Corresponding author: Olga M. Gulina (omgulina18@mail.ru)

Academic editor: Yury Korovin

Modern foreign computer codes predict a linear growth in the pipeline wall thinning with time due to the process of flow-accelerated corrosion (

Erosion-corrosion wear (

Early versions of the codes for the

A linear dependence of the thinning value on time and the constancy of the corrosion rate are not typical of the NPP pipeline operating conditions. At the same time, excessively conservative estimates lead to increased economic costs of repeated inspections, since, in this case, the predicted residual life is much shorter than with the decreasing

The purpose of the study is to investigate the problem based on the wall thickness inspection data, including the analysis of repeated measurements, and the data on the damaged pipelines, as well as to analyze the corrosion tester readings.

The analog for Russian software tools, EKI-02 and EKI-03, (the authors of this paper are the developers of these codes) is the CHECWORKS (Chexal-Horowitz Engineering Corrosion Workstation) code (ASME code Case N-480). The codes were developed using foreign experience and the results of domestic investigations (

_{ECW} = _{0} × _{1}(_{2}(_{3}(υ) × _{4}(O_{2}) × _{5}(pH) × _{6}(_{k}_{7}(α) × _{8}(_{9}(τ), (1)

where _{ECW} is the corrosion rate, mm/year; _{0} is the coefficient equal to 1 mm/year; _{1}(_{2}(_{3}(υ) is the coefficient defined by the medium rate; _{4}(O_{2}) is the coefficient that corrects for the oxygen concentration; _{5}(pH) is the coefficient that corrects for the рН value; _{6}(_{k}_{7}(α) is the coefficient that corrects for steam humidity (for a single-phase medium, _{7}(α) = 1); _{8}(_{9}(τ) is the coefficient that corrects for the component operating time; Δ_{0}, τ).

A parameter has been introduced into the EKI-02 and EKI-03 codes to take into account the influence of operating time on the corrosion rate and the wall thinning value which allows estimating the

The time parameter introduced into the EKI-02 and EKI-03 codes allows one to calculate

the ECW rate at the pipeline operation start time;

the ECW rate at the pipeline operation end time;

the average ECW rate for the estimated time interval;

the pipeline wall thinning for the estimated time interval.

It was assumed in determining the dependence which describes the influence of operating time on corrosion rate, that the concentration of iron in feedwater depends on the escape of iron in the process of the secondary circuit equipment and piping

The content of iron reflects just indirectly the

The corrosion rate is estimated based on inspection data as follows (

_{ecw} = (_{nom} – _{min})/τ, (3)

where _{ecw} is the _{nom} is the nominal wall thickness value, mm; _{min} is the minimum wall thickness value based on in-service inspection data, mm; and τ is the time between the component operation start and the inspection date.

In the process of follow-up measurements, pointwise estimates of the ^{b}_{ecw} is pointwise estimates, and _{trend} is their approximation.

Feedwater pipeline

No. | Date, year | τ, years | _{ecw}, mm/year |
_{trend}^{fw}, mm/year |
---|---|---|---|---|

1 | 1984 | 3 | 0.900 | 0.772 |

2 | 1988 | 7 | 0.400 | 0.396 |

3 | 1989 | 8 | 0.238 | 0.357 |

4 | 1995 | 14 | 0.286 | 0.230 |

5 | 2001 | 20 | 0.180 | 0.174 |

6 | 2001 | 20 | 0.185 | 0.174 |

7 | 2005 | 24 | 0.158 | 0.146 |

8 | 2006 | 25 | 0.136 | 0.141 |

9 | 2007 | 26 | 0.154 | 0.142 |

10 | 2007 | 26 | 0.112 | 0.142 |

11 | 2008 | 27 | 0.148 | 0.138 |

12 | 2009 | 28 | 0.154 | 0.134 |

Steam line

No. | Date, year | τ, years | _{ecw}, mm/year |
_{trend}^{sl}, mm/year |
---|---|---|---|---|

1 | 1985 | 4 | 0.700 | 0.882 |

2 | 1989 | 8 | 0.575 | 0.381 |

3 | 1995 | 14 | 0.200 | 0.194 |

4 | 1999 | 18 | 0.120 | 0.143 |

5 | 2000 | 19 | 0.158 | 0.134 |

6 | 2005 | 24 | 0.096 | 0.101 |

7 | 2006 | 25 | 0.092 | 0.096 |

For feedwater, the dependence for the

_{trend}^{fw} = 1.725· τ ^{–0.75},

and for steam lines, it is

_{trend}^{sl} = 3.307·τ^{–1.10}.

The following dependence is used to determine the pipeline operating time, Δτ, until the minimum allowable thickness (residual life) is reached

Δτ = (_{min} – _{all})/_{ecw},

where Δτ is the residual life; and _{all} is the minimum allowable thickness value, mm.

The smooth lines in Figs

Fig.

Corrosion testers (

To determine the nature of the _{pen} changed practically equally, from 5 to 0.6 μm/year, which made it possible to merge the statistics for all units to obtain the averaged result.

Corrosion penetration rate for CTs at VVER-440 NPPs

No. | Exposure, h | _{pen}, μm/year |
No. | Exposure, h | _{pen}, μm/year |
---|---|---|---|---|---|

1 | 7896 | 5 | 14 | 18456 | 2 |

2 | 8568 | 3 | 15 | 19128 | 1.2 |

3 | 8736 | 4 | 16 | 24960 | 2 |

4 | 8784 | 4 | 17 | 25848 | 0.6 |

5 | 8856 | 3 | 18 | 26304 | 3 |

6 | 8928 | 4.1 | 19 | 33168 | 1.5 |

7 | 9528 | 3 | 20 | 35208 | 2.5 |

8 | 10224 | 3.2 | 21 | 35808 | 2 |

9 | 17208 | 2 | 22 | 41472 | 1.1 |

10 | 17640 | 2 | 23 | 43416 | 1.1 |

11 | 17784 | 3 | 24 | 51624 | 0.97 |

12 | 17904 | 2 | 25 | 51672 | 0.6 |

13 | 18336 | 1 | 26 | 61224 | 0.9 |

27 | 61296 | 0.85 |

Fig. _{pen}, and shows the trend line and its equation in the form of a power dependence.

Corrosion penetration rate on CTs for components with a single-phase medium at VVER-440 NPPs (W_{pen} = 3.1575×τ ^{–0.7956}).

It should be noted that processing and interpretation of inspection data are one of the most complex issues (

The corrosion penetration rate was calculated for CTs of carbon and high-chromium steels installed in pipelines at RBMK-1000 NPP units. While carbon-steel CTs are subject to corrosive wear during operation in an aqueous medium, high-chromium steels and alloys corrode electrochemically which is defined by the presence of the electrode potential.

For steel 20, the exposure time in different pipelines at NPP 1 and NPP 2 is from 10 to 113 thousand hours. The corrosion penetration rate values lie in a range of 1 to 11.5 μm/year.

In a general form, the equations to calculate the corrosion penetration rate _{pen} are reduced to the form

_{pen} = _{o}×τ-^{n}

where _{o} is the coefficient; τ is the

The average error of the corrosion penetration rare calculation, according to the trend with respect to the inspection data, does not exceed 10%.

The _{pen} diagram for the saturated steam pipeline CTs is presented in Fig.

Corrosion penetration rate for a saturated steam pipeline.

Fig.

Corrosion penetration rate for the RBMK-1000 NPP feedwater pipeline (08Cr18N10T austenitic steel).

A similar function describes the dependence of the corrosion penetration rate for CTs in saturated steam pipelines.

_{pen}^{ss} = 0.4126×τ ^{-0.921}.

Data from corrosion testers in stainless-steel pipelines at NPP 2, also with the RBMK-1000 reactor, is presented in Table

Corrosion penetration rate in 08Cr18N10T steel

Exposure time, thsd h | 18 | 26 | 40 | 93 | 133 |

_{pen}, μm/year |
0.20 | 0.17 | 0.13 | 0.10 | 0.08 |

Confidence intervals for the stainless-steel _{pen} = 0.7052×τ ^{–0.4415}), μm/year.

Calculation of confidence intervals for the trend:

Δ_{s}_{t}_{n}_{,β} ·(^{2}/^{1/2},

where Δ_{s}_{t}_{n}_{,β} is Student’s inverted distribution (for _{n}_{,β} = 2.776); and ^{2} is the sampling variance:

where _{i}_{i}_{i}_{i}_{i}_{s}_{t}

In nuclear power, the corrosive impacts of the medium on the material of structural components in conditions of operation are taken into account using allowance _{2} which characterizes the influence of the medium on the structural material in operating conditions (_{2} for corrosion-resistant austenitic alloys and high-chromium steels is assumed to be equal to 0.1 m for an operating time of 30 years which corresponds to the general corrosion rate equal to 3.3 μm/year. This value exceeds the corrosion penetration rate for stainless steels obtained from the

The greatest value of the corrosion penetration depth for an exposure time of 60 years, predicted from the obtained functional dependences, is equal to 2.64 μm, that is, much below 0.1 mm.

The experience of operation, repeated measurements, and an analysis of corrosion tester data show a substantial reduction in the

Apart from the time function built in the EKI-02 and EKI-03 codes, one can use the dependence obtained from repeated measurements on components of a particular NPP. It has been shown that this dependence for different pipelines is the same: ^{b}

It has been shown that, for a time span of up to 20 to 25 years, the time function used in the EKI-02 and EKI-03 codes leads to more conservative residual life values than the functions obtained based on inspection results; the difference is nullified with longer operating times.

The processing of data for corrosion testers in one- and two-phase media has shown the same dependence for the corrosion penetration rate as for pipelines. However, unlike the

Using the functional dependences obtained based on inspection data makes it possible to reduce the conservatism in calculating the

* Russian text published: Izvestiya vuzov. Yadernaya Energetika (ISSN 0204-3327), 2021, n. 1, pp. 29–40.