Estimation of permissible nitrogen content in the protective gas of the sodium circuit. Rosatom

1. Estimation of permissible nitrogen content in the protective gas of the sodium circuit

Joint Stock Company “State Scientific Centre of the Russian Federation –
Institute for Physics and Power Engineering named after A.I. Leypunsky”
STATE ATOMIC ENERGY CORPORATION "ROSATOM"
Estimation of permissible nitrogen
content in the protective gas of the
sodium circuit
Alekseev V.V.

2.

With an increased concentration of nitrogen
in the protective gas of the sodium circuits of the
NPP, the nitriding of the steel occurs.
Nitrogen as an impurity introduction into
structural materials has a significant impact on the
mechanical and technological properties of steel.
The main negative effect that occurs with
increasing nitrogen concentration is the loss of
plastic properties of steels.
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3.

Thermodynamic analysis shows that the
formation of iron nitrides in nitrogen is almost
impossible. When nitriding of iron alloys, the
formation of nitrides of alloying elements is
energetically more advantageous.
Nitriding of most austenitic steels does not
allow to obtain a layer with a depth of more than
0.12–0.15 mm. With increasing temperature, the
total depth of the nitrided layer increases. The
hardness of the nitrated layer decreases with
increasing temperature.
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4.

There are some patterns of the effect of
nitrogen on the behavior of unstabilized steels in
sodium:
1) steel with a high (up to 0.12%) nitrogen content
has a maximum corrosion rate, assessed by
weight change, and an increased tendency to lose
nitrogen and carbon;
2) nitrogen in the steel increases the activity of
carbon and enhances the decarburization of
steel;
3) the introduction of molybdenum into the
composition of the steel (along with chromium)
reduces the transfer of both carbon and nitrogen.
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5.

In the atmosphere of technical nitrogen, weak
nitriding of steel at 973 K is noticeable after 150 h of
exposure.
Under the same conditions in sodium,
nitriding occurred more intensively, but especially
quickly in the gas phase over sodium.
Increasing the pressure of nitrogen over
sodium causes accelerated nitriding of steel in
sodium and in nitrogen in the presence of sodium
vapor.
The decrease in the effect of nitrogen on the
mechanical properties of samples immersed in
sodium, compared with the effect on samples over
sodium, is due to the low solubility of nitrogen in
sodium.
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6.

The allowable partial pressure of nitrogen in the gas
phase is determined based on the reduction of the plastic
properties of steel. The more nitrogen, the steel becomes
more fragile.
It is assumed that the change in the relative elongation
of steel samples also correlates with the depth of penetration
of nitrogen. Therefore, assuming that the allowable partial
pressure of nitrogen corresponds to the allowable relative
elongation of steel, the calculated dependence is obtained,
which for steel in protective gas saturated with sodium vapor
has the form
= 259 ехр(-75300/RT) (P )0.5
where: is the time, h;
T is temperature, K;
P is the partial pressure of nitrogen, atm.
Nevzorov B.A., Zotov V.V., Ivanov V.A. and others. Corrosion of structural materials in liquid alkali metals. / Ed. prof. Nevzorov
B.A. - M .: Atomizdat, 1977.
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Dependence of permissible nitrogen pressure in the gas
cavity above steel 0Х16Н15МЗБ on the exposure time,
taking into account the relative elongation of steel by
10%, at a temperature of 600 С: continuous line
(calculation). Stroke line - normalized by NPP nitrogen
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pressure.

8.

The permissible nitrogen content in the
protective gas of the reactor was normalized to a
value of 0.2 vol. % at the maximum
temperature of 600 ° C (taking into account its
real effect on the mechanical properties of
structural materials). With a total gas pressure of
0.2 MPa, the partial pressure of nitrogen in the gas
cavity of the reactor vessel should not exceed
0.0004 MPa.
Substantiation of the range of regulated impurities in sodium coolant of the primary and secondary
circuits of the reactor BN-600. Technical Note, IPPE, 2010.
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9.

The solubility of molecular nitrogen in sodium
is extremely low ((1–4) 10–9 wt.% At 450–600 ° C).
It was established experimentally that the content of
chemically bound nitrogen in sodium is also
insignificant: nitrite forms <1.10-3 wt.%; cyanide
<1.3.10-4 wt.%. Considering this circumstance, as
well as the stability of the results of chemical
analyzes on the nitrogen content, this impurity was
excluded from the list of standardized quality
indicators for the sodium coolant of the BN-600.
Zagorulko Yu. Status of sodium coolant in BN600, at present time // WG4 "Liquid Metal Technology"
Meeting, Cadarache 2007 October 15th to 19th.
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Joint Stock Company “State Scientific Centre of the Russian Federation –
Institute for Physics and Power Engineering named after A.I. Leypunsky”
Thank you for your attention!
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