Development of a conservative safety demonstration for loss of forced flow events in MYRRHA

The MYRRHA facility is a fast research reactor cooled by lead–bismuth eutectic (LBE). It is designed to operate in both subcritical and critical modes. In subcritical mode, coupled with an accelerator, it will demonstrate the full concept of Accelerator Driven Systems at reasonable power level to allow operation feedback, scalable to an industrial demonstrator and allows for the study of efficient transmutation of high-level nuclear waste. In this paper, an example of a robust safety demonstration covering all initiating events related to primary pump failures is given. The different steps to determine an enveloping event and to construct a conservative evaluation model are depicted. The analysis shows that even for a very conservative postulated event there is considerable safety margin, and no pump related failures can jeopardize the integrity of the cladding barrier. The analysis is performed using a model set up in the RELAP5/3D v4.3.4 system thermal–hydraulic (STH) code. The analysis shows that for the MYRRHA reactor there is ample of time available to scram the reactor and prevent the cladding failure. Since there is no cladding failure the radiological safety objective 1 for category C-2 (anticipated operational occurrences) is met for all primary pump related failure initiating events. Sensitivityanalyses were performed to identify parameters that have an important impact on the loss of forced flow transients.The result is that all important parameters have sufficient margin compared to their uncertainty.