Part 4 – Light Water Reactors

Before reviewing the new reactor designs it would be convenient to firstly look more closely at the LWR design so we can identify what wrong and what right about it as a reactor design goes.

4.1       A brief intro to light water reactors

 Light water reactors (LWR’s) come in two distinct flavours, the PWR (pressurised water reactor) and the BWR (boiling water reactor). In both cases we use water as both our coolant and neutron moderator. In the PWR, we keep the water liquid in the core, preventing it boiling and turning into steam by maintaining the pressure vessel at high operating pressures. It passes through a steam generator (basically a large heat exchanger connected to an unpressurised water/steam loop) which creates steam, which runs the turbines. In the BWR we purposely allow the water to boil into steam within the pressure vessel. This steam is then passed through the turbines, and back into the reactor itself. An illustration of the two systems from the Virtual Nuclear Tourist is included below:

Boiling Water Reactor (from virtual nuclear tourist)
Pressurised Water Reactor (from virtual nuclear tourist)

In general the BWR has the advantage of a lower operating pressure, less moving parts and a number of safety advantages over the PWR (generally related to the lower operating pressure of the BWR). However, BWR’s have much large pressure vessels than a PWR (and thus higher costs) and PWR’s have the safety advantage of a higher negative void co-efficient (i.e an increase in temperature results in a decrease in neutron production), although this ability breaks down if pressure is lost (as happened in TMI) as well as the fact that the control rods of a PWR can be easily inserted in an accident by simply cutting power to the Control rod clamping mechanisms and allowing them to fall into the core under they’re own weight, something that isn’t generally possible in a BWR. PWR’s tend to burn less fuel and also don’t need to circulate radioactive steam outside the containment dome.

Overall however, the BWR is considered safer (in most situations, the PWR crew would probably disagree with me on this one) and PWR is considered cheaper (again, I suspect the makers of BWR’s would disagree with me on that one!).

4.2       History and development of the LWR’s

 The history of both reactors can be traced back to the 1950’s. The US Navy, realising the huge potential strategic advantages that a nuclear powered warship would enjoy, in particular a submarine, embarked on a plan to build just such a vessel, a project led by US Adm. Rickover.

The PWR design was chosen for development to forfill the navy’s needs for a series of simple reasons. Thanks to water’s low density, high heat capacity and incompressibility, a PWR reactor would be compact, something important on a ship and it would use a coolant (water) that was easily available and cheap. Existing industrial experience at the time was more used to working with steam and water than any other medium, so that solved a whole load of technical problems and made a PWR much easier (and quicker) to develop, compared to other proposed reactor concepts (at the time). The world’s first nuclear powered submarine the USS Nautilus was launched in 1954, quite a technical achievement at the time.

The BWR came later, again as part of a US military reactor development program. This also led to the first ever fatalities in a nuclear reactor accident, when the SL-1 reactor suffered a criticality accident, killing three operators (one of whom wound up impaled by a cooling rod to the ceiling!).

One would have to question how it is that either of these reactor designs ended up jumping from the tightly controlled realm of military research into the civilian world. There are a variety of reasons. Firstly, commercial concerns. While the military may have been paying the bills, it was often private companies like Westinghouse, Combustion Engineering and GE doing the actual research. Thus it was always going to be easier to take one of their own off the shelf military designs and scale it up, rather than come up with something completely different. So to some degree shear bone idle laziness on the part of the corporations was to blame.

Also in these heady days of the early nuclear age there was an intense desire to be “first” of the mark. Wasting time trying to get alternative reactor designs working would have been both time consuming and expensive, and this time it would be the corporations paying the bills not uncle Sam (or in the Soviet Bloc, uncle Joe). Sure you could waste time trying to come up with something better, but who’s going to care if your rivals are already selling reactors like hot cakes and have taken over half the market!

Some advocates of alternative reactor designs, notably the LFTR mob have concocted various conspiracy theories suggesting that the MSR (and indeed many other reactor design proposals) were deliberately killed because the US government wanted to use civilian PWR’s to process uranium into plutonium for nuclear bombs. The evidence would suggest otherwise, as only a fraction of US civilian nuclear waste has ever been actually reprocessed and the vast majority of US weapons grade plutonium came from purpose build non-power reactors. I suspect there probably was a view at the time, taken by the corporations that yes, we could make plutonium and sell it to the government (or each other) and that could serve as a handy little revenue stream. But this was more an extra added bonus in the LWR’s favour, rather than the sole decisive factor. In truth, it all likely boiled down to cost. It was obvious at this stage that LWR’s, in particular PWR’s, would likely be a lot cheaper to build than any of the alternatives. And given the pressure at the time to maintain this “too cheap to meter” dream these arguments won out over everything else.

4.3       Safety issues affecting a LWR

Of course this PWR’s being “cheaper” paradigm only held through because also in these heady days nuclear safety was a concept that hadn’t really been invented yet! As I noted before, the nuclear industry laid a trap for themselves with the LWR’s. The scaled up plants were very different beasts from the small submarine reactor designs they were based on. The large cores gave them a very high thermal inertia. Think about a sink full of hot water and a bath full of it. The Sink will cool down quicker because of its smaller size. This applies doubly so for a nuclear reactor, as even if all the control rods go in, it doesn’t shutdown completely. Decay heat will still be produced from the core and for these large PWR’s such a continued production of decay heat could, in the event of a cooling failure, result in a meltdown scenario.

There are basically three dangers for a LWR reactor in the event of a LOCA (Loss of Cooling Accident). Firstly, the risk of a steam explosion. The pressure vessel of a reactor is basically a glorified boiler, if the pressure goes high enough and pressure relief valves fail (or can’t be utilised because that would exacerbate an already dangerous situation) it could burst, thought its generally more likely that the pipe work connecting the pressure vessel to other components of the plant will fail first (weakest link) and relive pressure. Of course the failure of pipework could well make it impossible to retain the radiation within the core and would make cool it down difficult if not impossible. There is a particular risk of such an explosive event if the water is allowed to flash rapidly into steam, something that a sudden drop in pressure (for a PWR) can potentially trigger.

The second major risk is that of a hydrogen explosion. At elevated temperatures and in the presence of molten reactor fuel and cladding material the water in the core can break down forming to hydrogen and oxygen. A build up of hydrogen presents can potentially lead to a self reinforcing mechanism, resulting in exposure of more and more of the core and further hydrogen production and further core melting. If concentrations of hydrogen and oxygen get high enough there is an increasing risk of an explosion. This is a particular risk if containment fails and air intrudes into the pressure vessel. At Fukushima, the operators appear to have taken a calculated risk by venting hydrogen from the core to the containment building. While this increased the risk of an explosion (as the containment dome contained air), it ensured that the explosion took place outside the pressure vessel. The danger is, had it taken place inside the pressure vessel, this might well have ruptured, and given the scale of the explosions, likely taken the containment vessel out with it too. So it was a gamble, but one that paid off.

The progressive meltdown and currently speculated end state of the Fukushima reactor cores

The third major danger is that of molten reactor fuel melting its way out of the base of the reactor pressure vessel and down into the earth, the so-called China Syndrome. While widely believed to be a risk at up until the 1970’s, the current consensus is that it’s not a major issue, as the core will likely pick up contaminants as it melts downwards, eventually disrupting the process. Indeed at Fukushima at least one of the reactors completely melted down yet there was no “China syndrome”…..or “South America syndrome” as it would be in this case! That said, decontaminating the core and removing this melted fuel is going to be a fairly difficult and expensive job. Also the situation at Fukushima is far from clear (at the time of writing they still haven’t be brought to a cold shutdown state), so its a little early for us to be getting out the cigars and saying all is well! It was several years after TMI before they realised just how badly damaged the core really was.

The various barriers to the biosphere a modern LWR presents, note the physically large size of containment vessel next to the reactor core itself

At the time of LWR roll out, the corporations devised a series of strategies for dealing with these scenarios. This included putting in backup cooling systems, ensuring a large excess of cooling water in the core (to increase the time it took for the reactor to overheat) and a large containment dome to contain any runaway core accident or explosion.

4.4       Close calls and the mega-LWR death spiral

 However, various close calls in the nuclear industry over the 1960’s and 70’s called into question whether these measures were adequate, culminating in the TMI accident of 1979.

One of the outcomes of the TMI accident was that the regulations around nuclear reactor designs were tightened substantially. Even before TMI there had been several other close calls (such as Brown Ferry in 1975, the 1952 Chalk river accident (although this latter one involved a heavy water reactor, it did have safety implications for the LWR as well) and those 3,333 incidents a year mentioned earlier (see here) which TMI now gave the impetuous to the authorities to act on. The nuclear industry was therefore forced to go back to the drawing board and hastily redesign its reactors to meet these new regulations. The cost of meeting them led to many plants in the US and around the world being abandoned part way through construction. The subsequent Chernobyl accident, while again a Heavy water reactor, had further design implications for the LWR designs meaning yet more design iterations and further rises in costs.

Other close calls, less well publicised, such as the 1982 Ginna core venting incident and the Davis-Besse 2002 near miss have further dented confidence and led to further design reviews.

A worrying near miss at Davis-Besse caused by Boric Acid corrosion. This hole was found just below the main control rod drive mechanism, meaning the control rods might have failed in the event of a breach!

Then 9/11 highlighted the risks of suicidal pilots crashing into reactors, several of the planes on the way to New York used the Indian Point reactor as an obvious landmark to look out for. Again, more design chances (the EPR now features a dual concrete dome, one layer to stop radiation getting out and another to stop anything, like a 737, getting in!), more cost.

This Mega-LWR death spiral continues with Fukushima which will almost certainly result in further escalations in cost, as designs are hastily reviewed and more plants forced to shutdown prematurely. I cover some of these issues in this post here. Furthermore, Fukushima highlights a critical flaw in all the previous design iterations, if the power fails a lot of these safety systems go with it. While more modern reactor designs such as the ABWR and the EPR have a few more options up there sleeve (both for example use natural convection to maintain circulation within the core, thought they ultimately need something to drive the secondary side of the cooling system as well as power the computers and control systems) and are not immediately at risk of meltdown if power fails (it would take many hours or days depending on the status of the control rods). But ultimately if power is not restored within a reasonable time period, even these designs too can go into meltdown. And Fukushima was no bolt from the blue, other accidents and near misses, notably the 1999 Blayais flooding incident and the 2007 Forsmark incident means this is a problem present in LWR’s that has long been known about. The 1986 Chernobyl accident was, ironically enough, triggered by the desire of the Russians to test strategies with how to cope with a loss of on-site power!

4.5       Future options

 So is there any way we can yank the LWR concept out its current tailspin? The industry seems to be pinning its hopes on the Super-critical water reactor, a version of the PWR which will operate at temperatures of +800 °C. To me this is a crazy idea. The one good think the LWR has going for it is the fact it uses relatively cheap and easily machined materials. This idea would thus wed our inherently unsafe PWR design to one dependant on expensive materials, it will have all the disadvantages of the other designs and none of the advantages.

To me the answer is to go the other way. Smaller BWR’s would be much safer than the current Mega-PWR’s on offer. They could be passively cooled and with this achieve a much lower accident risk rate. It noteworthy that the principle developer of the US PWR’s the US Navy has never had a serious nuclear accident, thanks to its rigorous “subsafe” program, and the fact that the US navy only uses relatively small nuclear reactors, 25-150 MWe against the 1,100 to 1,600 MWe behemoths on offer to the civilian market. It’s also possible that such small reactors could be mass produced. We will discuss the implications of such matters in the penultimate section; firstly let us look at the HWR design.

Previous Chapter                                         Main Page                                         Next Chapter

Advertisements