Comparison of SODIUM REMOVAL Technologies

In-situ Alkali Metal Remediation

Creative Engineers, Inc. specializes in the removal of alkali metals from tanks, piping, cold traps, and other components. Sodium, potassium, lithium, and various alloys such as NaK present a significant hazard to clean up. A number of techniques have been developed over the years, but only one is able to safely clean up both large and small quantities in a controlled manner without adding additional hazards.

CEI’s superheated steam (CEI-SHS™) method combines vessel preheating, inerted gas, and a controlled supply of superheated, dry steam.  Our extensive alkali metal experience has shown that combining these in a well-controlled and well-monitored system is the optimal means for the removal of alkali metal wastes — from thin films to bulk mixtures.

The methods available for alkali metal remediation include:

  •  Superheated Steam (CEI-SHS™)
  • Water Vapor Nitrogen (WVN)
  • Carbonation using moistened CO2
  • Water sprays (rarely used in-situ)
  • Water filling (rarely used in-situ)
  • Dissolving the residues in ammonia (adds a new hazard and possible side reactions)
  • Alcohol washing (adds a new hazard but often done on a very small scale in a lab)
  • Flowing a solvent with a reactant (alcohol) through the system (complex and adds new hazards)
  • Weathering (normally done on very small scale where sudden reactions will not create a hazard)

Of the techniques listed, only the first three are currently proposed for nuclear or industrial in-situ alkali metal removal applications.   The experts at CEI are available to discuss selection of alkali metal removal technology – email Richard VanLieshout at rich@ceina.pro for more information.

Each of the three principal methods has potential applications for sodium and other alkali metal removal.  Only the Superheated Steam method can be used in all situations.

CEI-SHS ™ | WVN | Carbonation Quick Comparison

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Detailed Process Descriptions

CEI-SHS™ Method

The system for the superheated steaming process exposes moisture in the form of superheated steam diluted in nitrogen (or argon) to sodium or other alkali metal.  The alkali metal reacts with the moisture in a controlled manner to produce sodium hydroxide (and potassium hydroxide when NaK is treated), hydrogen gas, and a significant amount of heat as shown in the following equation:

Na  +  H2O    →   NaOH   +  ½ H2   +  heat

A simplified sketch of the proposed process is shown in figure 1.  Some of the key equipment components are:

  • Metering system for accurately measuring and controlling the amount of steam
  • Metering system for accurately measuring and controlling the amount of inert gas
  • Hydrogen gas analyzer
  • Oxygen gas analyzer
  • High-efficiency particulate scrubber system.  Brink Mist Eliminator suitable for 99-99.95% collection efficiency or equivalent of NaOH particulate
  • Heater (15 -20 KW) for heating the inert gas and steam to 500-600oF
  • Control station which houses the controls and collects all the data (temperatures, pressures, gas analysis, etc.).  The controls for the entire system are localized here so that treatment can be monitored at a distance.
  • Chilled water heat exchangers designed to cool process gasses and condense excess steam in the vent stream
  • Electric heater (if needed) to raise exiting gas temperature to control relative humidity in the exit gasses before they come in contact with HEPA units.

Superheated steam (carried by inert gas) completely reacts all alkali metal residues in a controlled manner, allowing them to be neutralized to a non-hazardous salt and removed.

  • The key operating principles for the superheated steam process are:
  • The system to be processed is confirmed to be leak-tight using a pressure test.
  • The alkali metal-bearing vessel operates at low pressure (up to 2.5 kPa) to ensure no oxygen entry.
  • Hydrogen and oxygen levels are continuously monitored
  • No reaction is initiated until the oxygen content is less than 1%, by volume.  All reaction activities will be stopped in the event that the oxygen increases above 1%.
  • For bulk reaction processing, the reaction takes place at a higher temperature (600-1000⁰F) than typically found using the WVN or carbonation processes.  The benefits of higher temperature are:
    • At these high temperatures, liquid water cannot be present – neither as a sodium reactant, nor for water of hydration (on the reaction product, sodium hydroxide).  Thus, there is no chance of accumulating water that could result in an excursion or rollover.
    • The sodium hydroxide product is molten and therefore incapable of encapsulating unreacted sodium.
    • Sodium and potassium are immiscible in their hydroxides and have half their densities.  Because of this, the molten alkali metals will layer on the top of the molten hydroxides – constantly presenting a fresh surface for the alkali metal-water reaction.
    • Because all the alkali metal is on top and exposed to the incoming moisture (from the superheated steam) the end point of the reaction is very sharp and distinct and is easily confirmed by a steady rapid decrease in temperature and hydrogen content in the off-gas.
  • The reaction rate and temperature are very controllable; simple adjustments to steam flow rate are used to increase and decrease temperature, hydrogen content, and reaction rate.  The reaction can be halted within seconds if necessary since the amount of water present is all in the vapor phase.   No water is present at any time before the reaction is complete as liquid or hydrates.
  • A constant inert gas flow rate is always maintained throughout the reaction phase; typically 5, 10, or 20 scfm depending upon vessel size.

The benefits of the superheated steam process include:

 

  • The reaction proceeds to completion without stalling, meaning that no unreacted sodium remains and the system is safe for post-treatment flooding or other operations.
  • The process is very controllable without any pressure or temperature excursions, such as observed with other technologies.
  • The system is safe from hydrogen explosions.  Both Carbonation and WVN have experienced problems due to a lack of reaction control and the potential for unreacted sodium to remain after processing.
  • The alkali metal can be reacted at rates of several hundred kilograms PER HOUR, meaning the project will be completed quickly.

SPECIAL NOTE FROM CEI'S EXPERIENCE IN REMOVING SODIUM FROM NUCLEAR REACTORS

Planning for Sodium Removal

Understanding the interaction between moisture and the sodium surface is critical to planning the application of the CEI-SHS™ process. Proper planning is essential, and all CEI projects start with an engineering assessment of the system to be cleaned.

For pools or deep layers of sodium or other metals, the location and design of the steam injection nozzles to the alkali metal ensures good contact between reactants and successful initiation of the treatment. Without proper nozzle design for the specific situation, much of the steam could bypass the alkali metal and exit the process equipment without reaction.  This is an inefficient use of the steam for the normal CEI-SHS™ process and becomes critically important when initiating reaction at temperatures below the ideal, as has been done (e.g. for the Fermi-1 reactor vessel).  In such cases the heat of reaction is needed to achieve the critical anhydrous temperature of the metal hydroxide (318oC in the case of sodium).  Without proper nozzle location, an insulating alkali metal hydroxide barrier could form, preventing the progress of the steam/alkali metal reaction.  Proper nozzle location maximizes steam contact and can agitate the surface to provide an optimum steam-alkali metal reaction interface.

Management of Hydrogen

Management of the hydrogen fire risk is often quoted as an issue for sodium removal. WVN and Carbonation techniques attempt to manage the risk by limiting hydrogen below 3.5%, but in doing so create risks of unreacted sodium and uncontrolled reactions that have destroyed parts of reactors being treated (for example at EBR-II). CEI-SHS™ manages this risk by keeping oxygen levels below 1% through pre-operation leak testing, positive gas pressure, constant monitoring, cooling of the vent gases as they flow through the scrubber, grounding (earthing) of the vent piping, control of spark potential at the outlet of the vent system, and exploiting the exceptional controllability of the CEI-SHS™ process. There have been no hydrogen-related issues in more than 40 major cleaning projects, and this is a key reason that CEI-SHS™ is the technology of choice for all industrial applications.

CEI-SHS™ is capable of cleaning the following items containing alkali metals:

  • Reactors and tanks of simple or complex configuration
  • Other equipment of extremely complex geometry
  • Cold and Hot Traps
  • Valves
  • Heat Exchangers
  • Piping
  • Pumps

SAFE – EFFICIENT – EFFECTIVE – FLEXIBLE – RAPID

Wet Vapor Nitrogen (WVN) Method

The WVN method humidifies nitrogen gas to saturation by flowing it through a water bath. The amount of moisture delivered is significantly less than that delivered by the dry steam in the CEI-SHS™ method. As a result of the decreased moisture feed rate, the reaction rate, hydrogen generation rate, and heat generation rate are all substantially lower than the CEI-SHS™ process. Tests and applications of the WVN method show negligible temperature increase, no sodium hydroxide smoke generation, and only trace amounts of hydrogen in the vent gas while the process is controlled.

Time-lapse Video of the WVN Method

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An international nuclear facility using sodium in its heat exchange loop approached CEI to complete the first known apples-to-apples-to-apples comparison of our CEI-SHS™ process with the Wet Vapor Nitrogen (WVN) and Carbonation (often abbreviated as CO₂) methods. The time-lapse video above shows the progress made over two weeks of WVN treatment during bench-scale testing at CEI’s New Freedom, PA facility.

As with CEI-SHS™, WVN uses water as a reactant to convert alkali metals (Li, Na, K, etc.) to alkali hydroxides (LiOH, NaOH, KOH, etc.). But unlike CEI-SHS™, the WVN reaction is performed at ambient temperatures and the alkali metal remains solid. In CEI-SHS™ the alkali metal is heated above its melting point so that the injected steam can be thoroughly mixed with the liquid metal.

By the end of the testing, less than 5 cm of the sodium was reacted – leaving approximately 67% of the original. Even more worrisome are the hydrated sodium hydroxide pools which remain atop the sodium. In the real world, if pools like this were left in a vessel or piping containing alkali metals they could easily result in significant and violent reactions when the hydrates and alkali metals finally do interface. While WVN has hydrate pools forming almost immediately and growing throughout the treatment, CEI-SHS™‘s process ensures that hydrate pooling cannot occur by maintaining process temperatures well above the hydration point of sodium hydroxide until well after it can be confirmed that all the alkali metal has been reacted.

  • Advantages of the method are:

    • Low (ambient) temperatures
    • Low levels of hydrogen while the process is under control
    • Usable for very small amounts of alkali metals and alloys, with a maximum depth of a few millimeters.
  • Disadvantages are:

    • Formation of dilute hydrated caustic solutions that create a safety hazard and must be mechanically removed during processing.
    • Months may be required to treat even small systems.
    • End point determination is not possible (i.e. there is no way to know if all the alkali metal is reacted and the system is safe to flood or other post-processing treatment, without opening and visually inspecting the equipment being treated)
    • Large pressure excursions (pops and bangs) are possible and even probable when treating metal depths of more than a few millimeters.
    • High probability the process will stall for long periods of time.
    • Removal of the hydrated caustic can lead to large pressure excursions and are a processing safety hazard.

Carbonation (CO₂) Method

The Carbonation (CO₂) method aims to convert alkali metals to alkali bicarbonates (e.g., NaHCO₃) and alkali carbonates (e.g., Na₂CO₃) that would be suitable for subsequent grouting or disposal. Carbonation involves humidifying carbon dioxide in the same manner that the nitrogen is humidified in the WVN method. The alkali metal reacts with the moisture to form an alkali metal-hydroxide which is then converted into an alkali metal/carbonate or bicarbonate salt by reacting with the carbon dioxide.

Time-lapse Video of the CO₂ Method

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An international nuclear facility using sodium in its heat exchange loop approached CEI to complete the first known apples-to-apples-to-apples comparison of our CEI-SHS™ process with the Wet Vapor Nitrogen (WVN) and the Carbonation (often abbreviated as CO₂) methods. The time-lapse video above shows the progress made over two weeks of Carbonation treatment during bench-scale testing at CEI’s New Freedom, PA facility.

As with CEI-SHS™, Carbonation uses water as a reactant to convert alkali metals (Li, Na, K, etc.) to alkali hydroxides (LiOH, NaOH, KOH, etc.). The Carbonation method then has a secondary reaction that reacts with carbon dioxide with the resulting alkali hydroxide. The final reaction product is a mixture of alkali carbonate (Li₂ CO₃, Na₂ CO₃, K₂ CO₃, etc.) and alkali bicarbonate (LiHCO₃, NaHCO₃, KHCO₃, etc.) – both of which have the noted advantage of being non-hazardous and non-corrosive. Unlike CEI-SHS™, the Carbonation reaction is performed at ambient temperatures and the alkali metal remains solid. In CEI-SHS™ the alkali metal is heated above its melting point so that the injected steam can be thoroughly mixed with the liquid metal.

By the end of the testing, less than 2 cm of the sodium was reacted – leaving approximately 85% of the original. This underscores the primary limitation of Carbonation – it can only be successfully applied for thin layers of alkali metals. It should also be noted that the resulting sodium carbonate/sodium bicarbonate layer also takes up more volume than the original sodium did – which could lead to line plugging if Carbonation is used with small-diameter piping, valve internals, and the like.

  • Advantages of the method are:

    • Low (ambient) temperatures
    • Low levels of hydrogen
    • Inert by-products
    • Excellent for very small amounts of alkali metal, up to a few millimeters.
  • Disadvantages are:

    • Only effective for depths of 5 cm or less.
    • Months may be required to treat even small systems.
    • End point determination is not possible (i.e. there is no way to know if all the alkali metal is reacted without opening and visually inspecting the equipment being treated)
    • There is no proven process to treat a system that has been carbonated but still contains unreacted metal.  This creates a dangerous situation — see the video of the EBR-II incident which destroyed part of the EBR-II secondary system for an example.
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