Preliminary Assessment of Hot Hydrotest of MO34, Unit 3

From December 21, 2018 to March 15, 2019, hot hydrotest was carried out at Unit 3 of NPP Mochovce 3 & 4 (EMO34). Hot hydrotest is the last comprehensive test of unit equipment prior to its commissioning. During the hot hydrotest, the most important technological equipment of the Unit was tested: reactor (without nuclear fuel, with fuel assembly imitators loaded); cooling circuit of the reactor (main circulation pumps, steam generators, pressurizer and other); safety systems for cooling the reactor core (hydro-accumulators, pumps supplying water for the reactor core cooling); important secondary circuit equipment to safely cool down the reactor.

A large number of tests have been carried out during the hot hydrotest according to the pre-approved program. In the course of the hot hydrotest, the reactor and its cooling circuits were heated to an operating temperature of 260 °C and pressurized to 12.2 MPa. At the end of the test, the entire primary circuit was cooled and depressurized.

The subject of the hot hydrotest was also the strength pressure test of the reactor and of the primary circuit to a pressure of 19.2 MPa. Such a test is performed only once during the entire lifetime of the nuclear Unit, in the following operation the testing pressure to verify the strength is 16.8 MPa, and the normal operating pressure is 12.2 MPa.

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

ÚJD SR inspectors and ÚJD SR contracted external support checked the performance of systems and equipment tests and fulfilment of their success criteria directly on site during the entire hot hydrotest. Slovenské elektrárne submitted to ÚJD SR inspectors in Mochovce a preliminary evaluation of the success rate of the tests performed after the completion of the individual substages of the hot hydrotest (there were 5 substages in total), and a preliminary evaluation of the whole hot hydrotest shortly after its completion. At present, the assessment of the Final Report on the Hot Hydrotest is being conducted directly in Mochovce, as part of the ÚJD SR inspection. The Final Report was submitted by Slovenské elektrárne to the ÚJD SR inspectors in the first half of June. Upon completion of the evaluation, ÚJD SR will publish its results on its website.

The knowledge gained and the evaluation of the hot hydrotest carried out so far show the following:

  • All scheduled tests have been performed.
  • During the hot hydrotest, the ventilation and air-conditioning system in the hermetic zone was not set in the expected configuration. This was due to the fact that specialists of Slovenske elektrárne underestimated the impact of not completing some building modifications inside the hermetic zone on the operation of ventilation and air-conditioning system. Therefore, it was not possible to obtain reliable data on the temperature in the hermetic zone, and thus sufficiently demonstrate, what these temperatures will be during normal Unit operation. For this reason, Slovenské elektrárne suggested repeating the heating of the primary circuit from 20 °C to 260 °C and its re-cooling. ÚJD SR accepted this proposal. Repeated heating and cool-down of the Unit will be performed as part of the extended revision of Unit 3, and due to its scope it is not possible to talk about repeated hot hydrotest as a whole. ÚJD SR does not yet have a date of repeated heating and cooling, but it assumes that Slovenské elektrárne will be ready to carry out repeated heating and cool-down of Unit 3 after completion of all work in the hermetic zone, probably in September 2019. Note: The hermetic zone is made up of rooms containing the basic technological equipment of the Unit (reactor, steam generators, main circulation pumps, pressurizer and other). Ventilation and air-conditioning systems create a negative pressure over the outside atmosphere during normal operation of the Unit in the hermetic zone. In addition, ventilation and air-conditioning systems cool the air in the hermetic zone. However, the most important function of the hermetic zone is to create a barrier to prevent escape of radiation into the environment during leaks from the cooling circuit of the reactor in the conditions of emergency operation. The vast majority of measures resulting from the stress tests after the accident in Fukushima NPP, Japan (2011), was focused on increasing 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. Its proper functioning, including the and air-conditioning system, has an irreplaceable role for the safe operation of the Unit.
  • ÚJD SR inspectors requested Slovenské elektrárne to repeat some tests during repeated heating and cool-down, the documentation of which they do not consider sufficient. Namely, it is a repetition of the diesel generators test and the system of their automatic loading, to maximize the load of the diesel generators and convincingly prove their output reserve. Diesel generators provide power supply to important Unit equipment when other proprietary external power sources are lost. Their proper and reliable operation is extremely important for safe cool-down of the Unit in such a mode. ÚJD SR inspectors also requested to repeat also some less extensive tests.
  • The interim results of extended revision show that during repeated heating and cool-down of the Unit, the pressure test of the primary circuit with the pressure of 16.8 MPa will have to be repeated. The reason is the replacement of one electric heater of the pressurizer – its unsatisfactory electric insulation condition was detected during the extended revision.

At present, an extended revision of Mochovce Unit 3 equipment is under way according to a program assessed by ÚJD SR. Its objective is to perform all the necessary inspections of systems and equipment, assess their condition after the hot hydrotest and before the fuel is loaded to the reactor. However, the extended revision also includes the removal of a large number of backlog items and deficiencies in installations of Unit 3 equipment and their tests. It is this activity that is limiting in view of the completion date of the extended revision.

ÚJD SR has repeatedly expressed dissatisfaction with the large number of backlog items and deficiencies in Unit 3 of MO34. Given that a synergy effect of a greater number of seemingly unrelated minor deficiencies could occur, which could potentially cause complications during the commissioning of the Unit, ÚJD SR reiterates that it will not permit loading of nuclear fuel to the reactor and the start of commissioning of Unit 3 until these are removed.

Updated: 13.01.2022