System for storing a radioactive salt solution

An improved system for receiving and storing a radioactive salt solution includes a tank configured to receive the radioactive salt solution while preventing criticality accidents, a solution inlet for carrying the radioactive salt solution to the tank, an overflow bottle, and a cap sealing the top end of the tank. The cap includes a lateral wye fitting having a lateral pipe configured to direct the radioactive salt solution from the solution inlet into the tank, a vertical pipe configured to direct gases from the tank to a ventilation system, and an overflow line configured to carry excess radioactive salt solution from the tank to the overflow tank. An air gap between the lateral pipe and the solution inlet prevents backflow of the radioactive salt solution into the solution inlet. A control system includes a level switch configured to provide a signal that the tank contains a maximum volume of the radioactive salt solution, a first valve configured to terminate flow of the radioactive salt solution to the lateral pipe upon receipt of the signal from the level switch; and a second valve configured to allow flow of the radioactive salt solution from the tank to the overflow line.

TECHNICAL FIELD

Various exemplary embodiments disclosed herein relate generally to systems for storing toxic radioactive salt solutions.

BACKGROUND

Acid deficient uranyl nitrate solutions are used in a sol-gel process for fuel fabrication. However, such solutions are toxic, and a system for safely storing unused or waste uranyl nitrate solutions, as well as other radioactive salt solutions, is required. Additionally, acid deficient uranyl nitrate solutions have a uranium concentration of from 0.5 M to 3.5 M, and a pH of 0.5 to 2.8. The system for storing uranyl nitrate solutions must therefore be able to withstand exposure to highly acidic conditions.

Other radioactive salt solutions may be used in a sol-gel process for fuel fabrication, including various nitrate salts of radioactive metals. Ceramic fuel elements based on uranium, thorium, and plutonium are made from acidic solutions of UO2(NO3)2(uranyl nitrate), U(NO3)6(uranium nitrate), K2UO2(SO4)2(potassium uranyl sulfate), UO2(SO4) (uranyl sulfate), U(SO4)2(uranium sulfate), uranium phosphates, Th(NO3)4, or Pu(NO3)4. Thus, a system for safely storing unused or waste radioactive salt solution should be suitable for storing various radioactive metal salts.

In view of the foregoing, it would be desirable to develop improved methods and systems for storing toxic radioactive salt solutions.

SUMMARY

In light of the present need for storing toxic, radioactive, or otherwise dangerous liquid materials, a brief summary of various embodiments is presented. Some simplifications and omissions may be made in the following summary, which is intended to highlight and introduce some aspects of the embodiments disclosed herein, but not to limit the scope of the invention. Detailed descriptions of embodiments adequate to allow those of ordinary skill in the art to make and use the inventive concepts will follow in later sections.

Various embodiments disclosed herein relate to a system for receiving and storing a radioactive salt solution, including:a tank having a top and a bottom, the tank being configured to receive the radioactive salt solution while preventing criticality accidents, wherein the tank has a defined width;a solution inlet for carrying the radioactive salt solution to the tank;an overflow bottle;a cap sealing the top end of the tank; andan air gap between the lateral pipe and the solution inlet configured to prevent backflow of the radioactive salt solution into the solution inlet.

In various embodiments, the cap includes a lateral wye fitting having:a lateral pipe configured to direct the radioactive salt solution from the solution inlet into the tank,a vertical pipe configured to direct gases from the tank to a ventilation system; andan overflow line configured to carry excess radioactive salt solution from the tank to the overflow tank.

The system for receiving and storing a radioactive salt solution may also include a pickup for the ventilation system configured to receive the gases from the tank, and a second air gap between the pickup and the vertical pipe configured to prevent flow of the radioactive salt solution into the pickup.

In various embodiments, the system includes a control system having:a level switch configured to provide a signal that the tank is full, i.e., that the tank contains a maximum volume of the radioactive salt solution;a first valve configured to terminate flow of the radioactive salt solution from the solution inlet to the lateral pipe upon receipt of the signal from the level switch; anda second valve configured to:allow flow of the radioactive salt solution from the tank to the overflow line, andallow flow of the gases from the tank to the pickup.
The first valve is set to a Normally Closed condition to prevent overfilling of the tank in the event of failure; and the second valve is set to a Normally Open condition to prevent spilling of the radioactive salt solution in the event of failure when the tank is being filled. The second valve is configured to be closed when the tank is not being filled or emptied, thereby preventing liquids or gases in the tank from escaping. In various embodiments, the first valve and the second valve are solenoid valves.

The system for receiving and storing a radioactive salt solution may also include a solution outlet at the bottom of the tank, and a valve configured to allow the radioactive salt solution in the tank to flow through the solution outlet, thereby emptying the tank.

The system for receiving and storing a radioactive salt solution may include an input pump configured to pump the radioactive salt solution to the solution inlet, wherein the input pump stops pumping the radioactive salt solution upon receipt of a signal from the level switch, thereby preventing overfilling of the tank.

In various embodiments, the overflow bottle in the system for receiving and storing a radioactive salt solution includes a container having a mouth; and a cap including a breather vent configured to trap harmful vapors, an opening welded to the overflow line; and a means for removably securing the cap to the mouth of the container. The means for removably securing the cap to the mouth of the container may include:a tri-clamp closure,a clamp with C-shape clamping sections connected by a hinge, the clamp being configured to engage an outer circumference of the cap and an outer circumference of the mouth; ora male thread on the mouth of the container configured to mate with a corresponding female thread on the cap.

DETAILED DESCRIPTION

Scrap or waste acid-deficient uranyl nitrate (ADUN) solutions need to be stored safely. Safe ADUN storage requires a tank which is geometrically safe to prevent possibility of a criticality accident, i.e., a tank having an outer diameter of about 5 inches or less, 4.5 inches or less, or 4 inches or less. The tank must be chemically resistant to concentrated nitric acid.

While ADUN solution storage is of particular interest herein, other radioactive salt solutions which may be used as precursors for formation radioactive ceramic nuclear fuels by sol-gel processes may be stored with the systems disclosed herein. Suitable radioactive salt solutions include acidic solutions of UO2(NO3)2(uranyl nitrate), Th(NO3)4, Pu(NO3)4, and mixtures thereof. Radioactive salt solutions based on uranium, thorium, and plutonium may be acidic solutions of UO2(NO3)2(uranyl nitrate), U(NO3)6(uranium nitrate), K2UO2(SO4)2(potassium uranyl sulfate), UO2(SO4) (uranyl sulfate), U(SO4)2(uranium sulfate), uranium phosphates, Th(NO3)4, or Pu(NO3)4.

A solution of acid deficient uranyl nitrate, may be prepared by dissolving a uranium oxide in aqueous nitric acid to produce a uranium solution, placing the uranium solution under a pressure of 5 to 40 atmospheres in a sealed reaction chamber, and heating the uranium solution to a desired holding temperature of between 150° C. and 250° C. The uranium solution is maintained at the desired holding temperature in the sealed vessel for a desired hold time, and then the pressure and temperature of the uranium solution are reduced to obtain an acid deficient uranyl nitrate solution.

Once the acid deficient uranyl nitrate solution is prepared, it is converted into a ceramic nuclear fuel particle by sol-gel processes known in the art. Such conversion may be carried out immediately, or acid deficient uranyl nitrate solution may be stored for later use in an acid-resistant tank.

Storage of scrap acid-deficient uranyl nitrate (ADUN) requires a tank which is geometrically safe, to prevent possibility of a criticality accident. Generally, the tank may have any desired height, but must have a narrow width to avoid excessive buildup of nuclear material at any point along the height of the tank. Since acid-deficient uranyl nitrate is formed in a nitric acid solution, the tank must be chemically resistant to concentrated nitric acid. The tank must be designed to preclude the possibility of solution backflow into a solution inlet, which may be done through the inclusion of air gaps. The tank should be capable of sealing to simultaneously limit both gas emissions and overflow of ADUN solution from the tank.

The tank should include a pickup for ventilation to remove off gases from the tank during filling and emptying operations, where ventilation from the tank cannot blocked unless all flow into or out of the tank is ceased. Additional features of the system include:an overflow line able to carry excess fluid away from the tank;a level switch to shutoff flow of ADUN solution into the tank before the tank is full; andan optional demister internal to the tank to limit emission of solution droplets to the ventilation pickup.

The system for receiving and storing a radioactive salt solution disclosed herein contains the following improved design features:a backflow-safe inlet flange (BSIF) which prevents solution backflow, provides independent overflow path, interfaces with facility ventilation, and allows for sealing the tank to prevent vapor release;a criticality-safe tank support (CTS) which allows for mounting the tank to a process skid; anda criticality-safe tank overflow bottle (CTOB) which provides a final containment avenue for solution during an overflow event.

FIG.1illustrates a system for receiving and storing a radioactive salt solution, such as ADUN. The system includes a tank1having an outer diameter y and a height x. In various embodiments, the tank1may be formed from a vertically orientated stainless-steel pipe having an outer diameter y of from 5 to 12.5 cm, or 2 to 5 inches; or from 10 to 11.5 cm, or 4 to 4.5 inches. A tank of this diameter may be used in the nuclear fuel processing industry to prevent potential criticality accidents due to excessive buildup of radioactive material at any given depth in the tank. In various embodiments, the tank may have a height x of 0.5 to 5 meters, 1 to 4.5 meters, 2 to 4 meters, or 3 to 3.8 meters. The tank may have an outer diameter y of 11 to 11.5 cm, and a height x of 3.5 to 3.6 meters, and may be sealed at the top and bottom to create a tank that has a capacity of roughly 30 L. Tank1is geometrically safe to prevent possibility of a criticality accident during radioactive salt storage, and the stainless-steel material is chemically resistant to concentrated nitric acid.

The system disclosed herein is suitable for receiving and storing any fissile-bearing solution inside a fuel manufacturing facility. Any soluble salt of uranium, thorium, plutonium, or an oxidized form thereof may be used. The only limitation is material compatibility between the tank1and the solution. A solution of a salt of uranium, thorium, or plutonium in an aqueous sulfuric, nitric, or phosphoric acid medium is compatible with stainless steel tank1. A solution of a salt of uranium, thorium, or plutonium in aqueous HCl would not be compatible with stainless steel, as HCl corrodes stainless steel. Should storage of a salt of uranium, thorium, or plutonium in aqueous HCl be desired, tank1may be constructed from a glass pipe.

The upper end of tank1is sealed with the backflow-safe inlet flange, which includes cap2having a flange. The cap2includes a lateral wye fitting5with a vertical pipe and a lateral pipe4. The lateral pipe4is configured to direct radioactive salt solution from a solution inlet to the interior of tank1. The vertical pipe in the lateral wye fitting5is configured to vent gases in tank1to a ventilation system. Tank1may include a demister17, to enhance the removal of radioactive salt solution droplets from gases vented from tank1. Demister17may be a mesh-type coalescer to cause droplets to coalesce into larger drops. Demister17may be a knitted wire mesh pad mist eliminator, a woven mesh mist eliminator, a nonwoven mesh mist eliminator, or a mist eliminator formed from a plate or a series of plates with fine perforations. Demister17is positioned in tank1, immediately under cap2.

To prevent spilling radioactive salt solution in the event that tank1overflows during filling, an overflow line13carries excess radioactive salt solution from the lateral wye fitting5to criticality-safe tank overflow bottle12, shown in more detail inFIG.6. The pipes in the lateral wye fitting5may have an outer diameter which is greater than the diameter of the opening in the overflow bottle12. The outer diameter of the overflow line13may be the same as the inner diameter of the lateral wye fitting5when it exits fitting5. To compensate for the difference in the initial outer diameter of the overflow line13and the diameter of the opening in the overflow bottle, one or more pipe reducer couplings8and9may be used to reduce the diameter of the overflow line.

Between cap2and lateral wye fitting5, valve3may be positioned. When valve3is open while filling tank1, radioactive salt solution may pass from lateral pipe4to the interior of tank1, and gases in tank1may be vented through the vertical pipe in fitting5to the ventilation system. When valve3is closed, radioactive salt solution may be stored in tank1without allowing spillage of the solution or venting of gases. Valve3may be controlled by control3a.

Tank1may include a solution outlet14at the bottom of the tank, allowing the contents of tank1to be emptied and recovered. Valve7may be used to open or close solution outlet14. A sidestream15opened or closed by valve6may be used to sample the radioactive salt solution in tank1.

Overflow line13may be fitted with a sight glass16. The interior of overflow line13may be monitored with a camera or optical sensor10to detect radioactive salt solution in the overflow line. If radioactive salt solution is detected in the overflow line13, sensor10may send a signal to CPU11.

In various embodiments, tank1is manufactured from a stainless steel pipe with a nominal pipe size (NPS) of 4 to 5, and cap2at the top of tank1is a flange cap for an NPS 4 pipe or an NPS 5 pipe, as needed. Two ports are machined into the flange cap: a ¾″ port for a level switch, and a 2″ port for connection to the lateral wye fitting through valve3. In various embodiments, the inlet assembly includes the following components:valve3, which may be a 2″ solenoid valve; andlateral wye fitting5, which may be a 2″ lateral wye, modified with a 2″ overflow pipe and a lateral pipe4with a funnel-shaped opening.

FIG.2shows a control system for managing flow of input solutions and off gases in the system ofFIG.1. Cap2on tank1includes level switch31, configured to provide a signal that the tank1contains a maximum volume of the radioactive salt solution, i.e., that tank1is full. A first valve21is configured to terminate flow of the radioactive salt solution from the solution input20to the lateral pipe4upon receipt of the signal from the level switch. The second valve3, also shown inFIG.1, is configured to:allow flow of the radioactive salt solution from the tank1to the overflow line13(not shown inFIG.2), andallow flow of gases from the tank1to a ventilation system through ventilation pickup23.

When level switch31detects that tank1is full, a signal from level switch31is sent to CPU22, which may be the same as, or different from, CPU11inFIG.1. Upon receipt of the signal from level switch31, CPU22sends a first signal to an input pump pumping radioactive salt solution to the solution input20, where the first signal turns off the input pump to prevent overfilling tank1. CPU22also sends a second signal to valve21, closing valve21to terminate flow of the radioactive salt solution from the solution input20to the lateral pipe4, also to prevent overfilling tank1. CPU22sends a third signal to valve3, opening valve3to allow flow of the radioactive salt solution from the tank1to the overflow line13and allow flow of gases from the tank1to the ventilation pickup23.

Valve21is set to a Normally Closed condition so that, should valve21fail, it will fail in a closed position to prevent overfilling of the tank. Valve21is meant to cut off flow from the solution input. While filling the tank, CPU22sends a signal to valve21to open to allow flow of the radioactive salt solution into the tank1.

Valve3is set to a Normally Open condition so that, should valve3fail, it will fail in an open position to prevent spilling of the radioactive salt solution when the tank is being filled. When the tank is not being filled or emptied, valve3is closed to seal radioactive salt solution and vapors within tank1, so as to limit emissions that may be radioactive or dangerous. During normal operation, valves3and21work together to allow flow only under conditions which prevent backflow.

FIGS.3A to3Cshow an embodiment of the disclosed system for receiving and storing a radioactive salt solution. The system includes a tank1with an outer diameter y, e.g., 4.5 inches, and a height x, e.g., 141 inches.FIGS.3A and3Bshow the bottom of tank1, which includes a flange37and a radioactive salt solution outlet with a valve38.

FIGS.3A and3Cshow the top of tank1, which includes a cap2. Cap2includes a level switch31, which fits into a first opening in cap2for use in the control system ofFIG.2. Lateral wye fitting5is connected to a second opening in cap2, with valve36therebetween. When the tank is filled, a radioactive salt solution flows from a solution inlet34through valve35to the lateral pipe4of lateral wye fitting5. During a filling operation, both valve35and valve36are open, allowing radioactive salt solution to flow from inlet34through the lateral wye fitting and into tank1through valve36. After filling the tank, both valve35and valve36are closed to prevent spilling the radioactive salt solution.

An overflow pipe32extends from the vertical pipe of lateral wye fitting5. If tank1is overfilled during a filling operation, the excess solution may back up into the lateral wye fitting. This solution is drained through overflow pipe32to an overflow bottle (shown inFIG.1). This prevents excess solution from spilling out of the opening in lateral pipe4, where solution is received from inlet34, or from the ventilation opening in the vertical pipe of lateral wye fitting5.

As seen inFIG.3C, ventilation pickup33is shaped like in inverted funnel. Gases within tank1, which may be radioactive or contain nitric acid vapors, may escape tank1through valve36during a filling operation, and pass through the vertical pipe of lateral wye fitting5. Suction may be applied to ventilation pickup33, so that gases escaping from tank1are sucked into ventilation pickup33.

Lateral wye fitting5provides a solution input through the lateral pipe4, an independent ventilation pathway through the vertical pipe, and an independent overflow pathway through overflow pipe32. Positioning the wye fitting5above valve36allows for the sealing of tank1without providing the avenue for a blocked ventilation pathway. The ventilation pathway through the vertical pipe of fitting5must passively be open during operation. If the ventilation pathway and the solution input were separate ports in cap2, a valve on the ventilation pathway would allow for the valve to fail in an open position, and thus would not be passively safe. Since the ventilation pickup33and the solution inlet34are each above valve36, if valve36is closed, the tank is not in operation and gases cannot escape from the tank. Thus, the ventilation pickup and inlet filling are linked by this component.

The system ofFIGS.3A to3Cincludes a means for preventing back pressure backflow into the solution inlet34. Such backflow is caused by a downstream pressure that is greater than the inlet pressure. As seen inFIG.3C, backflow may be prevented by an air gap between inlet34and lateral pipe4. If desired, an air gap may be replaced with a mechanical backflow preventer to introduce a physical barrier to backflow. The mechanical backflow preventer may be a reduced-pressure principle assembly, a double check valve assembly, or a pressure vacuum breaker assembly.

A second air gap may be introduced between the ventilation pickup33and the opening in the vertical pipe of lateral wye fitting5. This air gap prevents radioactive salt solution in the lateral wye fitting5from being sucked into ventilation pickup33.

Referring toFIGS.3B and3C, during a filling operation, valves35and36are both open to allow incoming solution to enter tank1through valves35and36, while allowing ventilation of gases through valve36. During the filling operation, valve38is closed to hold the solution in tank1. After filling is completed, valves35and36are both closed to prevent overfilling the tank, or escape of cases or radioactive salt solution from tank1; and valve38is closed. When emptying tank1, valve38is opened to allow the radioactive salt solution to flow out of tank1, and valve36is opened to allow atmospheric gas to enter tank1and prevent formation of a vacuum inside the tank.

FIG.4shows a criticality-safe tank support including tank support plate44with length and width m, used for the system ofFIG.3A. The tank support plate includes a central hole41with a diameter y′, where y′=y+Δ. Diameter y′ is slightly larger than the outer diameter of tank1by distance Δ, so as to allow the lower end of tank1to slide through the opening of central hole41without interference. Holes42are configured to receive bolts passing through flange37ofFIG.3B, where holes42are arranged in a circle concentric with central hole41. In various embodiments, each hole42has a corresponding hole42on an opposite side of central hole41, where each pair of opposing holes42is separated by a distance p. Tank support plate44also includes holes43along the margins of plate44. Holes43are configured to receive bolts through a support skid (shown inFIG.5). Each pair of adjacent holes43is separated by a distance n. Each hole43is separated from an edge of plate44by a distance q.

FIG.5shows a tank ofFIG.3Bmounted to a tank support plate44ofFIG.4, where plate44is shown in cross section. Tank1has an outer diameter y′, and an inner diameter y. Tank1passes through central hole41of plate44. Flange37on tank1is bolted to plate44with bolts52having heads51. Bolts52pass through holes42in plate44, and are secured in position with nuts53. Tank support plate44is in turn bolted to support skids56with bolts54. Bolts54pass through holes43(shown inFIG.4) in plate44, and are secured in position with nuts53. This system including tank support plate44bolted to flange37provides a robust support for the tank. As seen inFIG.5, tank1includes an outlet57leading to valve38.

In various embodiments, an NPS 4 pipe flange or an NPS 5 pipe flange is used as flange37, and is bolted to plate44. Flange37is also secured to an outer surface of tank1, in much the same way that cap2is fixed to the upper end of tank1. Plate44is secured to the support skids56. This provides a robust support for tank1.

FIG.6shows an overflow bottle12for connection to overflow line13ofFIG.1. Bottle12is has a volume of 0.5 to 5 liters, 0.7 to 4 liters, 0.75 to 2.25 liters, or 0.8 to 1.2 liters. Bottle12has a closure64with a first opening configured to receive a breather vent61, where breather vent61vents gases from inside bottle12. Closure64has a second opening62adapted to be connected to overflow line13. In various embodiments, bottle12and closure64are made of an acid resistant material such as stainless steel. Opening62may be butt-welded to overflow line13. A means63is provided for securing bottle12to closure64. In various embodiments, closure64may have a skirt with a female thread, and bottle12may have an opening with a male thread corresponding to the female thread, so that the bottle12may be unscrewed from closure64. In various embodiments, closure64and bottle12may be secured together with a tri-clamp closure, or a clamp with C-shape clamping sections connected by a hinge, the clamp being configured to engage an outer circumference of the cap and an outer circumference of the mouth.