1. State Scientific Center– Research Institute of Atomic Reactors
Change in the Geometry of the VVER-1000 FAs TVS-2, TVS-2M and Their Components Under Operation
© G.V. Shevlyakov. А.V. Strozhuk. О.G. Sidorenko. S.S. Sagalov. А.S. Khrenov. А.А. Bokov, 2019
XI Conference on Reactor Materials Science,
in commemoration of 55 th anniversary of material science department of JSC “SSC RIAR”
Dimitrovgrad, May 27-31, 2019

2. INTRODUCTION
Safe operation of the VVER reactor facilities depends to a great extent on the dimensional stability of the FA design. Since 2003, Russian NPPs started adopting new generation fuel assemblies with a rigid skeleton TVS-2 and its retrofit TVS-2M. To date, PIEs were completed for three FAs TVS-2 and three FAs TVS-2M that were spent for 1-3 fuel campaigns. The PIEs were performed to confirm the dimensional stability of the fuel assemblies and included measurements of the geometric parameters of the FAs and their components.
Brief characteristics of the examined FAs TVS-2 and TVS-2М
FA reference number
FA type
Fuel average burnup. MW day/kgU
Service time , eff. days / number of fuel campaigns
Number of spacer grids /mixing grids 1
TVS-2
18.19
340.1 / 1 12 2
47.4
1075 / 3 3
47.8
/ 3 15 4
TVS-2М
40.96
/ 2 13 5
46.31
/ 3 6
51.17
/ 3
13/3

3. Elongation of TVS-2, TVS-2М, guide tubes and central tube
Elongation of TVS-2, TVS-2М, guide tubes and central tube. Stiffness of springs.
Change of the FA length is caused by the elongation of guide tubes and central tube owing to radiation growth and radiation-thermal creep. In the examined burnup range, the elongation of TVS-2 FA does not exceed 3.2 mm, that of TVS-2М – 6.5 mm, which is nearly the same as the elongation of the guide tubes and central tube.
Elongation of FAs TVS-2 and TVS-2М
Elongation of guide tubes and central tube
Difference in elongations of FAs TVS-2 and TVS-2М is caused by the different axial load from the head springs, which is transmitted from protective tubes when FA is tightened in the core.
Different stiffness coefficients are caused by the use of springs made of the wire of different diameters (5.6 mm for TVS-2 and 5.1 mm for TVS-2М).
Stiffness of springs

4. Warping of the spacer grids and guide tubes
TVS-2 and TVS-2М fuel assemblies use spacer grids with increased height of the cells (30 mm) and optimized shape of the cell indents. Spacer girds are attached to the guide tubes with the use of resistance spot welding.
Spacer grid warping
Guide tube warping
The results of measuring the TVS-2 and TVS-2M spacer grids warping showed no deformation in the spacer grids.
The lower lattice warping was measured on FA No.6 only (TVS-2M). The lower lattice bowing is caused by the skeleton manufacturing technique. Changes in the lower lattice shape during operation were not found.
Unchanged shape of the spacer grids and lower lattice during operation bears witness to the absence of the significant thermo-mechanical loads during fuel rods slipping through the spacer grid cells.

5. FA geometry measuring diagram Across-flats dimension
To illustrate changes in the across-flats dimensions, presented here are the results of measuring the reference fuel assembly TVS-2M (FA No.6) spent for three fuel campaigns.
Geometry measurements were performed at an automated facility by a contact method with the use of linear displacement sensors mounted on a carrier moving along the vertical axis of the fuel assembly.
The FA across-flats dimension after operation is increased as compared with the initial value. The maximum increase achieves 1.22 mm on spacer grid 11.
FA geometry measuring diagram
Across-flats dimension

6. FA geometry FA appearance
Hodograph
FA appearance demonstrates the absence of significant deformations.
The FA bowing is nearly arc-shaped. Direction of the bowing deflection in the initial state and after operation differ; i.e. the initial bowing has a little impact on the bowing direction during operation.
FA bowing modules

7. FA geometry. Summarized data
Maximum across-flats dimension is increased with the fuel burnup and exceeds the value of by ~0.5 mm. As a rule, this exceedance is registered in a small area.
Maximum across-flats dimension
FA bowing does not exceed 11 mm for TVS-2 and 6 mm for TVS-2М.
Bowing deflection
Twisting angle increases with the fuel burnup and reaches 1.6 angle degrees at the fuel burnup of MW∙day/kgU.
Twisting angle

8. Oxide film diameter and thickness
Guide tubes and central tube geometry.
Oxide film diameter and thickness
Guide tubes
Central tube
Changes in the diameter along the length of the guide tubes and central tube are due to oxidation of the channel tubes made from E635 alloy where a thick oxide film is formed on the tube surface. The corrosion film on the outer surface of the guide tubes and central tube is ~43 µm thick after operation during three fuel campaigns.

9. Conclusion
The results of PIEs of TVS-2 and TVS-2M fuel assemblies spent for 1-3 fuel campaigns indicate that fuel assemblies with the rigid skeleton have sufficient geometric stability during operation.
Maximum recorded elongation was 3.2 and 6.5 mm for TVS-2 and TVS-2М, respectively. Maximum across-flats dimension is mm, twisting angle – 1.6о, bowing deflection – mm for TVS-2 and 5.7 mm for TVS-2М.
Increased diameter of the guide tubes results from their oxidation; thickness of the oxide film on the outer surface of the guide tubes and central tube does not exceed 43 µm.
There is no warping in the spacer grids of the fuel assemblies of both designs.

10. Thank you for your attention!10