Tuesday, March 15, 2011

The Risks of Nuclear

The recent explosion and now likely meltdown at the Fukushima nuclear power plant run by TEPCO has generated a lot of press and re-sparked the debate on nuclear safety. This is an especially important discussion in the present period as many countries throughout the world are currently assessing which potential alternatives to natural gas can serve as feasible replacements.

The current reactor design is a BWR-type reactor which uses light water as a coolant. While it share some basic features with the notorious Chernobyl RMBK plant, it also has important differences. In addition to the RMBK's many mis-features which lead to the famous accident, RMBK also suffered from an almost complete lack of passive safety systems. It even lacked a primary containment vessel for the reactor. When it exploded it shot flaming graphite and radioactive products into the atmosphere which spread over a wide area - a seriously catastrophic event.

Because of the existence of a primary containment vessel, under normal conditions the design at Fukushima should keep even a complete meltdown from causing a serious radiation danger to the public, much in the same way the Three Mile Island reactor was able to do. However, the conditions that the reactor has so far encountered are not particularly normal. After a 9.0 earthquake, it's very difficult to be sure if your design is going to act in the way you intended.

Which leads us to the primary difficulty which has plagued the Fukushima reactor. The reactor design relies critically on an active coolant system. I have read in several places where people have wondered why the reactor wasn't scrammed (scramming means implementing emergency shutdown procedures). In fact it was scrammed. The problem is that it takes a long time to cool down. During this entire cool-off period, one needs to be flowing coolant past the core to avoid a meltdown. Unfortunately the pump system were unable to function because of a failure to power them. Without coolant the core melts and the problem becomes much more complicated and dangerous. In the worse case a complete liquification of the core could even lead to a return to criticality. This would be similar to the reactor core turning back on, except this time without the designed geometry. Essentially an uncontrolled and very difficult to control reactor. If this happens, things become much more complicated and dangerous.

The assessment of safety for the Fukushima units was based on the idea that redundancy would provide sufficient safety. However, they neglected to calculate the risk of some event in which both causes were common - that the same cause of electrical failure would also knock out the generators.

A passive safety coolant system should likely have been a requirement for any reactor design as this event shows. Reactors such as the Economic Simplified Boiling Water Reactor would not have been affected by a generator failure and would have been able to provide passive cooling for the period needed to cool the core to avoid meltdown. This would presumably lead to a greater margin of safety.

However, we should still wonder whether or not if it would be safe enough. The fact that some coolant failure could lead to a meltdown and consequently a return to criticality should give pause. A worse case scenario becomes very bad indeed.

There are many questions that are necessary to contemplate in evaluating the safety of various technologies. Nuclear designs as they currently stand, are somewhat peculiar compared to most of our other fuel technologies. Nuclear designs, have, per TWh proved to be extremely safe as compares other power generation technologies such as natural Gas. In Europe, nuclear is on the order of between 10 and 1000 times safer* in count of number of deaths per TWh from all causes than natural gas.

Should we count Chernobyl into our calculations? How do we assess risk from cataclysmic events? The assessment of risk from low probability but potentially massive events is very difficult. Very low risks are very difficult to measure accurately since their frequency is so low that our estimates tend be dominated by guesses.

In addition we need to compare the safety against other replacement technologies, or the possibilities of abandoning the technologies niche itself. In the case of nuclear power, this would be a search for baseload power replacements.

When we begin to look at technologies in comparison we find that even in this tragic and improbable event in Japan natural gas has itself not been free from problems. Many people in Japan were incinerated from natural gas explosions. There were also 1800 homes washed away by a dam failure. It's not clear how many died from that, but the number is likely to be very substantial. Which energy source turns out to be more deadly under such extreme conditions will have to wait until after the scale of the nuclear threat is fully understood.

Yet the nuclear power systems continue to drive more public fear. Some of this may have to do with the difficulty of providing an accurate risk assessment leaving us to guess exactly how bad things can get. When people look to the nuclear experts for opinion the best they can seem to do is say something along the lines of: We expect it will not be as bad as Chernobyl. Such statements are hardly very reassuring.

The character of the particular technology itself is not irrelevant in our calculations. To take a rather less charged subject than nuclear power we can instead turn to the question of Hydro power. Hydro power deaths per TWh if taken in summation over the entire world turns out to be one of the worst offending technologies. Worse than even natural gas or coal. However, almost all of the problems with hydro occurred in impoverished third world countries. A single catastrophe in 1975 at the Banqiao Dam in China left over 20,000 dead directly from drowning and somewhere around 100,000 dead from famine and disease.

No such legacy haunts Europe's dams. They have proved to be both safe and stable and hydro power in Europe deaths per TWh is effectively zero if we exclude eastern Europe. A similar truth holds for nuclear power.

Now we can perhaps say that large dams in Europe should be avoided on the off chance that some Typhoon or Earthquake hits - an event that while it may seem improbable - is not impossible. Since the potential death tolls would be tremendous, it's not totally unreasonable to overestimate the probability in order to provide some buffer of safety for ourselves.

However, this same reasoning should not cause us to avoid micro hydro power, since the possibility of massive disasters from a small water turbine is impossible to imagine (though some deaths would not be impossible). Similarly, it should not be the case that we reject all nuclear power based on specific applications of the technology in specific circumstances. The evaluations of the worst case scenarios need to be made on the basis of the implementation.

In order to understand nuclear safety, or the lack thereof, it helps to go back a bit in time to the creation of the US nuclear programme to see why we have the reactors that we do.

Light water reactors are not by any means the only type of reactor. During the course of development there were a large range of reactors which were tested. The number of types now in operation is much less diverse than when nuclear power was in its infancy.

One might suppose that this was because we have settled on what are effectively the most safe and reliable nuclear reactors with the best characteristics. Unfortunately, to assume this would be to assume wrongly.

The development of nuclear power has been closely coupled with the desire to develop nuclear weapons. Without understanding this fact it's impossible to understand the direction of nuclear development.

Several designs for nuclear reactors, including one of the first, the AHR (Aqueous Homogeneous Reactor) and a later design based on similar ideas, the MSR (Molten Salt Reactor) were dropped despite the fact that they had achieved similar potential viability as a comercial reactor technology to the now popular LWR (Light Water Reactors). Some of these designs were considered so safe that universities were given licenses to operate them for the generation of isotopes or neutron flux for experiments.

These reactors had many potential advantages including intrinsic passive safety features. They allowed designs ranging from the truly tiny, around .05MW up to large scale reactors, around 1GW. These designs allowed cheaper fuel production, since they used a fuel slurry, liquid or aqueous suspension, rather than complicated metal cladded fuel pellets. Most surprisingly, they also allowed arbitrarily high burnup of the nuclear fuel.

In a standard LWR, one can expect somewhere around 5% of the fissile material to be used. In some of the most sophisticated high temperature reactors that have been operated, solid core configurations can reach 20%. The end result of these low burnups are high production of waste, and low efficiency in the use of fuel. If you can exceed 99% then you are potentially producing very little waste.

Liquid reactors are also able to evacuate Xenon 135 by bubbling it out of the core. The Chernobyl accident was exacerbated by a lack of primary containment. However, the initial instability was due to a build up a of the neutron poison, Xe-135. This element stops neutrons in the chain reaction as its absorption profile is enormous compared to anything else. Nuclear fission can cause a buildup in a solid fuel leading to a sudden drop in neutrons. However, when the Xe-135 decays one can find a sudden return to neutrons and a consequent heating of the reactor. Xe-135 is a major difficulty in the operation of solid fuel reactors, since they are not able to evacuate it, but must wait for decay.

If that weren't enough, these reactor types could also use Thorium as a fuel. Thorium is much more prevalent in the Earth's crust than Uranium and much more evenly distributed

So why didn't the Atomic Energy Commission forge ahead with these reactor designs? As Kirschenbaum, who worked on the AHR, related, the design was rejected already in 1944 when they realised it would not produce Plutonium as quickly as the AEC wanted. The use of Thorium turns out to have been scratched for similar reasons. There is no good production pathway for Plutonium from Thorium.

The AEC was dedicated, not to finding the most efficient fuel source as the "Atoms for Peace" moniker might lead one falsely to believe, but was interested in the production of weapons grade plutonium. As such it was completely dedicated to the "Plutonium economy", which included an array of LWRs and fast breeder reactors which would allow the production of large quantities for the nuclear weapons program. LWRs were to become dominant despite their lack of inherent safety features.

During the 1960s, one of the great nuclear scientists, and lifelong proponent of nuclear power, Alvin Weinberg, was asked by the AEC to do safety assessments of LWR type reactors. What Weinberg and his team found in their assessments caused them some distress. The LWR designs indeed had very serious safety deficiencies. Weinberg then began attempting to warn the industry and the AEC about the shortcomings in the designs.

Eventually, Weinberg was sidelined. US Senator Chet Holifield, a proponent of the "Plutonium Economy", famously said: "Alvin, if you are concerned about the safety of reactors, then I think it might be time for you to leave nuclear energy."

Whether or not nuclear power should take centre stage, be a bit player, or not even make the cut is a question that can't be answered easily. As for myself, I'm sympathetic towards nuclear power as a fuel source for a world that will need ever more energy. The question of course, requires a careful evaluation of the options and the associated costs of these options.

In the last analysis however, more important even than this careful analysis of our options, are the following two points:

There is only one all important factor in which energy source we use, and that is humans. It isn't how much the plant cost and it isn't about the strict conversion efficiencies of thermal energy to electric or any other such technical parameter. It simply matters if it will improve or disimprove our lives compared to not using it.

Lastly, what makes the most sense from this perspective is irrelevant if we haven't the power to make it happen. As we see clearly with the choice to develop LWR technology, those with the power call the shots. If we want the over-riding important factor to be how things impact people, the people are going to need a lot more power.

* Figures for deaths per TWh are from ExternE, and modified to include some of the most pessimistic estimates for Chernobyl

1 comment:

  1. the storage pool appears to have caught fire this morning
    the only combustion possible is a zirconium fire from the cladding
    this safety quiz

    appears to be an exact roadmap for how this will pan out: a large distribution of caesium 137 into the area the wind takes it.
    I wonder if the same fate is next for the other storage pools