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

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.

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.

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.

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.

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
δ = 259 ехр(-75300/RT) (P τ)0.5
where: τ 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.

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 pressure.

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.

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.