Atalante research facility implementation of a rule of fractions for the management of reflecting materials in mass-limited units

Abstract

International audienceATALANTE,located in Marcoule,is one of the main Nuclear Facilities of the French CEA.In terms of criticality risk prevention, the facility is divided into work units mostly managed through a mass control mode.The limit authorized is 350 g of fissile materialwith 239Pu-H2O as reference fissile medium. This mass limit was determined considering a reflectionby 20 cm of water.Under these conditions, some neutronic reflectors, which are more efficient than watercan be authorized only in limited quantities. In ATALANTEthe reflecting materials identified as requiring a specific management are lead, uranium (235U/Utotal = 1%), graphite, heavy water and beryllium.Initiallya maximum permissible mass was determined for each of these materials taken separately. However, this method requires that when different reflectors are present simultaneouslythe sum of the masses of all these reflectors must be less than the limit specified for the most penalizing of them.This rule has proved to impose too many re-strictions on the operator.A new rule has therefore been implemented:the rule of fractions.To demonstrate that this rule is acceptable, criticality calculations have been performed. The geometric configurations studied are a sphere of 350 g of 239Pu moderated by water and re-flected by various masses of 2 to 6 reflectors (successions of concentric shells) followed by 20 cm of water.It has been concluded that compliance with the new rule makes it possible to ensurecriticality safety in the case of the simultaneous presence of different reflectors