Explosibility of nuclear graphite measured in a 1 m3 chamber

Introduction. Nuclear graphite poses a threat due to the formation of the graphite dust – air mixture (GDAM) during the dismantling of decommissioned nuclear reactors. However, there is no clear answer to the question on the GDAM explosibility. A review of international studies suggests that GDAM is either inexplosive or its explosibility is weak (Phylaktou H.N. et al., 2015). In this paper, the authors advance arguments for the explosion safety of GDAM.

Selected research result. The authors considered a well-known result of a study on the combustion of GDAM with an average particle size of 5 μm, the concentration of about 450 g/m³ in a 1.138 m³ chamber, and an ignitionsource made by Fr. Sobbe GmbH («Sobbe 10 kJ»). The maximum overpressure ΔP_max was 0.47 bar in the chamber, and it fitted the case of an explosive air suspension, according to EN 14034-3 (1 bar = 100 kPa).

Interpretation of the research result. Pressure oscillograms were compared for the following two cases: the case of the maximum manifestation of the GDAM explosion hazard (ΔP_max = 0.47 bar; dP/dt|_max = 3.8 bar/s) and the case of combustion of an ignition source in the absence of air suspension (ΔP_max = 0.027 bar; dP/dt|_max = 2.7 bar/s). The comparison shows that the first 20 ms of a pressure change inside the chamber is mainly due to the combustion of the ignition source: the characteristic values ΔP = 0.03 bar and (dP/dt) ≈ 3.8 bar/s are close to the «Sobbe 10kJ» combustion index in the absence of GDAM. A further increase in ΔP is accompanied by the constant or sharply decreasing value of (dP/dt), which means a monotonous decrease in the flame velocity and proves the incombustibility of GDAM.

Conclusions. Due to the smallness of ΔP_max, GDAM can be considered nonexplosive under normal atmospheric conditions. Dependency diagrams, relating the pressure of combustion products and its growth to time offer important information about the combustion of the air suspension in explosion chambers under the condition of a low dust explosion hazard.

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