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Inspectors Warn Faulty Valves In New-Generation EPR Nuclear Reactor Pose Meltdown Risk
Bruce66423 writes: Valves for the new generation of French reactors being built now have raised substantial safety concerns on top of the existing issues about the quality of the steel used for the containment vessel. Similar to the Three Mile Island nuclear accident, France’s nuclear safety watchdog found “multiple” malfunctioning valves in the Flamanville EPR that could cause its meltdown. The Telegraph reports: "The watchdog reportedly cited 'multiple failure modes' that could have 'grave consequences' on the safety relief valves, which play a key role in regulating pressure in the reactor. Owned by state-controlled French utilities giant EDF, Flamanville lies close to the British Channel Islands and about 150 miles from the southern English coast. Designed to be the safest reactors in the world and among the most energy-efficient, the €9 billion (£6.5 billion) EPR has suffered huge delays in models under construction in France, Finland and China. It is now due to enter service in 2017, five years later than originally planned."
The article is from the British newspaper The Telegraph so it is directed at a British audience.
What is the importance of being 5 years late?
Costs Of Nuclear Power Plants - What Went Wrong?
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I'm more concerned about the vessel steel problems mentioned in the article. If faulty, the vessel head could be replaced (at great expense), and the reactor vessel itself can be replaced during the construction phase (at even greater expense). I would hate to see the project put at risk over the issue.
Unfortunately, the articles are either vague or alarmist, so it's hard to be sure how serious of a problem it is. Being familiar with the nuclear industry, the 'problem' might be something like this:
1) Carbon content for the steel has been analyzed and tested as satisfactory between 0.50% and 1.25%.
2) Inspection reveals the carbon content at these two spots is 1.26%, outside the analyzed range.
3) New analysis and coupon testing is necessary to determine if 1.26% is safe.
It could even be general engineering knowledge that the steel is sufficient up to 2.00%, but since the properly documented analysis and tests haven't been done to that level, it doesn't count.
(I am not a metallurgist and my numbers are entirely made up)
Alcohol, Tobacco and Firearms should be the name of a store, not a government agency.
Then what happened in Fukushima?
The plant was deluged by a tsunami, it was never designed to handle that, and that was the central flaw. Cooling systems were not available, a necessity for this plant design. However, the melted fuel is still generally contained, but there are releases of contaminated coolant which is unacceptable, an outcome of placing a plant in the path of a tsunami when it is not designed to handle it, thus disabling the features that mitigate the things you discussed.
But, left completely with no mitigation, you are right in that the containment of older designs alone may not be enough to guarantee complete retainment under all circumstances, and newer passive designs or ones with core catching features are addressing this aspect.
There was a big change in design philosophy. Early reactor designs were intended to prevent meltdown and had limited mitigation. More recent designs now include substantial mitigation as well as more robust prevention strategies.
E.g. The fukushima accident occurred because of a "common cause" failure of multiple safety critical systems - the redundant diesel generators. This failure led to a "cliff edge" cascading failure of numerous safety systems, effectively meaning that core melt was inevitable. (This is in addition to the incorrect site risk assessment, where an incorrect tsunami risk was used when assessing the suitability of the site for a nuclear power plant, and the additional failure to mitigate that risk when the tsunami risk was recognised in the 1980s).
Most modern reactor designs (the EPR excepted) do not class their diesel generators as "safety critical", because they are not necessary to place the plant in a safe state and initiate adequate reactor cooling. In addition, nuclear regulators (Japan excepted) around the world started carefully investigating "cliff edge" scenarios following the 9/11 attacks, to see if deliberate sabotage could result in disproportionate failure of safety features. In the US, the NRC started mandating that "safety critical" diesel generators be heavily hardened against beyond design-basis natural events and other methods of attack, even if not originally conceived at design stage; that UPS batteries be upgraded to provide up to 24 hours of safety, in order to allow emergency assistance to be called in, and/or that additional electrical power sources (e.g. gas turbines) be installed in fortified near-site (to mitigate against local site damage) installations.
A similar set of upgraded mitigations have also been in place for a while - hydrogen catalytic recombiners (these are basically catalytic converters similar to those in a car exhaust which react hydrogen and oxygen at a low temperature and low hydrogen concentration, well below the minimum ignition level. Heat generated from the recombination is used to cause natural circulation of air through the combiner to accelerate hydrogen removal and stir up the air to ensure that hydrogen cannot pool away from the recombiners) have been installed in-containment, and in buildings close to hydrogen vent pipes. In Fukushima, no hydrogen recombiners were used, instead the main containment building was inerted with nitrogen. As a result, hydrogen (and steam) built up in the containment pressurising the building. In order to reduce pressure to prevent rupture, the containment building was vented into the main reactor building, where the hydrogen mixed with air and later ignited. More modern designs vent directly outside through filters, or vent through hydrogen recombiners.
The other complicating issue is that at Fukushima unit 1, the reactor core appears to have completely melted through the reactor vessel into the containment building, severely contaminating the water in the containment building which was being used for cooling (and also leaked through minor damage to the containment). Again, modern designs try to mitigate this. The AP1000 design fills the bottom of the reactor vessel with low-melting point, sacrificial material into which molten core material will melt, resulting in dilution, prevention of re criticality, and spreading of the decay heat. Then by flooding the containment building and submerging the reactor with water, "melt through" is prevented because of combination of external cooling water and the diluted core material, as a result the containment building itself is not contaminated. The EPR instead, has a special chamber beneath the reactor intended to spread and retain molten core material, in such a way that it would not contaminate the containment building.