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By Charles Rhodes, P.Eng., Ph.D.

Elsewhere on this website Fast Neutron Reactors (FNRs) have been identified as the primary source of energy for meeting mankind's future energy needs. This web page focuses on issues related to the sodium used for FNR primary cooling and secondary heat transport.

The sodium used as a FNR primary coolant needs to be free of contamination by carbon, calcium, chlorine and nickel. To the extent that these elements are present in the primary sodium over time they will absorb neutrons and become the radio isotopes C-14, Cl-36, Ca-41 and Ni-59 all of which have long half lives and hence are disposal problems. The best strategy is to minimize the C, Cl, Ca and Ni content in the liquid sodium before the reactor is turned on and to the extent possible to further remove these contaminants over time via a side arm filter system.

The cost of reactor grade sodium may be as much as $10.00 per kg. Thus 4600 tonnes of reactor grade sodium could cost as much as $46 million before shipping and handling costs.

A good reference with respeect to purification of sodium is: Sodium - NaK Engineering Handbook Vol. V.

In a sodium cooled reactor is is extremelyimportant to prevnt formation of particulates that can accumulate and block the sodium cooling channels in the fuel bundles. Sodium has a melting point of about 98 degrees C. However, it is difficult to totally exclude air from the primary sodium pool enclosure, so over time unwanted oxygen and water vapor in the argon atmospnere will react with the primary sodium to form oxides and hydroxides. Furthermore the Na will not be perfectly pure and may contain significant contents of Mg, K, Al, Ca, Cr, Fe and Ni.

We need to be concerned that the oxides of these elements will settle out as sludge and the hydroxides will not precipitate out onto cool heat exchange surfaces. Hence it is useful to tabulate the melting points and densities of these compounds.

K2O> 350?

Choose to operate the FNR cooling sodium within the temperature range 375 degrees C to 580 degrees C where the only solids suspended in the sodium are Ca(OH)2 plus the metal oxides. By operating above 375 degrees C CrO3 plus the hydroxides of Al,Cr, K, Mg, Na and Ni all remain as liquids and do not precipitate. At 580 degrees C the Ca(OH)2 melts.

The maximum primary sodium temperature is chosen to be 460 C to avoid fuel center line melting with 2 blocked sodium cooling channels.

Thus the permitted range of liquid sodium temperatures is 375 C to 460 C.

There should be nearby liquid sodium storage of sufficient capacity to hold all the primary liquid sodium so as to permit major maintenance/repairs to the FNR primary liquid sodium tank. There must be a means of easily transferring heat emitting fuel bundles from the main liquid sodium tank to a spare liquid sodium tank and vice versa.

The sodium might be delivered to the site as a solid in 55 US gallon steel drums with removeable covers. Assuming a total sodium requirement of:
Pi (10 m)^2 (15 m) = 4712.4 m^3
the required number of drums is:
[4712.4 m^3 X (1 US gal / .0037854 m^3)] / [55 US gal / drum] = 22,634 drums

These drums should be stored on pallets in groups separated by aisles that act as fire breaks and provide access for fire suppression. A safe drum pallet size is 30 inches X 30 inches. Hence exclusive of aisles the space requirement is:
(30 inches X 30 inches) / drum X (.0254 m / inch)^2 X 22,634 drums = 13,143 m^2

Allowing for aisle area equal to 3 X storage area implies a requirement for 52,569 m^2 = 0.0525 km^2 for single layer sodium drum storage.

The drums will likely arrive by rail. Hence the reactor site may need a dedicated rail siding.

The plan is to remelt the primary sodium by supplying heat to it from one or more of the intermediate sodium loops. These heat supply loops must be fitted with electric or fossil fueled heaters.

There are several ways of avoiding liquid sodium coolant channel blockage:
1) Do not use a hexagonal fuel tube configuration. The problem with that design is that as the fuel tube swells the coolant channel flow cross section seriously decreases. This issue has been avoided in modern CANFLEX fuel bundles by abandonment of a hexagonal fuel tube configuration. There must always be sufficient coolant channel cross section even after severe fuel tube swelling.

2) Stop trying to use spaghetti thin fuel tubes as were used in the EBR-2. Go to half inch OD fuel tubes with 9 mm OD core fuel rods. Making the core fuel rod thicker increases the average ratio of fuel to fuel + steel + sodium and hence improves the core reactivity, especially at the low end of the Pu concentration range. With higher core reactivity it is possible to increase the coolant channel cross section which makes the reactor less sensitive to particulates in the liquid sodium.

Live with the reality that this design change increases the required amount of start fissile. It remains my concern that attempts to reduce the fissile fuel start tonnage will trigger a reduction in coolant flow channel cross sectional area.

3) Adopt natural circulation of the primary sodium in place of pumped circulation. Then dirt particles will naturally settle out and sink to the bottom of the sodium pool where they can be extracted with a mechanism similar to a swimming pool vacuum cleaner.

4) Use a pool filter, again analogous to a swimming pool.

5) Use a fuel bundle inlet filter. This filter is intended to last the life of the fuel bundle and should do nothing if the aforementioned mechanisms all work properly.

6) Provide space behind the sloped fuel bundle inlet filter such that if part of the filter is blocked liquid sodium can flow horizontally behind the filter to serve all the tubes served by that filter.

7) Periodically run the reactor at low power so the bottom of the primary liquid sodium pool rises above 462 degrees C. The issue is that if there are any foreign metals in the sodium such as Li, K, Mg they can potentially combine with leakage air to form hydroxides that can deposit on the heat exchange surfaces or in the bottom of the fuel tubes. These hydroxides all melt at less than 462 degrees C. From time to time the entire primary sodium bath must be raised above 462 degrees C to melt off such deposits. In normal reactor operation most of these oxides and hydroxides should sink to the bottom of the primary sodium pool and should be removed with an apparatus similar to a swimming pool vacuum cleaner. That methodology works much better with natural primary sodium circulation than with pumped circulation.

8) The major ongoing issues are NaOH and MgOH which melt at 318 deg C and 350 deg C. In normal reactor operation keep the lowest temperature primary sodium at 330 degrees C and vacuum up the MgOH as it forms as a result of Na-24 decay. Removal of the NaOH requires occasional cooling of the liquid sodium down to about 310 degrees C. That requires a modest drop in steam pressure while NaOH is being removed.

9) The density of liquid sodium is less than the density of water. Most particulates will settle out if the liquid sodium flow velocity across the bottom of the pool is small. That means that the average reactor power density should be low. With the addition of a blanket and a guard band the primary sodium pool diameter becomes about 20 m. I am contemplating a reactor core zone diameter of about 12.6 m for 1000 MWt of thermal power capacity.

10) Each movable fuel bundle is narrow (approximately one foot square) and has individual discharge temperature monitoring, gamma monitoring, and vertical position monitoring. The purpose is to prevent local reactor hot spots occurring. The gammas will indicate the rate of local heat release and the temperature will indicate if the coolant flow for that fuel bundle is insufficient as compared to its operating power level.

11) I am concerned about other parties taking shortcuts that fail to address the causes of potential FNR flow channel blockage. Chief among these issues is pumped primary sodium which will likely prevent dirt psrticles from settling out.

12) During my SCUBA diving days I observed particles trapped in haloclines. This particle trapping results from natural density stratification of still water. That same method has been used by police services for determining the source of broken glass using dense liquid hydrocarbons. I believe that the same separation method will occur in liquid sodium if its flow velocity is small. Think of dust which tends to settle on horizontal surfaces. In the deep ocean in the tropics the whole sea floor is covered by fine dust.

13) The thermal conductivity of liquid sodium is very high. As a result there is no necessity of having turbulent flow through a liquid sodium heat exchanger. Hence with appropriate reactor design it is not necessary to use a primary circulation pump. Instead the reactor should be designed to operate with a high differential temperature.

14) Everything in the primary sodium pool must be non-reactive with liquid sodium. Hence there should be no corrosion products in the primary sodium.

15) We must do all necessary to filter out MgOH. The Mg forms as a result of Na-24 decay.

16) Adjust the secondary sodium flow so that normally regardless of the thermal load the lowest temperature in the primary sodium pool is 340 degrees C. That control strategy should stop precipitation of NaOH which melts at 318 degrees C.

17) If there is 4000 tonnes of liquid sodium it is almost impossible to keep LiOH, KOH, MgOH and NaOH and other metal hydroxides totally out of the sodium over the long term except by ongoing filtering. Moisture laden air will eventually creep into the argon cover gas. Hence it is essential to operate the system in a manner that continuously removes these metal hydroxides before they become particulate formation and deposition problems.

18) The fortunate issue with liquid sodium is that almost all particulates are denser than liquid sodium and given a chance will settle out. The operating temperatures will break down contaminant hydrocarbons.

19) Some pumped liquid sodium cooled reactor designs run the primary sodium above 462 degrees all the time. The problem with doing that is that the sodium flow velocity has to be very high for adequate heat transport. The high flow velocity makes sludge separation and removal difficult. There is no natural settling out of hydroxide sludge. The merit of such designs is that the heat is more suitable for industrial use. We can make steam sufficient for electricity generation at 320 degrees C and hence with attention to thermal stress we can operate a naturally circulated electricity generation reactor down to 340 degrees C with a 120 degree C temperature differential.

This web page last updated February 12, 2022.

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