SAFETY DISTANCES: DEFINITION AND VALUES Alessia Marangon 1 , Marco Carcassi 1 , Angunn Engebo 2 , Sandra Nilsen 3 1 Department of Mechanical, Nuclear and of Production, University of Pisa, Via Diotisalvi 2, Pisa, 56126, Italy 2 DNV Research, Det Norske Veritas AS, Veritasvn 1, Høvik, N-1352, Norway 3 Norsk Hydro Corporate Research Centre Porsgrunn, N - 0246 OSLO, Norway ABSTRACT In order to facilitate the introduction of a new technology, as it is the utilization of hydrogen as an energy carrier, development of safety codes and standards, besides the conduction of demonstrative projects, becomes a very important action to be realized. Useful tools of work could be the existing gaseous fuel codes (natural gas and propane) regulating the stationary and automotive applications. Some safety codes have been updated to include hydrogen, but they have been based on criteria and/or data applicable for large industrial facilities making the realization of public hydrogen infrastructures prohibitive in terms of space. In order to solve the above mentioned problems, others questions come out: how these safety distances have been defined? Which hazard events have been taken as reference for calculation? Is it possible to reduce the safety distances through an appropriate design of systems and components, or through the predisposition of adequate mitigation measures? This paper presents an analysis of the definitions of “safety distances” and “hazardous locations”, as well as a synoptic analysis of the different values in force in several States for hydrogen and natural gas. The above mentioned synoptic table will highlight the lacks and so some fields that need to be investigated in order to produce a suitable hydrogen standard. 1.0 INTRODUCTION The term “safety distances” has in technical field and in juridical field different shades of meaning, even if it is broadly used in both fields. Moreover, as for the term “Risk”, a safety distance is perceived in different ways depending on the person (culture, position and responsibility) using it. Such dichotomy does not belong only to the industrial field (hydrogen included), but more generally to all those fields where one, by applying physical distances, is trying to avoid disagreeable consequences that could be generated by the use of hazardous substances. The first notions about safety distances in industrial field were tied up to the level of ignorance concerning the behavior of some technologies. As a consequence a certain level of protection was established. As an example the safety distance problem in the nuclear energy pacific use, from which were derived the majority of the techniques and of the safety principles actually in force, was faced in the 1950 when the “Reactor Safety Committee” of the Atomic Energy Commission solved the problem of the safety distances (at that time the exact term was “exclusion distance”, i.e. without resident population) by providing a formula in which the distance (in miles) was proportional to the square root of the thermal power (in kW) of the reactor (R=0.01* √KW). By that formula, derived by qualitative and quantitative argumentations and expressed in a simple way, therefore easily comprehensible from the population, the intent was to express an easy concept: “also in the case of the worst accident scenario over such distance there was nothing to fear”. But still from the nuclear history we all well know that when such worst accident scenarios were analysed with greater details, such formula failed. 1