The properties of high burn-up fuel and the corresponding claddings, which are increasingly made from new alloys (M5, DUPLEX and others), have not been yet sufficiently investigated for Loss-of-Coolant Accident (LOCA) conditions. The current LOCA embrittlement criteria were developed in 1973. The protection goal of these criteria is the maintenance of a coolable geometry of the reactor core. According to these internationally accepted criteria, the following requirements shall be met: 1) the calculated maximum cladding temperature shall not exceed 1200 °C (PCT - Peak Cladding Temperature criterion); 2) the calculated oxidation degree of the cladding tube must not exceed the value of 17% (criterion ECR - Equivalent Cladding Reacted).
These criteria take into account only partially the effect of hydrogen absorbed by the cladding metal, the concentration of which increases with burn-up, thereby increasing the embrittlement of the material. Especially in the area of the largest ballooning of the tube and the burst region, stronger hydrogen absorption could be occurring in the cladding. Here it could be expected the so-called secondary hydrogenation of the cladding through a burst opening. In order to investigate this problem, several tests of individual rods have been carried out worldwide. For realistic simulation of the conditions in the reactor, it is especially necessary to use bundle experiments. Therefore, it was decided to conduct a series of integral LOCA tests in the QUENCH facility at KIT/IAM. The following phenomena were investigated: 1) the mutual influence of fuel rods; 2) realistic cross sections of bundle cooling channels; 3) axial forces through spacers; 4) large statistics of LOCA phenomena of several fuel rods that are exposed to very similar conditions in the bundle; 5) Confirmation / transferability of the results of single-rod experiments in other tests.
The experimental program is carried out in close cooperation between IAM institutes and includes the following follow-up examinations of the ducts:
1. Axial and azimuthal profile measurement of the cladding tube using special laser tube scanners and metallographic follow-up examinations (IAM-AWP);
2. Neutron radiography to determine the concentration of hydrogen in the cladding tube and metallographic post-test investigations (IAM-AWP);
3. Mechanical tensile and ring compression tests (IAM-WBM).