Gov’t: Shutdown at U.S. nuclear plant after 10 control rods accidentally fall into reactor core — Caused by ‘smoldering’ event, Fire Brigade on scene — Incident of this type could lead to overheating, potentially resulting in ‘supercriticality’

U.S. Nuclear Regulatory Commission Event Notification Reports, Dec 7, 2015 (emphasis added): MANUAL REACTOR TRIP INITIATED DUE TO MULTIPLE DROPPED CONTROL RODS — At 1731 [EST] on December 5, 2015, Indian Point Unit 2 Control Room operators initiated a Manual Reactor Trip due to indications of multiple dropped Control Rods. The initiating event was a smoldering Motor Control Center (MCC) cubicle in the Turbine Building that supplies power to the Rod Control System… The affected cubicle has ceased smoldering and is being monitored by on-site Fire Brigade trained personnel… The cause of the smoldering MCC is being investigated and a post reactor trip evaluation is being conducted by the licensee… The licensee has notified the NRC Resident Inspector and appropriate State and Local authorities.
The Journal News, Dec 7, 2015: One of Indian Point’s two nuclear reactors will remain shut down for the next couple of days following a power loss on Saturday, a company spokesman said Sunday. Unit 2 was powered down around 5:20 p.m. Saturday by operators after about 10 control rods “dropped” into the reactor core, according to the Nuclear Regulatory Committee… Neel Sheehan, a spokesperson for the NRC, said a preliminary investigation indicated the problem stemmed from a sudden power loss to the mechanism holding the rods. The plant’s control rods, made of materials that can absorb neutrons, are used to control the fissioning of atoms that generate power… Government inspectors visited Indian Point on Saturday and Sunday. “No immediate concerns were identified,” Sheehan said in an e-mail, adding that inspectors would return to the site “to follow up on troubleshooting, repair activities and restart planning.”
Gothamist, Dec 6, 2015: In a statement, Gov. Cuomo… said he’s sending a team from the Department of Public Service to investigate the incident and monitor the process of bringing the reactor back online.
Several control rods also appear to have dropped at Virginia’s North Anna nuclear power plant after an earthquake in 2011:

• U.S. NRC Event Notification, Aug 29, 2011: EMERGENCY DECLARED… North Anna Power Station declared an Alert due to significant seismic activity
• U.S. NRC email, Sept 2011: RE: North Anna… At this point, it appears that the grippers for several of the control rods deenergized and dropped.
• Westinghouse patent: A dropped rod… will initially result in a reduction in the total power generated by the core. The reactor will then attempt to meet the load… by increasing power in the remainder of the core which could lead to local overheating elsewhere in the core.
• Satoshi Sato, Nuclear Engineer, April 28, 2015 (emphasis added): “I don’t know if you remember, there was the earthquake in 2011 in Virginia. That was a big one… enough to cause vibration to the North Anna reactor resulting in a big spike of neutron flux — or reactor power… and somehow caused the additional radioactivity into the core. So that’s something scary, potentially resulting in a supercriticality.” (IEER: “The neutron spike accompanying a sudden supercriticality can lead to an explosion of the reactor core. It is this sort of event which occurred at the Chernobyl reactor”)