About leak detection systems in the framework of LBB concept application at Russian NPPs

Abstract

Application of "leak before break" concept to reactor coolant circuit is obligatory for RF NPPs and the success depends also on fulfilment of requirements to the leak detection systems specified in RF national standard. During 1999-2020 requirements to the leak detection systems were permanently improved in regulatory documents. The most important changes in requirements have been done according to the Federal law on ensuring the uniformity of measurements. Paper gives comparative analyses of these evolutionary changes of requirements as well as details of their implementation during design and manufacturing of leak detection systems to supply NPP Units with VVER-440/1000/1200 and RBMK-1000 reactor facilities. Recently approved in the Russian Federation, the Federal norms and rules (FNR) in the field of atomic energy use ensure the continuity of the general requirements for reactor coolant leak monitoring (detection) systems (LDS) at nuclear power plants (NPPs) in relation to the previously valid regulatory documents.

1. Introduction

Actual Russian Federal Norms and Rules (FNR) in the field of nuclear energy use [ 1-3 ] ensure continuity of general requirements for reactor coolant leak monitoring (detection) systems (LDS) at nuclear power plant (NPP) in relation to former regulatory documents [ 4-6 ].

Specific requirements for LDS were first defined in normative guidelines on the "leak before break" (LBB) concept [ 7 ] for use at Russian NPPs. The guideline [ 7 ] had been developed by the analogy with international practice of the LBB concept application: the publications [ 8-9 ] contain references to LDS manual [ 10 ].

2. Requirements to LDS in 1999-2018

The Federal Law on the unity of measurements No.102 [ 11 ] (2008) led to changes in the technical policy of Russian nuclear power industry. Indicator-type LDS (as was actually the case in [ 7 ]) gradually began to be replaced by measurement-type LDS with technical requirements providing explicitly the necessary characteristics of measurement accuracy.

It should be noted that the regulatory documents on LBB concept applications [ 7, 12-13 ] did not take into account the requirements for the LDS of the primary coolant, which were specified in paragraph 5.2.5 PNAE G01-036-95 (NP-006-98) [ 6 ] in relation to the detection of minimum leak through the pressure boundaries of the first circuit of the VVER type reactor: "specify the minimum amount of leakage that can be detected by the methods used." This requirement is fully contained in the current document NP-006-16 [ 3 ] in subsection 5.2.6 of the mandatory Annex No. 3.

The application of this requirement under the conditions of a complete ban on the operation of the NPP Unit with through wall defects in equipment and pipelines [ 14 ] provided the way to effective LDS transfer from the indicator status to the measuring one by metrological confirmation of the LDS characteristics: sensitivity Q0 (one of the main parameters used in the algorithms for proving the LBB concept in [ 7, 12-13 ]) and the minimum leakage detected value Qmin (should be evaluated for the LDS application).

The relationship of these parameters through the relative measurement error of leak flow rate δ can be written as Qmin = (1 + δ)Q0.

The normative guideline [ 12 ] specified, as metrological requirement, that "the relative error in measuring the leak flow rate (relative to the measured value) is not more than ± 50 %".

Also [ 12 ] contained the requirement that "the LDS must ensure the reliability of leak control at a level not lower than 0.9. The reliability indicator of the LDS leak control must be justified in the design documentation for the system". The document [ 13 ] defined the indicator value equal to 0.8.

In contrast to [ 12 ], [ 13 ] the national standard on the LBB concept application (GOST R 58328-18 [ 15 ], valid from 01.01.2019), has no any mention of the reliability indicator of the LDS leak control.

The requirements for the lower limit of LDS sensitivity Q0 – 1.9 kg/min for pipelines with 150 ≤ Dnom < 750 mm and 3.8 kg/min for pipelines with Dnom ≥ 750 mm – are coincide in the regulatory documents [ 12-13, 15 ].

Results of comparing the LDS requirements specified by documents on LBB concept applications are presented in detail in table 1.

3. Characteristics of VVER type LDS

For NPP Units with VVER-440/1000, JSC IPPE developed LDS for the primary (SACT and SCTV) and secondary (SACT-2K, SCTV-2K and SCTV-2P) circuits in accordance with the technical specifications [ 16-17 ]. The LDS are characterized by relative error of ±50% for

LDS status

Requirement for intervention of

the NPP Unit control panel operator in the operation of LDS

Requirement to separate leaks from identified and unidentified sources

Number of subsystems in LDS

Should leak monitoring be

performed based on the radiation activity parameter?

Requirements for LDS Integral

Level when NPP unit operating at rated power: - leak rate sensitivity is not worse - the upper range of leak flow measurement - the time interval of the detection and measurement of leaks - the relative error of a leak flow measurement - error in measuring the leak location coordinates

The requirement to LDS at the lowest recorded flow rate of unidentified leakage

The requirement to LDS to function in the mode of hydraulic p.8.1, paragraph 2 (pneumatic) tests

The requirement to LDS to

function in regimes of NNUE and accident situation The requirement to LDS to detect and identify leaks of equipment and pipelines when the unit is the leak flow rate measurements (table 2). The listed LDS subsystems provide monitoring of leaks for pipelines under thermal insulation (except for SCTV-2P) using acoustic sensors and humidity control sensors, determining the leak location or the distance from the acoustic sensor to the leak location with absolute error ±2 m. The SCTV-2P subsystem is able to locate leak position up to the compartment of the NPP Unit. Certificates of approval of the type of measuring instrument (CATMI) have been issued for LDS subsystems of acoustic control and humidity control (table 2). These certificates confirm the minimum leakage detected value Qmin at least 1.5 l/min. So requirements of the [ 1-6 ] and [ 12-13, 15 ] are fulfilled. - -139rep,8la./p8cmlic.t/p5omiirne,rcilinciu(tnfeoio(etmof)rsol)is3rnt:etghaem --c31ir,,c98ukklpagpgti/./ip5mmoe.n3ilinin.pn1(ie(.fp5fsooe:)rrlionmtehasei)nr -(31,580 k≤p-g.1DD/m,n9.m2oimnk.m2g<(,)/Dm“7na5io”nm0: ≥m7m5)0 not established

not established p.8.5, item 3: no more than 1 hour don’t need to

specify p.8.5, item 3: recommendation ± 2 m no no no

p.5.3.1.5: no more than 1 hour (as possible)

p.5.3.1.5: reduced (modified) to the measured value, no more than ± 50% p.B.2.5: yes no no p.6.2.3 p.B.2.1 p.B.2.2 no no no no operating in start-up (stop) modes)

Need to approve the type of

measuring instrument on LDS

Separate requirement for approval

of the type of measuring instrument to measuring channels

Requirements for reliability of leak detection

Requirement to LDS to specify

the minimum leakage value that must be detected using the methods used (from NP-006-98 and NP-006-16)

The method for measuring leak

parameters should be developed in accordance with the requirements of GOST R 8.9322017

LDS software must be verified

and passed the procedure for confirming compliance with the requirements of GOST R 8.6542009

Requirement for certification

LDS software/software tool in Rostechnadzor bodies in accordance with current requirements Seismic qualification requirement for each LDS subsystem p.8.6.1: requirements to

minimize: - the probabilities of false operation; - probability of missing an event no no no no no no p.8.7.1, item 4:

yes; p.8.9 - see below1:

Requirement for identification

p.8.7.1, item 6: yes and seismic qualification of AC p.8.9 - see below2 power sources for each LDS

The requirement to ensure the possibility of calibration of measuring channels in the operating mode

p.8.10:

yes 1 The drainage level monitoring system and at least one of the three leak monitoring subsystems must be qualified to perform the design functions corresponding to the maximum design earthquake for NPP Unit; 2 The earthquake-resistant LDS must be powered from a earthquake-resistant AC power source 3 LDS in the table 2 meet the requirements of NP-006-98, NP-006-16 and p.6.2.3 of GOST R 58328-2018. 4 Qmin – Minimum detectable amount of coolant flow through a leak; δ – Limits of permissible relative error in determining the flow rate of the coolant through a leak, %

LDS with relative humidity and temperature 55687- measurement channels 13 Manufacturer (Vendor): IPPE JSC https://info.metrologu.ru/grsi/grsi_226346.html 28.11.2018; 11.04.2023/

2 years

Characteristics of RBMK type LDS

Full-scale LDS for RBMK NPP Units - ASOTT - were developed by NIKIET JSC. The details of certified measuring procedures (CMP) for full-scale LDS at four RBMK NPP Units are provided in table 3.

The CMPs (FR.1.29.2016.24188 ÷ FR.1.29.2016.24191) contain formulas to calculate flow rate measurement error as applied to individual subsystems and LDS as a whole. The formulas are the same for Units of the first and second generations of RBMK NPPs, which have differences in layout of technological equipment. The formulas do not take into account presence or absence of thermal insulation on

Registration ID

Name of the CMP Valid until/ Verification interval

SCTV, SCTV-2K

/ Technical specification E.091.7326.01 TU 1,0 l/min

SCTV-2P / ±50%

Technical specification E.091.7326.01 TU ±2m «-» accurate to the room piping outer surface. Calculations using these formulas, as applied to full-scale LDS give for the lower flow rate boundary 114 kg/h the corresponding relative error equal to ±75%, and for the upper boundary 1140 kg/h the corresponding relative error equal to ±65%. In other words, the threshold of ±50% specified by the regulatory documents [ 12-13, 15 ] has been exceeded.

For LDS of other two RBMK NPP Units (Smolensk NPP Unit 1, Kursk NPP Unit 3) CATMI have been issued also (tables 4 and 5). Formulas to calculate flow rate measuring errors of individual subsystems and LDS as a whole specified in the CATMI (tables 4 and 5) are similar to the ones specified in CMP (table 3). Therefore, the estimates given above are valid and also do not meet the requirements of regulatory documents [ 12-13, 15 ].

Number of measuring channels1 4 4 3 3 4 1 Measuring channel which characterized the specific way to leak detection (for example, humidity etc.) Subsystem of ASOTT-V (ASOTT-humidity) of Kursk NPP Unit 3 / TU 4389-007-73555750-2012 https://fgis.gost.ru/fundmetrology/registry/4/items/371903 Subsystem of ASOTT-T (ASOTT-Temperature) of Kursk NPP Unit 3 / TU 4389-008-73555750-2012 https://fgis.gost.ru/fundmetrology/registry/4/items/371904 Subsystem of ASOTT-A of Kursk NPP Unit 3 / TU 4389006-73555750-2012 https://fgis.gost.ru/fundmetrology/registry/4/items/371905 Subsystem of ASOTT-Ac (ASOTT-acoustic) of Kursk NPP Unit 3 / TU 4389-009-73555750-2012 https://fgis.gost.ru/fundmetrology/registry/4/items/371906 Full-scaled automated LDS of Kursk NPP Unit 3 / TU 4389-010-73555750-2012 https://fgis.gost.ru/fundmetrology/registry/4/items/371907

Information and formulas provided in CMPs (table 3) for each LDS measuring channel to assess flow rate measurement error and values of Qmin as applied to individual subsystems and LDS as a whole are given in table 6. Estimations of flow rate measurement error and values of Qmin for a set of G values are provided in table 7.

Measuring channel / ASOTT-P

+ 3,38 ∙ ( В − Н) * G – value of the measured mass leak flow rate, kg/hour; GB and GH – upper and lower level of measurement range of mass leak flow rate, kg/hour; P - confidence probability.

The time between the leak occurrence and the output of the measurement result with the specified error does not exceed 1 hour ± 74,0 % ± 70,09 % ± 65,0 %

Information and formulas provided in CATMI (tables 4 and 5) for each LDS measuring channel to assess flow rate measurement error and values of Qmin as applied to individual subsystems and LDS as a whole are given in

The results discussed above indicate existing deviations from the requirements of [ 15 ] at seven RBMK NPP Units and absence of official data for LDS status of Leningrad Units 2-4 with RBMK.

The non-compliance of LDS at NPP Units with the requirements of GOST R 58328-2018 [ 15 ] violates compliance of these NPP Units with the requirements of paragraph 3.3.3 of NP-001-15 [ 1 ] and, consequently, noncompliance with the requirements of paragraph 21 of NP010-16 [ 18 ] remains uncompensated by improper LBB application. This situation as well as other deficiencies in the LBB concept applications at NPP Units as stated in the report [ 19 ] seems worth to be discussed with the Utility and NPP staff.

5. Conclusion

The article considers the evolution of requirements to LDS in Russian regulatory documents in 1999-2020.

The article provides details about LDS status at VVER and RBMK NPPs as certified measuring instrument for LBB application according to GOST R 58328-2018.

Some deviations from the actual requirements are highlighted and recommended for further discussion and improvement.

Acknowledgment

The reported study was funded by RFBR according to the research project № 19-07-00455. δ δ δ δ δ

About the authors

References

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[14] PNAE G- 7 - 008 - 89 . Rules of arrangement and safe operation of equipment and pipelines for nuclear facilities . http://docs.cntd.ru/document/1200044519.

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[17] JSC IPPE. Technical specification E. 091 .7326.01 TU.

[18] NP-010-16 . Rules of design and operation of localizing devices for NPP . https://files.stroyinf.ru/Data2/1/4293755/429375579 3.htm.

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