Preliminary Assessment of the Implementation of the Reheating Program of Unit 3 of NPP Mochovce

ÚJD SR, in its report dated 3 September 2019 (Final evaluation of hot hydrotest of Unit 3 of NPP EMO3&4) provided a detailed opinion on the reasons for requesting reheating of Unit 3 on its website.

In the preparatory phase before reheating, ÚJD SR inspectors carried out inspections of the preparedness of Unit 3 of NPP Mochovce with emphasis on the following areas, in particular:

• Preparedness of hermetic zone and airtight zone of Unit 3 (rooms that make up the hermetic zone enclosure) – a thorough inspection of a full completion of civil works, installation works and housekeeping in the rooms was carried out,
• Preparedness of HVAC systems of hermetic zone and airtight zone – inspection of full completion of HVAC system tests and protocol confirmation of their results,
• Completion of replacement of poor quality signal cables, testing of their functionality and protocol confirmation of performed tests after replacement of cables – replacement of signal cables was carried out to the extent specified by ÚJD SR (205 pcs of cables). In the phase after reheating, the remaining 48 pcs cables, which are related to secondary equipment and do not cross the main cable routes, will be replaced. ÚJD SR approved the list of these additionally replaced cables,
• Preparedness of the cable rooms in the main generating unit and the electrical buildings with the emphasis on meeting fire protection requirements,
• Status of implementation of programs to assess electromagnetic compatibility for Unit control systems and equipment controls – during repeated heating, measurements of electromagnetic fields induced by the operation of the equipment and systems of the Unit will continue.

The result of all inspections carried out confirmed compliance with the conditions of Unit 3 preparedness for reheating according to the approved program.

During the reheating itself, ÚJD SR inspectors and its external contractual support specialists performed inspections at Unit 3 on a daily basis, attended the most important tests and performed preliminary evaluation of their results.

Program of reheating of Unit 3 Mochovce was realized from 14 December 2019 till 13 January 2020.

Time course of the most important milestones of reheating:

Preparatory stage of reheating (before starting to increase the temperature of Unit 3 primary circuit):

11 Dec. 2019

• Testing of the operation of high pressure pumps of emergency make-up water for the primary circuit in all three safety systems (without any deficiencies).

12 Dec. 2019

• Testing of automated gradual loading of diesel generators performed on all three safety systems – program of loading diesel generators at primary circuit temperature lower than 140 °C. The preliminary evaluation of the test is satisfactory. During the test, a large range of electric parameters of diesel generators was captured on request of ÚJD SR. Reading of these parameters was performed by non-standard apparatus, the results are being evaluated. The purpose of these measurements is to demonstrate the proper operation the start-up and on-power regulators of diesel generators and their power margin in the given mode.

14 Dec. 2019

• Leak test of primary circuit after its full filling with overpressure of 0.5 MPa (without any deficiencies).
• Leak test of primary circuit with overpressure of 2.5 MPa (without any deficiencies).

Start of Reheating:

14 December 2019 at 11:36 a.m., start-up of the first main circulation pump, start of the primary circuit temperature increase, initial temperature of primary circuit of 30 °C.

Course of Reheating:

16 Dec. 2019

• The mean primary circuit temperature reached 130 °C.
• Inspection of primary circuit equipment at nominal pressure of 12.26 MPa (without any deficiencies).
• Leakage and strength pressure test of the primary circuit (pressure in the primary circuit at the strength pressure test of 16.8 MPa). The results of the strength test were in accordance with the criteria (compliant test), the leak pressure test (overpressure of 13.7 MPa) was evaluated as failed – minor leaks through the gasket at 4 valves of the primary circuit were found by inspection. The identified leaks had to be removed in order to continue the reheating process.

17 Dec. 2019

• Pressure tests of steam generators with overpressure of 5.0 MPa performed (without any deficiencies).
• Interrupted implementation of the reheating program, start of lowering the temperature of the primary circuit before repairing leaks on the valves of primary circuit, identified during the primary circuit leak pressure test.

18 Dec. 2019

• Primary circuit cooled down, preparation for elimination of leaks on the valves identified during pressure test, shut down of the main circulation pump and primary circuit depressurized.

26 Dec. 2019

• Completed repair of leaks on the valves identified during the pressure test on 16 December 2019. Main circulation pump start-up, primary circuit heating started.

27 Dec. 2019

• Primary circuit heated to 130 °C.
• Repeated leak pressure test of the primary circuit by overpressure of 13.7 MPa after the repair (re-sealing of separating plates) of valves – satisfactory.

28 Dec. 2019

• Continued heating of the primary circuit after a successful leak pressure test of the primary circuit.
• Primary circuit heated to 140 °C.
• Temperature of the primary circuit reached 200 °C.

29 Dec. 2019

• Program implemented to verify cool-down of the primary circuit through the discharge stations into the atmosphere, the steam reduction station for station-service consumption and reduction station for the cool-down – without any deficiencies.
• Primary circuit heated to 245 °C.

30 Dec. 2019

• Primary circuit heated to 260°C, primary circuit temperature stabilized at 260 °C and pressure of the primary circuit at 12.25 MPa. Start of the program implementation to measure temperature fields and negative pressure in the hermetic zone. Proven setting of normal operating mode for all HVAC systems in the hermetic zone

5 Jan. 2020

• Completed measurements of temperature fields and negative pressure in the hermetic zone at primary circuit temperature of 260 °C.
• Preliminary evaluation of achieved results (in accordance with the criteria).
• Start of cool-down of the primary circuit after realization of all planned tests and measurements at primary circuit temperature of 260 °C.
• Primary circuit temperature reduced to 245 °C.

6 Jan. 2020

• Primary circuit temperature reduced and stabilized at 160 °C to implement the tests at this temperature

7 Jan. 2020

• Start of testing automated gradual loading of diesel generators on all three safety systems – program of diesel generator loading at primary circuit temperature higher than 140 °C. The tests also include flow rate measurement of the essential service water through the coolers of the spraying system on all three safety systems.

9 Jan. 2020

• Completed test of automated gradual loading of diesel generators on all three safety systems – program of loading diesel generators at primary circuit temperature higher than 140 °C. The preliminary evaluation of the test is satisfactory. During the test, a large range of electric parameters of diesel generators was captured based on a request from ÚJD SR. Reading of these parameters was performed by non-standard apparatus, the results are being evaluated. The purpose of these measurements is to prove the proper operation of the excitation and power controllers of diesel generators and their power margin in the given mode. During the tests of automated gradual loading of diesel generators, the flow rate of essential service water through the spraying system coolers was measured on all three safety systems (flow rates achieved in accordance with the success criteria).

11 Jan. 2020

• Continued cooling down of the primary circuit after completion of all the tests that have been planned at 160 °C temperature.
  Primary circuit temperature reduced to 130 °C, start of implementation of the steam generator passivation program (passivation – surface treatment).

13. Jan. 2020

• Completion of the steam generator passivation program.
• Continued cooling down of the primary circuit from 130 °C.

End of reheating:

13 January 2020, at 3:50 p.m. – the primary circuit cooled down to 38 °C, completed implementation of the reheating program of Unit 3.

Based on the knowledge gained from the inspections carried out during reheating, the results of the tests carried out and other documents, the following can be stated:

a) All scheduled tests have been carried out.

b) The strength pressure test of the primary circuit with an overpressure of 16.8 MPa was successfully carried out.

c) The leak pressure test of the primary circuit with an overpressure of 13.7 MPa was successfully realized. This pressure test was performed on 27 December 2019 after removal of minor leaks on the separating plates of the primary circuit valves that were detected during the failed first leak test of the primary circuit performed on 16 December 2019.

d) Pressure tests of all steam generators with an overpressure of 5.0 MPa were successfully performed.

e) The program of reheating was interrupted from 17 December 2019 until 26 December 2019. The reason for the interruption was the cooling of the Unit, its preparation for repair of detected leaks on the valves of the primary circuit and the repair itself.

f) Testing of automated gradual loading of diesel generators on all three safety systems – program of loading diesel generators at the primary circuit temperature lower than 140 °C. Preliminary evaluation of the test is satisfactory. During the test, a large range of electric parameters of diesel generators was captured based on a request from ÚJD SR. Reading of these parameters was performed by non-standard apparatus, the results are being evaluated. The purpose of these measurements is to prove the proper operation of the excitation and power controllers of diesel generators and their power margin in the given mode.

g) Testing of automated gradual loading of diesel generators on all three safety systems – program of loading diesel generators at the primary circuit temperature higher than 140 °C. Preliminary evaluation of the test is satisfactory. During the test, a large range of electric parameters of diesel generators was captured based on a request from ÚJD SR. Reading of these parameters was performed by non-standard apparatus, the results are being evaluated. The purpose of these measurements is to prove the proper operation of the excitation and power controllers of diesel generators and their power margin in the given mode.

h) During the tests of automated gradual loading of diesel generators, the flow rate of essential service water through the spraying system coolers was measured on all three safety systems (flow rates achieved in accordance with the success criteria).

i) The normal operating mode of the HVAC system of the hermetic zone and the airtight zone (rooms that make up the enclosure of the hermetic zone) was set. During 6 days, measurements of temperature and negative pressure in the hermetic zone were taken, the operation parameters of HVAC systems were recorded and also the impact of their configuration on the temperature and negative pressure in the hermetic zone. Preliminary evaluation of these measurements is satisfactory.

j) During the reheating, data for validation of the operating procedures for the personnel were obtained. It has been proven that the operating procedures are developed at a very good standard and are fully applicable to the Unit heating and cooling stages. All identified shortcomings in the documentation are duly registered and will be used for further improvement of the procedures.

k) Data were obtained for validation of the automatic reactor protection system and the safety assurance system. During reheating, these systems run fully in accordance with the design.

The following deficiencies were identified during Unit reheating:

l) Stated reduced insulation resistance of 4 electric heaters of the pressurizer – the failure is being analysed by experts of Slovenské elektrárne in order to take necessary action to prevent its recurrence.

m) Detected reduced air quality in the hermetic zone -probably due to vapours from applied coatings. A sample of air mass was taken and is being analysed in a specialized laboratory. Appropriate action will be taken based on the results of the analysis.

Slovenské elektrárne, a. s., and its contractors are currently working on detailed evaluations of individual tests of the reheating of Unit 3, analysing the failures and detected deficiencies and taking corrective actions.

After submitting the test results (in particular the tests referred to in paras f, g, h, i, j, k), submitting a cause analysis and appropriate corrective actions to the deficiencies found (as stated under paras l) and m), and evaluating the submitted documents, ÚJD SR will issue its final assessment of the reheating of Unit 3 of NPP Mochovce.