deactivating a structural element of a nuclear reactor" Patent RU №2711292, International patent application PCT/RU2019/000816, European patent application EP 19888171.6.
[4] A.S. Petrovskaya, A.Yu. Kladkov, S.V. Surov, A.B. Tsyganov, AIP Conference Proceedings 2179, 020020 (2019)
