3. The unique properties of sodium cooled reactors

LWRs are considered “thermal” spectrum reactors because the neutrons that are involved in the chain reaction are slowed down by collisions with a “moderator”, typically the hydrogen in the water coolant so that they (the neutrons) are in thermal equilibrium with their surroundings.⁵ Since the hydrogen nucleus has an atomic number of one, it has a mass about equal to a neutron, and as a result neutrons can lose a great deal of energy with each hydrogen collision. This slowing down of neutrons is desirable since U²³⁵ has a very high cross section for fission with thermal neutrons.⁶ Thus the chain reaction can be made to occur with a relatively low concentration of the fissile material, U²³⁵. However, fission with thermal neutrons comes at a price. The number of secondary neutrons produced per fission is lower with thermal neutrons than is the case with fission resulting from fast neutrons. While there are enough neutrons produced per thermal fission in a thermal reactor to sustain the chain reaction, there are not enough left over for breeding. While some transformation of U²³⁸ occurs in a thermal reactor, the number of transformations per fission is less than one.⁷

If it is desired to breed, viz. to produce more transformations of U²³⁸ than there are fissions, a coolant must be used that does not slow down the neutrons appreciably on the occasions of collisions. Sodium with an atomic number of 23 meets this requirement. The law of conservation of momentum requires that elastic collisions between neutrons and sodium atoms at rest result in just a 2% neutron energy loss on average compared with over 60% neutron energy loss per collision that occurs with hydrogen atoms.

At ambient conditions, sodium is a soft metal that is chemically very reactive, particularly with water or oxygen. In its elemental form, it does not exist in nature, although its compounds are common. In its elemental form, it has a melting point of 208° F. and a boiling point of 1616° F. which are in the operating range of a thermal steam plant. If the reactor is cooled with sodium⁸ and the heat is then transferred to water generating steam, the reactor can be used as a heat source for an electric power generating station.

• Sodium has certain features that make it very desirable for use as a reactor coolant.
• Since its boiling point is 1616° F. the reactor coolant system can be operated at low pressure greatly simplifying the design of the reactor vessel primary coolant system components and piping. The wall thickness of sodium piping, even at sizes up to 48 in. in diameter, will typically be no greater than ½ in.
• The reactor can be operated at much higher temperatures than water-cooled reactors, greatly improving thermodynamic efficiency. Reactor outlet temperatures in the 1000-1100°F range are readily obtainable in contrast to maximum reactor outlet temperatures of around 600°F for water-cooled reactors.
• Superheated steam can be produced from a sodium cooled reactor plant in contrast to LWRs which produce only saturated steam.⁹ Superheat of approximately 250-300°F is readily achievable. Because of the steam conditions that are obtainable with sodium cooled reactors, they can operate with a thermodynamic efficiency in excess of 40% — much better than LWRs can attain. Since much of a power plant’s size is determined by its thermal power and greater efficiency translates into lower thermal power for a given electric output, greater efficiency is a positive cost driver. Higher efficiency also means that less fuel is consumed for a given quantity of electricity generated and less heat must be rejected to the environment.
• A coolant system breech does not result in the immediate pressurization of the containment system which encloses the reactor coolant system as is the case for water-cooled reactors. As a result, containment systems can be designed to much lower pressures than are typically required in water-cooled reactors. Also, the containment size is dictated by the physical size of the primary system and is not influenced by considerations of loss of coolant accidents. Novel containment design concepts such as ice condensers need not be considered.
• There is no need for post accident emergency containment cooling systems such as those typically found on LWRs to cope with flashing steam from a coolant leak.
• The sodium coolant is chemically benign with respect to the metal systems that contain it. At reasonably high purities, sodium is non-corrosive of steels or other metals likely to form any sodium system boundary. This is an important feature since corrosion is a continuing problem in power plant design and must be accounted for in the design and operation of the plants. This feature also makes sodium systems attractive if very long plant design lives are an objective.
• Since the coolant is electrically conductive, electromagnetic (EM) pumps with no moving parts can be used. Such pumps have been used at small scales in sodium auxiliary systems since the early days of LMFBR development and have more recently been demonstrated at large scale for use in the primary coolant system. EM pumps have no penetrations into the sodium system reducing the possibility of coolant leakage and eliminating rotating seals. Moreover, the same principle can be used in reverse for flow meters, further reducing the number of coolant penetrations.
• Coolant boundary breech can be accommodated by incorporation of guard vessels into the design. There is therefore no need for emergency injection systems as are required in LWRs.
• There is no deposition of corrosion products on reactor core components, eliminating the problem of crud¹⁰ deposits and crud bursts in the primary system.
• The steam generators are located outside containment improving their accessibility and maintainability.
• The limitations on scaling up the size of the reactor plant are much less restrictive when sodium is used as the coolant than would be the case for pressurized systems. In 1966, Argonne National Laboratory performed a feasibility study of a 10,000 MWth loop-type LMFBR plant with a net electrical output of 3880 MWe.¹¹ There were no obvious technical obstacles. The scale-up of pressurized systems to this size is likely to encounter limitations on the wall thickness of the pressurized vessels such as the reactor vessel or the containment vessel.
• Sodium is light in weight having a specific gravity around 0.9. At temperatures of interest for reactor coolant applications, its viscosity is comparable to water. It therefore is relatively easy to pump and does not weigh down structural components the way heavier liquid metals e.g. lead/bismuth or mercury would. Unlike mercury, it does not form amalgams with other metals.
• Sodium is an excellent conductor of heat. Its thermal conductivity is approximately 30 Btu/hr-ft-°F vs. 0.3 Btu/hr-ft-°F for water; sodium being about 100 times greater.
• The coefficient of thermal expansion of sodium at operating temperatures is about 0.16 X 10^-3 per °F which compares with about 0.9 X 10^-3 per °F for water in the 400-500 °F temperature range. While this coefficient is lower for sodium than for water, it is still sufficiently high for the purposes described in the next item.
• The high thermal conductivity of sodium enables it to effectively remove heat without dependence on turbulent flow. Its reasonably high thermal expansion coefficient combined with a ~250 °F temperature rise across the reactor promotes natural circulation provided the heat exchanger removing heat is sufficiently elevated above the reactor core. The resulting natural circulation is adequate for decay heat removal from core surfaces following shutdown.
• When compounded with other elements, sodium is abundant in the earth’s crust and relatively inexpensive. Sodium can be manufactured to exacting purity standards.
• Sodium readily removes heat from reactor surfaces. There is no boiling on core surfaces thus any need for concern about departure from nucleate boiling (DNB) which is a major design consideration for LWRs.
• There is no need to inject hydrogen or alkalis to control pH as is required in water-cooled reactors.
• The solubility of most contaminants, mainly sodium oxide (NaO₂), in sodium decreases with decreasing temperature allowing for their removal by cold trapping. Cold trapping the coolant can maintain NaO₂ concentration below 10 ppm, which is well below the concentration that it would become corrosive to the metals forming the system boundaries.¹² The cold traps turn out to be effective in removing most of the fission product contaminants from the coolant. There is no need for filters or resin beds and no concomitant need to change out and dispose of spent filters or resins, although the internals of the cold traps will eventually require replacement, particularly if the traps become obliged to remove high levels of oxide contamination in the coolant.
• The specific heat of sodium is 0.32 at 100°C or about 1/3 that of water. The temperature rise across the reactor is typically 250°F or about five times that of a typical PWR. The reactor flow rate is therefore 3/5 that of a PWR of equivalent thermal power. Accounting for the difference in thermodynamic efficiency the primary system pumping power is less than half that of an equivalently sized PWR given equal system head losses.
• Since the Control Rod Drive Mechanisms are not pressurized, it is possible to provide features that eliminate control rod ejection accidents from the design basis (see Appendix 2D).

For the above reasons, sodium was seriously considered for use in thermal spectrum reactors during the 50s and 60s. There were two naval reactors that were sodium cooled. However, sodium cooled reactors were considered inappropriate for use in submarines and their development for land based applications could not be justified as a competitor with sodium cooled fast reactors.¹³

Sodium does introduce challenges into the design.

• Since sodium will freeze at 208° F., if it is desired to keep it in the liquid state during plant shutdown, piping systems and components must be separately heated.
• As a consequence of the relatively high temperature rise across the reactor, transient behavior in sodium cooled systems can be more challenging than in water-cooled plants unless design features are present ameliorating the possible effects.
• In the reactor coolant system, the radioactive isotopes Na²² and Na²⁴ are formed from interactions between the reactor neutrons and the coolant. ¹⁴ Na²⁴ has a half life of 15 hours and Na²² has a half life of 2.6 years.¹⁵ As a result, the coolant becomes very radioactive following operation. This is a contrast with water, which is not radioactive shortly after shutdown¹⁶ other than through a slow buildup of tritium, which is relatively benign. Although contaminants in the water do become radioactive, they generally do not pose a serious problem – at least not as serious as that posed by Na²⁴, in particular. To put this in perspective, LWRs with no fuel element failures may have a coolant activity of around 10^-3 μCi/cm³ attributable to some fission product recoil that penetrates the cladding and activated corrosion products, mainly Co⁶⁰. This number might increase an order of magnitude or two following a large crud burst or a fuel element failure. In contrast, a LMFBR can experience coolant activity as high as high as 50,000 μCi/cm³ attributable to Na²⁴. (Although the coolant is highly radioactive, the heat it produces is less than 0.1% of full reactor power.) Sodium cooled thermal reactors have even higher levels of activation due to the greater absorption cross section of sodium at thermal energies. The SRE was designed for primary coolant activity of twice this number, and actual experience disclosed the coolant activity to be 0.3 Ci/cm^{3 17}. Na²² activity is much lower, but still somewhat high by LWR standards at around 0.5-1.0 μCi/cm³. As a result, sodium-cooled reactors generally that have been designed and built to date have a shielded operating floor inside containment with sodium containing systems located below the floor. The spaces containing primary coolant sodium generally cannot be accessed by personnel until after adequate Na²⁴ decay has occurred, typically 10 days. It is primarily for this reason sodium cooled reactors make poor candidates for marine applications. Were some event to occur in the reactor compartment of a submarine that required personnel entry, it would be unsatisfactory to be obliged to wait 10 days before someone could safely enter. Because of the coolant activation issue, two reactors in separate compartments would have been needed to achieve acceptable reliability for naval applications including submarines. Water reactors don’t have this problem.
• The fast fission cross section for fissionable isotopes is two orders of magnitude lower than equivalent thermal fission cross sections resulting in the neutron flux being nearly two orders of magnitude greater in a fast reactor in comparison to a thermal reactor. The higher neutron flux can create problems with the structural members of the Reactor Vessel. Stainless steel is susceptible to radiation induced swelling at levels above about 4 x 10²² n/cm² (E > 0.1 MEV). Such swelling would introduce dimensional anomalies and compromise the material strength of the structures. Structures near the reactor core need to be adequately protected against such fluence levels over their lifetimes by adequate shielding.
• Operating temperatures in the range of 1000°F. require that thermal creep be evaluated as a part of structural design.
• Cesium is very near one of the peaks in the fission yield curve and is also an alkali metal like sodium. While most fission product contamination is removed by the cold traps, cesium is not and will gradually build up in the primary coolant unless some specific means is provided for its removal. At the end of operation of the Russian BN-350 reactor, Cs¹³⁷ contamination was 6-7 μCi/cm³. Cs¹³⁷ has a half life of 37 years so its build up in the coolant creates an operational and maintenance headache which also serves to discourage any prolonged operation with significant cladding breaches. Potential solutions for this problem will be addressed later.
• The coolant cannot be exposed to air, which complicates operations, such as refueling, when the primary system must be opened.
• It is necessary to transfer the primary system heat to a water system for power generation. Since water and sodium react violently if they come in contact with one another, it is necessary to take measures to prevent any leakage, even in minute quantities, across the sodium water boundary. When sodium reacts with water, hydrogen is produced along with NaOH. A means must be provided for dealing with potential sodium water reaction products in the steam generators.
• There is no convenient way to temporarily poison the coolant as is routinely done in PWRs, which add a boron compound to the coolant for reactivity control, then reduce its concentration as the core burns down.
• Sodium will remove any oxide layer on the metals that contain it. As a result, if metals are in contact, such as in a valve, there is a tendency for the metals to self weld. The tendency for self welding increases with the contact pressure between the surfaces and may be greater with increasing sodium temperature.
• Certain metals including copper, magnesium, tin, lead, and antimony are soluble in sodium and must be avoided as alloying materials in any metals used for the sodium containment boundary.

The designer’s challenge is to accentuate the positives and if it is not possible to eliminate the negatives, at least accommodate them in a fashion that minimizes their economic impact on the plant.

There are additional neutronics considerations needing acknowledgement in the design by virtue of the neutron spectrum in fast reactors. These matters are treated in Appendix 1.

endnotes

5 Some thermal spectrum reactors have used carbon as a moderator. Beryllium is also a suitable moderator.

6 The “cross section” of a nucleus is a measure of its rate of reaction with neutrons.

7 The other “price” that is paid is much greater absorption by fission products, structural materials, and the coolant during slowing down and while at thermal energies.

8 Other liquid metals may be used as a coolant. The EBR-1 used a eutectic mixture of sodium and potassium commonly referred to as NaK as the primary system coolant. NaK has the important advantage of a lower melting point (70°F) then sodium, but it was found to be much more chemically reactive and hazardous to handle. In addition, K⁴², formed by neutron capture from K⁴¹ is a strong gamma emitter with a 12.4 hr. half life. The Russians have used a mixture of lead and bismuth as a coolant in some reactor applications. Mercury has been used on at least one reactor.

9 The B&W design LWR NSSS uses once through steam generators and produces steam with about 35°F. superheat. While this modest superheat contributes somewhat to the performance of the B&W designs, it is small in comparison to that which can be achieved in a LMFBR.

10 Crud is an acronym for “Chalk River Undetermined Deposits”, Chalk River being a Canadian plant from the 1950s where the deposits were first observed. They are now known to be corrosion products which tend to selectively deposit on core surfaces in LWRs. The corrosion products typically contain cobalt, an alloying metal used in stainless steel, which becomes radioactive while residing on the core surfaces and later deposits on other parts of the primary system following “crud bursts”.

11 Koch, L. J., Reactor Engineering Division Annual Report, July 1, 1965 – June 30, 1966; ANL-7290; April 1967.

12 One exception to this is zirconium, for which oxide concentrations need to be maintained lower than 10 ppm. In plants where zirconium has been used, hot traps are installed which operate around 1200 degrees F. and use zirconium as a sacrificial material.

13 The Hallam reactor, which was the last sodium cooled thermal reactor in the U.S., experienced difficulties with the canning material surrounding the graphite moderator blocks. Development of a solution to this problem couldn’t be justified in light of the breeder option that did not need a moderator. It turned out that the contractor for Hallam had independently developed a solution to prevent the failure mode, butit was too late to save the Hallam project.

14 An (n,γ) reaction in the case of Na²⁴ and an (n,2n) reaction in the case of Na²².

15 The Na²⁴ decays to Mg²⁴ which is one of the metals soluble in sodium. The Na²² decays to Ne²² which will collect in the cover gas.

16 While operating, N¹⁶ is produced in LWRs, which causes the coolant to be highly radioactive during operation, but its 7.3 second half-life renders it inconsequential five minutes after shutdown.

17 R.E. Durand, Sodium Reactor Operating Experience, Chemical Engineering Progress, Vol. 57, No. 3, March 1961