Long Term Effects and Numerical Simulation of Radiolytic Gas, Non-Condensable Gas, and Boron Transport for Small Modular Light Water Reactors

In the past decade, nuclear industries and governments worldwide have developed interests in designing and deploying small modular reactors (SMRs) as viable energy source options to reduce carbon dioxide emissions and help resolve climate change issues. According to the IAEA), there are approximately 70 small modular reactor designs under investigation in 17 countries. These new SMR designs are at different stages of research, development, licensing, and commercialization. The collaborations coordinated by IAEA and driven by major industrial countries have put several SMR designs as the front runners in this phase of technology commercialization. Some of these SMR designs under development have evolved from large light water reactor (LWR) designs and use light water as both a coolant and neutron moderator with passive gravity driven systems for normal operation and accident mitigation. The design goals of these passive safety systems are to maintain reactor core cooling for at least 72 hours without any on-site or off-site power supply or operator intervention for a broad range of hypothetical accident scenarios including loss of coolant accidents (LOCA) and station blackout. These SMR designs normally have significantly higher coolant inventory/reactor power ratios than that of conventional large LWRs and are expected to keep the reactor core covered under a two-phase water level, or, at a minimum, preclude the fuel from experiencing prolonged heat-up for at least 72 hours. The reliance of such passive SMR designs on gravity driven buoyancy flow and natural circulation for their long term emergency core cooling system (ECCS) operation makes it possible to eliminate the need for higher cost, active pumping systems, simplify the containment design, and reduce the initial capital investment. The gravity driven ECCS designs can be reliable because of their reliance on inherent features and natural phenomena. However, the gravity driven buoyancy flow and natural circulation change the system mass and energy distribution during the long term cooling period after the initial transient. Depending on the design, the systems may become sensitive to some physical phenomena, such as (1) radiolytic gas generation and migration, (2) non-condensable gas effects, and (3) boric acid transport if used in the primary circuit to control reactivity. These phenomena were not considered to be significant concerns for most current LWR designs utilizing active ECCS, so they were not explicitly modelled in detail historically. Detailed evaluation of the accumulated effects of these phenomena may become necessary for passive LWR SMRs designs as part of design basis analyses. In this paper, the authors summarized the information on these three phenomena and identified their potential safety implications for passive LWR SMRs. The state of the art computer simulation tools commonly used by both the industry and regulatory agencies are discussed for their applications to evaluate the accumulated effects of these phenomena, including the challenges, limitations of these computer codes and future development needs.