Sodium as coolant

As a part of the power demonstration program of the AFC in the 1950s, the Enrico Fermi fast breeder reactor (Fermi-1) was built near Detroit by a consortium of companies led by Detroit Edison. Fermi-1 used enriched uranium as fuel and sodium as coolant, and produced 61 MWe. It suffered a partial fuel melting accident in 1966 as the result of a blockage of core coolant flow by a metal plate. The reactor was repaired but shut down permanently in November 1972 because of lack of binding. Valuable experience was gained from its operation, however (58). 

It was found necessary to define this sub item in order to remind for the future what were in the past the reasons to select sodium as coolant for fast reactors and not another coolant (such as gas or lead or whatever). And also to remind what were the comparisons between sodium and other coolants with the choice criteria. 

In elaborating this idea of Fermi s, specialists of the ANL (USA) chose metallic fuel and lightweight sodium as coolant (Na—K in the EBR-I reactor). In the USSR, Leipunsky considered Pb-Bi and gas but decided on Na (Pb-Bi found its first application in reactors of nuclear submarines in the 1950s.) As regards fuel, he gave preference to heat-resistant ceramics. 

The use of sodium as coolant in the DRC loops meets the thermal-hydraulic requirements economically and enables the application of common sodium auxiliaries for DRC and secondary loops. 

Lead or lead-bismuth alloy can be used as alternatives to sodium as coolant. They have the advantages of higher boiling point and a coolant void reactivity which is much less positive than that of sodium, or even negative. They also lack the high gamma activity of Na-24, and as they are also inflammable in air or water a reactor cooled with Pb or Pb-Bi would not have to have an intermediate circuit between the primary coolant and the steam. 

Sodium is a possibiUty, as is the use of a sodium/potassium alloy (alloys have a lower melting point than their main constituent). The DFR used a sodium/potassium alloy and the later PFR used 1,500 tonnes of liquid sodium as coolant. Liquid sodium is not an easy material to work with and a very great deal of development work was needed. 

Both sodium and potassium are so reactive that they have to be stored under oil to prevent them from coming into contact with water or air. However, because they have low melting points and are good conductors of heat, they are used as coolants for nuclear reactors. 

In a fast-breeder reactor it is impractical to use water as coolant because it is too effective a moderator for neutrons. Liquid sodium is the coolant most extensively investigated for fast... 

Large experience has been already gained with sodium cooled fast reactor operation. The use of sodium as a coolant poses fire danger in case of its leakage and interaction with air or water. Operating experience testifies the possibility of coping with the mentioned problem, but the quest for excellence calls for future improvement in LMFRs technology. 

However, the choice of liquid sodium as a coolant and principal design features of fast reactors were mainly determined in the 1960s, as already mentioned, by the requirement of high power densities in the reactor core (about 500 kW(th)/l for MOX fuel), and the need of a weakly moderating material with good heat transfer properties. The important fact was also that sodium is practically non-corrosive to stainless steal. 

Techniques to counter the heavy metal coolant disadvantages are being developed, but in spite of this work and the apparent disadvantages of sodium, the consensus in favour of sodium remains strong. This is demonstrated by fact that before lead-cooled fast reactor BREST-300 is built, MINATOM will first build a sodium-cooled LMFR BN-800 (E. Adamov, NW, 23 September 1999). Moreover, in the last few years sodium has been chosen in both China and the Republic of Korea for the respective fast reactor development project. This is a significant endorsement for sodium as a fast reactor coolant. 

By now, comprehensive studies have been performed and reliable industrial experience has been gained on material corrosion in sodium. Corrosion intensity in sodium is significantly lower than that in water or lead-based coolants [5.9], In sodium, as well as in the other liquid metals, corrosion rate depends on many factors (temperature level, coolant velocity, impurity content, temperature difference, time, etc.). When evaluating corrosion rate, the major part of researchers took into account only the most contributing factors. Empirical equations for corrosion rate were most commonly derived for 316 steel at the coolant velocity of > 4 m/s and oxygen content of < 10 ppm. The most reliable results were obtained in [5.10, 5.11] for corrosion rate K, mg/cm h, that can be expressed as follows ... 

The disadvantage of using sodium as a coolant is a recurring question of plant design because of its chemical reactivity. 

A two-loop heat removal system uses a eutectic alloy of sodium and potassium as coolant. The first loop coolant, heated up to about 1,000 K, is supplied into the TEG located in a cylindrical casing. The second loop coolant removes the excess heat to the radiator. 

A fast neutron spectrum allows production of more fissile material than that consumed for heat generation. In a fast reactor liquid metal such as sodium is normally used to remove the heat, and it has a minimum effect on the moderation of fission neutrons. Sodium as a coolant has an excellent heat capacity, low operating pressure and natural convection capability. 

The sodium-cooled fast reactor (SFR) uses liquefied sodium as the primary coolant. The major advantage is that the reactor can operate at atmospheric pressure in the primary coolant loop. The ultimate inherent safety of this reactor, when using metallic (not oxide) uranium fuel, was demonstrated in 1986 with the Experimental Breeder Reactor 11 at the Idaho National Laboratory, under the direction of Argonne National Laboratory. 

BR-5 was the first reactor in the world using sodium as a coolant and plutonium oxide as a fuel (Status of liqued metal cooled fast breeder reactors. Technical Reports Series, No. 246, IAEA, Vienna, 1985, p. 89). The main purpose of the reactor was to gain bumup data on Pu02 and other fuel types, to obtain experience in operation of radioactive sodium systems. 

A single critical point of sodium cooled reactors of the BN type is the possibility of water-to-sodium leakage and sodium-water reaction. The BN GT-300 retains all positive features of previous sodium cooled reactors but makes use of a gas turbine, which eliminates leakage as a problem. In addition, primary circuit parameters (such as coolant flow rate, neutron flux, fuel burn-up and others) of the BN GT-300 were selected lower than those in conventional BN type reactors and also improve the BN GT safety in transient modes. 

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