Climate change

September 15, 2010

IFR FaD 6 – fast reactors are easy to control


by Barry Brook

There are many topics in the IFR FaD series that I want to develop in sequence — and in some detail. But for the moment, here’s a little diversion. People often complain that sodium-cooled fast reactors are about as easy to control as wild stallions — at least compared to the docile mares that are water-moderated thermal reactors. The experience on the EBR-II (which I’ll describe further in future posts) certainly belies this assertion, but for now, I want to go to another source.

Here are comments from Joël Sarge Guidez, written in 2002, who Chairman of International Group Of Research Reactors (IGORR), Director of Phénix fast breeder reactor (a 233 MWe power plant which operated in France for more than 30 years, with an availability factor of 78 % in 2004, 85% in 2005 and 78% in 2006), and President of the club of French Research Reactors:

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A reactor that’s easy to live with

Pressurised water reactor specialists are always surprised how easy it is to run a fast reactor: no pressure, no neutron poisons like boron, no xenon effect, no compensatory movements of the rods, etc. Simply, when one raises the rods, there is divergence and the power increases. Regulating the level of the rods stabilises the reactor at the desired power. The very strong thermal inertia of the whole unit allows plenty of time for the corresponding temperature changes. If one does nothing, the power will gradually decrease as the fuel ages, and from time to time one will have to raise the rods again to maintain constant power. It all reminds one of a good honest cart-horse rather than a highly-strung race horse.

Similarly, the supposed drawbacks of sodium often turn out in practice to be advantages. For example, the sodium leaks (about thirty so far since the plant first started up) create electrical contacts and produce smoke, which means they can be detected very quickly. Again, the fact that sodium is solid at ambient temperature simplifies many operations on the circuits. More generally, because of the chemical properties of sodium, the plant is designed to keep it rigorously confined, including during handling. During operation, all this provides a much greater “dosimetric convenience” than conventional reactors. In particular, a very large part of the plant is completely accessible to staff whatever power the reactor is at, and the dose levels are very low.

Because of the very high neutron flux (more than ten times as high as with water reactors), there is great demand for experiments. These experiments are performed using either rigs inside carrier sub-assemblies or using special experimental sub-assemblies with particular characteristics. All experiments are run and monitored in the core like the other subassemblies.

Since the origin Phénix irradiated around 1000 sub-assemblies, on which 200 were experimental sub-assemblies. It is true that the Phénix is not as flexible as an experimental water reactor, in which targets can easily be handled and moved. But, with a minimum of preparation – which is necessary anyway for reasons of safety and quality – numerous parameters such as flux, spectrum and duration can be adjusted to the needs of each experiment.

Furthermore, the reactor was designed by modest people who thought in advance of everything that would be needed for intervention on the plant: modular steam generators, washing pits, component handling casks etc. All of which has been very useful and has made possible numerous operations and modifications in every domain. All this has meant that a prototype reactor built in the early 1970s is still operational in 2004, and will continue so for several years yet.

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Some further useful information can be had from Guidez’s presentation at the 2008 International Group on Research Reactors conference. Download and read over this 19-page PDF, which is the easy-to-read slides of his presentation, called “THE RENAISSANCE OF SODIUM FAST REACTORS STATUS AND CONTRIBUTION OF PHENIX”.

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