Potassium metal and NaK eutectic K
K · CAS 7440-09-7
Potassium is a soft silvery alkali metal that melts at 63.5 degC and boils at 759 degC.
Evidence level
2 of 5
Lab-measured
- 0Predicted only. No one has made it, or no measurement exists.
- 1Calculated. Only calculated or predicted values from sources.
- 2Lab-measured. Measured properties with their conditions stated.
- 3Space-relevant tests. Measured in conditions relevant to space, such as vacuum or atomic oxygen.
- 4Ground-qualified. Qualified for an application on the ground.
- 5Flown or field-proven. Flight or field results exist.
Melting point
63.5 °C
Liquid at low temperature
-12.6 °C
Heat carrying
0.23 cal/(sec degC cm) at 21 °C
Vapor pressure
8 mm Hg at 432 °C
Recipe, share of the weight
- K 100.0%Potassium
Best for
Liquid-metal heat transport in space fission reactor loops, where NaK stays liquid at room temperature
Weakest at
Any contact with water or oxygen: it reacts violently with water and forms a surface crust in air
Risk at a glance
- People Severe. Violently reactive with water and corrosive
- If it fails High. Water entering the metal or the loop
- Corrosion Severe. Potassium reacts violently with water, releasing hydrogen and forming a caustic hydroxide.
Quick facts: K
| Property | Value | Basis |
|---|---|---|
| Density | 0.86 g/cm³ | 2 sourced values across conditions |
| Thermal conductivity | 0.23 cal/(sec degC cm) | 1 sourced value |
| Melting point | -12.6 to 64 °C | 3 sourced values across conditions |
| Specific heat | 0.18 cal/(g deg) | 1 sourced value |
What is the density of Potassium metal and NaK eutectic?
Potassium metal and NaK eutectic has a density of 0.86 g/cm³. The range covers 2 sourced values across the tempers, forms and test conditions the sources report; room-temperature values are used where a source states a temperature.
What is the thermal conductivity of Potassium metal and NaK eutectic?
Potassium metal and NaK eutectic has a thermal conductivity of 0.23 cal/(sec degC cm). The source and test conditions are listed in the properties table on this page.
What is the melting point of Potassium metal and NaK eutectic?
Potassium metal and NaK eutectic has a melting point of -12.6 to 64 °C. The range covers 3 sourced values across the tempers, forms and test conditions the sources report; room-temperature values are used where a source states a temperature.
What is Potassium metal and NaK eutectic used for?
Uses recorded in the sources include Liquid-metal reactor coolant loop for a space fission power system; Primary test circuit for fission surface power (ground test).
Why it exists
Potassium is a soft alkali metal. It is used as a metal and, as the eutectic NaK-78, as a liquid-metal coolant in space fission reactors, where the liquid metal carries reactor heat to a radiator or a converter. NaK stays liquid down to -12.6 °C, so a loop can be filled and circulated without first heating the metal, and it boils at 785 °C. Its difficulty is chemistry: it reacts violently with water and forms an oxide crust in air. NASA used NaK in the SNAP-10A space reactor and in thermoelectric power systems, which is why it has a service record in space that most metals lack.
Where it's used
Space fission power
NaK was the coolant of the SNAP-10A space reactor. A 1973 NASA and AEC summary describes a 5-kWe thermoelectric system in which reactor heat was carried by NaK to a radiator. The NaK in that system was a 22% sodium and 78% potassium eutectic.
Ground test circuits
NASA Marshall chose NaK as the primary coolant for its fission surface power primary test circuit, and the stainless steel circuit was tested at NaK temperatures up to 650 °C.
Recipe
K
Potassium is the element K, with one atom per formula unit. The NaK-78 eutectic is a liquid alloy of 22% sodium and 78% potassium by weight, as printed in the 1973 NASA and AEC summary report.
| Ingredient | Atoms per unit | Share of the weight | What it does |
|---|---|---|---|
| Potassium K | 1 | 100.0% | The only element in potassium metal. In NaK-78 it is the majority component (78% by weight), which sets the eutectic melting point. |
Weight shares are calculated from standard atomic weights.
What it can do
Melting point
63.5 °C
Potassium melts at 63.5 °C, which is close to the temperature at which it can be handled as a liquid with mild heating. The supplier SDS gives 64 °C.
Liquid at low temperature
-12.6 °C
The NaK-78 eutectic stays liquid down to -12.6 °C. NASA notes that the low melting point helps ground testing, because the liquid metal does not need to be heated before it can be circulated.
Heat carrying
0.23 cal/(sec degC cm) at 21 °C
Thermal conductivity is 0.23 at 21 °C and 0.956 at 400 °C, in the unit PubChem prints. The rise with temperature matters for a coolant that operates hot.
Vapor pressure
8 mm Hg at 432 °C
The vapor pressure reaches 8 mm Hg at 432 °C, which is the figure to check when a loop runs hot.
How it compares
Potassium is compared here with the NaK-78 eutectic as a coolant. Potassium melts at 63.5 °C and boils at 759 °C, while NaK-78 stays liquid down to -12.6 °C and boils at 785 °C. Potassium is less dense than water (0.856 g/cm3), so it floats, and both metals react violently with water. The choice of NaK is mainly about keeping the coolant liquid at room temperature.
The same measurements for each material, so you can compare them. Each row names the key condition under the value; the full conditions and methods are in the numbered notes under Technical details. Other materials are shown beside it; each was measured under its own conditions, so open its page before comparing closely. Compare every grade in the family.
| Measurement | K | Cs | Ba |
|---|---|---|---|
| Thermal behavior and safety | |||
| Melting point, potassium metal | 63.5 °C1 | 28.44 °C | 727 °C |
| Specific heat at 0 degC, potassium metal | 0.176 cal/(g deg)4 | no data | 0.204 J/(g K) |
| Thermal conductivity at 21 degC, potassium metal | 0.23 cal/(sec °C cm)5 | no data | 18.4 W/(m·K) |
| Thermal conductivity at 400 degC, potassium metal | 0.956 cal/(sec °C cm)6 | no data | 18.4 W/(m·K) |
| Vapor pressure at 432 degC, potassium metal | 8 mm Hg7 | 1.5×10^-6 mmHg | no data |
| Melting point, supplier SDS (potassium metal) | 64 °C8 | 28.44 °C | 727 °C |
| Vapor pressure at 260 degC, supplier SDS (potassium metal) | 0.12 hPa11 | 1.5×10^-6 mmHg | no data |
| Melting point, NaK-78 eutectic | -12.6 °C12 | 28.44 °C | 727 °C |
| Lifetime and stability | |||
| Volume increase of NaK heated from 25 degC to 650 degC | 17 %14 | no data | no data |
| Physical and electrical | |||
| Density, potassium metal | 0.856 g/cm³3 | 1892 kg/cu m | 3.51 g/cm³ |
| Relative density, supplier SDS (potassium metal) | 0.86 g/cm³10 | 1892 kg/cu m | 3.51 g/cm³ |
| Other measurements | |||
| Boiling point, potassium metal | 759 °C2 | 671 °C | 1897 °C |
| Boiling point, supplier SDS (potassium metal) | 774 °C9 | 671 °C | 1897 °C |
| Boiling point, NaK-78 eutectic | 785 °C13 | 671 °C | 1897 °C |
Why choose it, and why not
Good for
- Liquid-metal heat transport in space reactor loops, where NaK stays liquid at room temperature.
- A coolant with a service record in NASA space-reactor systems, including SNAP-10A.
Watch out for
- Water. The reaction releases hydrogen, which may ignite spontaneously, and the SDS warns of a possible flash fire.
- Oxygen. A crust of potassium oxide, peroxide and superoxide forms on the surface of NaK.
Potassium and NaK are proven coolants in space fission systems. Their usefulness depends on keeping water and oxygen out of the loop.
Technical details
For scientists and researchers. Everything above comes from the record below: every number with its method, its conditions and its source.
Notes
The method, full conditions and any caveat for the numbered rows above.
| # | Measurement | Method | Conditions | Caveat |
|---|---|---|---|---|
| 1 | Melting point, potassium metal | Experimental value listed in the PubChem compound record | form potassium metal, pure element as listed in PubChem | — |
| 2 | Boiling point, potassium metal | Experimental value listed in the PubChem compound record | pressure 760 mmHg | A supplier SDS gives 774 degC at 1013 hPa. Both values are kept as printed; see the supplier SDS rows. |
| 3 | Density, potassium metal | Experimental value listed in the PubChem compound record | temperature not stated in the record | — |
| 4 | Specific heat at 0 degC, potassium metal | Experimental value listed in PubChem, kept in the unit it prints (not converted to J/(kg K)) | temperature 0 °Cunit in source cal/g deg | — |
| 5 | Thermal conductivity at 21 degC, potassium metal | Experimental value listed in PubChem, kept in the unit it prints (not converted to W/(m K)) | temperature 21 °Cunit in source cal/(sec °C cm) | — |
| 6 | Thermal conductivity at 400 degC, potassium metal | Experimental value listed in PubChem, kept in the unit it prints (not converted to W/(m K)) | temperature 400 °Cunit in source cal/(sec °C cm) | — |
| 7 | Vapor pressure at 432 degC, potassium metal | Experimental value listed in the PubChem compound record | temperature 432 °C | — |
| 8 | Melting point, supplier SDS (potassium metal) | Value printed in the supplier safety data sheet, Section 9 | form potassium metal, supplier SDS | Rounded value. PubChem gives 63.5 degC. |
| 9 | Boiling point, supplier SDS (potassium metal) | Value printed in the supplier safety data sheet, Section 9 | pressure 1013 hPa (760 mmHg) | Differs from the PubChem value of 759 degC. Both are kept as printed. |
| 10 | Relative density, supplier SDS (potassium metal) | Value printed in the supplier safety data sheet, Section 9 | temperature not stated in the SDS | — |
| 11 | Vapor pressure at 260 degC, supplier SDS (potassium metal) | Value printed in the supplier safety data sheet, Section 9 | temperature 260 °C | — |
| 12 | Melting point, NaK-78 eutectic | Value stated in a NASA paper on a NaK-cooled circuit | material NaK-78 eutectic, 22% Na and 78% K by weight | — |
| 13 | Boiling point, NaK-78 eutectic | Value stated in a NASA paper on a NaK-cooled circuit | material NaK-78 eutectic, 22% Na and 78% K by weight | — |
| 14 | Volume increase of NaK heated from 25 degC to 650 degC | Value stated in a NASA paper on the fission surface power test circuit | material NaKfrom temperature 25 °Cto temperature 650 °C | — |
Corrosion and compatibility
What happens when it meets other materials and environments.
| Environment | Concern | What happens |
|---|---|---|
| Liquid water and wet air | severe | Potassium reacts violently with water, releasing hydrogen and forming a caustic hydroxide.1 |
| Oxygen in air (NaK in a loop) | moderate | NaK forms a crust of potassium oxide, peroxide and superoxide on its surface.2 |
| Stainless steel in NaK up to 650 degC | low | NaK has shown good compatibility with stainless steel up to 650 degC.3 |
| # | Environment | Details | Test conditions |
|---|---|---|---|
| 1 | Liquid water and wet air | NOAA CAMEO Chemicals records that potassium reacts violently or explosively with water. The PubChem entry for the alloy form says the reaction forms potassium hydroxide, a corrosive material, and hydrogen, a flammable gas that the heat may ignite. | exposure contact with liquid or moist airform potassium metal |
| 2 | Oxygen in air (NaK in a loop) | Exposure to oxygen makes NaK form potassium oxide, potassium peroxide and potassium superoxide, which create a crust on the surface of the liquid metal. NASA lists this as a disadvantage of NaK. | form NaK-78 eutectictemperature not stated in the paper |
| 3 | Stainless steel in NaK up to 650 degC | NASA reports good compatibility between NaK and stainless steel over the temperature range tested, with a maximum of 650 degC. The paper gives no corrosion rate. | temperature up to 650 °Cmaterial stainless steel |
Risks and side effects
Hazards to people, what happens when a cell overheats, and how the material is handled and stored.
For people
Potassium metal is classified Danger. It reacts violently with water and causes severe skin burns and eye damage.
- Classification
- The GHS classification is H260, which covers flammable gas release on contact with water, and H314, severe skin burns and eye damage. PubChem and the supplier SDS both report them.
- Corrosion
- It is corrosive to skin and eyes. Its reaction with water forms potassium hydroxide, a caustic solution, and hydrogen.
- Fire
- Keep it away from any contact with water, because of violent reaction and possible flash fire, the SDS warns. Dry sand, dry chemical or alcohol-resistant foam are the listed extinguishing media.
If a loop leaks
These risks come from the chemistry of the metal and from NASA test reports, not from field incidents.
- Water
- Water contact releases hydrogen, which may ignite. NASA's loop reports treat water as the main risk for a NaK system.
- Oxide crust
- Oxygen forms a crust of potassium oxide, peroxide and superoxide on the NaK surface, which NASA lists as a disadvantage of NaK.
- Stainless steel
- NaK has shown good compatibility with stainless steel up to 650 °C in NASA's test, but no corrosion rate is given.
Handling and storage
- Storage
- Store and handle under inert gas in a dry, closed container, away from water. The SDS asks for protection from moisture.
- Fire
- The SDS lists dry sand, dry chemical or alcohol-resistant foam for a fire, and notes that potassium oxides are formed in a fire.
Test details
The original wording and numbers from the sources, for readers who want them.
Failure risks
| Failure | Trigger and consequence |
|---|---|
| Water entering the metal or the loophigh severity | Leak, wet surface, or moisture during storage, handling or fillingViolent reaction releases hydrogen, which may ignite spontaneously. The SDS warns of possible flash fire. |
| Crust formation on the NaK surfacemoderate severity | Exposure of NaK to oxygenPotassium oxide, peroxide and superoxide form a crust on the liquid surface, which NASA lists as a disadvantage of NaK. |
Side effects on surrounding parts
| Affects | Effect |
|---|---|
| Anything the reaction products contact, including the loop or enclosure | Reaction with water forms hydrogen gas and potassium hydroxide, a corrosive solution. |
Handling and storage
- Handle under inert gas and protect from moisture. Keep away from any possible contact with water.
- Never allow the product to contact water during storage. Handle and store under inert gas.
Mitigations
- Use stainless steel, which NASA found compatible with NaK up to 650 degC in its test circuit.
- Fight fires with dry sand, dry chemical or alcohol-resistant foam, as the SDS directs.
Supply and geography
How common the raw materials are and where they come from.
Potassium (as potash, K2O equivalent)
- Top producers (2025)
- Canada 30.6%, Russia 20.4%, China 12.9%, Belarus 12.2%, Germany 6.1%, Laos 4.9%
- Critical mineral lists
- U.S. Final 2025 List of Critical Minerals (potash added in 2025): listed
Documents
Datasheets, safety data sheets, papers and standards you can open.
Full record
The complete spore, as stored. Free to reuse under CC-BY-4.0. Images keep the licence shown with each one.
Show the full YAML
spore: profile
id: potassium
spore_version: 0.2
family: alkali-and-alkaline-earth-metals
name: Potassium metal and NaK eutectic
abbreviation: K
role: liquid-metal coolant
category: metals
type: heavy-metals
summary: |
Potassium is a soft silvery alkali metal that melts at 63.5 degC and boils at 759 degC. Mixed with sodium as the eutectic NaK-78 (22% sodium and 78% potassium by weight), it stays liquid down to -12.6 degC, which made it a working coolant for space fission reactors. The metal reacts violently with water, and that is the main hazard in handling and in any loop leak.
best_for: Liquid-metal heat transport in space fission reactor loops, where NaK stays liquid at room temperature
weakest_at: 'Any contact with water or oxygen: it reacts violently with water and forms a surface crust in air'
identity:
formula: K
cas: 7440-09-7
source: pubchem-k
where: Names and Identifiers > Other Identifiers > CAS (7440-09-7). NaK-78 composition (22% Na and 78% K by weight) from the 1973 NASA and AEC summary report, Introduction.
tags:
environments: []
stresses: []
applications:
- thermal-control
properties:
- benchmark: melting_point
label: Melting point, potassium metal
value: 63.5
unit: degC
scope: material
conditions:
form: potassium metal, pure element as listed in PubChem
method: Experimental value listed in the PubChem compound record
status: ground
source: pubchem-k
where: Chemical and Physical Properties > Experimental Properties > Melting Point (63.5 degC entry)
- benchmark: boiling_point
label: Boiling point, potassium metal
value: 759
unit: degC
scope: material
conditions:
pressure: 760 mmHg
method: Experimental value listed in the PubChem compound record
status: ground
note: A supplier SDS gives 774 degC at 1013 hPa. Both values are kept as printed; see the supplier SDS rows.
source: pubchem-k
where: Chemical and Physical Properties > Experimental Properties > Boiling Point (759 degC at 760 mmHg)
- benchmark: density
label: Density, potassium metal
value: 0.856
unit: g/cm3
scope: material
conditions:
temperature: not stated in the record
method: Experimental value listed in the PubChem compound record
status: ground
source: pubchem-k
where: Chemical and Physical Properties > Experimental Properties > Density (0.856 g/cm3 entry)
- benchmark: specific_heat
label: Specific heat at 0 degC, potassium metal
value: 0.176
unit: cal/(g deg)
scope: material
conditions:
temperature: 0 degC
unit_in_source: cal/g deg
method: Experimental value listed in PubChem, kept in the unit it prints (not converted to J/(kg K))
status: ground
source: pubchem-k
where: Chemical and Physical Properties > Experimental Properties > Other Experimental Properties (specific heat, 0 degC)
- benchmark: thermal_conductivity
label: Thermal conductivity at 21 degC, potassium metal
value: 0.23
unit: cal/(sec degC cm)
scope: material
conditions:
temperature: 21 degC
unit_in_source: cal/(sec degC cm)
method: Experimental value listed in PubChem, kept in the unit it prints (not converted to W/(m K))
status: ground
source: pubchem-k
where: Chemical and Physical Properties > Experimental Properties > Other Experimental Properties (thermal conductivity, 21 degC entry)
- benchmark: thermal_conductivity
label: Thermal conductivity at 400 degC, potassium metal
value: 0.956
unit: cal/(sec degC cm)
scope: material
conditions:
temperature: 400 degC
unit_in_source: cal/(sec degC cm)
method: Experimental value listed in PubChem, kept in the unit it prints (not converted to W/(m K))
status: ground
source: pubchem-k
where: Chemical and Physical Properties > Experimental Properties > Other Experimental Properties (thermal conductivity, 400 degC entry)
- benchmark: vapor_pressure
label: Vapor pressure at 432 degC, potassium metal
value: 8
unit: mm Hg
scope: material
conditions:
temperature: 432 degC
method: Experimental value listed in the PubChem compound record
status: ground
source: pubchem-k
where: Chemical and Physical Properties > Experimental Properties > Vapor Pressure (8 mm Hg at 432 degC)
- benchmark: melting_point
label: Melting point, supplier SDS (potassium metal)
value: 64
unit: degC
scope: material
conditions:
form: potassium metal, supplier SDS
method: Value printed in the supplier safety data sheet, Section 9
status: ground
note: Rounded value. PubChem gives 63.5 degC.
source: sa-k
where: Section 9, Melting point/freezing point
- benchmark: boiling_point
label: Boiling point, supplier SDS (potassium metal)
value: 774
unit: degC
scope: material
conditions:
pressure: 1013 hPa (760 mmHg)
method: Value printed in the supplier safety data sheet, Section 9
status: ground
note: Differs from the PubChem value of 759 degC. Both are kept as printed.
source: sa-k
where: Section 9, Initial boiling point and boiling range
- benchmark: density
label: Relative density, supplier SDS (potassium metal)
value: 0.86
unit: g/cm3
scope: material
conditions:
temperature: not stated in the SDS
method: Value printed in the supplier safety data sheet, Section 9
status: ground
source: sa-k
where: Section 9, Relative density (0.860 g/cm3)
- benchmark: vapor_pressure
label: Vapor pressure at 260 degC, supplier SDS (potassium metal)
value: 0.12
unit: hPa
scope: material
conditions:
temperature: 260 degC
method: Value printed in the supplier safety data sheet, Section 9
status: ground
source: sa-k
where: Section 9, Vapor pressure (0.12 hPa at 260 degC)
- benchmark: melting_point
label: Melting point, NaK-78 eutectic
value: -12.6
unit: degC
scope: material
conditions:
material: NaK-78 eutectic, 22% Na and 78% K by weight
method: Value stated in a NASA paper on a NaK-cooled circuit
status: ground
source: nasa-2006
where: Introduction, paragraph on liquid metals for space reactors (melting point of -12.6 degC)
- benchmark: boiling_point
label: Boiling point, NaK-78 eutectic
value: 785
unit: degC
scope: material
conditions:
material: NaK-78 eutectic, 22% Na and 78% K by weight
method: Value stated in a NASA paper on a NaK-cooled circuit
status: ground
source: nasa-2006
where: Introduction, paragraph on liquid metals for space reactors (boiling point of 785 degC)
- benchmark: volume_expansion
label: Volume increase of NaK heated from 25 degC to 650 degC
value: 17
unit: '%'
scope: material
conditions:
material: NaK
from_temperature: 25 degC
to_temperature: 650 degC
method: Value stated in a NASA paper on the fission surface power test circuit
status: ground
source: nasa-2007
where: Circuit design section, paragraph on expansion reservoir (NaK expands to 17% above its volume at 25 degC)
corrosion:
- environment: Liquid water and wet air
headline: Potassium reacts violently with water, releasing hydrogen and forming a caustic hydroxide.
effect: NOAA CAMEO Chemicals records that potassium reacts violently or explosively with water. The PubChem entry for the alloy form says the reaction forms potassium hydroxide, a corrosive material, and hydrogen, a flammable gas that the heat may ignite.
rating: severe
conditions:
exposure: contact with liquid or moist air
form: potassium metal
source: cameo-k
where: Reactivity profile, water entry
- environment: Oxygen in air (NaK in a loop)
headline: NaK forms a crust of potassium oxide, peroxide and superoxide on its surface.
effect: Exposure to oxygen makes NaK form potassium oxide, potassium peroxide and potassium superoxide, which create a crust on the surface of the liquid metal. NASA lists this as a disadvantage of NaK.
rating: moderate
conditions:
form: NaK-78 eutectic
temperature: not stated in the paper
source: nasa-2006
where: Introduction, paragraph on the disadvantages of NaK
- environment: Stainless steel in NaK up to 650 degC
headline: NaK has shown good compatibility with stainless steel up to 650 degC.
effect: NASA reports good compatibility between NaK and stainless steel over the temperature range tested, with a maximum of 650 degC. The paper gives no corrosion rate.
rating: low
conditions:
temperature: up to 650 degC
material: stainless steel
source: nasa-2007
where: Introduction, paragraph on NaK properties
risks:
hazards:
- text: Classified Danger. In contact with water it releases flammable gases which may ignite spontaneously (H260), as reported in PubChem and the supplier SDS.
ghs:
- H260
source: pubchem-k
where: Safety and Hazards > GHS Classification (H260)
- text: Causes severe skin burns and eye damage (H314), as reported in PubChem and the supplier SDS.
ghs:
- H314
source: sa-k
where: Section 2, Hazards identification (H314)
- text: UN number 2257 (potassium) appears in the PubChem record.
ghs: []
source: pubchem-k
where: Names and Identifiers > Other Identifiers > UN Number
failure_risks:
- mode: Water entering the metal or the loop
trigger: Leak, wet surface, or moisture during storage, handling or filling
consequence: Violent reaction releases hydrogen, which may ignite spontaneously. The SDS warns of possible flash fire.
severity: high
source: sa-k
where: Section 2 (precautionary statement P223) and Section 10 (reacts violently with water)
- mode: Crust formation on the NaK surface
trigger: Exposure of NaK to oxygen
consequence: Potassium oxide, peroxide and superoxide form a crust on the liquid surface, which NASA lists as a disadvantage of NaK.
severity: moderate
source: nasa-2006
where: Introduction, paragraph on the disadvantages of NaK
side_effects:
- text: Reaction with water forms hydrogen gas and potassium hydroxide, a corrosive solution.
affects: Anything the reaction products contact, including the loop or enclosure
source: pubchem-k
where: Chemical and Physical Properties > Physical Description (alloy entry)
handling:
- text: Handle under inert gas and protect from moisture. Keep away from any possible contact with water.
source: sa-k
where: Section 2 (P223, P231 plus P232) and Section 7
- text: Never allow the product to contact water during storage. Handle and store under inert gas.
source: sa-k
where: Section 7, Conditions for safe storage
mitigations:
- text: Use stainless steel, which NASA found compatible with NaK up to 650 degC in its test circuit.
source: nasa-2007
where: Introduction, paragraph on NaK properties
- text: Fight fires with dry sand, dry chemical or alcohol-resistant foam, as the SDS directs.
source: sa-k
where: Section 2 (P370 plus P378) and Section 5 (dry powder)
uses:
- application: thermal-control
device: Liquid-metal reactor coolant loop for a space fission power system
example: The 1973 NASA and AEC summary describes a 5-kWe thermoelectric system in which reactor heat was carried by NaK, a eutectic of 22% Na and 78% K by weight, to a radiator.
source: nasa-1973
where: Description of the 5-kWe system, text near Figure 1
- application: thermal-control
device: Primary test circuit for fission surface power (ground test)
example: NASA Marshall selected NaK as the primary coolant for its fission surface power primary test circuit, because NaK was the coolant of the SNAP-10A fission reactor.
source: nasa-2007
where: Abstract
supply:
- subject: Potassium (as potash, K2O equivalent)
producers:
- country: Canada
share_percent: 30.6
year: 2025
- country: Russia
share_percent: 20.4
year: 2025
- country: China
share_percent: 12.9
year: 2025
- country: Belarus
share_percent: 12.2
year: 2025
- country: Germany
share_percent: 6.1
year: 2025
- country: Laos
share_percent: 4.9
year: 2025
critical_lists:
- list: U.S. Final 2025 List of Critical Minerals (potash added in 2025)
listed: true
substitutes: []
source: usgs-mcs2026-potash
where: World Mine Production and Reserves table, 2025e, K2O equivalent, world total 49,000 thousand t (shares calculated by Space Mushroom as country production divided by the world total); critical minerals paragraph
documents:
- title: 'Stanford Advanced Materials safety data sheet: potassium metal (revised 19 January 2023)'
type: sds
url: https://samaterials.com/tds/1697120713-1699-potassium-k-metal-sds.pdf
year: 2023
- title: 'PubChem: Potassium (CID 5462222), GHS classification and experimental properties'
type: database
url: https://pubchem.ncbi.nlm.nih.gov/compound/Potassium
year: 2026
- title: 'USGS Mineral Commodity Summaries 2026: Potash'
type: report
url: https://pubs.usgs.gov/periodicals/mcs2026/mcs2026-potash.pdf
year: 2026
- title: 'NASA: Design, Fabrication and Integration of a NaK-Cooled Circuit'
type: paper
url: https://ntrs.nasa.gov/citations/20060025550
year: 2006
- title: 'NASA: Capabilities and Testing of the Fission Surface Power Primary Test Circuit'
type: paper
url: https://ntrs.nasa.gov/citations/20070031971
year: 2007
- title: 'AEC and NASA: Space Nuclear System Thermoelectric NaK Pump Development, Summary Report (NASA-CR-121231)'
type: report
url: https://ntrs.nasa.gov/citations/19730016794
year: 1973
- title: 'NOAA CAMEO Chemicals: Potassium'
type: database
url: https://cameochemicals.noaa.gov/chemical/4316
- title: 'NIST: Atomic Weights and Isotopic Compositions for Potassium'
type: database
url: https://physics.nist.gov/cgi-bin/Compositions/stand_alone.pl?ele=K
gaps:
- No crustal abundance for potassium was read in a source for this profile, so the abundance field is left out.
- The density of NaK-78 at 20 degC was not read in a source opened for this run. Wikipedia and a mirror give 0.866 g/cm3, but Wikipedia is used here only for composition and history, and the 1972 NaK handbook was not found.
- Thermal conductivity, specific heat and electrical resistivity of NaK-78 were not found in a source opened for this run.
- Electrical resistivity of potassium metal was not found. The flash point is not applicable in PubChem (combustible solid) and reads No data available in the supplier SDS.
- Vapor pressure and volume_expansion are not canonical names in spores/property-canon.yaml. They are used here as short snake_case names.
- No alkali-metal type exists under the metals category in src/data/taxonomy.ts. The type heavy-metals is the nearest available choice and should be reviewed.
- The EU critical raw materials list was not checked for potassium or potash.
- Producer shares are calculated from potash K2O-equivalent mine production, not from elemental potassium. The USGS world total is rounded.
- No recycling or substitute data for potassium were read in a source for this run.
- Boiling point differs between PubChem (759 degC) and the supplier SDS (774 degC). Both are kept as printed.
- Radiation, vacuum and flight-result data for NaK-78 were not found beyond the 1973 summary report and the 2006 and 2007 NASA papers. No NaK-specific corrosion rate was found; NASA reports compatibility only.
sources:
pubchem-k:
title: PubChem compound record, Potassium (CID 5462222)
id: CID 5462222; CAS 7440-09-7
url: https://pubchem.ncbi.nlm.nih.gov/rest/pug_view/data/compound/5462222/JSON
year: 2026
kind: database
license: public domain (NCBI)
sa-k:
title: 'Safety data sheet: Potassium metal (Stanford Advanced Materials)'
id: revised 19 January 2023
url: https://samaterials.com/tds/1697120713-1699-potassium-k-metal-sds.pdf
year: 2023
kind: datasheet
license: supplier document, linked not reproduced
usgs-mcs2026-potash:
title: Mineral Commodity Summaries 2026, Potash
id: USGS MCS, February 2026
url: https://pubs.usgs.gov/periodicals/mcs2026/mcs2026-potash.pdf
year: 2026
kind: report
license: public domain (USGS)
nasa-2006:
title: Design, Fabrication and Integration of a NaK-Cooled Circuit (NASA Marshall Space Flight Center, Paper 6389, 2006)
id: NTRS 20060025550
url: https://ntrs.nasa.gov/citations/20060025550
year: 2006
kind: paper
license: public domain (NASA)
nasa-2007:
title: Capabilities and Testing of the Fission Surface Power Primary Test Circuit (FSP-PTC), Space Nuclear Conference 2007, Paper 2030
id: NTRS 20070031971
url: https://ntrs.nasa.gov/citations/20070031971
year: 2007
kind: paper
license: public domain (NASA)
nasa-1973:
title: Space Nuclear System Thermoelectric NaK Pump Development, Summary Report (AEC and NASA-CR-121231)
id: NTRS 19730016794; AEC-AI-13091
url: https://ntrs.nasa.gov/citations/19730016794
year: 1973
kind: report
license: public domain (NASA)
cameo-k:
title: 'NOAA CAMEO Chemicals: Potassium'
id: CAMEO chemical record 4316
url: https://cameochemicals.noaa.gov/chemical/4316
kind: database
license: public domain (NOAA)
nist-k:
title: Atomic Weights and Isotopic Compositions for Potassium (NIST Physical Measurement Laboratory)
id: NIST standard atomic weight 39.0983(1)
url: https://physics.nist.gov/cgi-bin/Compositions/stand_alone.pl?ele=K
kind: database
license: public domain (NIST)
license: CC-BY-4.0
updated: '2026-10-26'
review:
status: draft
method: assistant
checked_values: 14
unreachable: 0
caveats: 3
note: Read by an AI assistant on 2026-10-26 from the cited pages. Three values differ between sources (boiling point, melting point rounding, density) and both are kept with notes.
story:
recipe:
line: Potassium is the element K, with one atom per formula unit. The NaK-78 eutectic is a liquid alloy of 22% sodium and 78% potassium by weight, as printed in the 1973 NASA and AEC summary report.
parts:
- name: Potassium
symbol: K
atoms: 1
atomic_weight: 39.0983
role: The only element in potassium metal. In NaK-78 it is the majority component (78% by weight), which sets the eutectic melting point.
source: nist-k
where: Standard atomic weight 39.0983(1)
why: 'Potassium is a soft alkali metal. It is used as a metal and, as the eutectic NaK-78, as a liquid-metal coolant in space fission reactors, where the liquid metal carries reactor heat to a radiator or a converter. NaK stays liquid down to -12.6 °C, so a loop can be filled and circulated without first heating the metal, and it boils at 785 °C. Its difficulty is chemistry: it reacts violently with water and forms an oxide crust in air. NASA used NaK in the SNAP-10A space reactor and in thermoelectric power systems, which is why it has a service record in space that most metals lack.'
uses:
- where: Space fission power
line: NaK was the coolant of the SNAP-10A space reactor. A 1973 NASA and AEC summary describes a 5-kWe thermoelectric system in which reactor heat was carried by NaK to a radiator. The NaK in that system was a 22% sodium and 78% potassium eutectic.
- where: Ground test circuits
line: NASA Marshall chose NaK as the primary coolant for its fission surface power primary test circuit, and the stainless steel circuit was tested at NaK temperatures up to 650 °C.
can_do:
- label: Melting point
value: 63.5 °C
line: Potassium melts at 63.5 °C, which is close to the temperature at which it can be handled as a liquid with mild heating. The supplier SDS gives 64 °C.
- label: Liquid at low temperature
value: '-12.6 °C'
line: The NaK-78 eutectic stays liquid down to -12.6 °C. NASA notes that the low melting point helps ground testing, because the liquid metal does not need to be heated before it can be circulated.
- label: Heat carrying
value: 0.23 cal/(sec degC cm) at 21 °C
line: Thermal conductivity is 0.23 at 21 °C and 0.956 at 400 °C, in the unit PubChem prints. The rise with temperature matters for a coolant that operates hot.
- label: Vapor pressure
value: 8 mm Hg at 432 °C
line: The vapor pressure reaches 8 mm Hg at 432 °C, which is the figure to check when a loop runs hot.
compare: Potassium is compared here with the NaK-78 eutectic as a coolant. Potassium melts at 63.5 °C and boils at 759 °C, while NaK-78 stays liquid down to -12.6 °C and boils at 785 °C. Potassium is less dense than water (0.856 g/cm3), so it floats, and both metals react violently with water. The choice of NaK is mainly about keeping the coolant liquid at room temperature.
compare_rows:
- label: Melting point
benchmark: melting_point
- label: Boiling point
benchmark: boiling_point
- label: Density
benchmark: density
handling:
- Handle under inert gas and protect from moisture. The safety data sheet calls for this in its handling section.
- Keep it away from any contact with water. Never let it contact water during storage.
- Store in a dry place in a closed container.
good_for:
- Liquid-metal heat transport in space reactor loops, where NaK stays liquid at room temperature.
- A coolant with a service record in NASA space-reactor systems, including SNAP-10A.
watch_out:
- Water. The reaction releases hydrogen, which may ignite spontaneously, and the SDS warns of a possible flash fire.
- Oxygen. A crust of potassium oxide, peroxide and superoxide forms on the surface of NaK.
verdict: Potassium and NaK are proven coolants in space fission systems. Their usefulness depends on keeping water and oxygen out of the loop.
risk_groups:
- title: For people
level: 4
verdict: Violently reactive with water and corrosive
lead: Potassium metal is classified Danger. It reacts violently with water and causes severe skin burns and eye damage.
points:
- label: Classification
line: The GHS classification is H260, which covers flammable gas release on contact with water, and H314, severe skin burns and eye damage. PubChem and the supplier SDS both report them.
- label: Corrosion
line: It is corrosive to skin and eyes. Its reaction with water forms potassium hydroxide, a caustic solution, and hydrogen.
- label: Fire
line: Keep it away from any contact with water, because of violent reaction and possible flash fire, the SDS warns. Dry sand, dry chemical or alcohol-resistant foam are the listed extinguishing media.
- title: If a loop leaks
level: 3
verdict: Water leaks and oxide crust are the risks
lead: These risks come from the chemistry of the metal and from NASA test reports, not from field incidents.
points:
- label: Water
line: Water contact releases hydrogen, which may ignite. NASA's loop reports treat water as the main risk for a NaK system.
- label: Oxide crust
line: Oxygen forms a crust of potassium oxide, peroxide and superoxide on the NaK surface, which NASA lists as a disadvantage of NaK.
- label: Stainless steel
line: NaK has shown good compatibility with stainless steel up to 650 °C in NASA's test, but no corrosion rate is given.
- title: Handling and storage
points:
- label: Storage
line: Store and handle under inert gas in a dry, closed container, away from water. The SDS asks for protection from moisture.
- label: Fire
line: The SDS lists dry sand, dry chemical or alcohol-resistant foam for a fire, and notes that potassium oxides are formed in a fire.
References
Every number above carries a reference number. Click one to see the source and where in it the value comes from.
| # | Source | Type | Year | Used for |
|---|---|---|---|---|
| PubChem compound record, Potassium (CID 5462222) | database | 2026 | 11 values | |
| Safety data sheet: Potassium metal (Stanford Advanced Materials) | datasheet | 2023 | 9 values | |
| Mineral Commodity Summaries 2026, Potash | report | 2026 | 1 value | |
| Design, Fabrication and Integration of a NaK-Cooled Circuit (NASA Marshall Space Flight Center, Paper 6389, 2006) | paper | 2006 | 4 values | |
| Capabilities and Testing of the Fission Surface Power Primary Test Circuit (FSP-PTC), Space Nuclear Conference 2007, Paper 2030 | paper | 2007 | 4 values | |
| Space Nuclear System Thermoelectric NaK Pump Development, Summary Report (AEC and NASA-CR-121231) | report | 1973 | 1 value | |
| NOAA CAMEO Chemicals: Potassium | database | 1 value | ||
| Atomic Weights and Isotopic Compositions for Potassium (NIST Physical Measurement Laboratory) | database | 0 values |