Final evaluation of the hot hydrotest by ÚJD SR

The hot hydrotest of Unit 3 of NPP Mochovce 3 & 4 (MO3&4) was carried out from 21 December 2018 until 15 March 2019. During the hot hydrotest, the reactor and its cooling circuits were heated up to an operating temperature of 260 °C and pressurized to 12.2 MPa. At the end, the entire primary circuit was cooled and depressurized. The most important technological equipment of the Unit was tested: reactor (without nuclear fuel, with loaded imitators of fuel assemblies), reactor cooling circuit (main circulation pumps, steam generators, pressurizer and other), safety systems for reactor core cooling (hydro accumulators, pumps that supply water for cooling the reactor core) and equipment for safe cooldown of the reactor. Part of the hot hydrotest was also the strength pressure test of the reactor and the primary circuit to a pressure of 19.2 MPa. Such a test is performed only once during the whole life of the nuclear unit, in the following operation the test pressure to verify the strength is 16.8 MPa, the normal operating pressure is 12.2 MPa.

The hot hydrotest was interrupted twice (from 18 January till 7 February, and from 21 February till 25 February) for reasons not directly related to the equipment under test. In both cases the cause of the interruption was a collision of the finishing construction work with the tests of the hot hydrotest.

ÚJD SR inspectors checked the performance of equipment and system tests and the fulfilment of their success criteria on the spot during the whole hot hydrotest. Slovenské elektrárne submitted to the ÚJD SR inspectors in Mochovce a preliminary evaluation of the success rate of performed tests after the completion of individual sub-stages of the hot hydrotest (there were 5 sub-stages in total), and the preliminary evaluation of the whole hot hydrotest shortly after its completion. The final report was submitted by Slovenské elektrárne to ÚJD SR inspectors in the first half of June.

Lessons learned from the hot hydrotest, the submitted test results and the preliminary and final evaluation of the hot hydrotest, show the following:

• All planned tests have been performed.
• During the hot hydrotest the hermetic zone HVAC system was not set to expected configuration. The reason for this was that the specialists of Slovenske elektrárne did not fully appreciate the impact of incomplete conditions of some structural modifications within the hermetic zone on the operation of the ventilation system. Therefore, it was not possible to obtain reliable temperature data in the hermetic zone and sufficiently demonstrate what these temperatures would be during normal Unit operation. For this reason, Slovenské elektrárne suggested repeating the primary circuit heating from 20 °C to 260 °C and then cooling it down again. ÚJD SR accepted this proposal. Repeated heating and cooling down of the Unit will be carried out as part of the extended revision of Unit 3, and due to its extent, it is not possible to talk about repeated hot hydrotest as a whole. ÚJD SR will inform about the course of the repeated heating of Unit 3 on its website.

Note: Hermetic zone consists of rooms containing the basic technological equipment of the Unit (reactor, steam generators, the main circulation pumps, pressurizer and others). The most important function of the hermetic zone is to provide a barrier to prevent leakage of activity into the environment in case of releases from the reactor cooling circuit under emergency operation conditions. The vast majority of post-accident actions resulting from the stress tests after the accident in Fukushima, Japan (2011) were aimed at enhancing the reliability and robustness of this barrier. The hermetic zone is the last and the most important barrier that separates the reactor and its systems from the environment. HVAC systems create negative pressure in the hermetic zone against the outside atmosphere during the normal operation of the Unit. In addition, HVAC systems cool the air in the hermetic zone. The proper functioning of the hermetic zone, including ventilation systems, has an irreplaceable role for the safe Unit operation.

ÚJD SR inspectors asked Slovenské elektrárne to provide a more detailed analysis of the results of some tests that were carried out during the hot hydrotest of Unit 3. The reasons for such analyses were as follows:

• Hot hydrotest conditions differed in some aspects from normal Unit operation. The reasons for these differences were objective (e.g. the reactor core was loaded with imitators of fuel assemblies and not the real nuclear fuel), but also related to the uncompleted state of some work on the Unit (e.g. uncompleted construction work in the hermetic zone that ultimately influenced the regime of heat removal from the hermetic zone by using HVAC systems).
• The measured and recorded parameters during some tests required more detailed evaluation. The subject of evaluation was, in particular, whether the whole set of parameters is sufficient to fully demonstrate the proper operation of the given system or the correct test run.

Slovenské elektrárne submitted to ÚJD SR the required detailed analyses. ÚJD SR inspectors evaluated them and on the basis of this evaluation, ÚJD SR:

• Accepted test result of the system for Unit cooldown after a seismic event (earthquakes). The subject of the required analysis was the fact that the achieved cooling rate of the Unit was somewhat higher than the test program stated in a certain range of primary circuit temperatures. The SE analysis convincingly demonstrated that the cause was an objective difference between the hot hydrotest conditions and the conditions of future operation of the Unit, for which the system for cooldown after a seismic event is designed – the reactor core was not loaded with real fuel, but only by its imitators. For this reason, the balance of produced and removed thermal energy during the tests was different than expected during the future operation of the Unit (the residual core power was missing). This fact led to a slight excess of the primary circuit cooling trend (the Unit cooled somewhat faster than required by the test program). However, the test showed full functionality of the system for cooldown of the Unit after seismic event and its sufficient sizing. The SE analysis shall be annexed to the test results report as a justification for the cause of the cooling trend to exceed the one required by the program.
• Did not accept the result of the flow rate test of the essential service water through the cooler of the spraying system, in the “large release“ mode. During the tests, the flow rate of the essential service water through the cooler was slightly lower than the set success criteria. The reason was a higher parasitic flow through other consumers of essential service water. ÚJD SR ordered the test to be repeated during repeated heating under conditions that are identical with normal operation of the essential service water system.
• Did not accept the method of documenting the results of the test of automatic loading of diesel generators. Tests of automatic loading of diesel generators were performed at primary circuit temperature below 140 °C and above 140 °C. All the drives that are part of diesel generators automatic load program have been put into operation without faults, and the diesel generators have worked reliably in the given automatic sequential load mode. However, some electrical parameters have not been capture in sufficient quality to prove the correct operation of the automatic diesel generators load regulator and its excitation system (especially voltage drop and frequency change of diesel generators immediately after load connection). Capturing such parameters requires the installation of non-standard measuring apparatus capable of recording hundreds of data per second. ÚJD SR requested to repeat the test of automatic diesel generators loading together with such a measuring apparatus.

Note: Diesel generators belong to the most important Unit safety systems. In case of loss of own and external sources of power supply, they are automatically activated and provide power supply for important systems and equipment of the Unit. Proper and reliable operation of diesel generators is of paramount importance to ensure important safety functions

• Rrequested to elaborate a report on the validation of the automatic reactor protection system and the safety assurance system (SZB or ESFAS) on the basis of detailed documentation of their activity during changes in technological parameters expected during repeated heating and cooldown (change in primary circuit temperature from approx. 50 °C to 260 °C, pressure change in primary circuit from atmospheric to 12.2 MPa, or up to 16.8 MPa during pressure test).
• Requested the validation of the correctness of the operating procedures for Unit 3. The hot hydrotest was carried out under a specific program. Repeated heating and cooling of Unit 3 will be performed based on request of ÚJD SR according to operating procedures, which will be minimally adjusted to reflect different conditions compared to normal operation.

Intermediate results of extended revision indicate that during repeated heating and cooldown of the Unit it will be necessary to repeat the primary circuit pressure test with a primary circuit pressure of 16.8 MPa. The reason is the replacement of one electric heater of the pressurizer, because during the extended revision it was found that it has an inadequate electric insulation condition.

At present, Unit 3 of Mochovce 3 & 4 is undergoing extended revision of Unit 3 equipment according to a program assessed by ÚJD SR. During the extended revision, all prescribed inspections of systems and equipment were performed to assess their condition after the hot hydrotest and before the fuel loading to the reactor. However, part of the extended revision is also removal of a large number of backlogs and deficiencies from installations of Unit 3 equipment and from their tests. It is this activity, together with repeated heating and cooldown of the Unit, as the limiting factor in terms of the deadline for completion of the extended revision.

ÚJD SR has repeatedly expressed its dissatisfaction with the large number of backlogs and deficiencies on Unit 3 of MO3&4. Considering that a synergic effect of a large number of seemingly unrelated minor deficiencies could occur, potentially causing complications during commissioning of the Unit, ÚJD SR stresses that it will not permit loading of nuclear fuel into the reactor and start of commissioning of Unit 3 without their removal.