A possible solution for the MYRRHA-FASTEF critical core with the 30 wt. % Pu mass content: the increasing of fissile length

This work was carried out at the end of the CDT/FASTEF EU FP7 project. The main outcome of the WP2 Task 2.2 is represented by the Deliverable D2.2 (ENEA Technical Report UTFISSM-P9P0-020) that describes the rationale of the MYRRHA-FASTEF FA / core design, together with its detailed analyses (neutronic, TH, shielding and activation) performed in both critical and sub-critical operational modes. While the critical mode was investigated by adopting a MOX fuel with the 34.5 wt.% Pu mass content (EPu), the ADS mode was analysed by using the 30 wt.% EPu. This is because the sub-critical core characterisation was performed in the last year of CDT activities when further information became available (from SCK-CEN) about the most feasible / reasonable fuel procurement for MYRRHA. In spite of the incongruence with the critical core analyses, all the CDT partners agreed that FASTEF had to follow the MYRRHA evolution design. The present report tries to identify a possible solution for the critical mode with the 30 wt.% MOX EPu. As a matter of fact, by maintaining the CDT FA architecture, more than 100 FA would be needed at the equilibrium. This would imply some drawbacks as: the impossibility to maintain 7 IPS in the centre, the degradation of the flux level and the increase of the barrel damage. A possible mitigation of them was addressed by increasing the fissile length: e.g. from 60 up to 70 cm. Some benefits were obtained, even without significant modifications of the architecture of the FA (i.e.: same pin length) and of the other core components (e.g.: CR). The impact of the fissile length increase on the sub-critical operational mode was not evaluated: nevertheless, no significant drawbacks are expected.