Science Interviews


Tue, 27th Mar 2012

Keeping Reactors Running

Jonathan Morrison, University of Birmingham

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Ben -   Once a nuclear power plant is commissioned, itís important to keep it running well and efficiently, and keeping the water flowing is just as important as maintaining the radioactive fuel. 

Heat exchanges are necessary to take the heat from the nuclear reaction and put it into water to make steam which will then turn a turbine and generate your electricity.  So heat exchanges rely on having hot water on one location and piping it through very small pipes which have a large surface area over which the heat can pass into the cooler water.  Now that in itself creates a novel set of problems, that scientists such as Birmingham Universityís Jonathan Morrison are trying to solve.

Jonathan -   Well the nut that weíre trying to crack is the problem that's been seen in the nuclear industry for many, many years which is deposition of corroded material of very small restrictions in the systems.  So, as you go from a large flow, as you would, having a flow directly from the core of the nuclear reactor and into the steam generator, you go from a very large flow into very small tubes and you often see deposition at these very small tubes in the most inconvenient spot which is right at the entrance of them as the waters entering the small flow regime.

Ben -   How big a problem really is it?  Does it cause a reactor to stop functioning properly?

Jonathan -   Not so much stop functioning.  Itís more of a money issue.  The worse it gets, the more often you have to shut the reactor off and clean it because obviously, you want to improve your efficiency and the best way to do so is to have the best heat transfer from your steam generator, and from your fuel rods.  And you often have to switch the reactor off and clean it by hand in order to get this off.  So this isn't something that would stop the reactor from functioning completely.  This is merely money and the actual efficient operation of the plant.  And if we can solve this problem, we can prevent people from having to clean it that often and obviously, the less time people have to clean it, the less material they're exposed to, and the less dangerous Sizewell Benvironments they're exposed, and that's a good thing all around.

Ben -   So what is the material that's actually being deposited?

Jonathan -   Well that depends on what you've built your reactor out of.  At the very beginning of the nuclear age, we used something called admiralty brass I believe.  The type of copper brass alloy in them that obviously wouldíve deposited copper oxide and all sorts of things like that. 

In more recent years, weíve moved from using that into using stainless steel which has been an excellent material all around and is still used in many plants and itís still used in conventional plants as well. 

But in more recent years, the most commonly used materials have been the nickel based alloys which are extremely good.  They're very good at corrosion resistance, they have lifetimes well in excess of many of the other materials weíve used for the past 40 or 50 years, and they are being increasingly found to be far superior to what weíve used.  

My research is into stainless steel based material.  Mainly because many plants around the world still have stainless steel steam generators.  Replacing them with the new ones is really expensive.  These things are hundreds of millions of pounds each.  So, if we can solve the problem and people don't have to replace the steam generator because of a failure, all the better.

Ben -   Power plants that rely on other types of fuel, essentially work in the same way by heating a water to generate steam to turn a turbine.  So do we see the same problems in gas-fired plants and in coal-fired power plants?

Jonathan -   Not so much.  The issue with conventional plants and by conventional plants, I mean oil and coal, and gas.  They generally use something called supercritical water which means the water has been heated up above 374 degrees Celsius and itís at a very specific pressure, which I can't remember off the top of my head, but what it means is that the water is no longer either liquid, nor is it gas.  Itís actually in a completely new phase of its own! This new phase means that each unit of this water can carry a lot more heat than either steam or liquid water can carry on its own.  So, as you increase the temperature which will burn in your fuel, and as you increase the temperature of your coolant, you're actually increasing the efficiency of the fuel you've burned.  So you're getting more energy out of the same ton of coal or oil that you've burned.  Nuclear power stations don't work in the same way precisely.  They do work by heating up water but they don't work in that supercritical region because there is still a question of what will radiation do to water when itís heated up and when itís in the supercritical regime.

Ben -   So it is really a nuclear bespoke problem that you're looking at but what do you think is leading to this deposition being in those particular places where obviously, itís going to disrupt the flow, probably as much as it could do anywhere?

Jonathan -   Well, there are many problems that it could be, but I've been charged with looking at a single specific one which is a problem called the streaming current. The streaming current comes from a corroded surface being exposed to flowing water that contains corroded particles.  Now, the particles will sit very close to the wall and as we flow water very quickly across the surface, these particles that are loosely adhered to the wall will be forced downstream.  As they're forced, they are flowing across the surface and they're generating something of a current.  If you change the flow regime, that current is imbalanced.  And in order to satisfy physics and the law of charge conservation, something has to happen in order for these particles and this charge to be balanced.  So normally, the particles will find themselves pulled back towards the surface and quite tightly adhered, because they'll be re-oxidised into another state.  Now at this point, you find that you have quite hard deposition around this point.  Now, where we normally see this is when you change the flow regime from being a large flow and into a small flow.

sellafieldBen -   Do we have, in the works, any better ways to solve it rather than just cracking it open, and cleaning them off?

Jonathan -   No.  Unfortunately, the only way to clean these things is to get in there and manually do it.  Not necessarily on your hands and knees with a pipe cleaner, there are obviously methods to do it, but people have to be involved in cleaning the thing.  Thereís very few sections around.  You can't simply clean the pipes by putting some chemicals through it and shoving some bleach through the steam generator. That isn't going to work.

Ben -   And if you confirm that it is this, this induced current that attracts the particles in there, can we then use that to maybe aid, future reactor design in order to reduce this problem?

Jonathan -   Yes, the reactor design itself is not necessarily at fault but it will need to be changed, should this prove to be a really consequential problem.  Reactor design over the course of the last 50 years has changed in order to redesign out a problem that they see and water chemistry has been changed numerous times because of simple little things that we think, we can change that by changing the pH or just running it at a different temperature.  This is just another one of those problems.  Once this has been proven, there is probably more than likely a way to design the steam generator or to design any small flow restriction, in a way that it will no longer be affected by this kind of thing.

Ben -   Jonathan Morrison from the school of chemical engineering at the University of Birmingham.


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