Corresponding author: Igor N. Gusev (gusevin@nvnpp1.rosenergoatom.ru)

Academic editor: Yury Korovin

for developing proposals for adjusting the operation control algorithms in case of malfunctions and emergency modes with the main equipment shutdown and power unit protection actuation; and

for verifying design solutions for updating the power unit systems, which are associated with the use of new equipment or changes in flow diagrams.

The dynamic stability of a power unit in transient modes is the ability of its systems and equipment to provide the design limits for changing process parameters without actuating the reactor protection system and disconnecting the power unit from the network (

An urgent task is to ensure and improve the power unit dynamic stability (

Along with improving the power unit dynamic stability, it is no less important to check the correctness of design solutions for upgrading the power unit system, involving the use of new equipment or changes in plant flow diagrams.

The correctness of the proposed adjustments to the algorithms of process protections and interlocks or changes in design process engineering solutions can be confirmed by the use of current NPP power unit simulations based on various software and hardware tools.

During the work, calculations were made for the mode with two electric feed pumps disabled and the backup pump not enabled at NvNPP II-1 and 2. The calculations were carried out on the basis of the mathematical model of NvNPP II-1 developed by LLC IF SNIIP ATOM (

Then the results of the performed calculations were compared with the results of tests carried out at the stages of pilot commercial operation of NvNPP II-1 and 2:

at Unit 1, when two electrical feed pumps (EFP-1, 5) were disabled and the backup one was not enabled at a power level of 97.8% Nrated with the high pressure heaters (HPH) disconnected (01.27.2017); and

at Unit 2, when two electrical feed pumps (EFP-1, 5) were disabled and the backup one was not enabled at a power level of 99.9% Nrated with the HPHs connected (08.26.2019).

Modes with deviations in the feed water system operation belong to the groups of modes with deviations of heat removal by the second circuit. In accordance with the NvNPP II design (

In the transient process, when one

The mathematical model of NvNPP II-1 is implemented on the basis of the multi-platform version of the Kruiz software environment.

The power unit model (

a distributed dynamic neutronic model of the core;

a one-dimensional two-phase thermophysical model of the processes occurring in the main systems of the power unit; and

a model of an automated process control system.

A characteristic feature of calculations related to reactor facilities is the mutual influence of neutronic and thermophysical processes in the core. This leads to the need for conjugate calculations, when the results of the neutronic calculation become the output parameters for the thermophysical calculation and vice versa. In addition to this, various regulatory influences from the automated process control system (

To carry out the predictive calculations, it is necessary to initialize the initial state of the power unit model. Initialization is based on data packages generated by the upper unit level system (

The operation speed of the mathematical model depends on the performance of the equipment based on which the power unit model is implemented; however, in any case, the operation speed is an order of magnitude higher than real time.

Table

Main parameters of NvNPP II-1 and 2

Parameter | Unit 1 | Unit 2 |
---|---|---|

Pumps disabled during the experiment | 1, 5 | 3, 5 |

Backup pump | 3 | 1 |

State of |
Disabled | Enabled |

Reactor power, % | 97.8 | 99.9 |

Average thermal power, MW | 3132 | 3197 |

TG power,% | 92.0 | 94.9 |

Automatic power controller ( |
T | T |

Turbine governor electrical part ( |
PC | PC |

Above-core pressure, MPa | 15.87 | 15.94 |

Average hot leg temperature, °C | 326.2 | 326.0 |

Average cold leg temperature, °C | 294.8 | 295.3 |

Pressurizer level, m | 7.9 | 7.8 |

Average SG level, m | 2.71 | 2.70 |

LPH-2 water level, m | 3.44 | 3.34 |

LPH-4 water level, m | 0.158 | 0.286 |

Deaerator water level, m | 2.63 | 3.11 |

Total pump head flow rate, m^{3}/h |
6298 | 7162 |

Pump head pressure, MPa | 9.3 | 8.7 |

Average main level controller ( |
39 | 60 |

Average startup level controller ( |
28 | 27 |

Total SG feed water flow rate, m^{3}/h |
6403 | 7698 |

Average SG inlet feed water temperature, °C | 172 | 227 |

Main steam header ( |
6.8 | 6.9 |

Fast acting steam dump valve with discharge to the deaerator (BRU-D) opening, % | 5 | 0 |

Average turbine stop-control valve ( |
29 | 46 |

Fast acting steam dump valve with discharge to the auxiliary header (BRU-SN) opening, % | 12 | 0 |

The table shows that the main differences between the initial state of Unit 1 and Unit 2 are as follows:

reactor power increased by 2.1%;

turbine generator power increased by 2.9 %;

HPH-5, 6 are in operation;

SG inlet feed water temperature increased by 55 °C;

total pump head feed water flow rate increased by 864 m 3/h;

total SG feed water flow rate increased by 1295 m 3/h;

SG MLC opening higher by 21%;

SCV opening higher by 17%; and

closed BRU-D and BRU-SN.

Numerical simulation of the transient state with disabled

For a comparative analysis of the model data, the archival parameters of the

In the course of the experiment, after the EFPs were disabled, the reactor thermal power decreased from 3150 to 1467 MW due to the

Reactor power dynamics.

The feed water temperature at the steam generator inlet was kept constant about 174 °С.

Figure ^{3}/h at the head of ^{3}/h at the head of ^{3}/h.

After

Figure ^{3}/h in ^{3}/h in the calculation model. After ^{3}/h and 2060 m^{3}/h, respectively. In the model, the maximum flow rate was 2098 m^{3}/h.

Prior to the test, the total feed water flow to the steam generators, according to the ^{3}/h; after the two EFPs were disabled, it was stabilized at 4180 m^{3}/h. The calculations of the transient mode in the power unit model with ^{3}/h to 4200 m^{3}/h (Fig.

Feed water flow to the steam generators.

It can be argued that both in the model and in the real power unit, the dynamic stability is preserved, i.e., the process parameters are within the limits established by the design and do not reach the protection operation trip set points. The changes in the transient mode parameters correspond to those observed in the real tests. During numerical simulation, the power unit parameters are stabilized at the same values as the real power unit parameters during testing.

Using the mathematical model of NvNPP II-1 in the Kruiz SHC, the authors simulated a transient process with

To bring the parameters of the initial states into conformity in the numerical model, the reactor power was increased to 3205 MW (Fig.

Reactor power dynamics with the modified parameters of the initial states.

After two EFPs out of the four operating ones were disabled in the same way as it was done at Unit 1, Unit 2 was unloaded from 3200 MW to 1600 MW due to the

Differences in the transient process parameters manifest themselves when the flow rate through the ^{3}/h. In the power unit model, before the transient process, the average flow rate at the head of ^{3}/h.

Figure ^{3}/h in ^{3}/h in the calculation model. After ^{3}/h and 2148 m^{3}/h, respectively. In the model calculations, the maximum flow rate was 2176 m^{3}/h.

The total feed water flow to the steam generators, according to the ^{3}/h; after the EFPs were disabled, it was stabilized at 4317 m^{3}/h. The total feed water flows to the steam generators in the power unit model before and after the transient mode were 7737 m^{3}/h and 4551 m^{3}/h, respectively.

It can be noted that during the tests with high-pressure heaters involved in operation, both in the real process at the power unit and in the calculations in the power unit model at the head of the pumps remaining in operation, higher flows are realized. The main danger of an increase in the flow rate is that the achievement of the protection trip set points for disabling the ^{3}/h) will lead to the disabling of the EFPs remaining in operation, further reactor unloading, a decreased level in the steam generators, shutdown of the reactor coolant pump set (

Feed water flow to the steam generators.

The paper considered the course of transient processes during tests on disabling two electrical feed pumps without enabling the backup pump at NvNPP II-1 and 2. The authors analyzed the transient process by means of numerical simulation of NvNPP II-1 and 2 in the power unit model software-hardware complex (

The tests at Unit 1 and 2 were carried out under different initial conditions, which influenced the achieved process parameters. In particular, Unit 2 had a higher reactor power (by 2.1%), ^{3}/h) in a stationary state.

It was shown that in both cases the dynamic stability of the power unit was preserved. Its operational parameters were within the design limits and did not reach the protection operation set points. In particular, the protection trip setting for disabling the

A comparative analysis of the results of numerical simulation in the

It can be assumed that the

^{th}International Scientific and Technical Conference on Ensuring the Safety of NPP with VVER, Podolsk, 2005.

* Russian text published: Izvestiya vuzov. Yadernaya Energetika (ISSN 0204-3327), 2021, n. 2, pp. 16–26.