Japan Nuclear Engineer: US reactor at risk of ‘supercriticality’ during recent emergency…
from ENENews “That’s something scary” — Damaged nuclear fuel rods and fuel fragments found at plant, conditions reported as “seriously degraded”
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”)
U.S. NRC Event Notification, Aug 29, 2011: EMERGENCY DECLARED… North Anna Power Station declared an Alert [“actual or potential substantial degradation of the level of safety”] due to significant seismic activity… This notification is to report… a condition existed which met the emergency plan criteria but was not declared… Design Basis Earthquake limits may have been exceeded.
U.S. NRC, 2012: Reactor trips at both North Anna 1 and 2, were caused by a “Power Range High Negative Neutron Flux Rate”… four possible causes [were identified] (1) loss of power to the control rod motor generator sets, (2) a dropped control rod, (3) movement of the nuclear instrumentation detectors, and (4) core barrel movement… Analysis included a third-party evaluation of the potential response of the core nucleate boiling… [Physics Forum: “nucleate boiling… reduces the heat transfer capability of the coolant… which leads to even more boiling, etc, until the fuel becomes ‘dry’… the cladding will burn and/or fuel will will melt”]
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.
ASEE 2003 Conference: If a critical reactor is over moderated… the reactor can rapidly reach and exceed the prompt critical condition… Chernobyl started at a low power level, which corresponded to a positive void coefficient.
North Anna Root Cause Evaluation, Oct 2011: The last power decreases prior to the trips were of sufficient magnitude and rate to generate the negative rate trip… Root Cause Evaluation Team has postulated that the major cause of the power oscillations was void defect created by bursts of bubbles… which caused the power increases which followed the power decreases… It is postulated that [as] the seismic waves shake… a large portion of the [fuel] cladding surface is suddenly in the nucleate boiling region [and] causes the bubble burst… [I]t may not be plausible to generate sufficient void to decrease the core power up to 10% [therefore it] can be concluded that the bubble burst theory alone does not explain the magnitude of change [in] Units 1 and 2.
Enformable, Oct 2011: “It is likely that the North Anna Operators are going to be shocked… when they open the reactor to attempt to refuel it, and find that they cannot retrieve the fuel”
3 years later >> Richmond Times-Dispatch, Sept 30, 2014: Dominion Virginia Power has found two damaged nuclear fuel rods in its North Anna 2 power plant [NRC in 2011: “It is noteworthy that the power oscillations were more pronounced on Unit 2“]… 15 uranium fuel pellets came out of two rods… [CEO David Heacock said] rods rubbed against the support structure, cutting grooves in them and eventually causing their tops to crack off… “We have accounted for about eight of the pellets”… The failed fuel assembly had been used during three 18-month operating cycles at North Anna…
U.S. NRC Event Notification Report, Sept 15, 2014: FAILED FUEL ASSEMBLY IDENTIFIED DURING CORE OFF-LOAD… North Anna Unit 2… Video inspection of the reactor vessel identified debris that has the potential to be fragments of fuel pellets resting on the core plate… this event is being reported [as an] event or condition that results in the condition of the nuclear plant, including its principle safety barriers, being seriously degraded.