6.4.3 – Fire risk and mitigation

Indeed it is fire that is very much my concern as regards these reactors. That big graphite core is a serious worry, it is listed as one of the concerns here. As one nuclear physics quipped to me graphite is basically just high grade coal. Obviously enough, building a nuclear reactor core out of coal doesn’t sound like a sensible idea! Its worth remembering that part of what made Chernobyl the disaster that it was, and why Fukushima is likely to have a much smaller level of fallout (despite 4 reactors involved, one fuelled with MOX against a single reactor fuelled with only lightly enriched uranium) is because the graphite moderated core at Chernobyl caught fire. It was this fire and the smoke it generated that allowed the radioactive material from the core to spread over such a large area.

There is some dispute within nuclear energy circles as to how significant the risk of fire is to graphite cored reactors. The evidence from Chernobyl suggests that it did catch fire, as the NEA official accident report (10 years on) discusses.  Analysis of the fission products from here and the Windscale accident suggests some combustion of graphite occurred in both cases. Dr Moormann (same as before)  suggests that such concerns over fire should be applied to a HTGR too, especially if the core is breached in some way by a large impact (Earthquake or a suicide attack with a jet airliner). This impact issue was recently a cause for concern regarding assessments of the safety of the UK’s AGR reactors. Dr Moormann also brings up the risk of water ingress (mentioned earlier) which could cause a temporary flip in the reactors void co-efficient from negative to positive. Moir and Teller (2004, yes that Teller) also bring up the matter of fire risk (briefly!) and it is mentioned back in a 1982 ed of the New Scientist.

However, on the other hand we have a recent report from the Windscale decommissioning team suggesting that no fire occurred and that damage to the core was localised to the region around fuel assemblies. Also Richards et al (1985) suggests that pure “nuclear” grade graphite is very difficult to set alight. General atomics, promoters of the GT-MHR reactor (a prismatic block type HTGR) say that graphite fires are of little concern. However I would note that in the above case the MHR has a layer of SiC to protect the graphite (Penner et al, 2007) and it is to be built underground (which limits our concerns regarding “large impacts” although as will be discussed in chapter 10.2.2 it opens up other problems).

Weighing up all the above evidence suggests, that the graphite fire risk is probably not as bad as it was long understood to be. However, we cannot conclusively conclude that there is no fire risk (as some mistakenly do), particularly given the evidence from Chernobyl. Indeed the precautionary principle would require that we assume that there is a fire risk, until it can be conclusively be proved otherwise. So clearly any FMEA process would zero in on this as a major issue that needs tackling. We need to take care in our design to make sure that any potential fire can be safely contained. How far the regulating authorities (and the general public) would require us to go is difficult to say, but we’ll now speculate on a few measures.

Obviously the above means that any ideas we have about building HTGR’s without containment domes, as some supporters of these reactors suggest we can (and indeed the UK’s AGR’s and Magnox reactors were also built without containment domes), wouldn’t be a good idea. I should note that the containment dome over a HTGR wouldn’t need to be build to the same exacting standards as one over a LWR as our goal is to contain a fire, not a melting down reactor core. This is important as it’s largely been the delays and difficulties in pouring concrete for these cores that is responsible for the messy cost overruns on the various new LWR reactor projects that are ongoing. Designing this containment dome to withstand some form of aircraft impact might also prove to be a requirement. Unfortunately, that would likely be expensive.

Our HTGR’s containment dome would need to be fitted out with some form of automatic fire detection and suppression system, specifically one that can cope with a high temperature graphite fire. As the Windscale power plant fire showed proper planning and equipment would be essential. At Windscale the initially attempt to put out the fire using CO2 failed, as the high temperatures of the fire simply stripped the oxygen from the CO2. The operators finally gambled and poured in water, knowing that this risked setting off an explosion, which fortunately didn’t happen. So clearly we’d need to be better prepared, an inert gas (Nitrogen, Argon or Xenon) or Halon gas could do the trick, if we have enough of it on site. I would note that a number of Halon’s have some potentially nasty environmental issues, such as being known carcinogens and mucking up the ozone layer, so inevitably storing a large quantity of them on site (never mind using them!) would have some serious environmental implications.I would note that the German HTG plants had connectors attached to the outside of the containment building (Kroger et al 1989) compatible with fire hose connectors, to allow fluid drain (in the event of a water ingress scenario) or the flooding of the core with water (as with Windscale, risky but a last ditch resort measure) to put out the fire.

And while talking about fire brigades, we don’t want to be relying, as at Chernobyl, on the local fire crew showing up and doing a Matrosov. Having a dedicated on-site fire crew covering the plant at all times (or nearby covering several plants in a geographical area), as is standard practice for airports, would be sensible. This fire crew, would be specifically trained in dealing with a high temperature graphite fire and be properly equipped to tackle such an event (i.e have working radiation suits! Unlike the situation at Chernobyl).

Post-Fukushima the authorities seem to have recognised the risks fire posses to nuclear power plants (graphite cored otherwise) and there is talk about requiring just such dedicated fire crews at the plants. Another option being considered is to have some sort of dedicated global “Thunderbirds” team (see here) which will fly in robots from worldwide to a plant to put out the fire. I suspect the fire crew option will re-emerge thought once the practicalities of this are brought up, but watch this space.

These measures outlined about should ultimately close off this “fire risk” loop hole, both for HTGR’s and other graphite cored reactors. However, how many of these measures would be necessary and how much it would cost is difficult to assess at this time.

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