Patent Publication Number: US-2019172595-A1

Title: Sticky neutron moderator core pellets

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     This invention relates to field of nuclear radiation contamination cleanup. 
     Description of the Background 
     A recent nuclear power plant accident has brought to light significant challenges relating to the cleanup and waste management of radioactive materials at accident sites. These issues include, but are not limited to the following: the processing of large volumes of contaminated water, debris, soil, secondary wastes, potentially damaged spent fuel within reactor spent fuel pools, and damaged fuel and fuel debris within reactors and primary containment structures. 
     After progress has been made in the cooling of the reactors at an accident site mid-term to long-term waste management issues will continue to be a major technical concern that must be addressed and overcome as any recovery actions continue toward an acceptable end state. As such, there are considerable waste management challenges associated with the treatment of contaminated water, the resulting filter and equipment waste storage, the disposal of secondary waste, contaminated soil, vegetation, and debris decontamination. 
     SUMMARY OF THE INVENTION 
     The shortcomings of the prior art are overcome and additional advantages are provided through the provision of a neutron absorbing decontamination pellet that comprises an innermost core of a predetermined neutron absorbing material and an insoluble polymer coating that forms a continuous outer casing for the core of neutron absorbing material. Further, the outer casing decontamination pellet will become sticky, yet retain its integrity, upon exposure to water or upon exposure to a predetermined temperature range. 
     A further exemplary embodiment of the present invention comprises a method for the formation of neutron absorbing decontamination pellets. The method comprises the steps of; creating a slurry comprising a predetermined neutron absorber material and a binding agent; transporting the slurry through a feeder hose, the hose terminating at a deposit nozzle; employing the deposit nozzle to form the slurry into neutron cores of predetermined sizes; immersing the neutron cores formed by the deposit nozzle into a bath of a predetermined insoluble polymer solution to create an outer casing upon each immersed neutron core in order to construct a neutron absorbing decontamination pellet; and exposing the neutron absorbing decontamination pellets to air to dry, and thereafter transferring the dry decontamination pellets to storage. 
     Additional features and advantages are realized through the techniques of the present invention. Other embodiments and aspects of the invention are described in detail herein and are considered a part of the claimed invention. For a better understanding of the invention with advantages and features, refer to the description and to the drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The subject matter that is regarded as the invention is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The foregoing and other objects, features, and advantages of the invention are apparent from the following detailed description taken in conjunction with the accompanying drawings in which: 
         FIG. 1  is a cross-section view of a neutron moderator pellet. 
         FIG. 2  illustrates an apparatus for the formation of neutron moderator pellets. 
         FIG. 3  illustrates a nuclear reactor container with the melted remains of a nuclear fuel element assembly. 
         FIG. 4  illustrates a close-up view of the neutron moderator pellets upon the application of the neutron moderating pellets to the melted nuclear fuel elements. 
         FIG. 5A  illustrates a contaminated structure and the delivery of a neutron moderator pellet application onto the contaminated surface of the structure. 
         FIG. 5B  illustrates a congealed coating of neutron moderator pellet material being removed from the contaminated structure. 
     
    
    
     The detailed description explains the preferred embodiments of the invention, together with advantages and features, by way of example with reference to the drawings. 
     DETAILED DESCRIPTION OF THE INVENTION 
     One or more exemplary embodiments of the invention are described below in detail. The disclosed embodiments are intended to be illustrative only since numerous modifications and variations therein will be apparent to those of ordinary skill in the art. In reference to the drawings, like numbers will indicate like parts continuously throughout the views. Herein, the use of the terms first, second, etc., do not denote any order or importance, but rather the terms first, second, etc., are used to distinguish one element from another. Furthermore, the use of the terms a, an, etc., do not denote a limitation of quantity, but rather denote the presence of at least one of a referenced item. 
     Exemplary embodiments of the present invention comprise pellets fabricated from neutron moderating/absorbing materials that are coated with a water insoluble polymer. As shown in  FIG. 1 , the neutron core moderator pellet  100  comprises a core matrix of a neutron absorbing material  102  and a polymer coating  104  that in some instances can serve as a binding agent for the construction of the core  102 . The neutron moderating/absorbing core material  102  can be selected from a variety of chemical element materials that have been proven to exhibit established neutron capture/absorbing capabilities. For example, such materials could comprise (but are not limited to) boron, hafnium, silver, indium, cadmium, cobalt, samarium, europium, ytterbium, dysprosium, erbium, holmium, thulium, terbium, and gadolinium. The neutron moderating/absorbing core material  102  can also comprise a compound or alloy, e.g., titanium diboride, boron carbide, silver-indium-cadmium alloy, zirconium diboride, and gadolinium titanate. 
     As presented above, the pellets  100  within the exemplary embodiments of the present invention have a core  102  comprising a predetermined neutron moderating material and an outer shell coating  104  that is comprised of a predetermined polymer material. The polymer chosen to comprise the outer shell  104  must have the intrinsic property of being insoluble in water and being semi-rigid to rigid in structure when exposed to air. The semi-rigid to rigid hard coating of the outer shell  104  is necessitated to facilitate the storage and transportation of the neutron core moderator pellets  100 . 
     The polymer coating employed within the exemplary embodiments of the present invention can have slowly degradable water insoluble, poor water soluble, or partial water soluble characteristics depending upon the area of usage in which the pellets  100  will be deployed and the ambient temperature range within the area of usage. Thus pellets configured with a degradable poor or partial water soluble polymer coating  104  can be configured to become tacky or sticky when introduced to water yet still retain their structural integrity. Further, pellets  100  compromising a water insoluble polymer coating can be constructed to retain their hard shell structural integrity in water until introduced to high temperatures ranges (any temperatures approximately above 60° C.) and thereafter become tactilely tacky or sticky. 
       FIG. 2  illustrates a system for forming neutron moderating pellets. A hopper configuration  200  is shown that tapers downward and the materials deposited within the hopper  200  are gravitationally transported through and discharged at the bottom of the hopper  200 . Within exemplary embodiments a slurry mixture  202  comprised of a neutron moderating material and a binding agent  202  are deposited into a storage cylinder  204  that is configured with a nozzle  206  that is used to form the pellets  100 . The binding agent for the slurry mixture  202  is selected based upon the binder substance comprising a density that that is higher than the polymer coating  104  that is used to construct the pellets  100 . The binder holds the neutron moderating/absorbing material together and allows for the moderating material to harden once the pellet core  102  formed from the slurry mixture  202  has cured. Within exemplary embodiments of the present invention the pellet  100  forming nozzle  206  has the capacity to be adjusted to predetermined dispensing settings in order to change the size of pellets  100  that are expelled from the storage cylinder  204 , depending on the type of pellet that is necessitated. 
     The pellet  100  forming nozzle  206  deposits the formed neutron moderator cores  102  into a bath of a predetermined pellet  100  outer coating polymer  208 . The characteristics necessitated by the polymer to be utilized will depend on the type of pellet  100  that is to be produced. For pellets  100  that are to be utilized in high temperature settings the pellets  100  can comprise a hard thermoplastic polymer shell at ambient temperatures-thus making the storage and transport of said pellets  100  relatively easy—wherein upon exposure to high temperatures the hard thermoplastic polymer shell will soften and become tacky, thus making it easier for the pellet  100  to adhere to whatever surface it is being applied to whether in an aqueous or dry environment. Also, since the pellets  100  will start to harden again when the temperature of the pellets  100  lowers, the ease of cleaning up the pellets  100  will be greatly enhanced. 
     Alternately, for pellets  100  that are to be applied to surfaces at or around ambient temperatures, the polymer  104  selected to coat the core  102  will have the characteristic of remaining solid yet pliably tacky or sticky at ambient temperatures while allowing the core  102  of the pellet  100  to completely dry and cure. In this instance the pellets  100  can be stored in a liquid stasis for the convenient storage and transportation of the pellets  100 . 
     Within the hopper  200  the liquid neutron moderator pellet  100  and the polymer  208  are not miscible, thus within the polymer coating  208  solution of the hopper  200  the denser neutron moderating material does not mix with the less dense polymer material. As the neutron modulating pellets  100  are pulled by gravity to the lowermost portion of the polymer outer coating  208  solution each pellet is subsequently bathed in the polymer solution  208 . As the pellets  100  exit the formation segment  210  of the hopper  200  they are deposited into a collector (not seen). The coating of the pellets  100  dry once they are exposed to air upon exiting the formation container and the coating solidifies around their neutron moderator cores  102  prior to the pellets  100  landing in the remotely situated collection container. 
       FIG. 3  shows a nuclear reactor vessel  310  wherein the melted remains of the unit&#39;s fuel element assembly  312  are pooled on the floor of the reactor vessel. Coolant water  314  has been supplied via coolant pipes in order to cool the melted remains of the fuel elements  312 . In this example a neutron moderating pellet delivery system  302  is shown, the delivery system  302  comprising a pellet  100  storage unit and delivery mechanism. Depending upon the emergency service, the water insoluble or partially soluble neutron moderating pellets  100  can be deposited directly upon the melted fuel element  312  in either a dry form or as part of a previously prepared water slurry mixture  304  (the dry pellets will form a slurry mixture  304  with the coolant water  314  upon being deposited onto the melted fuel elements  312  that are immersed within the coolant water  314 ). 
     In either form, the pellets  100  are or would become tacky/sticky upon being introduced to the water  314  and fuel elements  312  and as such will easily adhere to the surface of the melted fuel elements  312  when deposited at the site and slow any reactions that may be occurring with the damaged fuel elements  312  ( FIG. 4 ). Any pellets  100  that have not adhered to the surface of the melted fuel element  312  will be suspended in the aqueous slurry mixture  304  and being that they are in the proximity of the melted fuel element  312  the suspended pellets  100  will absorb any resulting radiation and thus slow the reaction occurring within the nuclear fuel elements. Additionally, since the neutron modulating pellets  100  can be produced in a variety sizes ranging from the micro to macro, the pellets  100  can be sufficiently sized so that they can penetrate and adhere to any fissures that may have occurred within the damaged nuclear reactor core materials. 
       FIGS. 5A and 5B  illustrate a further exemplary embodiment of the present invention detailing the delivery of a neutron moderator pellet  100  application onto the surface of a contaminated structure  500 . In this example a contaminated structure  500  is approached by a remotely controlled vehicle  502 . The vehicle is equipped to carry a supply of the neutron modulator pellets  100  that have been suspended in a gel-solution  504 . In this embodiment the neutron modulator pellets  100  are configured into elongated fiber strand shaped pellets  102  encased within the polymer coating  104  and thereafter mixed into the gel solution  504 . 
     The gel-solution is formulated to congeal upon coming into contact with air. Thus, as the gel-solution  504  containing the neutron modulating pellet strands  100  is sprayed onto a patch of contaminated wall the sprayed material will coagulate and form a thin cover over the sprayed patch of contaminated structural area. The congealing gel  504  serves to entrap and absorb the resulting radiation from the contaminated particles beneath it on the wall whereon the gel has been sprayed. The congealed gel covering can be removed from the wall and disposed of in a safe manner at a later period of time. 
     While the invention has been described with reference to exemplary embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiments disclosed for carrying out this invention, but that the invention will include all embodiments falling within the scope of the claims.