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This paper presents a methodology and its application to a Level 2 Probabilistic Safety Assessment (PSA-2), to evaluate the impact of the Passive Autocatalytic Recombiners (PARs) on the hydrogen risk in the reactor containment in case of a severe accident. Among the whole set of accidental scenarios calculated in the framework of the PSA-2, nine have been selected as representative in terms of the in-vessel hydrogen production rate and in-vessel total produced hydrogen mass. Five complementary scenarios have been added as representative of the core reflooding situations. For this set of selected scenarios the evolution of the conditions in the containment (i.e., pressure, temperature, and composition) during the in-vessel phase of the accident has been evaluated by means of a lumped parameter approach. The use of spray systems in the containment has also been considered as well as the presence of recombiners. Moreover, the ignition by recombiners of the flammable atmosphere has been considered.

Most cheese varieties are considered as allowing the growth of L. monocytogenes, and producers must guarantee its non-detection in 25 g before commercialization. Besides safety concerns, detection of L. monocytogenes can lead to harmful moral and economic consequences for producers. Defining more accurately the growth of L. monocytogenes in different types of cheese is a priority. Currently, challenge studies are the most appropriate method for this purpose. Here, the growth potential of L. monocytogenes was assessed in 32 cheese varieties using challenge studies: unripened acid-curd cheeses, mold-ripened soft cheeses, smear-ripened soft cheeses, and Gouda-type and Saint-Paulin-type semi-hard cheeses. The number of batches to include in challenge studies was determined using Sym'Previus, a predictive microbiology software. Following guidelines provided by European Union Reference Laboratory for L. monocytogenes, a cocktail of three strains was inoculated in cheese, targeting a level of 100 cfu/g. Samples were stored at 8±1°C during respective shelf-life. Growth potential was considered as the difference between contaminations at the use-by-date and at day-0. Twenty-three varieties did not allow the growth (growth potential ≤0.5 log cfu/g). Nevertheless, the calculation method recommended by guidelines led to erroneous conclusions when inter-sample variability in L. monocytogenes levels was > 0.5 log cfu/g. Alternative calculation methods could increase accuracy. ; Peer reviewed

Introduction Listeria monocytogenes is the bacterium responsible for listeriosis, a foodborne disease for which the number of cases increased during the recent years in the European Union (1). Various types of cheese have already been identified as eventual vectors of L. monocytogenes (2). Cheese is considered as suitable for the growth of the pathogen, following Regulation (CE) N°2073/2005, if it presents a pH > 4.4, a water activity > 0.92 or a combination of a pH > 5.0 with a water activity > 0.94 (3). To assess the real growth potential of L. monocytogenes in a panel of 32 artisanal cheeses, challenge-tests were performed. Materials and Methods Between July 2018 and January 2019, 32 challenge-tests were performed on Belgian artisanal cheeses, sampled to cover the variability of process met in Belgium (4). Three batches were sampled directly after production for each of the 32 types of cheeses, excepting if theoretical growth potential calculated using the online tool Sym'Previus was ≤ 0. In this case, only one batch was studied. A cocktail of three strains of L. monocytogenes was used to inoculate the cheeses. Samples were stored at 7°C during the two first thirds of shelf life, and were then transferred to 9°C for the remaining third. The growth potential of the bacterium in each product was calculated on the basis of enumerations at beginning and end of shelf life. Discussion The results were contrasted between types of cheese. A growth of L. monocytogenes was never observed in fresh cheeses. It seems thus that these products should not represent a danger for food safety. In mold-ripened and smear-ripened soft cheese, the pathogen was able to grow, sometimes for more than 5 log cfu/g. Only one exception was observed, for a Herve cheese, with a decrease of at least 0.5 log cfu/g for each of the three batches. Regarding semi-hard/hard cheeses, the growth of L. monocytogenes was generally not allowed. However, in 3 batches out of 32, a limited growth was observed, always around 1 log cfu/g. Statistical analyses were not able to explain the divergence in the results for this cheese family.

International audience ; In case of a severe accident in a light water nuclear reactor, hydrogen would be produced duringreactor core degradation and released into the reactor building. The stratification of the released hydrogen in the reactor containment could lead to local pockets of gas mixtures of high hydrogen concentration and, in case ofcombustion, to high pressure loads which might challenge the containment structural integrity.The objectives of ERCOSAM and SAMARA projects, co-funded by the European Union and the Russia, are to investigate hydrogen concentration build-up and break-up due to safety components operations, as sprays, coolers and Passive Auto-catalytic Recombiners (PARs).For this purpose, various experiments addressing accident scenarios scaled down from existing plant calculations to different thermal-hydraulics facilities (TOSQAN, MISTRA, PANDA, SPOT) are considered. This paper describes the work performed in framework of the workpackage WP1 of the ERCOSAM project and presents theadopted methodology to scale down the real plant calculations results, provided by the projects partners, to the experimental facilities.

International audience ; In case of a severe accident in a light water nuclear reactor, hydrogen would be produced duringreactor core degradation and released into the reactor building. The stratification of the released hydrogen in the reactor containment could lead to local pockets of gas mixtures of high hydrogen concentration and, in case ofcombustion, to high pressure loads which might challenge the containment structural integrity.The objectives of ERCOSAM and SAMARA projects, co-funded by the European Union and the Russia, are to investigate hydrogen concentration build-up and break-up due to safety components operations, as sprays, coolers and Passive Auto-catalytic Recombiners (PARs).For this purpose, various experiments addressing accident scenarios scaled down from existing plant calculations to different thermal-hydraulics facilities (TOSQAN, MISTRA, PANDA, SPOT) are considered. This paper describes the work performed in framework of the workpackage WP1 of the ERCOSAM project and presents theadopted methodology to scale down the real plant calculations results, provided by the projects partners, to the experimental facilities.

International audience ; In case of a severe accident in a light water nuclear reactor, hydrogen would be produced duringreactor core degradation and released into the reactor building. The stratification of the released hydrogen in the reactor containment could lead to local pockets of gas mixtures of high hydrogen concentration and, in case ofcombustion, to high pressure loads which might challenge the containment structural integrity.The objectives of ERCOSAM and SAMARA projects, co-funded by the European Union and the Russia, are to investigate hydrogen concentration build-up and break-up due to safety components operations, as sprays, coolers and Passive Auto-catalytic Recombiners (PARs).For this purpose, various experiments addressing accident scenarios scaled down from existing plant calculations to different thermal-hydraulics facilities (TOSQAN, MISTRA, PANDA, SPOT) are considered. This paper describes the work performed in framework of the workpackage WP1 of the ERCOSAM project and presents theadopted methodology to scale down the real plant calculations results, provided by the projects partners, to the experimental facilities.