Nuclear data needs for reactor graphite radiological characterization and recycling

Graphite is widely used as material to moderate neutrons in nuclear reactors and graphite recycling opportunity depends on the concentration of radioactive contaminants. The activity of radionuclides in irradiated graphite depends on the concentration of impurities in virgin graphite and on the characteristics of the neutron flux. Concentrations of impurities in virgin graphite are usually unknown and have to be determined experimentally. As there are tens of important nuclides which have to be measured for modelling medium- and long-lived waste, it is not cost effective to measure all of them by expensive neutron activation analysis. Comparison of results on impurity concentration obtained with X-ray fluorescence technique, ICP-MS technique and neutron activation analysis for graphite irradiated in RBMK-1500 reactor has shown that precise enough determination of concentrations of elements Na, V, Zn, Ge, Ag, and Ta in graphite can be done only by neutron activation analysis. Precise evaluation of the neutron fluence in different locations of the reactor graphite stack and other graphite constructions has been performed using Monte Carlo MCNPX code. The MCNPX calculation of the ¹³C/¹²C ratio in irradiated graphite has been validated against the ¹³C/¹²C ratio measured by stable isotope ratio mass spectrometry. For the calculation of ¹⁴C activity in graphite microscopic cross-sections of reactions ¹²C(n, γ)¹³C, ¹³C(n,γ)¹⁴C, and ¹⁴N(n,p)¹⁴C have been used. Data analysis of these microscopic cross-sections in different databases has revealed their large uncertainty and this uncertainty must be reduced to get an accurate evaluation of neutron fluence in the graphite stack and of ¹⁴C activity