about 0 N, or sometimes leading to a gap between the fuel rod and the spacer spring
4. It was determined on the basis of the fretting tests’ results that the larger motion than spacer spring preload’s
spring’s increase leads to larger wear.
spring’s increase leads to larger wear.
It was demonstrated that the test rod with a clearance fit is It was demonstrated that the test rod with a clearance fit is
able to move more able to move more
freely and more extensively at the same level of excitation.
freely and more extensively at the same level of excitation.
The effective vibration The effective vibration
amplitude in the direction of excitation was at clearance fit amplitude in the direction of excitation was at clearance fit
about one and a half and about one and a half and
two times greater than one at interference fit. In addition to two times greater than one at interference fit. In addition to the intensity of rod motion is not only in the direction of the intensity of rod motion is not only in the direction of excitation but also perpendicularly to it significant larger excitation but also perpendicularly to it significant larger than at interference fit. In the case of a clearance fit of 0.10 than at interference fit. In the case of a clearance fit of 0.10 mm, the effective vibration amplitude perpendicular to the mm, the effective vibration amplitude perpendicular to the direction of excitation was about four times greater than direction of excitation was about four times greater than
one at interference fit one at interference fit. .
399399
THERMAL-HYDRAULIC IN NUCLEAR REACTOR
XXIII.2. Failure mechanism XXIII.2. Failure mechanism XXIII.2.1. Fretting wear
XXIII.2.1. Fretting wear
Nuclear fuel assemblies are exposed to severe thermal, Nuclear fuel assemblies are exposed to severe thermal,
mechanical and radiation conditions during operation.
mechanical and radiation conditions during operation.
Global core and local fuel assembly flow fields typically result Global core and local fuel assembly flow fields typically result in fuel rod vibration. Under certain conditions, this in fuel rod vibration. Under certain conditions, this vibration, when coupled with other factors, can result in vibration, when coupled with other factors, can result in excessive cladding fretting wear. This phenomenon is of excessive cladding fretting wear. This phenomenon is of significant concern for nuclear fuel designers, especially in significant concern for nuclear fuel designers, especially in light of a growing need for higher burnup, longer cycle light of a growing need for higher burnup, longer cycle lengths, and operational safety margins. Understanding lengths, and operational safety margins. Understanding fretting wear margins, the probability of a fuel assembly fretting wear margins, the probability of a fuel assembly being at risk of excessive wear, and the factors affecting being at risk of excessive wear, and the factors affecting fretting wear resistance are important in order to better fretting wear resistance are important in order to better
guide design, testing, and operational flexibility guide design, testing, and operational flexibility
THERMAL-HYDRAULIC IN NUCLEAR REACTOR
Fretting wear is governed by a set of complex physical Fretting wear is governed by a set of complex physical phenomena, which do not remain constant during nuclear phenomena, which do not remain constant during nuclear fuel operation in reactor cores. Based on various field data fuel operation in reactor cores. Based on various field data for grid to rod fretting wear, we believe that the amount of for grid to rod fretting wear, we believe that the amount of
grid to rod fretting wear depends on:
grid to rod fretting wear depends on:
The extent of flow induced vibration caused by fuel design The extent of flow induced vibration caused by fuel design
and/or plant specific operating conditions;
and/or plant specific operating conditions;
Grid to rod support conditions;
Grid to rod support conditions;
The initial grid to rod contact area;
The initial grid to rod contact area;
Grid materials.
Grid materials.
A larger flow induced vibration, larger grid to rod gap, smaller A larger flow induced vibration, larger grid to rod gap, smaller initial grid to rod contact area and softer cladding tube initial grid to rod contact area and softer cladding tube contribute to a higher probability of grid to rod fretting contribute to a higher probability of grid to rod fretting wear. In general, it is known that the fuel rod has less wear. In general, it is known that the fuel rod has less amplitude than 0.2 mm under normal operating conditions amplitude than 0.2 mm under normal operating conditions
of the reactor
of the reactor. . 401
THERMAL-HYDRAULIC IN NUCLEAR REACTOR
Evolution of grid spring force under irradiation Evolution of grid spring force under irradiation
The calculation of the evolution of grid spring force under The calculation of the evolution of grid spring force under
irradiation and the eventual rod to grid gap is applied to:
irradiation and the eventual rod to grid gap is applied to:
- Inconel 718 grid (bottom grid and top grid), - Inconel 718 grid (bottom grid and top grid), - Zy-4 grids (mid grids).
- Zy-4 grids (mid grids).