KEY FINDINGS OF SAFETY REVIEW OF NPP IN REGARD TO PASSIVE SYSTEMS

This section presents the example of key findings regarding passive systems, found during the fulfilment of safety review of Novovoronezh NPP-2 and Leningrad NPP-2 based on regulatory approach described in Section 3.

Today the safety review of Novovoronezh NPP-2 and Leningrad NPP-2 are in different stage of implementation. For Novovoronezh NPP-2 review of preliminary version of Final SAR [11] is finished – the unit got a license for operation. For Leningrad NPP-2 review of preliminary safety analysis report (PSAR) is finished [12] and review of preliminary version of final SAR is just started. According to this the different level of issues were revealed in the course of safety review.

4.1. Novovoronezh NPP-2

Findings about aerial PHRS SG

evaluation of the impact of non-condensable gases inside PHRS’s steam condensate tract on the efficiency of system heat exchangers;

PHRS efficiency in case of considerable coolant loss, active ECCS failure, and non-condensable gases entering heat exchange area of the steam generator;

PHRS efficiency in case of on-site fire (multifactor impact of the fire including high temperature and air humidity decrease on the entrance of the PHRS air intake, possible downflow air movement streaming around the containment, reverse flow in the PHRS air ducts on the outer side of the compartment opposite the fire can deteriorate performance of a PHRS heat exchangers);

PHRS efficiency in case of external impacts (e.g. wind impacts);

substantiation of the PHRS start-up characteristic. Substantiation of the PHRS start-up time under the minimum and maximum external temperatures. Substantiation of reliability of the PHRS lock valve and air controller.

Findings about HA-2:

substantiation of the efficiency of the HA-2 system, taking into account joint operation of HA-2 and PHRS (calculated and experimental substantiations of the efficiency of passive safety systems operation in case of LOCA, taking into account the interaction of primary circuit, passive systems (HA-2 and PHRS), containment and the impact of non-condensable gases. Substantiations should prove the effectiveness of the passive safety systems, as well as show the available reserves of time before drying of the reactor core);

substantiation of the HA-2 flowrate characteristic, taking into account the possible range of places and size of the primary circuit pipe rupture, and the impact of other systems (e.g. HA-1, active ECCS).

D. ROGATOV

36

Current state

As a result of the large scale complex of the calculated and experimental substantiations of the HA-2 and PHRS performance efficiency fulfilled by the Licensee significant part of issues was eliminated or significance of issue to safety was reduced to level of recommendations.

Positive conclusions of the final review were based on assessment of the topical reports which contained results of background calculations, tests and experimental justification based on specially constructed experimental stands that were submitted by the Licensee to the Regulatory Body, along with additional information as provided in Chapter “Accident analysis” of SAR.

4.2. Leningrad NPP-2 4.2.1. Findings about PHRS SG

experimental validation or calculations of PHRS SG capacity (test rig /mockup experiments data).

computational analysis does not include information about closing relations, drag coefficients, etc. As a result, it is not possible to assess correctness of calculation results.

reliability analysis of PHRS SG is not fully adequate: there is no information about the values for probability of system components failures used while the analysis.

it is not shown by the licensee what signals and devices will be used to add the water inventory in emergency heat removal tanks after 24 hours of PHRS SG operation.

4.2.2. Findings about containment PHRS

experimental validation or calculations of containment PHRS capacity (test rig /mock-up experiments data).

reliability analysis of containment PHRS is not fully adequate: there is no information about the values for probability of system components failures used while the analysis.

it is not shown by the Licensee what signals and devices will be used to add the water inventory in emergency heat removal tanks after 24 hours of Containment PHRS operation (it is a common issue for PHRS SG and containment PHRS as they use one tank as water source).

4.2.3. Current state

The safety review shows that the PSAR submitted by the licensee is incomplete, however, the revealed shortcomings are not critical and can be rectified in further stages of design realization. The preminaliry version of FSAR is under consideration now.

5. CONCLUSION

Today passive safety systems are one of the important elements (along with active systems) of defence-in-depth of NPPs and provide significant contribution to the safety of the NPP, especially in case of the accidents involving SBO and loss of UHS. In this regard, an important issue is assessing their compliance with existing regulatory requirements, as well as the issue about necessity of developing special regulatory requirements for their assessment.

The experience of safety review of new passive systems has shown that the available regulatory requirements are sufficient to assess their safety and the effectiveness of their performance of safety functions. At the same time, the main attention should be paid to the verification of sufficiency and completeness of the justifications (experimental and calculated) of these systems.

In addition, an important issue that should remain in the sphere of the regulator's attention is the evaluation of the experience of operating passive systems (the presence of failures, test results, the absence of negative influence on the operation of other systems), the results of which, as expected, will be received and included in the SAR within the next years.

D. ROGATOV

37 FIG. 1. Configuration of passive safety systems. Novovoronezh NPP-2.

FIG. 2. Configuration of passive safety systems. Leningrad NPP-2.

D. ROGATOV

38

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http://www.rosenergoatom.ru/resources/8166b5004aef9f04b6c0f7ec7604272f/

broshure_nw_aep_site.pdf

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[12] Safety review of the construction of Unit 1 of Leningrad NPP-2 in regard with the adjustment of the PSAR (DNP-5-3465-2016). SEC NRS, Moscow (2016).

[13] Common position addressing Fukushima-related issues. MDEP Design-Specific Common Position No VVER-01.

VVER Working Group, 2017.