METHOD FOR PREPARING A CEMENTITIOUS SHIELDING MATERIAL AGAINST IONIZING RADIATION

Abstract

The invention relates to a method for preparing a cementitious material for shielding against ionizing radiation, which comprises at least the steps of: a) preparing a cementitious paste comprising at least one cement and water, the cement being a hydraulic cement or a phosphomagnesium cement, and then b) hardening the cementitious paste obtained in step a), and is characterized in that the cementitious paste further comprises a powder consisting of triuranium octoxide depleted in uranium-235. Applications: immobilization by cementation of radioactive waste, manufacture of radioactive waste packaging containers, transport and / or storage packaging for radioactive materials, components of radioactive waste storage structures, components of shielded or radiation-protected rooms, etc.

Description

Title of the invention: METHOD FOR PREPARING A CEMENT MATERIAL FOR SHIELDING AGAINST IONIZING RADIATION Technical field

[0001] The invention relates to the field of shielding against ionizing radiation.

[0002] More specifically, it relates to a process for preparing a cementitious material for shielding against ionizing radiation in which a powder of triuranium octoxide (U3O8) depleted in uranium-235 is used as a component of the cementitious paste from which this material is obtained, by hardening.

[0003] The invention finds application in all fields where it is desired to have shielding against ionizing radiation and in particular: - in the immobilization by cementing, that is to say by embedding in a cement matrix, of radioactive waste such as waste from the processing of spent nuclear fuel or from operating, maintenance and / or dismantling activities of nuclear installations, with a view to the storage and / or disposal of this waste; - in the manufacture of primary or secondary containers for packaging radioactive waste of the type mentioned above, having been previously or not confined in a cement matrix, also for the purpose of storing and / or stockpiling this waste; - in the manufacture of transport and / or storage packaging for radioactive materials such as spent nuclear fuel assemblies; - in the manufacture of structural elements for radioactive waste storage sites such as plates, doors, cladding segments, seals or plugs for tunnels or galleries dedicated to the storage of this waste; and - in the manufacture of structural elements such as slabs, walls, ceilings, etc., of shielded or radioprotected rooms, whether mobile or permanent, used, for example, for industrial or medical radiography or nuclear medicine activities.

[0004] The invention also has these applications as its subject. State of the prior art

[0005] The fuels used in most of the nuclear reactors currently in operation around the world are prepared from enriched uranium produced by enrichment facilities.

[0006] The enrichment of uranium generates a so-called depleted uranium because its isotopic rate of uranium-235 is lower than that of natural or reprocessed uranium having been subjected to enrichment.

[0007] Generally, the 235U isotopic content of depleted uranium is less than 0.5% and, more specifically, between 0.2% and 0.4%.

[0008] Uranium enrichment facilities have been in operation for more than half a century and their operation has led to the accumulation of large stocks of depleted uranium which, depending on the enrichment facility, can be stored in different forms: uranium metal, uranium hexafluoride (UF6), uranium tetrafluoride (UF4), uranium dioxide (UO2), uranium trioxide (UO3) and triuranium octoxide (U3O8, also called triuranium octaoxide or triuranium octoxide).

[0009] Although depleted uranium is used in particular in the manufacture of MOX-type nuclear fuels (a mixture of uranium and plutonium oxides), this use is limited and the nuclear industry players, who hold these stocks, have therefore launched ambitious research programmes with a view to finding ways of exploiting depleted uranium and thus reducing the volumes of depleted uranium already accumulated and to come.

[0010] As early as the 1980s, it was proposed in patent JP-S-6191598, hereinafter reference [1], to use depleted uranium metal or depleted uranium dioxide in the form of aggregates in a concrete intended to serve as a screen against radiation, in particular type y.

[0011] Uranium dioxide being chemically reactive - as are most of the uranium compounds - it tends to transform into uranium trioxide, or even triuranium octoxide when present in concrete and this transformation leads to degradation of this concrete.

[0012] This is the reason why it was proposed in US patents 5,786,611 and US 6,166,390, hereinafter referenced [2] and [3], to include, in a Portland cement-based concrete, depleted uranium in the form of aggregates which measure from 3 mm to 10 cm in diameter and which are made up either of a depleted uranium silicide (U3Si or U3Si2) or of a stable ceramic form of depleted uranium previously obtained by sintering (1,000 °C - 1,500 °C) or even of a depleted uranium compound (oxide in particular) previously covered with a coating impermeable to water and air, of the glass, clay or polymer type.

[0013] In references [2] and [3], it is more specifically recommended to use aggregates comprising a depleted uranium oxide, UO2 or U3O8, or a mixture of depleted uranium oxides having previously been subjected to liquid phase sintering, the liquid phase preferably consisting of a natural or artificial basalt and being intended to form, on the oxide or the mixture of oxides, a coating which is both protective and stabilizing.

[0014] If some of the concretes conforming to the teaching of these references present interesting properties - which is the case of the concrete known under the commercial reference Ducrete™ and in which the aggregates are made of uranium dioxide sintered with basalt - it turns out that they are not, however, entirely satisfactory.

[0015] Indeed, the use of depleted uranium in the form of aggregates does not allow to benefit from a filler effect, that is to say a filling of the voids left by the aggregates used in the concretes (i.e. sands and / or gravel), and, thereby to have concretes having a compactness capable of giving them satisfactory mechanical performance and dimensional stability. Moreover, as shown by LR Dole and WJ Quapp in the report ORNL / TM-2002 / 111 published in August 2002, hereinafter reference [4], the compressive strength values ​​of Ducrete™ are only between 20 MPa and 41 MPa at 28 days of maturation.

[0016] The temperature resistance of concretes conforming to the teaching of references [2] and [3] is also not satisfactory. Thus, as also shown in reference [4], a decrease in mechanical performance with increasing temperature is observed for samples of Ducrete™ exposed to temperatures up to 250°C, accompanied by disintegration of these samples after 14 days of exposure, which are partly explained by the oxidation of uranium dioxide to U3O8.

[0017] Furthermore, the teaching of references [2] and [3] has the defect of being limited to the use of Portland cement. However, Portland cements are incompatible with certain radioactive waste such as that comprising aluminum in the metallic state, so that it is not possible to apply this teaching for the production of packaging packages for this type of waste.

[0018] Finally, the preparation of concretes conforming to references [2] and [3] is long and costly given that it systematically involves preliminary operations of treatment of the depleted uranium used, whether to convert it into silicide or into a stable ceramic form (by sintering) or to cover it with a coating impermeable to water and air.

[0019] In view of the above, the inventors set themselves the goal of developing a cementitious material from depleted uranium which, while being capable of providing very effective shielding against ionizing radiation, has very satisfactory mechanical properties, temperature resistance, dimensional stability, workability and setting time.

[0020] They further set themselves the goal that this cementitious material could be prepared using a cement which was not limited to Portland cements.

[0021] They also set themselves the goal of making the preparation of this material as simple and inexpensive as possible.

[0022] However, in the course of their work, the Inventors noted that contrary to the teachings According to the state of the art, the use of triuranium octoxide in powder form in cement pastes makes it possible to obtain cement materials with all the desired properties, namely: - on the one hand, without this powder or the uranium octoxide which constitutes it being subjected to any prior transformation treatment and, in particular, to a sintering or coating type treatment as described in references [2] and [3], and - on the other hand, that the cement used is a phospho-magnesium cement or a Portland cement.

[0023] And it is on these experimental observations that the invention is based. Statement of the invention

[0024] The subject of the invention is therefore a process for preparing a cementitious material for shielding against ionizing radiation, which comprises at least the steps of: a) preparation of a cement paste comprising at least one cement and water, the cement being chosen from hydraulic cements and phospho-magnesium cements, then b) hardening of the cement paste obtained in step a), and which is characterized in that the cement paste further comprises a powder consisting of triuranium octoxide depleted in uranium-235.

[0025] In the context of the invention, the terms ionizing radiation are used in their usual meaning, namely that they designate any form of energy released by atoms and which propagates by means of electromagnetic waves, i.e. γ or X rays, or particles, i.e. neutrons or α or [3.

[0026] Furthermore, the term powder consisting of triuranium octoxide depleted in uranium-235 is understood to mean a solid in the form of grains whose size (the term size being here synonymous with largest dimension) is at most equal to 1 mm and which comprise nothing other than triuranium octoxide depleted in uranium-235, apart from impurities linked to the process for producing triuranium octoxide such as uranyl fluoride (UO2F2), or corrosion products from the installations, the content of which can be up to 5,000 ppm.

[0027] In the following, this powder will be referred to more simply as depleted U3Os powder or even depleted U3OS.

[0028] As previously mentioned, the cement present in the cement paste may, firstly, be a hydraulic cement.

[0029] Hydraulic cement means a cement whose hardening is the result of the hydration by water of a finely ground material, consisting in whole or in part of a clinker, that is to say a product resulting from the firing of a mixture of limestone and clay. Thus, hydraulic cements do not include so-called geopolymer cements. whose hardening is the result of polycondensation of a finely ground aluminosilicate material, free of clinker, in an alkaline solution, nor cements whose hardening is the result of a chemical reaction between the constituent material(s) of these cements and an acid or basic solution (magnesium cements, alkali-activated slags, etc.).

[0030] When the cement is a hydraulic cement, then it can in particular be chosen from: - cements classified CEM I by the European standard NF EN 197-1, corresponding to the Portland cements previously mentioned, which comprise at least 95% by mass of a clinker and at most 5% by mass of secondary constituents; - cements classified CEM II by the aforementioned standard, also called composite Portland cements, which comprise at least 65% by mass of a clinker, at most 35% by mass of a component chosen from blast furnace slag, silica fume, natural pozzolan, calcined natural pozzolan, siliceous or calcic fly ash, calcined shale or limestone, and at most 5% by mass of secondary constituents; - cements classified CEM III by the aforementioned standard, also called blast furnace cements, which comprise from 5% to 64% by mass of a clinker, from 36% to 95% by mass of a blast furnace slag and at most 5% by mass of secondary constituents; - cements classified CEM IV by the aforementioned standard, also called pozzo-lanic cements, which comprise from 45% to 89% by mass of a clinker, from 11% to 55% by mass of a component chosen from silica fume, natural pozzolan, calcined natural pozzolan, siliceous or calcic fly ash, and at most 5% by mass of secondary constituents; and - cements classified CEM V by the aforementioned standard, also called composite cements, which comprise from 20% to 64% by mass of a clinker, from 18% to 50% by mass of a blast furnace slag, from 18% to 50% by mass of fly ash and at most 5% by mass of secondary constituents.

[0031] All these cements are available in particular from the companies Holcim and its subsidiary Lafarge, Heidelbergcement, Cemex and Eqiom.

[0032] Among these cements, it is preferred to use a Portland cement (i.e. CEM I) or composite Portland cement (i.e. CEM II) and, even more so, a Portland cement, which is advantageously chosen according to the properties, in particular mechanical, which one wishes to confer on the cementitious material with regard to the use for which it is intended.

[0033] Thus, for example, for the preparation of a cementitious material which is desired to have high mechanical properties, preference is given to a high-performance Portland cement of the CEM I 52.5 R or CEM I 52.5 N type, whereas, for the preparation of a cementitious material whose destination does not require it to have high mechanical properties, then a Portland cement CEM I 32.5 R or CEM I 32.5 N may be perfectly suitable.

[0034] The cement present in the cement paste may also be a phospho-magnesium cement, that is to say a cement which is composed of at least one source of magnesium in the oxidized state, that is to say a compound comprising magnesium in the oxidation state +11 or a precursor thereof, and at least one source of phosphate.

[0035] Any source of magnesium in the oxidized state known to those skilled in the art can be used within the framework of the invention.

[0036] Thus, the source of magnesium in the oxidized state may in particular be a magnesium oxide, a magnesium hydroxide, a magnesium carbonate, a magnesium hydroxycarbonate, a magnesium chloride, a magnesium bromide or a mixture thereof.

[0037] According to the invention, the source of magnesium in the oxidized state is, preferably, a magnesium oxide (MgO), in which case this oxide can be: - uncalcined MgO; - a soft burnt or light burnt MgO, i.e. an MgO resulting from the calcination of a magnesium carbonate or a magnesium hydroxide at a temperature typically between 600°C and 1000°C; - a hard burnt MgO, i.e. an MgO resulting from the calcination of a magnesium carbonate, a magnesium hydroxide or a soft burnt MgO at a temperature typically between 1,000°C and 1,500°C; or - a dead burnt MgO, i.e. an MgO resulting from the calcination of a magnesium carbonate, a magnesium hydroxide or a soft burnt MgO at a temperature typically above 1,500°C and up to 2,300°C.

[0038] Furthermore, the MgO may be a pure MgO (i.e. having a purity, expressed by mass, greater than 95% and, preferably, greater than 99%) or, on the contrary, comprise at least one other element at a level of at least 5% by mass such as silicon, calcium, iron or aluminum, this (these) element(s) generally being in the form of oxide(s) and / or hydroxide(s).

[0039] Any source of phosphate known to those skilled in the art can also be used in the context of the present invention. Thus, the source of phosphate can in particular be: - an acid such as phosphoric acid, orthophosphoric acid, pyrophosphoric acid or polyphosphoric acid; or - a phosphoric acid salt, which can be: * a salt of phosphoric acid and a metallic element such as a phosphate of an alkali or alkaline earth metal, aluminum phosphate, a mono- hydrogen phosphate of an alkali or alkaline earth metal, aluminium monohydrogen phosphate, a dihydrogen phosphate of an alkali or alkaline earth metal, aluminium dihydrogen phosphate, an orthophosphate of an alkali or alkaline earth metal, aluminium orthophosphate, a pyrophosphate of an alkali or alkaline earth metal, aluminium pyrophosphate, a tri-, tetra- or pentapoly-phosphate of an alkali or alkaline earth metal, or an aluminium tri-, tetra- or pentapoly-phosphate; or * a salt of phosphoric acid and a non-metallic element such as ammonium phosphate, ammonium monohydrogen phosphate, ammonium dihydrogen phosphate, ammonium orthophosphate, ammonium pyrophosphate, or an ammonium tri-, tetra-, or pentapolyphosphate; or * a mixture of these.

[0040] Preferably, the phospho-magnesium cement is composed of: - a calcined MgO, preferably hard butt to avoid too rapid setting of the cement paste, such an MgO being, for example, that marketed by the company Martin Marietta Magnesia Specialties under the reference MagChem™ 10 CR, and - a phosphoric acid salt, which is advantageously sodium phosphate (Na3PO4), sodium monohydrogen phosphate (Na2HPO4), sodium dihydrogen phosphate (NaH2PO4), potassium phosphate (K3PO4), potassium monohydrogen phosphate (K2HPO4), potassium dihydrogen phosphate (KH2PO4), aluminum phosphate (A1PO4), aluminum monohydrogen phosphate (A12(HPO4)3), ammonium phosphate ((NH4)3PO4), ammonium monohydrogen phosphate ((NH4)2HPO4) or ammonium dihydrogen phosphate (NH4H2PO4) and, preferably, KH2PO4.

[0041] The phospho-magnesium cement advantageously has a magnesium / phosphorus (Mg / P) molar ratio which is between 0.1 and 12, preferably between 0.8 and 3 and, better still, equal to 1 ±0.1.

[0042] According to the invention, the depleted U3O8 powder is a powder of which 50% by volume of the grains (d50) have a size which is advantageously at most equal to 5 qm, preferably at most equal to 3 qm and, better still, equal to 2.35 qm, and of which 90% by volume of the grains (d90) have a size which is advantageously at most equal to 10 qm, preferably at most equal to 7 qm and, better still, equal to 6.15 qm.

[0043] Furthermore, the depleted U3O8 powder can be present in the cement paste at an extremely variable mass content, this content being preferentially chosen according to the cement used.

[0044] Thus, the depleted U3O8 powder preferably represents: - from 1% to 80% and, better still, from 10% to 45% by mass, for example 12.5%, 24.5% or 42.5% by mass, relative to the total mass of the cement paste, in the case of a hydraulic cement, and, in particular, of a CEM I or CEM II cement, and - from 1% to 80% and, better still, from 35% to 75% by mass, for example, 73.5% by mass, relative to the total mass of the cement paste, in the case of a phospho-magnesium cement and, in particular, of a cement composed of a calcined MgO and a phosphoric acid salt.

[0045] In addition to comprising the cement and the depleted U3O8 powder, the cement paste may comprise one or more additives for cementitious material, capable of allowing adjustment of the properties of this material in the fresh state (rheological properties, setting time, hardening time, etc.) and / or in the hardened state (durability for example).

[0046] Thus, this (these) adjuvant(s) may in particular be chosen from plasticizers (water reducers or not), super-plasticizers (high water reducers or not), setting time modifiers, hardening time modifiers and adjuvants which combine several properties such as high water reducer / setting retarder super-plasticizers.

[0047] In particular, the cement paste may comprise a super-plasticizer and / or a setting retarder.

[0048] Suitable superplasticizers include high water-reducing superplasticizers such as polycarboxylic ether polymers and polynaphthalene sulfonates.

[0049] Suitable setting retarders include hydrofluoric acid (HF) and its salts (e.g. sodium fluoride), phosphoric acid (H3 PO4) and its salts (e.g. sodium phosphate), boric acid (H3BO3) and its salts (e.g. sodium borate of the borax type), citric acid and its salts (e.g. sodium citrate), malic acid and its salts (e.g. sodium malate), tartaric acid and its salts (e.g. sodium tartrate), sodium carbonate (Na2CO3) and sodium gluconate.

[0050] Among these, preference is given to boric acid and sodium borate.

[0051] When the cement paste comprises a superplasticizer, the latter preferably represents no more than 4.5% by mass of the mass of cement, whereas when the cement paste comprises a setting retarder, the latter preferably represents no more than 10% by mass of the total mass of said cement paste.

[0052] According to the invention, the cement paste may also comprise, depending on the use for which the cement material is intended: - sand, which can be siliceous, calcareous or silico-calcareous, in which case the cement paste is then called mortar and the sand / cement mass ratio can be up to 6, and / or - gravel, which can also be siliceous, calcareous or silico-calcareous, in which case the cement paste then takes the name of concrete and the mass ratio of gravel to cement can be up to 4.

[0053] The terms sand and gravel are used here in their usual meaning in the field of mortars and concretes (see in particular standard NF EN 12620 relating to aggregates for concrete), namely that: - a sand is an aggregate whose upper dimension D is at most equal to 4 mm; while - a gravel is an aggregate whose lower dimension d is at least equal to 2 mm and whose upper dimension D is at least equal to 4 mm, it being understood that, within the framework of the invention, the upper dimension D of the gravel(s) is, preferably, at most equal to 16 mm.

[0054] The cement paste may also comprise, depending on the use for which the cement material is intended: - inorganic fibers such as metal fibers (e.g. steel, amorphous cast iron or stainless steel), glass fibers or carbon fibers, and / or - organic fibers, such as acrylic fibers, polyamide fibers, aramid fibers, Kevlar fibers, polyethylene fibers, polypropylene fibers or polyacrylamide fibers.

[0055] According to the invention, the cement paste typically comprises a water / cement mass ratio ranging from 0.10 to 1, preferably from 0.20 to 0.60 and, better still, from 0.30 to 0.55.

[0056] According to the invention, the cement paste may further comprise radioactive waste if the cement material is used as a matrix for immobilizing this waste.

[0057] In which case, if the radioactive waste is liquid waste, then part of the water present in the cement paste can be provided by this waste.

[0058] As shown in the examples described below, the cementitious materials prepared by the process of the invention have been found to have very satisfactory properties both in terms of mechanical properties, temperature resistance, dimensional stability, workability, setting time and durability and shielding against ionizing radiation.

[0059] In particular, the materials obtained from Portland cement showed a compressive strength close to 60 MPa, or even higher than this value, at 28 days of maturation as well as a temperature resistance of up to 232°C without cracking.

[0060] This method is therefore likely to find numerous applications and the invention also has the following object: - a process for immobilizing radioactive waste by cementing, which is ca- characterized in that it comprises the preparation of a cementitious material for shielding against ionizing radiation by a process as previously defined. - a method of manufacturing a container for packaging radioactive waste, - a method of manufacturing a packaging for transporting and / or storing radioactive materials, - a method of manufacturing a structural element of a radioactive waste storage site, and - a method of manufacturing a structural element of a shielded or radio-protected room, all these processes being characterized in that they comprise the preparation of a cementitious material for shielding against ionizing radiation by a process as previously defined.

[0061] Other characteristics and advantages of the invention will appear on reading the additional description which follows and which is given with reference to the appended figures.

[0062] It goes without saying that this additional description is given only as an illustration of the subject of the invention and in no way constitutes a limitation of this subject. Brief description of the figures

[0063] [Fig-1] illustrates the results of compression tests carried out with samples of cementitious materials obtained from two different pastes, called respectively CPM-U3O8 and CPM-CV, of the same phospho-magnesium cement, the two pastes being stoichiometric (molar ratio Mg / P = 1) but differing from each other in that the CPM-U3O8 paste comprises 73.5% by mass of depleted U3O8 while the CPM-CV paste comprises 39.3% by mass of fly ash.

[0064] [Fig.2] illustrates the results of temperature compressive strength tests carried out with samples of two cementitious materials obtained from two different pastes of the same phospho-magnesium cement, namely the aforementioned CPM-CV paste and a paste, called CPM-REF2, differing from the CPM-CV paste in that it is over-stoichiometric (molar ratio Mg / P = 6) and free from fly ash.

[0065] [Fig.3] corresponds to photographs of samples of cementitious materials obtained from the CPM-REF2 (left photographs) and CPM-CV (right photographs) pastes as they appear after heating tests up to 232°C, just before the compressive strength measurements, the results of which are illustrated in [Fig.2].

[0066] [Fig.4] illustrates the results of dimensional stability tests carried out with samples of cementitious materials obtained from two different pastes of the same phospho-magnesium cement, namely the aforementioned CPM-CV paste and a paste, called CPM-REF1, differing from CPM-CV paste in that it is over-stoichiometric (Mg / P molar ratio = 6) and free of fly ash.

[0067] [Fig.5] illustrates the results of compressive strength tests carried out with samples of cementitious materials obtained from four different mortars, called respectively CP-24.5%U3O8, CP-42.5%U3O8, CP-7.2%CV and CP-15%CV, of the same Portland cement but differing from each other in that the mortars CP-24.5%U3O8 and CP-42.5%U3O8 comprise respectively 24.5% by mass and 42.5% by mass of depleted U3O8 while the mortars CP-7.2%CV and CP-15%CV comprise respectively 7.2% by mass and 15% by mass of fly ash.

[0068] [Fig.6] illustrates the results of compression tests carried out with samples of two different dimensions of cementitious materials obtained from three different mortars of the same Portland cement, namely the CP-15%CV mortar as well as two mortars, called respectively CP-3.3%CV and CP-REF, differing from the CP-15%CV mortar in that the CP-3.3%CV mortar comprises 3.3% by mass of fly ash while the CP-REF mortar is free of fly ash.

[0069] [Fig.7] illustrates the results of temperature compression tests carried out with samples of cementitious materials obtained from six different mortars of the same Portland cement, namely the mortars CP-24.5%U3O8, CP-42.5%U3O8, CP-3.3%CV, CP-15%CV and CP-REF as well as a mortar, called CP-12.5%U3O8, comprising 12.5% ​​by mass of U3O8.

[0070] [Fig.8] illustrates the results of dimensional stability tests carried out with samples of cementitious materials obtained from mortars CP-3.3%CV, CP-7.2%CV, CP-15%CV and CP-REF.

[0071] [Fig.9] illustrates the results of leaching tests carried out with samples of cementitious materials obtained from mortars CP-12.5%U3O8, CP-24.5%U3O8e and CP-42.5%U3O8 as well as with samples of a depleted U3O8 powder.

[0072] [Fig. 10] and [Fig. 11] and [Fig. 12] and [Fig. 13] and [Fig. 14] and [Fig. 15] and [Fig. 16] and [Fig. 17] illustrate the ionizing radiation shielding properties of cementitious materials obtained respectively from CPM-U3O8 and CPM-CV pastes and CP-12.5%U3O8, CP-24.5%U3O8, CP-42.5%U3O8 and CP-REF mortars.

[0073] Detailed description of particular embodiments

[0074] Example I: Mechanical properties, dimensional stability, workability and setting time of a cementitious material of the invention with phospho-magnesium cement 1.1 - Preparation of cement pastes:

[0075] Four different phospho-magnesium cement pastes - hereinafter referred to as CPM-U3O8, CPM-CV, CPM-REF1 and CPM-REF2 - are prepared using: - a “hard burnt” magnesium oxide (MagChem™ 10 CR from Martin Marietta Magnesia Specialties, LLC); - potassium dihydrogen phosphate KH2PO4 (ChemLab) as a phosphoric acid salt; and - boric acid H3BO3 as a setting retarder.

[0076] Among these 4 cement pastes, only the CPM-U3O8 paste complies with the invention.

[0077] As a result, it comprises, in addition to the aforementioned constituents, 73.5% by mass of depleted U3O8 which is provided in the form of a powder, which has, in the present case, the following granular distribution: di0 = 0.80 pm; d50 = 2.35 pm and d90 = 6.15 pm.

[0078] The CPM-CV paste, which is intended to obtain a cementitious material suitable for simulating in certain tests (for reasons of radioprotection) the cementitious material obtained from the CPM-U3O8 paste, is free from depleted U3O8 but comprises, as an analogue of the latter, 39.3% by mass of fly ash (silico-alumino-calcic material), with a granular distribution similar to that of the U3O8 powder used to prepare the CPM-U3O8 paste.

[0079] The CPM-REF1 and CPM-REF2 pastes, which are intended to obtain cementitious materials suitable for use as references from scientific literature, do not include depleted U3O8 or fly ash.

[0080] All these pastes are prepared according to the same operating protocol, namely that: 1. Dissolve boric acid in water, 2. KH2PO4 is dissolved in the aqueous boric acid solution, 3. MgO is added to the solution thus obtained and, in the case of CPM-U3O8 and CPM-CV pastes, U3O8 powder for the former and fly ash for the latter, and 4. Mix everything until smooth.

[0081] The CPM-U3O8 and CPM-CV pastes are stoichiometric, i.e. they have a magnesium-to-phosphate (Mg / P) molar ratio of 1 and a water-to-cement (W / C) mass ratio of 0.51 (with C = MgO + KH2PO4). The CPM-U3O8 paste has a U3O8-to-cement mass ratio of 4.2 while the CPM-CV paste has a fly ash-to-cement mass ratio of 1. However, the CPM-CV paste includes a volume of fly ash that is identical to the volume of depleted U3O8 present in the CPM-U3O8 paste.

[0082] The CPM-REF1 and CPM-REF2 pastes are super-stoichiometric and both have an Mg / P molar ratio of 6. On the other hand, the CPM-REF1 paste has a W / C mass ratio of 0.20 while the CPM-REF2 paste has a W / C mass ratio of 0.255.

[0083] The qualitative and quantitative composition of the 4 cement pastes is specified in table 1 below, per 100 g of paste.

[0084] [Tables 1] Pastes MgO (g} KHsPCh (g) UsOs (g) Vocal ashes (g) Water (g) CPM-Ü3Û8 4.0 13.5 0.5 Vl —- 9.0 CPM-CV 9.0 30.4 1.2 — 39.3 20.1 CPM-REF1 52.0 293 2.4 —- — 16.3 CPM-REF2 49.8 28.0 2.4 — 19.8

[0085] Samples of the cement pastes are poured into molds and are subjected to endogenous curing (watertight curing) at 25°C except in the case where these samples are subjected to workability and setting time tests.

[0086] The cementitious materials thus obtained are named in the following and in figures 1 to 4 and 15 to 22 in the same way as the cementitious paste from which they are derived. 1.2 - Workability and setting time:

[0087] Samples of the CPM-CV cement paste simulating the CPM-U3O8 paste and samples of the reference CPM-REF1 cement paste are subjected to spreading and setting time tests.

[0088] The spreading tests are carried out with an Abrams micro-cone while the setting tests are carried out with an automatic Vicat prisometer.

[0089] The spread of the CPM-CV paste is 30 ± 1 cm, while that of the CPM-REF1 paste is 16 ± 1 cm, i.e. twice as low.

[0090] The setting time of the CPM-CV paste is 3 hours and 45 minutes, while the setting time of the CPM-REF1 paste is around 20 minutes, i.e. 11 times shorter. The setting time of the CPM-REF1 paste is very rapid and not very suitable for practical use.

[0091] 1.3 - Compressive strength over time:

[0092] Compressive strength tests are carried out at 1 day, 7 days and 28 days of curing with samples of the cementitious materials CPM-U3O8 and CPM-CV.

[0093] For reasons of radiation protection, the samples of the CPM-U3O8 material are miniature samples in the form of cylinders 0.75 cm in diameter and 1.3 cm in height, cored in prisms of 1.3x1.3x5.2 cm3 (analogous to the standard prisms of 4x4x16 cm3).

[0094] The samples of the CPM-CV material are presented in the form of 4x4x4 cm3 cubes.

[0095] The tests are carried out under uniaxial press.

[0096] The results are presented in [Fig. 1] which illustrates the evolution of the compressive strength, noted Rc and expressed in MPa, of the materials tested as a function of time, noted t and expressed in days.

[0097] This figure shows that both CPM-U3O8 and CPM-CV materials have a compressive strength of 25 ± 2 MPa at 28 days.

[0098] 1.4 - Compressive strength at temperature:

[0099] Compressive strength tests at temperature are carried out to assess the resistance of cement pastes, without or with the addition of depleted U3O8. The case of thermal stress is a possible scenario for the use of these materials for shielding against radioactive materials which also release heat.

[0100] These tests are carried out with samples: - on the one hand, CPM-CV material simulating CPM-U3O8 material (for the radioprotection reasons previously mentioned), and - on the other hand, the reference material CPM-REF2.

[0101] These samples are standardized prisms of 4x4x16 cm3.

[0102] The treatment to which the samples are subjected consists of a temperature treatment typical for the intended application: 1. subject the samples, after 28 days of curing at 25°C, to an initial 24-hour temperature increase until reaching 107°C, 2. leave the samples at 107°C for 28 days, then 3. subject the samples to a second 24-hour temperature increase until reaching 232°C, and 4. Leave the samples at 232°C for 40 hours.

[0103] Six samples of each material are subjected to uniaxial compression at 1 day, 7 days and 28 days of treatment, then 7 days after the start of treatment at 107°C and at the end of this treatment (after complete cooling to room temperature), then finally at the end of treatment at 232°C (also after complete cooling to room temperature).

[0104] The results are presented in [Fig.2], which illustrates the evolution of the compressive strength, noted Rc and expressed in MPa, of the materials tested as a function of time, noted t and expressed in days, and temperature, noted 0 and expressed in °C, as well as in [Fig.3], which corresponds to photographs of 3 samples of each of the tested materials, taken at the end of temperature resistance tests.

[0105] [Fig.2] shows that the mechanical performances increase over time for a cure at 25°C (up to 28 days), then, after 9 days of heating at 107°C, a significant drop in compressive strength is obtained: -52% (compared to the value measured after 28 days at 25°C) for the CPM-CV material and -66% (compared to the value measured after 28 days at 25°C) for the CPM-REF2 material. This drop in compressive strength is attributed to the decomposition of the cement binder phase (i.e. k-struvite). The mechanical performances remain stable for the following 21 days at 107°C, but fall even further after heating for 2 days at 232°C. Whatever the material, the compressive strength at the end of the treatment at 232°C is of the order of 8 ± 1 MPa.

[0106] In addition, as shown in [Fig.3], the samples of the CPM-CV material do not exhibit cracking, unlike the CPM-REF2 material, without fly ash.

[0107] It can therefore be concluded that the addition of fines to a paste of a phospho-magnesium cement improves the resistance to cracking of the cementitious material obtained from this paste, when heated up to 232°C. This has been observed for fly ash, and is also assumed to be true for materials made with the analogous powder of U3O8. 1.5 - Dimensional stability:

[0108] Dimensional stability tests are carried out with samples: - on the one hand, CPM-CV material simulating CPM-U3O8 paste (for the radioprotection reasons previously mentioned), and - on the other hand, the reference material CPM-REF1.

[0109] The samples are standardized prisms of 4x4x16 cm3 and are equipped with measuring plots at each end. The tests are carried out according to the standards NF P15-433 (Cement testing methods - Determination of shrinkage and swelling) and NF P18-427 (Concrete - Determination of dimensional variations between two opposite faces of hardened concrete specimens) for 46 days of endogenous curing (in a sealed bag) at 21°C.

[0110] The results are presented in Figure 4 which illustrates the evolution of the relative variation in length of the samples al _ (where Lt is the length at time t and Lo the length at time t=0), noted e and expressed in pm / m. Time t is expressed in days, e is an expansion (when it is positive) or a shrinkage (when it is negative).

[0111] This figure shows that the highest increase in e is observed between 1 and 7 days, up to more than 800 pm / m for the CPM-CV formulation. After 7 days, the Both formulations stabilize dimensionally and have similar expansions. This expansion at early age (up to 7 days) comes from a reaction pathway that is different for the CPM-CV material compared to the CPM-REF1 material. For the CPM-REF1 paste, the k-struvite formation reaction is rapid, such that only the presence of k-struvite and MgO is detectable (by TGA and XRD). For the CPM-CV paste, the k-struvite formation reaction is preceded by the formation of intermediate hydrates, still present at 28 days of maturation. These phase assemblies, although different, have dimensional variations of the same order after 7 days.

[0112] The value of 1200 pm / m for the CPM-CV formulation, measured at 45 days, remains comparable to that of phospho-magnesium cement pastes in the literature, matured under different conditions.

[0113] Example II: Mechanical properties, dimensional stability, workability and durability of cementitious materials of the invention with Portland cement II. 1 - Preparation of mortars:

[0114] Three different Portland cement-based mortars are prepared - hereinafter referred to as CP-12.5%U3O8, CP-3.3%CV and CP-REF respectively - using: - a CEM I 52.5 R cement (Lafarge); - a high water-reducing superplasticizer (MasterGlenium™ 27 - BASF); and - 96% silica sand with a particle size distribution that follows the NF EN 196-1 standard.

[0115] This distribution is specified in Table 2 below, for 100 g of sand.

[0116] [T ables 2] Grain heap < 160 gm .160-315 qm 315-630 pm 0.630-1.25 mm 1.25-2.5 mm Mass proportion 13.8% 14.2% 16.7% 35.8% 19.5%

[0117] Among these 3 mortars, only the CP-12.5%U3O8 mortar complies with the invention.

[0118] As a result, it comprises, in addition to the aforementioned constituents, 12.5% ​​by mass of depleted U3O8 which, here too, is provided in the form of a powder, which is identical to that used in point 1.1 above. It has a U3O8 -cement mass ratio of 63% and a W / C mass ratio of 0.41.

[0119] The CP-3.3%CV mortar, which is intended to obtain a cementitious material suitable for simulating in certain tests the cementitious material obtained from the CP-12.5%-U3O8 mortar, is free from depleted U3O8 but includes, as an analogue of this last, 3.3% by mass of fly ash identical to that used in point 1.1 above. It has a fly ash-to-cement mass ratio of 15% and a W / C mass ratio of 0.41. However, it includes a volume of fly ash which is identical to the volume of depleted U3O8 present in the CP-12.5%U3O8 mortar.

[0120] The CP-REF mortar, which is intended to serve as a reference from the literature, does not contain U3O8 or fly ash. It has a W / C mass ratio of 0.40.

[0121] These 3 mortars are prepared according to the operating protocol described in standard NF EN 196-1, namely that: 1. the superplasticizer is mixed with water until homogenized, 2. this mixture is added to the cement and the whole is mixed at slow speed for 30 seconds, 3. the sand is added to the dough for the next 30 seconds, still at slow speed, then for 30 seconds at fast speed. 4. mixing is temporarily stopped for 1 minute 30 seconds, then resumed for 60 additional seconds at high speed.

[0122] Their qualitative and quantitative composition is specified in table 3 below, per 100 g of mortar.

[0123] [Tables3] Mortars Cement (g) (g) Superficializer (g) UsOa (g) Fly ash ig' Water {g) CP-12.5%U3Ôa 1.9.7 594 O 12.5 — 84 CP-33%CV 21, S 65.2 0.7 — 3.3 9.0 CP-REF 22.5 67.5 0.7 — —

[0124] Four other mortars are also prepared - hereinafter referred to as CP-24.5%U3O8, CP-42.5%U3O8, CP-7.2%CV and CP-15%CV - following an operating protocol identical to that described above but replacing in the formulations indicated above for the mortars CP-12.5%U3O8 and CP-3.3%CV: the grains of standardized sand which have a size less than 160 qm by an identical volume of depleted U3O8 in the case of CP-24.5%U3O8 mortar and fly ash in the case of CP-7.2%CV mortar, and the grains of standardized sand which have a size less than 315 qm by an identical volume of depleted U3O8 in the case of mortar CP-42.5%U3O8 and fly ash in the case of mortar CP-15%CV.

[0125] The qualitative and quantitative composition of these 4 mortars is specified in table 4 below, per 100 g of mortar.

[0126] [Tables4] Mortars Cement (g) Sand (g) Superstifier (g) U3Os {g} Fly ash Water (g[ CP-24„5%UîO8 18.7 48.5 0.6 24.5 —- 7.7 CP-42.5%U30s 15.9 34.5 0.5 42.5 6.5 CP-7.2%CV 23.0 a-? un 0.7 —- 7.2 9.4 CP-15%CV 23.5 51.1 0.7 — 15.0 9.7

[0127] Samples of the 7 mortars thus prepared are poured into molds and undergo endogenous curing (watertight curing) at 25°C, except in the case where these samples are subjected to workability and setting time tests (we then speak of mortars in the so-called fresh state, for a period of the order of 30 minutes after manufacture for the workability test and 24 hours after manufacture for the setting time test).

[0128] The cementitious materials thus obtained are named in the following and in Figures 5 to 22 in the same way as the mortar from which they are derived. II.2 - Workability and setting time:

[0129] Samples of the CP-3.3%CV mortar simulating the CP-12.5%U3O8 mortar as well as samples of the CP-REF reference mortar are subjected to spreading and slump tests.

[0130] These tests are carried out using the Abrams micro-cone.

[0131] Both mortars have similar spread and slump values.

[0132] Indeed, the spread of the CP-3.3%CV mortar is 13.5 cm and its slump is 7.5 cm while the spread of the CP-REF mortar is 14 cm and its slump is 8 cm.

[0133] Concerning the setting time, this was not determined by tests with the automatic Vicat prisometer but, on the basis of observations made during the first hours after mixing, in conjunction with data from the literature. It appeared that this time is at least equal to 40 min.

[0134] n.3 - Compressive strength over time:

[0135] Two series of compressive strength tests are carried out under uniaxial press.

[0136] The first series of tests is carried out at 14 days and 28 days of treatment with samples cementitious materials CP-12.5%U3O8 and CP-3.3%CV.

[0137] The 2nd series of tests is carried out at 1 day, 4 days, 14 days and 28 days of curing with samples of the cementitious materials CP-24.5%U3O8, CP-42.5%U3O8, CP-7.2%CV and CP-15%CV.

[0138] For reasons of radiation protection, all these tests are carried out with miniature samples in the form of cylinders 0.75 cm in diameter and 1.3 cm in height, cored in prisms of 1.3x1.3x5.2 cm3 (analogous to the standard prisms of 4x4x16 cm3).

[0139] The compressive strengths obtained at 28 days in the first series of tests are of the order of 60 MPa for the CP-12.5%U3O8 material and of the order of 70 MPa for the CP-3.3%CV material. Measurements on standardized prisms (4x4x16 cm3) gave similar values ​​(without significant difference) for the CP-3.3%CV formulation.

[0140] The compressive strengths obtained in the 2nd series of tests are presented in [Fig.5] which illustrates the evolution of the compressive strength, noted Rc and expressed in MPa, of the materials CP-24.5%U3O8, CP-42.5%U3O8, CP-7.2%CV and CP-15%CV as a function of time, noted t and expressed in days.

[0141] This figure shows an increase in the compressive strength of all materials as a function of time. It also shows that it is possible to incorporate up to 42.5% by mass (based on 100 g of mortar) of depleted U3O8 into a mortar while obtaining a cementitious material which has an average compressive strength of 63 + / - 11 MPa at 28 days of curing.

[0142] n.4 - Compressive strength at temperature:

[0143] Two series of compressive strength tests are carried out after temperature curing.

[0144] The first series of tests is carried out with samples of the cementitious materials CP-3.3%CV, CP-15%CV and CP-REF, the samples being: - on the one hand, miniature samples in the form of cylinders 0.75 cm in diameter and 1.3 cm in height, cored in prisms of 1.3x1.3x5.2 cm3, and - on the other hand, samples in the form of 4x4x4cm3 cubes previously obtained from 4x4x16 cm3 prisms.

[0145] The 2nd series of tests is carried out with samples of the cementitious materials CP-12.5%U3O8, CP-24.5%U3O8, CP-42.5%U3O8, CP-3.3%CV, CP-15%CV and CP-REF, the samples all being miniature samples (cylinders of 0.75 cm in diameter and 1.3 cm in height cored in prisms of 1.3x1.3x5.2 cm3).

[0146] All samples are subjected, after 28 days of curing at 25°C, to a heat treatment identical to that described in point 1.4 above.

[0147] The samples of the first series of tests are subjected to uniaxial compression at 28 days of treatment, at the end of the 28-day treatment at 107°C (after complete cooling to room temperature) as well as at the end of the 40-hour treatment at 232°C (also after complete cooling to room temperature).

[0148] The samples of the 2nd series of tests are subjected to uniaxial compression at 1 day, 4 days, 14 days and 28 days of curing at 25°C, then at the end of the 28-day treatment at 107°C (after complete cooling to room temperature) as well as at the end of the 40-hour treatment at 232°C (also after complete cooling to room temperature).

[0149] The results of these tests are presented in [Fig.6] for the 1st series and in [Fig.7] for the 2nd series, the compressive strengths, noted Rc and expressed in MPa, being indicated in these two figures as a function of time, noted t and expressed in days, and temperature, noted 0 and expressed in °C.

[0150] [Fig.6] shows that all samples have an average compressive strength of at least 51 MPa after 28 days at 25°C. The standard-size samples see their compressive strength increase further during the next heating, for 28 days at 107°C, to an average of 70 + 6 MPa; after 40 hours at 232°C, the strength drops little, and remains at 67 + 5 MPa. The miniature samples see their strength decrease after 28 days at 107°C, to an average of 44 + 6 MPa; this value drops little, with an average of 41 ± 9 MPa after heating for 40 hours at 232°C.

[0151] The mechanical performances obtained for the samples with the largest dimensions are higher than those obtained for the miniature samples. This is linked to the difference in volume existing between the two types of samples which is a factor of 29, and which greatly influences the physicochemical phenomena of thermal degradation. In the case of samples with larger dimensions, drying due to heating improves the mechanical performances of the materials whereas, for miniature samples, the materials degrade more easily due to their small volume.

[0152] [Fig.7] shows that during the first 28 days at 25°C, the U3 O8 mortars develop their performance a little more slowly than those with fly ash; at 28 days, they have similar performances, as well as after heat treatment at 107°C then 232°C. A decrease in the compressive strength of the materials is observed with increasing temperature, with a compressive strength reaching respectively 33 + 5 MPa for the CP-42.5%U3O8 material and 45 + 5 MPa for the CP-24.5%U3O8 material at the end of the treatment at 107°C then 232°C. For the latter material, if we extrapolate, from the data in [Fig.7], the compressive strength obtained to larger samples, then we obtain a compressive strength greater than 60 MPa (typical value compressive strength of so-called high-performance concrete) at the end of treatment at 232°C. II.5 - Dimensional stability:

[0153] Dimensional stability tests are carried out with samples: - on the one hand, CP-7.2%CV and CP-15%CV materials simulating respectively the CP-24.5%U3O8 and CP-42.5%U3O8 materials, and - on the other hand, the CP-REF reference material.

[0154] The samples are standardized prisms of 4x4x16 cm3 and are equipped with measuring plots at each end. The tests are carried out according to standards NF P15-433 (Cement testing methods - Determination of shrinkage and swelling) and NF P18-427 (Concrete - Determination of dimensional variations between two opposite faces of hardened concrete specimens) for 7 days of endogenous curing (in a sealed bag) at 21°C.

[0155] The results are presented in Figure 8 which illustrates the evolution of the relative variation in length of the samples al _ LL) (where Lt is the length at time t and Lo is the length at time t=0), noted e and expressed in pm / m. Time t is expressed in days, e is an expansion (when it is positive) or a shrinkage (when it is negative).

[0156] This figure shows that the three materials tested exhibit shrinkage, which is lower for the CP-7.2%CV and CP-15%CV materials than that observed for the CP-REF material. It is linked to the hydration of Portland cement (Le Châtelier contraction). II.6 - Sustainability:

[0157] The durability of the materials CP-12.5%U3O8, CP-24.5%U3O8 and CP-42.5%U3O8 is assessed by leaching tests.

[0158] To do this, samples of these materials in the form of cubes of 1.3x1.3x1.3 cm3 are immersed, after 28 days of curing at 25°C, in 30 mL of one of the following 2 leaching solutions: - demineralized water, noted ED, with a pH of around 6, or - a saturated solution of calcium hydroxide (Ca(OH)2), called lime water and noted EC, with a pH of around 12.5.

[0159] At different intervals of 3, 9, 15 and 30 days of immersion, the leachates are analyzed by plasma torch atomic emission spectrometry (ICP-AES) to measure their uranium content and the leaching solutions are renewed after each measurement interval.

[0160] For comparison, identical tests are carried out with 1 g samples of the depleted U3O8 powder identical to that used to prepare the mortars of which materials CP-12.5%U3O8, CP-24.5%U3O8 and CP-42.5%U3O8 are derived.

[0161] The results of these tests are presented in [Fig.9] in which the ordinate axis corresponds to the quantity of leached uranium, noted QG and expressed in g per kg of cementitious material, while the abscissa axis corresponds to the time, expressed in days, of immersion of the samples.

[0162] This figure shows that, in the case of U3O8 powder, the leaching of uranium in demineralized water is significantly higher than the leaching in lime water (100 g / kg versus 3 g / kg at 30 days of immersion).

[0163] It also shows that, in the case of cementitious materials incorporating U3 O8, the leaching of this uranium by lime water is much lower than that observed for U3O8 powder, regardless of the material.

[0164] It should be noted that, concerning the leaching of cementitious materials by demineralized water, only the result obtained after 30 days of immersion for the CP-42.5%U3O8 material is indicated in [Fig.9], the quantity of uranium leached by the demineralized water being, in fact, lower than the detection limit of the ICP-AES device for the other materials tested. Similarly, only the results from 15 days in lime water could be measured (they are lower than the detection limit below 15 days).

[0165] Example III: Ionizing radiation shielding properties of cementitious materials of the invention

[0166] The shielding properties of the materials CPM-U3O8, CP-12.5%U3O8, CP-24.5%U3O 8 and CP-42.5%U3O8 are evaluated as a function of the incident energy Ej of the photon emitted during ionizing radiation.

[0167] In this regard, it is recalled that the incident energy of an ionizing beam of photons (X-rays) is attenuated through a given material according to the following exponential attenuation law:

[0168] [Math.l]

[0169] in which: Ej is the incident energy of the ionizing beam, is the linear attenuation coefficient of the material (in cm1), while x is the thickness of the material through which the ionizing beam passes.

[0170] From this equation, it is possible to calculate: (1) the half-attenuation layer, noted HVL (from Half Value Layery and expressed in cm, corresponding to the thickness of material to be crossed so that the incident energy of the ionizing beam is divided by two, i.e. the value of x such that E(x) / Ei = 0.5, and (2) the tenth attenuation layer, noted TVL (from Tenth Value Laver), and expressed in cm, corresponding to the thickness of material to be crossed so that 90% of the incident energy of the ionizing beam is attenuated, i.e. the value of x such that E(x) / E i = 0.1.

[0171] These two quantities are thus calculated for each photon energy by applying the following equations (deduced from E(x) / Ei = 0.5 and E(x) / Ei = 0.1):

[0172] [Math.2] HVL - — = (2) UX p

[0173] [Math.3]

[0174] in which: pp is the mass absorption coefficient (in cm2 / g) of the material, which depends on the atom considered and the energy of the incident photon, E, p is the density (in g / cm3) of the material, while p is as defined above.

[0175] The mass absorption coefficients pp of the cementitious materials of the invention are determined as a function of the energy of the incident photon, E, using the XCOM software (XCOM: Photon Cross Sections Database, https: / / physics.nist.gov / PhysRefData / Xcom / html / xcoml.html) on the basis of the linear absorption coefficients of the elements present in the cementitious matrix (mainly Ca, Si, Al and Fe for Portland cement and Mg, K, P for the phospho-magnesian matrix, as well as H and O in both cases) and the mass composition of the materials.

[0176] The linear mass absorption coefficients p are then calculated from these values ​​taking into account the density of the materials and the relation pp= p / p.

[0177] Since cementitious materials are porous, part of their composition is made up of voids assumed to be filled with air, which do not contribute to the attenuation of ionizing rays. The apparent density (with voids) was measured in the present study as the ratio between the mass of the sample (obtained by weighing) and the volume (obtained by geometric measurement).

[0178] Alternatively, the apparent density of each material was deduced from the absolute density with different hypotheses on the porosity of the material from literature data (B. Xu et al., Cem. Concr. Res. 2019, 123, 105781, hereinafter reference [5]; H. Lahalle et al., Cem. Concr. Res. 2019, 122, 42-58, hereinafter reference [6], and T. Tracz and T. Zdeb, Materials 2019, 12(192), 1-20, hereinafter reference [7]) and the theoretical densities of the constituents of the cementitious materials as provided by the suppliers.

[0179] These theoretical densities are presented in Table 5 below.

[0180] [Tables5] Constituents of dementasres materials Theoretical densities (g / cm3J) MgO KH2PO4 234 1.44 H 2.0 1.0 Voting ash 2.0 UsO-s 8.38 Portland cement 3.14 Sandblasted 2.6 Super plasticizer 1.01

[0181] For a phospho-magnesium cement material, the porosity is between 14 ± 1% and 20 ± 1% (see reference [6]) and between 9+1% and 13 + 1% (see reference [5]) depending on its maturation and formulation.

[0182] For a Portland cement material with a W / C ratio of 0.4, the porosity at 90 days is typically 21 + 1%. It decreases over time to reach a value of 18 + 1% after two years (see reference [7]).

[0183] The porosity assumptions retained are therefore 10% (minimum value) and 20% (maximum value).

[0184] Table 6 below shows, for each of the materials CPM-U3O8, CP-12.5%U3O8, CP-24.5%U3O8 and CP-42.5%U3O8, the absolute densities respectively calculated without taking into account, the apparent densities calculated for 10% porosity and for 20% porosity, as well as the apparent densities measured.

[0185] For comparison, this table also shows the absolute and apparent densities, calculated and measured for the CPM-CV and CP-REF materials.

[0186] [Tableauxô] Cementitious materials Apparent densities (g / cm3) Absolute densities calculated without taking porosity into account [g / cm3] Measured Calculated for 10% porosity Calculated for 20% porosity CPM-UjOs 3.691 3.383 4.060 CP-12;5%U3OS 2.451 2.318 2.125 2.550 CP-24.5%üàOs 2.706 2.536 2.325 2.790 CP-42.5%UsOs 3.073 2.982 2.730 3.280 CPM-CV 1.372 1.547 1.418 1.702 CP-REF 2.324 2.113 1,937 2,357

[0187] The HVL and TVL thicknesses are calculated as a function of the energy of the incident photon, E, for each of the materials CPM-U3O8, CP-12.5%U3O8, CP-24.5%U3O8, CP-42.5%U3O8, CPM-CV and CP-REF and this, for each of the densities, absolute and apparent, which are indicated in table 6.

[0188] The results are illustrated in Figures 10 to 13 for HVL and in Figures 14 to 17 for TVL, where HVL and TVL are expressed in cm and the energy of the incident photon, E, is expressed in keV.

[0189] Figures 10 and 14 correspond to the HVL and TVL respectively obtained for the absolute densities without taking into account porosity.

[0190] Figures 11 and 15 correspond to the HVL and TVL respectively obtained for the apparent densities calculated for 10% of the porosity.

[0191] Figures 12 and 16 correspond to the HVL and TVL respectively obtained for the apparent densities calculated for 20% of the porosity.

[0192] Figures 13 and 17 correspond to the HVL and TVL respectively obtained for the measured apparent densities.

[0193] Figures 10 to 13 show that, in all cases, at a given photon energy and between 80 keV and 1000 keV, the presence of depleted U3O8 in the cementitious materials makes it possible to significantly reduce their HVL compared to those obtained for cementitious materials free of depleted U3O8. This reduction is 63% in the case of the material based on phospho-magnesium cement (i.e. CPM-U3 O8) and 33% in the case of the cementitious material based on Portland cement. comprising the highest mass content of depleted U3O8 (i.e. CP-42.5%U3O8).

[0194] They also show that, as expected, the reduction in the HVL of a cementitious material comprising depleted U3O8 is all the higher the higher its mass content of U3O8 is itself.

[0195] Figures 14 to 17 show that the same trends are observed for TVLs. References cited

[0196] [1] JP-S-6191598 [2] US-B-5,786,611 [3] US-B-6,166,390 [4] LR Dole and WJ Quapp, report ORNL / TM-2002 / 111, 2002 [5] B. Xu et al., Cem. Concrete Res. 2019, 123, 105781 [6] H. Lahalle et al., Cem. Concrete Res. 2019, 122, 42-58 [7] T. Tracz and T. Zdeb, Materials 2019, 12(192), 1-20

Claims

Claims

1. A method for preparing a cementitious material for shielding against ionizing radiation, which comprises at least the steps of: a) preparing a cementitious paste comprising at least one cement and water, then b) hardening the cementitious paste obtained in step a), characterized in that the cement is a phospho-magnesium cement and in that the cementitious paste further comprises a powder consisting of triuranium octoxide depleted in uranium-235 of which 50% by volume of the grains have a size at most equal to 5 qm and of which 90% by volume of the grains have a size at most equal to 10 qm.

2. The method of claim 1, wherein the phospho-magnesium cement comprises calcined magnesium oxide and a phosphoric acid salt.

3. A method according to claim 2, wherein the phosphoric acid salt is sodium phosphate, sodium monohydrogen phosphate, sodium dihydrogen phosphate, potassium phosphate, potassium monohydrogen phosphate, potassium dihydrogen phosphate, aluminum phosphate, aluminum monohydrogen phosphate, ammonium phosphate, ammonium monohydrogen phosphate or ammonium dihydrogen phosphate, preferably potassium dihydrogen phosphate.

4. A method according to claim 2 or claim 3, wherein the phospho-magnesium cement has a magnesium / phosphorus molar ratio of between 0.1 and 12, preferably between 0.8 and 3 and, more preferably, equal to 1 ± 0.

1.

5. A method according to any one of claims 1 to 4, wherein the depleted U3O8 powder is a powder of which 50% by volume of the grains have a size at most equal to 3 qm, and of which 90% by volume of the grains have a size at most equal to 7 qm.

6. A method according to claim 2, wherein the depleted U3O8 powder represents from 1% to 80%, preferably from 35% to 75%, by mass of the total mass of the cement paste.

7. A method according to any one of claims 1 to 6, wherein the cementitious paste further comprises at least one adjuvant chosen from superplasticizers and setting retarders.

8. A method according to any one of claims 1 to 7, wherein the cement paste further comprises at least one selected from sand, gravel and fibers.

9. A method according to any one of claims 1 to 8, wherein the cement paste comprises a water / cement mass ratio ranging from 0.10 to 1, preferably from 0.20 to 0.

60.

10. A method according to any one of claims 1 to 9, wherein the cement paste further comprises radioactive waste.

11. A method of immobilizing radioactive waste by cementing, characterized in that it comprises the preparation of a cementitious material for shielding against ionizing radiation by a method according to any one of claims 1 to 9.

12. Method for manufacturing a container for packaging radioactive waste or a packaging for transporting and / or storing radioactive materials, characterized in that it comprises the preparation of a cementitious material for shielding against ionizing radiation by a method according to any one of claims 1 to 9.

13. Method for manufacturing a structural element of a radioactive waste storage site or of a shielded or radioprotected room, characterized in that it comprises the preparation of a cementitious material for shielding against ionizing radiation by a method according to any one of claims 1 to 9.

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