NC experiments have also been performed in ITF that simulate the BWR system performance. Relevant NC data have also been recorded from the operation of BWR NPP and used for benchmarking system code performance. The flow map for the operation of BWR systems shows the core power as a function of the core flow rate. A parabola like curve in the considered plane is derived from experiments and confirmed from code applications. A steep power increase up to about 50% of nominal core power can be observed from the mentioned diagram when core flow rate achieves roughly 30% of the its nominal value (i. e. value at 100% core power). This implies that the BWR systems can operate at 50% power in NC. However, in these conditions the system is prone to instabilities, identified in the literature as density wave oscillations (DWO). A wide literature exists related to the DWO that can be considered as a NC phenomenon. A state-of-the-art report on this topic has been recently issued by OECD/CSNI, Ref. [14].

Lesson learned from the application of system codes to BWR related NC experimental situations are summarized in the following, again including references where details for the analyses can be found.

(a) The curve core power versus core flow rate in NC conditions predictable by system codes is close to the experimental values in relevant ITF, Ref. [15]. The code is also capable of predicting the same curve related to the BWR operation. The capability in predicting the NC flow map in BWR is not affected by the scaling of the system.

(b) The code has been successfully used in predicting the NC measured between core and a heat exchanger installed in a pool outside the main vessel in a configuration that is typical for the SBWR equipped with the IC (isolation condenser), Ref. [16].

(c) The code has capability to predict DWO occurring recorded during the NC in typical BWR conditions, Ref. [17]. However, the prediction is largely affected by the user choices.

3.4.2. Final remarks

The system codes have been widely applied to the prediction of NC in situations relevant to PWR and BWR conditions. The scaling problem has been addressed by demonstrating that the accuracy of the prediction is not affected by the geometric dimensions of the involved systems. No major deficiencies have been detected. It can be concluded that codes are suitable in predicting NC phenomena in conditions relevant to the present generation reactors. An exception is constituted by the predictive capability of DWO. In this case the user may substantially affect the predictive capabilities.

The above conclusions can be extended to a number of systems that are part of the design of advanced reactors like the PRHR and the CMT in the case of the AP-600 and the IC in the case of the SBWR, in relation to which a suitable experimental database exists. However, in this last case, more rigorous procedures may be established where:

— Precision requirements are established (e. g. core flow rate in NC must be predicted with an error of 3%);

— Measurements are shown to comply with the fixed error-threshold;

— Accuracy of the predictions, adopting well established input decks, is shown to lie within established error bands.

— Needs for experiments and for development of new models are derived from deficiencies found from the above process.