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Hunterston B sheep and grid streetview
The electrical grid, sheep at Hunterston B – about 30 miles, 50 km, from Glasgow, Scotland.

Dependence upon external sources of power is one of the most bizarre aspects of nuclear power stations. In the event of electrical grid failures, nuclear cooling systems can be powered with backup generators, which sometimes fail to start up. They also require diesel fuel to run. This is a major achilles heel of nuclear power.

As explained by the US NRC: “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.http://www.nrc.gov/reactors/bwrs.html Hunterston B is CO2 cooled, but is apparently still dependent upon the electrical grid to circulate the coolant. As well, the spent fuel at Hunterston probably uses water cooling, as at other nuclear reactors.

Multiple failures can and have occurred. In this 1998 near miss, severe winds knocked out all external power lines; emergency diesel generators failed to start; computer system failure due to lack of electrical power caused “considerable confusion and delay“.

In December 1998 a INES 2 incident occurred after severe winds and sea spray disabled all four power lines to the site.

After multiple grid failures in a short period of time, emergency diesel generators failed to start.

Normally, in the absence of power for the reactor cooling pumps, the reactor would be passively cooled.” [This a CO2 gas-cooled not water-cooled reactor.] “However, the emergency control systems which would have initiated passive cooling failed to act, as it had not been reset. Reactor cooling was reinstated after 4 hours.

There was considerable confusion and delay in restoring power as plant schematics and security systems were computerised but were rendered inoperable due to lack of electrical power.

Due to the very large inherent safety margins of the AGR” [Advanced Gas-Cooled Reactor, which uses graphite as neutron moderator and CO2 as the coolant. CO2 cooled reactors can and have suffered nuclear meltdowns, e.g. Lucens in Switzerland. ] “reactor design, there was no reactor damage, and the plant would have tolerated loss of cooling for 20 hours.” [This is probably under ideal conditions. Note that Three Mile Island Nuclear Disaster was well underway within 2 hours (6 am) of the reactor trip (4 am).]

The subsequent investigation made several recommendations: redesign of the insulators on the 400 kV power lines, installation of an additional 132 kV power line for emergency power, a second diesel generator building remote from the first, installation of an uninterruptible power supply for the reactor safety systems and for essential computer equipment, provision of hard copy plant schematics and emergency protocols, and revised staff training procedures including simulation of multiple simultaneous system failures.” Creative Commons: https://en.wikipedia.org/wiki/Hunterston_B_nuclear_power_station (Emphasis our own; Our comments added in brackets.)

BBC Reports on the incident here: “Nuclear Alert at Scottish Plant“: Wednesday, December 30, 1998 Published at 14:44 GMT http://news.bbc.co.uk/2/hi/uk_news/244765.stm

An Advanced Gas-cooled Reactor (AGR) is a specific type of nuclear reactor. These are the second generation of British gas-cooled reactors, using graphite as the neutron moderator and carbon dioxide as coolanthttps://en.wikipedia.org/wiki/Advanced_gas-cooled_reactor

The Lucens CO2 gas-cooled reactor had a loss-of-coolant accident and partial meltdown. In the event of a major accident, the graphite at Hunterston B (and other graphite reactors) could burn and emit radioactive materials for days. Hunterston B has cracks in its graphite, which if they break through, could impede reactor shut-down, leading to meltdown. As explained in The Guardian: Cracks in graphite bricks at Hunterston B, Hinkley Point, Hunterston B, Heysham 1, and Hartlepool could cause misalignment of the holes, thus significantly increasing the risk of an accident: “fuel rods could become jammed in the reactor, and misalignment could make it hard, if not impossible, to lower in control rods to close the power plant down.” See: “Why cracks at the cores of ageing AGRs worried safety inspectorhttps://www.theguardian.com/science/2006/jul/05/energy.greenpolitics

The Lucens reactor at Lucens, Vaud, Switzerland, was a small pilot nuclear reactor destroyed by an accident in 1969. In 1962 the construction of a Swiss-designed pilot nuclear power plant began. The heavy-water moderated, carbon dioxide gas-cooled, reactor was built in an underground cavern[1] and produced 30 megawatts of heat (which was used to generate 8.3 megawatts of electricity).[2] It became critical in 1966 and the plant was decommissioned in 1988.[3] It was fueled by 0.96% enriched uranium alloyed with chromium cased in magnesium alloy (magnesium with 0.6% zirconium) inserted into a graphite matrix… It was intended to operate until the end of 1969, but during a startup on January 21, 1969, it suffered a loss-of-coolant accident, leading to a partial core meltdown and massive radioactive contamination of the cavern, which was then sealed.

The accident was caused by water condensation forming on some of the magnesium alloy fuel element components during shutdown and corroding them. The corrosion products from this accumulated in some of the fuel channels. One of the 73 vertical fuel channels was sufficiently blocked by it to impede the flow of carbon dioxide coolant so that the magnesium alloy cladding melted and further blocked the channel. The increase in temperature and exposure of the uranium metal fuel to the coolant eventually caused the fuel to catch fire in the carbon dioxide coolant atmosphere. The pressure tube surrounding the fuel channel split because of overheating and bowing of the burning fuel assembly, and the carbon dioxide coolant leaked out of the reactor.[4][5] … the cavern containing the reactor was seriously contaminated…

Note that Wikipedia, especially English Wikipedia, appears to have become largely corrupted by the nuclear industry, which is able to pay people to write articles. Thus, much written appears to be nuclear PR and one can be almost certain that they understate risk. Sometimes lazy pro-nuclear hacks add only one or two false sentences, which undermine an otherwise excellent article, written by honest, hard-working people. Nonetheless, often Wikipedia remains the only source available, especially the only Creative Commons source. Sometimes articles in other languages are better. Frequently, information from US nuclear labs, which is public domain, is actually more honest. If US nuclear labs, or Wikipedia, say something about nuclear/ionizing radiation is bad, it means that it is probably very, very, very bad.

Coordinates from Wikipedia: https://en.wikipedia.org/wiki/Hunterston_B_nuclear_power_station

Report of The President’s Commission On the Accident at Three Mile Island (Kemeny et. al. Oct. 1979) found here: http://pbadupws.nrc.gov/docs/ML1216/ML12167A050.pdf and here: http://www.threemileisland.org/downloads/188.pdf