Analysis on Water Ingress Accident of a Gas Cooled Reactor

The gas cooled reactor is featured with high inherent safety, small size, and a simple start-up process. However, the water ingress accident might occur due to the influence of the working environment or operating status. Based on the design scheme of the Submersion-Subcritical Safe Space(S⁴) reactor, this work simulated and analyzed the water ingress accident caused by the rupture of the heat transfer tube of the condenser under normal operating conditions, and studied the accident consequences such as the introduction of positive reactivity, the overpressure of the Brayton cycle. This work calculated the influence of spectral shift materials on reactivity introduction during the water ingress process with the Reactor Monte Carlo code RMC. And the temperature and Brayton cycle pressure were calculated during the water ingress process with the gas cooled reactor transient analysis code, HXRTRAN. The results show that when a water ingress accident occurs, 0.5 kg water ingress causes the pressure of the Brayton cycle to exceed 10 MPa, which may lead to larger damage to the condenser pipeline and secondary seawater injection. Meanwhile, water ingress may lead to a large amount of positive reactivity introduction. If spectral shift absorbers, Ir, are added to the fuel surface in the reactor, the core may reduce power or even subcritical shutdown spontaneously in the water ingress accident. When the amount of water vapor exceeds 5 kg, the core power quickly decreases to about 2.2% FP and gradually approaches shutdown. Therefore, the spectral shift materials, Ir, have a significant inhibition effect on the introduction of positive reactivity caused by water ingress of the core.