Typhoon Soulik stuck Taiwan over the weekend with winds over 100 miles per hour, equivalent to a Category 2 hurricane, so strong in fact that it took out a nuclear reactor twice at the Jinshan Nuclear Power Plant.
Early Saturday morning, the strong winds knocked out emergency systems at the Unit 2 reactor which are designed to reduce the likelihood of lightning directly striking the reactor building, which lead to an automatic shutdown of the reactor. After restarting the reactor, neutron readings in the core began increasing to levels so high they led to another emergency shutdown while repairs were still being carried out on the lightning protection system. Reactor operators are generally trained to react and manually shutdown a reactor if they see any issues with operation or external conditions which could affect normal operations before any of the emergency systems automatically scram the reactor,
The strong winds and waves associated with the typhoon battered the island and left a significant amount of detritus blocking the water intake. Detritus is organic material suspended in water, also referred to as marine snow.
Only two years removed from the Fukushima Daiichi nuclear disaster, the safety of nuclear power plants is a matter of great concern for officials and citizens in Taiwan. Before Taipower is able to restart the nuclear power plant, they will have to submit a report to the Atomic Energy Council and unblock the water inlet.
The shutdown due to increased neutron levels in the reactor is one of the most concerning parts of this report and two scenarios have been provided which could explain the sequence of events. During the course of normal operations, neutron levels should only increase as the power of the reactor core increases. Had the facility been struck by lightning, the increase in neutron levels registered by the monitors in the reactor could have indicated a potential grounding problem, but the lighting protection system was disabled by the strong winds.
The other scenario does not involve grounding problems, rather is related to the amount of water in the reactor. In nuclear reactors, water is used as a neutron moderator, to control the chain reaction in the reactor and absorb decay heat. When water levels fall too much, or become too hot, the water in the reactor begins to boil too fast. This would lead to an increase in the amount of neutrons detected by the monitors. According to experts, the reactor would likely need to lose nearly 15 feet of water for the neutron readings to jump significantly, which would only take some 30 minutes after a reactor shutdown, but Taipower reported that the water intake was blocked due to the accumulation of organic material from the typhoon. If the lack of water intake had affected the reactor water levels, operators should have had many other indicators of problems prior to the neutron levels increasing, such as problems with the condenser or high backpressure causing a turbine trip.
The events at Jinshan leave the reader with many questions. Did the typhoon cause the nuclear power plant to suffer a loss of ultimate heat sink, as witnessed at Fukushima Daiichi? Did the emergency systems adequately cool the reactor core while the intake was blocked or clogged, or was the increase in neutron levels due to increased boiling or rapidly decreasing water levels in the reactor? While Taipower has not indicated the reason for the neutron level increase, Tsai Fu-feng, a spokesman for the national utility admitted that there was “room for improvement” in plant operations at Jinshan.
Source: China Post