Fulfilled R&D projects

Finite element 3D structural analysis of the driving wheel of the main circulation pump containing constructional lacks of welding. Estimation of working capacity of the structure

Keywords Main circulation pump, Tianwan NPP, constructional lack of welding, 3D curved crack, 20 billions of pulsations, 3D thermo-structural analysis, multiple contact interaction, static and fatigue strength, substantiation of the working capacity of the main circulation pump for 40 years of maintenance
Programs in use CAD system, ANSYS

Computational Mechanics Laboratory (CompMechLab) by Energonasos Design Bureau and AtomStroyExport Corporation request executed multivariate analysis of 3D thermal and thermal-stress state of the driving wheel of the main circulation pump (MCP) containing constructional lacks of welding. Static and cyclic strength estimation, estimation of driving wheel's (containing constructional lacks of welding, modeled as 3D curvilinear cracks) working capacity are executed.

For execution of this work CompMechLab has special capabilities:

  • License of the Federal service on ecological, technological and nuclear supervision «Possibility of maintenance of blocks of nuclear plants with regard to works' performance and to services accordance to the maintaining organization»
  • Termless commercial license for finite element analysis system (CAE-system) ANSYS (we shall remind that ANSYS is termless certified in scientific and technological centre on nuclear and radiating safety of Russian Federal supervision on nuclear and radiating safety (software registration number - 490, date of registration - 9/10/2002, software certification passport registration number - 450, date of issue - 10/31/2002). 

The main circulation pumps are intended for heat-transfer agent's reliable stable circulation maintenance through the reactor and the basic heat-exchange equipment. It is a necessary condition of the reliable heat discharge from the reactor's active zone, heat transfer to the heat-exchange equipment and its further use according to the technological scheme.

At nuclear power plants with pressurized water reactor (PWR) vertical rotary pump with shaft seal is used. It hasthe induction motor's (with short-circuit rotor) drive (see pump's general view). The pump and the electric motor are connected using elastic coupling. By configuration conditions MCP are combined in groups in four (three working, one reserve).

MCP-1391 general view
MCP-1391 general view

In this work on the basis of developed 3D finite element models using program system of finite element analysis ANSYS are executed:

3D thermal and stress state analysis for various MCP driving wheels (set on Tianwan nuclear power plant) operation modes;

driving wheels verifying strength analysis and strength estimations in the presence of constructional lacks of welding - non-meltedzones in welding's (the blade to the basic and covering disks) weld seams. As constructional lacks' of welding models 3D curvilinear cracks of variable width are considered.

Analyses are provided for the 40 years life cycle.

All analyses executed in this work are based on:

  • "Codes for strength analysis of nuclear power plants' equipment and pipelines"  (Rules and norms in nuclear engineering);
  • "Reactor plants' at the operation stage equipment and pipelines strength analysis manual"  (is applied to the equipment's and pipelines' strength estimation with metal 's temperature at the operation stageup to 350оС);

MCP driving wheel 3D solid model (1/6 part of the structure is marked with yellow)

MCP driving wheel 3D solid model
(1/6 part of the structure is marked with yellow)

The basic computational modes of operation of the plant for 40 working years are:

Normalmode of operation - 1 ("Normal Operating Conditions", "start–stop") – general number of starts  N1 = 1 000:

  • N1х= 0,23 ×N1 = 230 starts on cold water;
  • N1г= 0,77 ×N1 = 770 starts on hot water.

Normalmode of operation - 2 ("torque pulsatons"  –   ±5 % Мкр)

  • N2 =  2,10 ×1010 – general number of cycles:
  • N2х= 0,42 ×1010 – number of cycles on cold water;
  • N2г = 1,68 ×1010 – number of cycles on hot water.

Besides these two modes, four more generalized modes of operation are considered:

Normalmode of operation – 3 (working environment's temperature turndown is 290 - 275 °C,  working environment's temperature changerate is 1 °C/c)

  • N3 =  35 000 – general number of cycles

Violation of normalmode of operation (working environment's temperature turndown is 290 – 280 °C, working environment's temperature changerate is 2,5 °C/c)

  • N4 =  653 – general number of cycles

Emergency regime - 1 (working environment's temperature turndown is 290 – 268 °C, working environment's temperature changerate is 1,5 °C/c)

  • N5-1 =  28 – general number of cycles

Emergency regime - 2 (working environment's temperature turndown is 290 – 70 °C, working environment's temperature changerate is 0,22 °C/c)

  • N5-2 =  20 – general number of cycles

Defect (constructional lack of welding) schematization (accepted in the computational model) has to provide estimated conservative strength as a result. Defect schematization is based on the following principles:

  • as a computational defect for possible discontinuitiesin weld seams the computational crack is accepted (as the most dangerous defect type);
  • as computational defects subsurface defects are accepted;
  • as computational defects’ dimensions the maximum possible dimensions are accepted.

The crack-like defect (modeling constructional lack of welding) is presented with spatially-oriented stripe (crack). It has 4 - 7 mm width (depth) and it is located along the welded seam's root trajectory which is defined by manufacturing techniques. Curvilinear crack-likedefect extension is 700 mm.

Crack-like defects (constructional lacks of welding) arrangement in weld seams between the blade and the basic and the covering disks

Crack-like defects (constructional lacks of welding) arrangement in weld seams between the blade and the basic and the covering disks

In figures are presented 3D finite element model of the whole driving wheel and driving wheel's 1/6 part's 3D FEmodel different views taking cyclic symmetry of geometry and loads into consideration.

The crack is considered as 3D curvilinear cut. On the crack face contact interaction conditions are fixed. It allows to describe complex crack's behavior during straining - plural contact interaction when faces can enter in contact interaction with each other (the crack is closed) or to uncover (the crack is opened).

The whole driving wheel 3D finite element model
The whole driving wheel 3D finite element model

Driving wheel 1/6 part 3D finite element model
Driving wheel 1/6 part 3D finite element model

The MCP driving wheel (it consists of the basic disk, the covering disk and the blade situated between them) basic computational loads are:

  • hydraulic pressure acting on blades (counterbalanced by the driving wheel's torque) and pulsation 3D loads acting on blades (they are face-value's +/-5 %and are caused by hydro-dynamic forces;
  • driving wheel's rotation centrifugal loads;
  • varying in time according to set dependences thermal actions;
  • 3D loads distributed to the basic and covering disk's internal and external surfaces.

On the basis of developed FE models:

MCP wheel (for various operating modes) 3D thermal and thermal stress analyses were executed. Strength estimations were made according to special normative documents.

Total displacement, m

Total displacement, m

Equivalent stress distribution, MPa
​Equivalent stress distribution, MPa

At driving wheel's static strength computation Sigma1, Sigma2 andSigmaRV stress categories were considered. As maximum Sigma1andSigma2 stresseswere taken driving wheel's conservative (in addition to the safety factor) maximum membrane stresses and maximum membraneand bending stresses sums (from centrifugal forces and 3D loads except for singular stresses on crack fronts).

So the executed analysis prove that 

- driving wheel static strength is provided according to normative documents.

- driving wheel cyclic strength is provided during 40 years lifecycle according to normative documents.

On the basis of 3D stress state analysis taking possible crack faces (constructional lacks of welding) contact interaction in curvilinear cracks fronts neighborhood in the most dangerous areas into consideration stress intensity factors KI, KII, KIII are computed.

On the basis of Paris equation possible cracks fronts growths (during 40 years after putting into operation) are computed. For this in Paris equation for chromium-nickel rust-resisting steel of the austenitic class 18-8 and welded joints we used the factors considering environment corrosion influence.

Cracks dimensions growth at driving wheel static and cyclic loading (if constructional lacks of welding in blades welded seams with the basic and covering disks are considered as cracks) computation in conformity with all Russian standards prove that:

  • crack growth at static loading is insignificant;
  • the maximum crack growth (taking place on the blade's external part) taking basic computational modes (Normal Operation, Violation of Normal Operation, Emergency Regime) during 40 years lifecycle into consideration is insignificant.

Stress intensity in the lack’s of welding area

Stress intensity in the lack’s of welding area

With the legal after the operation lifecycle ending defects' (constructional lacks' of welding) dimensions (revealed in the MCP driving wheel's basic metal made from rust-resisting austeniticsteel) estimation purpose thewelded connection's elasto-plastic behavior computationalanalysis and the plastic instability conditions attain analysis are executed. It is important to note, that in this analysis we considered two plastic instability arise opportunities: general plastic instability, leading to destruction of  welded connection's (containing lack of welding ) full section and local plastic instability, leading to destruction of the crosspiece situated between the defect (lack of welding, crack) and the external welded connection's surface. It is proved that neither local, nor general plastic instability does not arise for cracks 7 mm depthand even for hypothetical cracks up to 20 mm depth.

In addition to this analysis (executed on the Russian normative documents basis) by Chinese oversight body request the R6-diagram “Integrity-Fracture” analysis was executed. It allows to conclude that according to Great Britain's CEGB-norms the crack-like defect (constructional lack of welding) does not sprout at all operation modes.

R6-diagram Integrity - Fracture

R6-diagram Integrity - Fracture

In this work MCP driving wheel (containing  constructional lacks of welding) 3D thermal and thermal stress state finite element analysis is executed, static and cyclic strength estimations are lead,  fracture failure resistance at static and cyclic loading estimations and also plastic instability arise opportunity (in structure with cracks) estimations are executed .

Executed in this work analyses allow to conclude that according to normative documents requirements considered MCP driving wheel strength and working capacity are provided during the 40 years life cycle in all operation modes.

Other materials on topic:
Nuclear power generation