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“AUTOMATIC REACTOR SCRAM ON MAIN STEAM ISOLATION DUE TO ELECTRICAL FAULT
On 1/9/16 at 0237 [CST], River Bend Station sustained a reactor scram during a lightning storm. An electrical transient occurred resulting in a full main steam isolation [MSIV] (Group 6) and a Division II Balance of Plant isolation signal. During the scram, level 8 occurred immediately which tripped the feed pumps. A level 3 signal occurred also during the scram. Subsequent level 3 was received three times due to isolated vessel level control. The plant was stabilized and all spurious isolation signals reset, then the MSIVs were restored. The plant is now stable in Mode 3 and plant walkdowns are occurring to assess the transient.”
During the scram, all rods inserted into the core. The plant was initially cooled down using safety relief valves. Offsite power is available and the plant is in its normal shutdown electrical lineup.
The licensee has notified the NRC Resident Inspector.”
Reactor offline since thunderstorm: http://www.nrc.gov/reading-rm/doc-collections/event-status/reactor-status/2016/
Weirdly, nuclear power stations must get power from an outside grid or diesel generator. Thus, it appears that the lightening strike could have come from the grid, rather than a direct strike:
“In a typical design concept of a commercial BWR, the following process occurs:
The core inside the reactor vessel creates heat.
A steam-water mixture is produced when very pure water (reactor coolant) moves upward through the core, absorbing heat.
The steam-water mixture leaves the top of the core and enters the two stages of moisture separation where water droplets are removed before the steam is allowed to enter the steamline.
The steamline directs the steam to the main turbine, causing it to turn the turbine generator, which produces electricity.
The unused steam is exhausted to the condenser, where it is condensed into water. The resulting water is pumped out of the condenser with a series of pumps, reheated, and pumped back to the reactor vessel. The reactor’s core contains fuel assemblies that are cooled by water circulated using electrically powered pumps. These pumps and other operating systems in the plant receive their power from the electrical grid. If offsite power is lost, emergency cooling water is supplied by other pumps, which can be powered by onsite diesel generators. Other safety systems, such as the containment cooling system, also need electric power. BWRs contain between 370-800 fuel assemblies.” http://www.nrc.gov/reactors/bwrs.html. Riverbend is a Mark III Reactor.
List of Mark I, II, III worldwide: https://en.wikipedia.org/wiki/List_of_boiling_water_reactors
Less than one week prior:
Potential Uncontrolled Radiation Release, Secondary Containment Declared Inoperable so they appear to have vented it, presumably releasing radiation. Notification came 2 days too late to be useful to people:
“10 CFR Section:
50.72(b)(3)(v)(C) – POT UNCNTRL RAD REL
SECONDARY CONTAINMENT DECLARED INOPERABLE
“At  CST, on January 5, 2016, with the plant operating at 100 percent power, the main control room alarm indicating high pressure in the auxiliary building actuated. Operators confirmed that the building pressure, corrected for temperature, indicated slightly positive, whereas the building pressure limit in Technical Specifications is 0.0 – 3.0 inches of water negative pressure. Secondary containment was declared inoperable, and the Division 2 standby gas system was started. This action restored building pressure to the acceptable range, and the building was declared operable at [0027 CST] on January 6.
“This condition is being reported in accordance with 10 CFR 50.72(b)(3)(v)(C) as an event that caused the secondary containment to be potentially incapable of performing its safety function. The NRC Senior Resident Inspector was notified.”
“A secondary containment structure, or reactor building, surrounds the primary containment. The reactor building is maintained at a slight negative pressure during normal operations. The top most part of the reactor building houses the refueling floor. The spent fuel pool is located within the reactor building.”
Mark III has a slightly larger, but still too small, containment. The following may or may not apply: “In order to protect the Mark I containment from a total rupture it was determined necessary to vent any high pressure buildup. As a result, an industry workgroup designed and installed the “direct torus vent system” at all Mark I reactors. Operated from the control room, the vent is a reinforced pipe installed in the torus and designed to release radioactive high pressure steam generated in a severe accident by allowing the unfiltered release directly to the atmosphere through the 300 foot vent stack. Reactor operators now have the option by direct action to expose the public and the environment to unknown amounts of harmful radiation in order to “save containment.” As a result of GE’s design deficiency, the original idea for a passive containment system has been dangerously compromised and given over to human control with all its associated risks of error and technical failure.” Read more and see original documents here: http://www.nirs.org/factsheets/bwrfact.htm