Damage of the bearing grid in the course of reactor operation and loss of bearing capacity may lead to an emergency of the reactor. Damage of bearing grids (BG) was revealed at dismantling of channels of reactor IVG-1 after various start-ups in the aspect of linings’ separation from adjoining rows of rods. We remind that the BG are made in the form of sintered block of twisted—four blade rods from ZrC, NbC carbides equimolar composition soldered to lateral surfaces of the block segment facings from pure carbide ZrC.

Though the load-bearing capacity of support grids (SG) was retained, the fact of its fracture confirmed an uncontrollable reliability loss of reactor operation. This neces­sitated the analysis of the stress fields of SG during operation and the need for deeper understanding of possible fracture mechanisms in order to develop recommendations

Table 7.1 Possible strengthening methods for rod-shaped fuel elements

Designated method A о jo (%)

1 Thermal cycling in temperature range from TBP to 2,700K 20

2 Quench hardening by thermal radiation cooling from 2,700K ~30

3 Quench hardening by forced cooling from 2,700K ~60

4 Thermal healing of initial cracks at 2,500K ~40

5 Reactor radiation creation of surface compressive stresses ~80

Combined method 2.4.5 100

for improving SG performance [13]. The results of stress analysis show that SG may be fractured during unstable start stage (before reaching steady-state operation mode) or, under certain conditions, also during cooling stage. When approaching the steady — state regime highest thermal stresses may be reached: up to 400+MPa for tangential tensile stresses on the SG periphery, and up to -70 MPa for compressive stresses near the SG center. Under such stresses, segment linings may crack and delaminate from soldered rod block.

Massive segment linings are proposed to be replaced with thin-walled split-type casing with semi-cylindrical fillers to avoid generation of dangerously high residual stresses. The splitting angle depends primarily on the prerequisite ofretaining the rods in a package during assembly and subsequent processing. Angle value may fluctuate over a wide range: from 5 to ~90° relative to generating line. Implementation of such casing for grid assembly presents no problems, while being also less labor­intensive as compared to lining design. Testing of an experimental batch of proposed SG design has confirmed triple increase of thermal stability of split-type casing SG, as compared to lining design.