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In conventional pressurized water reactor (PWR) designs, soluble boron is used for reactivity control over core fuel cycle. As high boron concentrations have significant impact on reactivity feedback properties and core transient behaviour, design changes to reduce boron concentration in the reactor coolant are of general interest in view of improving PWR inherent safety. In the framework of an investigation into the feasibility of low boron design, two PWR core configurations based on fuel with higher gadolinium and erbium utilization have been developed permitting to reduce the natural boron concentration at begin of cycle by approx. 50% resp. 30% compared to current PWR technology. Their performance was compared with the one of a standard German PWR core in a loss-of-feedwater anticipated transient without scram, boron dilution scenario and a hypothetical beyond design basis accident. Moreover, this paper deals not only with the potential advantages, but also addresses the drawbacks of the new PWR configurations — complex core design, increased power peaking, significant amount of non-burnable gadolinium and erbium isotopes, reduced cycle length, etc. The analyses presented here were supported by detailed cell, diffusion and thermal-hydraulic simulations, performed with the well known codes HELIOS, QUABOX/CUBBOX and ATHLET.Copyright © 2008 by ASME

In conventional pressurized water reactor (PWR) designs, the concentration of boron in primary coolant is limited by the requirement of having a negative moderator density coefficient. As high boron concentrations have significant impact on reactivity feedback properties, design changes to reduce boron concentration in the reactor coolant are of general interest in view of improving PWR inherent safety. In the framework of an investigation into the feasibility of low boron design, a PWR core configuration based on fuel with higher gadolinium (Gd) content has been developed which permits to reduce the natural boron concentration at begin of cycle (BOC) by approx. 50% compared to current German PWR technology. For the assessment of the potential safety advantages, a Loss-of-Feedwater Anticipated Transient Without Scram (ATWS LOFW) has been simulated with the system code ATHLET for two PWR core designs: a low boron design and a standard core design. The most significant difference in the transient performance of both designs is the total primary fluid mass released through the pressurizer (PRZ) valves. It is reduced by a factor of four for the low boron reactor, indicating its improved density reactivity feedback.Copyright © 2006 by ASME