This is the second of a four-part series of extracts from the book Plentiful Energy — The story of the Integral Fast Reactor by Chuck Till and Yoon Chang.
Reproduced with permission of the authors, these sections describe and justify some of the key design choices that went into the making the IFR a different — and highly successful — approach to fast neutron reactor technology and its associated fuel recycling.
These excerpts not only provide a fascinating insight into a truly sustainable form nuclear power; they also provide excellent reference material for refuting many of the spurious claims on the internet about IFR by people who don’t understand (or choose to wilfully misrepresent) this critically important technology. Click here for part 1 (metal fuels and plutonium).
The second extract, on coolant choice and reactor configuration, comes from pages 108-111 of Plentiful Energy. To buy the book ($18 US) and get the full story, go to Amazon or CreateSpace. (Note that the images below do not come from the book).
Liquid sodium was the choice of coolant from the beginnings of fast reactor development, because the neutron energies must remain high for good breeding and sodium doesn’t slow the neutrons significantly. (Water does, and so nullifies breeding.) But sodium has other highly desirable properties too—it transfers heat easily and removes heat from the fuel quickly; it has a high heat capacity which allows it to absorb significant heat without excessive temperature rise; its boiling point is far too high for it to boil at operating temperatures, and importantly, even to boil at temperatures well above operating; and finally, although a solid at room temperature, it has a low enough melting point to stay liquid at temperatures not too far above that. In addition, there is no chemical reaction at all between the sodium and the structural materials making up the core (such as steel and zirconium). It is chemically stable, stable at high temperatures, stable under irradiation, cheap, and commonly available.
Further, as a metal, sodium does not react at all with metal fuel either, so there is no fuel/coolant interaction as there is for oxide fuel exposed to sodium. In oxide fuel, if the cladding develops a breach such reactions can form reaction products which are larger in volume than the original oxide. They can continue to open the breach, expel reacted product, and could possibly block the coolant channel and lead to further problems. Metal fuel eliminates this concern.
For ease of reactor operation, sodium coolant has one supreme advantage. Liquid at room pressures, it allows the reactor to operate at atmospheric pressure. This has many advantages. Water as a coolant needs very high pressures to keep it liquid at operating temperatures. A thousand- to two-thousand-psi pressure must be maintained, depending on the reactor design. Thick-walled reactor vessels are needed to contain the reactor core with coolant at these pressures.
The diameter of the vessel must be kept as small as possible, as the wall thickness necessary increases directly with diameter. With the room-pressure operation of sodium coolant, the reactor vessel, or reactor tank as it is called, can be any diameter at all; there is no pressure to contain. And leaks of sodium, if they happen, have no pressure behind them, they drip out into the atmosphere, where generally they are noticed as a wisp of smoke. The important thing is that there is no explosive flashing to steam as there is when water at high pressure and temperature finds a leakage path.
Its principal disadvantage is that it is highly chemically reactive with oxygen, in water or in air. It must not be exposed to either so it must be maintained in an inert gas environment. Argon, a relatively common noble gas, which itself is completely non-reactive and heavy enough to blanket surfaces and keep them blanketed, is the obvious choice to do this. Its opacity is little more than a nuisance; techniques have been developed over the years to deal with it.
Editor’s Note: Click here to download a PDF of an excellent 50-slide presentation from Argonne National Labs, called “Sodium as a Fast Reactor Coolant“. The two Argonne images shown above are slides from that presentation.
The Reactor Configuration in the IFR
The reactor configuration—that is, the arrangement of the vessel containing the core and the necessary piping—is important too. The piping that carries sodium coolant exposed to radiation in the core, the primary sodium as it is called, mustn’t ever leak to the atmosphere. Primary sodium is radioactive; short-lived Na-24 is formed, which has a fifteen-hour half-life and decays with two hard gammas, at 1.38 and 2.75 MeV. Gamma radiation at energies this high is penetrating and hazardous to humans. Its half-life is short enough that it dies away reasonably quickly. But as sodium burns in air with a heavy white smoke, radioactivity would spread from a leak.
A pool configuration eliminates this possibility. It keeps all primary sodium and its associated piping inside a double-walled tank. Radioactive sodium is never exposed to the atmosphere, if the primary system does leak it merely leaks sodium back into the pool. The heat in the primary sodium is transferred in a heat exchanger inside the tank to a secondary cooling circuit. Only non-radioactive sodium from the secondary cooling circuit is brought out of the vessel. This piping may develop a leak, but there can be no spread of radioactivity from it. Radioactivity from sodium leaks is a non-existent problem in the pool reactor configuration.
The pool configuration is a conscious choice, just as the fuel and coolant materials choices are. The reactor tank is sized large enough to accommodate all the primary system components. The core itself, the primary piping, and the primary heat exchanger (where the heat is transferred from the radioactive primary sodium) are submerged in the pool of primary sodium. The tank boundary has no penetrations; it is a smooth walled tank, and it in turn sits in another larger diameter tank. This guard vessel provides double assurance that there will be no leaks to the room. Unpressurized, a leak of sodium from the primary vessel would go into the space between the two vessels. That space is “inerted” with argon gas, and instrumentation is provided to monitor the space for any leaks into it. (There were none in the thirty-year lifetime of EBR-II.)
It should be noted that of the two possible reactor configurations, pool or loop, each is suited to one particular coolant type. The water-cooled reactor, because of its high pressures, needs a small-diameter reactor vessel and the loop design is almost mandatory. The sodium-cooled reactor, because of its low pressure coolant can have any sized vessel. The primary coolant is radioactive, so it’s best to have primary components, piping, and connections inside the primary tank.
The pool is a natural choice, and it was the choice ofArgonne’s designers of EBR-II in the late 1950s. The loop design, of course, is possible, and in fact it became the choice for theU.S.breeder development in the late sixties and seventies, and several of the breeder reactors built around the world were given this configuration, but for a number of reasons it is not the natural choice for sodium cooling.
As will be seen in the chapter on safety, sizing the pool to provide enough bulk sodium to absorb the heat of accident conditions adds some remarkable extra safety properties to the system. It allows safe regulation of the reactor power even under conditions where an accident has disabled the control and safety systems (see image above). In such an accident the massive pool of sodium provides ballast—heat can be absorbed until the natural reactivity feedbacks of a metallic-fueled core come in strongly enough to reduce the reactor power to harmless levels.
These “natural reactivity feedbacks” reduce reactivity as the core expands from the increased temperatures of an accident. Neutron leakage is much more important to reactivity in a fast reactor than a thermal reactor. In a fast reactor, neutron cross-sections are small and neutrons typically travel tens of centimeters before being absorbed, compared to distances of fractions of a centimeter in thermal reactors.
The core dimensions are small too, so a large fraction of the neutrons are born close to the boundaries and many leak from the reactor. Small increases in the core diameter due to temperature increase in turn increase neutron leakage in the axial direction and give reactivity reductions sufficient to reduce reactor power.