The effect of power ramping on fuel rod behaviour

The behaviour under irradiation of a typical proposed small P.W.R. (100 MWth) fuel rod has been studied with the COMETHE Code-Code for predicting mechanical and thermal fuel performance the fuel rod has been assumed to be irradiated in low power rated core zone during three cycles of operation. The COMETHE results at the end of cycle two (E0C2) have been saved in order to study the power ramping effect on the fuel rod behaviour stress induced in the clad-and to optimize the best way of reaching the full power. Also, in the design of fuel pins, a precise knowledge of the thermal and mechanical behaviour of the rods during irradiation time is required. This involves a realistic evaluation of a number of interrelated parameters, e.g., the temperature distribution, the reconstructing and swelling rates of fuel pellets, the dimensions, the stresses and the strains in the clad, the inner gas pressure, etc. This complex problem can only be properly handed by a computer programme which analyses the fuel pin thermal and mechanical behaviour at successive steps of its irradiation life. COMETHE III-J is a multipurpose code which can provide the user with a realistic solution of this complex problem. The COMETHE III-J has been tested and calibrated with experimental results and here after, there will come some of applications of this code: 1. Evaluation of fuel vendor proposal. 2. Comparison between different proposed designs. 3. Analysis of the effects of tolerances. 4. Analysis of the effects of power history, power ramping, core management, effects anticipated by vendor and operation limitations imposed by the vendor, etc. 5. Help for selection of PIE and interpretation of the results. 6. Analysis of steady state input values for LOCA analysis. 7. Improvement of plant availability and manoeuvrability through reduction of pin failure probability and minimization of limitations from LOCA. (Author)