Beyond Design Basis Accidents

[quokka]

Since the Fukushima accident, there has been a fair bit of talk of mitigation of the consequences of beyond design basis accidents.

WNN reports that Areva is to supply filtered containment venting systems for all Japanese BWRs.

www.world-nuclear-news.org/RS-Areva_venting_systems_for_Japanese_BWRs-0406134.html

This, in addition to hydrogen recombiners.

My question for those that know more about these things than I do is whether a much smaller radiation release would have been the likely outcome of the Fukushima accident had these been fitted?

And the associated question of how much does this reduce the risk of major radiation release even in the event of serious core damage?

[edireland]

If hydrogen recombiners had been fitted the primary benefit would have been that large scale venting of gas from the secondary containment would have been unneccesary. This would have reduced releases of radioiodine, krypton and other volatile fission products.

Additionally the secondary containment of one of the units could not have exploded.

[quokka]

Yes, I get the picture about the hydrogen recombiners. The question was really more about the filtered containment venting.

As I understand it the major radiation release at Fukushima was due to venting the RPVs to avoid excessive and dangerous pressure. How much could filtered containment venting have reduced radiation release to the outside world? Are we talking order of magnitude? Or is that much too optimistic?

[edireland]

The filtered containment venting should have prevented any isotopes other than gasses like iodine and krypton from escaping the containment.
No caesium, no strontium and no technetium.

[quokka]

As I should have done before starting this thread, I had a bit of a quick look around and found a few things:

Westinghouse marketing material: www.westinghousenuclear.com/Products_&_Services/docs/flysheets/NS-IMS-0054.pdf

Areva marketing material: www.areva.com/globaloffer/liblocal/docs/Brochures/AREVA_Filtered-Containment-Venting-System_vEN.pdf

And an Areva presentation to the US NRC: pbadupws.nrc.gov/docs/ML1220/ML12206A263.pdf

Both Areva and Westinghouse claim a high level of removal of aerosols and iodine (>99%).

From which I conclude that it seems possible that if such devices were fitted to the Fukushima BWRs radiation release would have been much lower, and in particular cesium and iodine release would have been much lower, and evacuations other than short term may not have been necessary. Anybody disagree with that?

[cyrilr]

Most of the releases at Fukushima weren't from the venting operations, but due to containment bypass. Basically, leaks through seals in the containment due to overpressure, overtemperature (these are organic seals that don't like high temperature), plus bypass via backflow of the offgas system.

The reason for the overpressure and overheating was loss of cooling for the reactor. The heat produced by decay heating in the reactor was pushed out to the water in the containment, a so called pressure suppression system. This system has a certain amount of water so it can only condense so much steam (ie heat) from the reactor. When the capacity was exceeded (ie water becomes saturated, near boiling), the pressure suppression system stops functioning. Then the steam can't be condensed so you get higher pressure when venting the steam to the containment.

What makes this worse is hydrogen, in case of Fukushima coming from the overheating fuel that reacts the zircalloy cladding with water, to produce hydrogen and oxidized cladding, which fails the cladding, releasing radionuclides to the vessel, and then to containment. The hydrogen is also pushed out to the containment, one big problem with hydrogen is that it can't be condensed in the pressure suppression system, so even if suppression pool cooling was available, there would still be a large pressure rise just from the hydrogen. Without cooling for the suppression pool, which was the case at Fukushima, there's both excess steam and hydrogen which means a big pressure rise, failing the seals of the containment.

There are various design remedies, all focus on two things, passive cooling to prevent steam pressurization in a station blackout, and hydrogen control. Hydrogen control could be either recombiners or just increase the pressure capability of the containment (so that all of the hydrogen can be accomodated in the containment).

Modern reactors such as ESBWR use both passive cooling and inerted containments with 100% hydrogen storage capability.

I work in the field of industrial safety analysis. From a safety analysis perspective, preventing is better than accomodating, and accomodating is better than mitigating, so with reliable passive cooling you don't need to accomodate hydrogen, and accomodating hydrogen is better than filtered venting.

In addition, the main issue for the venting operations at Fukushima is that the venting systems needed power and instrument air. Pretty stupid, like designing an airbag of a car to only work when the car is fully intact. When the chances are pretty fat, that if you need the airbag, the car isn't intact anymore. It was well known, even in the 1970s, that the primary cause of core damage for these reactors is loss of electricity. So you don't design mitigating systems (filtered venting) to need power. That's just bad design.

So, yes, in my opinion, filtered venting is not that great an idea. Cooling capability is priority number one. Without cooling, you need to vent a lot, and have huge plant damage and cleanup costs. With cooling, you don't need venting systems, avoid plant damage, and cleanup costs. Importantly you also avoid entire countries forcing to shut down all you nuclear plants, kind of nice to have if you're a nuclear utility.