The nuclear industry continues to rise above challenges resulting from major plant events around the world. It is important to study the significant events, and find solutions to overcome the vulnerabilities identified, and retain the lessons while technology evolves to the next generation of SMRs. The plant events discussed are from Light Water reactors (LWRs) but he derived lessons could apply to LWR–SMRs and it could be extrapolated for other designs.
The historical lessons on Station-Black-Out needs to be re-examined further in a new dimension in the light of “Fukushima type” events where AC & DC power was lost to the entire station from a common cause and the recovery in a reasonable period was not practical.
The SMRs are specially considered for smaller grid size and for remote locations where a large reactor is unsuitable. The specific advantages for SMRs are its smaller foot print, modular type construction and flexibility for incremental units to meet rising demand in power. These features are very attractive to countries with small grids and for countries that are in the early stages of nuclear power technology development. In anticipation of these opportunities, when we launch into new technology of evolutionary and revolutionary advancements, the reactors have to be more rugged with ample diversity in critical reactor safety areas through both passive and active means to account for a conditions that may inhibit electrical energy as a source of motive power.
An electrical event in Forsmark plant that propagated from an electrical switchyard resulted in two core cooling divisions disabled and consequently exacerbating the plant condition by opening the steam relief system for reactor coolant system and that significantly increased the probability for core damage. A minor spark in an electronic control system card in a US plant caused inadvertent actuation of emergency core cooling and disabled the capability of control room operators to intervene and prevent the primary cooling system from completely going water solid in the primary loop of Reactor Coolant System. A renewed assessment is needed to address the following questions for advancing reactor safety in the new evolving generation of LWR-SMR plants and to other types as applicable to preserve the historic lessons in safety.
(1) Evaluate the diversity in emergency core cooling systems following loss of all AC power onsite;
(2) Ensure diversity in reactor trip & depressurization;
(3) Evaluate the suitability of logic/control system failure mode resulting from power supply failures in instrument channels and/or divisions, i.e. by conducting failure mode and effects analysis (FMEA) for system, power supplies and component level;
(4) Confirm independence in reactor trip and emergency core cooling systems for sensors, power supplies and actuation systems.