Investigation of Impurities of RBMK Graphite by Different Methods

Abstract

Samples of graphite from a RBMK-1500 reactor at the Ignalina Nuclear Power Plant from different construction elements (stack, sleeve, and bushing) were analyzed by the instrumental neutron activation analysis (INAA) method (LVR-15 experimental reactor of the Research Centre Řež, Ltd.) using the prompt gamma activation analysis (PGAA) method (Heinz Maier-Leibnitz Zentrum) and with an inductively coupled plasma mass spectrometer (ICP-MS) (CPST, Lithuania). These measurements were performed with the aim of obtaining the missing information on the impurity distribution in the RBMK-type nuclear graphite constructions as well as for intercomparison purposes, with the results measured in the graphite sleeve samples previously obtained by INAA & GDMS (Glow Discharge Mass Spectrometry) at CEA Saclay, France, and ICP-MS (CPST, Lithuania) methods. Validation of the ICP-MS method for the nuclear graphite impurity concentration determination was proven. The experimentally obtained RBMK-1500 graphite impurity values in different graphite constructions were compared with other measurements and new limits of the possible maximal concentrations of nuclear RBMK graphite impurity concentrations were obtained.

Determination of nuclear graphite impurities by prompt gamma activation analysis to support decommissioning operations

“…A comparison between ICP-MS and PGAA results obtained for bulk sample, together with literature data benchmarks, are provided in Table 1. [9,28,29].…”

“…[9] Moreover, both for ICP-MS and PGAA, overall obtained results often fall within the literature references values collected through several elemental characterizations of different nuclear grade graphite stocks. [9,28,29] Consequently, it is possible to confirm that AGOT graphite is highly pure, as it contains low amounts of impurities such as other analogue nuclear graphite types. In particular, for most of the considered elements, the measured values are closer to the lower limit of the literature range.…”

“…The values of deeper samples keep nearly constant within the measuring uncertainty, confirming literature data and reflecting the stabilisation of 14 N and 35 Cl concentrations just few mm from the graphite rod surface. [28,29] According to these outcomes, as a first approximation, the concentration of these precursors in the activation model could be assumed to be equal to the just determined bulk values. This simple and reasonable assumption would be useful during preliminary radiological characterization campaigns since it would allow obtaining sufficiently accurate values of 14 C and 36 Cl activity concentrations with low computational costs.…”

“…A Compton-suppression spectrometer was used for gamma detection (60% HPGe detector surrounded by a BGO scintillator and connected in anticoincidence mode). [28] All specimens were analysed by PGAA for four to ten hours. For the more accurate determination of impurities, a background measurement was performed Fig.…”