Theoretical and experimental studies on the stresses and strains in claddings of annular boiling-superheat fuel elements

The high temperature difference between exterior and interior cladding in an annular combined boiling-superheat fuel element results in considerable axial stresses and strains in both claddings during reactor operation. Each start-up or shut-down of the superheat reactor causes cyclic changes of these stresses and strains.

In the first part of this report the equations necessary to predict the behaviour of boiling-superheat fuel elements are deduced and the resulting stresses and strains are given for several examples with various cladding materials.

The second part of this paper reports the results of experimental studies with boiling-superheat fuel elements in an out-pile thermal cycling test facility. Full size fuel element specimens have been subjected to up to 800 severe thermal cycles. The observations made, including some defects which have occurred, are discussed. The results are critically reviewed and compared with the analysis of part one.

Even considering the fact that the mechanical properties under irradiation at high temperature of the cladding materials of interest are not very well known today, it is felt that the axial stresses and strains in a boiling-superheat fuel element do not present an unsurmountable problem in the development of this type of fuel elements.