Author's Reply to Comments by P. Ingham et al. on "Code Validation and Uncertainties in System Thermal-hydraulics"

Thank you very much for the comments given and for the interest toward this subject. Before giving a direct answer to the comments, the following two remarks are necessary: a) the paper is devoted to the general problems of code validation and uncertainty estimation; various sets of data are used as examples to support the conclusions of the authors ~id are not presented with enough detail to allow comments or implications other than those given in the paper; b) in relation to Fig. 4 (i.e. the figure commented by P. Ingham et al.), the only sentence included into the paper is " - comparison among natural circulation scenarios in PWR, WWER and CANDU type reactors, Fig. 4." Definitely, the authors did not draw any conclusion from the figure in the paper or in relation to the natural circulation in CANDU. Nevertheless the comments are appreciated very much and deserve the following answer. i) CANDU is a Pressurised (Heavy) Water Reactor equipped with Steam generators and producing decay power; Steam Generators (SGs) are installed at a higher elevation related to core presumably to allow natural circulation between core and SGs themselves. This is the reason why we tried to compare the amount of natural circulation flowrates in CANDU and in PWR; we also realise that there are mechanisms of natural circulation in CANDU (i.e. channel to channel), not present in a PWR; 2) we got the CANDU data (i.e. those given in Fig. 4) from an 'official' presentation of an AECL scientist at an OECD/CSNI meeting in Paris, June 1994 (obviously, more precise information can be provided in relation to this presentation); 3) in the case of PWR, we basically considered the flow through the SGs: this would be twice the value reported as "T9211 Flow through boiler 2" in the Fig. 1 of the P. Ingham et al. comment; in this case, even the 'quantitative meaning' of the data in Fig. 4 of the paper keeps its validity. Please note that the reason why CANDU natural circulation between core and SGs is expected to be lower than in the case of PWR having the same power and, roughly the same pressure, comes from the larger pressure drops in CANDU in the core active region and in the zones upstream and downstream the core (i.e. between core and headers). However, we wish to repeat that the evaluation of CANDU natural circulation was not at all the purpose of the paper.