(30c) Study Design of ATEX Formed in Case of Leakage of H2-N2 Mixtures From Pressure Piping

The processes involved in nuclear power generation require the use of hydrogen under pressure is likely in case of leakage, lead to the formation of explosive atmospheres (ATEX) in the operating areas.

INERIS has made a study for estimating risks in the premises of a nuclear plant that was part of an application for the nuclear French safety authority who had led a study of various cases of leaks hydrogen, supposed to occur in some local and coming from different pressure piping, implementing pure hydrogen or mixtures H₂-N₂-grade defined in H₂, in C_{H2 ?N2} (% vol.) to assess the volume of ATEX (Vatex) and the distance to the LIE (X_LIE).

INERIS has conducted a further study and consisted primarily to establish a simplified calculation model for determining the parameters and V_ATEX (m³) and x_LIE (m) for leaks defined mixture of H₂-N₂.

It was to establish a simplified model for calculating the following quantities that allow to assess the dimensions of the ATEX resulting from the leak :

●        distance from the LIE, X_LIE (m),

●        volume formed ATEX, V_ATEX (m³).

from the parameters following definition of a leak:

- data on the leakage orifice assumed circular :

●        its nominal diameter, D (m),

●        its external diameter, D_ext (m),

·        the thickness of the pipe, e (mm),

·        the diameter of the hole leakage, d (m) ; in the event of a guillotine rupture of this pipe, d is connected to D_ext and to e by the relationship d = ,

·        the diameter of the half leakage, d (m) ; in the event of a half rupture of this pipe, d is connected to D_ext and to e by the relationship d = ,

- H₂ concentration in the mixture H₂-N₂, C_{H2 ?N2} (%),

- absolute pressure in the pipeline, P(bar).

For information, an additional calculation was performed to determine the distances to the LIE and ATEX volume associated with a fraction of the LIE, χ_LIE noted.

We can thus define the following quantities :

- Xχ_LIE which corresponds to the distance the jet axis of flight, which is reached at a concentration χ_LIE ,

?         V_ATEXχ_LIE which corresponds to the volume of the locus of points in space where the concentration is greater than or equal to χ_LIE.

Finally, we established the mathematical relations for calculating the magnitudes Xχ_{LIE and}  V_ATEXχ_LIE, calculations made by considering the following value χ = 0.8, χ = 0.6 and χ = 0.4.