A NEUTRON ABSORPTION MEANS, A METHOD OF PRODUCTION THEREOF AND A USE THEREOF

Abstract: A method of producing a neutron absorption means for a control rod in a nuclear plant, the neutron absorption means comprising a compound of at least one neutron absorbing agent and an additional component, characterized in that a powder, comprising said compound, and a powder, comprising the neutron absorbing agent in an essentially pure state, are mixed and subjected to a pressing at an elevated temperature, in such a way that a solid body is formed, the content of the neutron absorbing agent in relation to the additional component being higher in the solid body than in the original compound. Furthermore, the invention relates to a neutron absorbing means for a control rod in a nuclear plant, the means comprising a compound with the composition BxCy, where x/y is greater than 4. The invention relates also to the use of such a neutron absorbing means for a control rod in a nuclear plant.

Description: A NEUTRON ABSORPTION MEANS, A METHOD OF PRODUCTION THEREOF AND A USE THEREOF TECHNICAL FI ELD OF THE INVENTION AND PRIOR ART The present invention relates to a method of producing a neutron absorption means for a control rod in a nuclear plant, the neutron absorption means comprising a compound of at least one neutron absorbing agent and an additional component. The invention also relates to a neutron absorption means for a control rod in a nuclear plant, the absorption means comprising a compound with the composition B x C y . Furthermore, the invention relates to the use of a compound with the composition B x C y in a neutron absorption means for a control rod in a nuclear plant. Control rods by means of which neutron absorption may be accomplished and control of the process thereby may be procured in a nuclear plant are previously well known in reactors of the type BWR, PWR and AGR as well as fast Breeder reactors. For example, by a BWR reactor such a control rod usually consists of a number of, usually four, control rod blades which are joined together to form the rod itself, which forms an elongated element provided to be pushed in between cladding tubes for fuel rod bundles arranged in the nuclear plant as required, with the purpose to control the speed by which the fission process occurs in the fuel rods located in the cladding tubes. By for example BWR reactors, the control rods are often of a considerable length, usually around 4 m, and they are also relatively heavy. Furthermore, they comprise closeable cavities in which one or more types of neutron absorption means are positioned. Said control rod blades extend radially outwards from the central axis of the control rod and along essentially the whole length thereof. Thereby, the cavities often comprise cylindrical holes extending inside the control rod blades in an axial or radial direction and are present in a great number and separated only by short distances. In some types of reactors the absorber material is located in vertical tubes or channels. A neutron absorption means frequently used in the control rods is boron carbide, and more precisely boron carbide with the composition B 4 C, which has a very good mechanical resistance at the same time as the neutron absorption is good thanks to the presence of boron, which is the neutron absorbing agent in this absorption means. Boron carbide is obtained through reduction of B 2 0 3 with carbon at about 2500°C. Furthermore, the natural boron present in the compound comprises the two isotopes 10 B and 1 1 B, 10 B comprising about 20% by weight of the amount of boron and 11 B comprising about 80% by weight of the amount of boron. Above all it is 10 B of these that presents a neutron absorbing capacity according to the fission 10 B + n → 7 Li + He In order that the boron carbide shall have a neutron absorbing capacity as good as possible, it is therefore desirable that its content of 10 B is as great as possible. However, prior art does not solve the problem of increasing the amount of 10 B in boron carbides to a satisfying degree. The present techniques for increasing 10 B in boron through isotope concentration are complicated and require too much effort to be economically justifiable. Therefore, B 4 C with the naturally present distribution of 10 B and 1 1 B of boron therein is usually used as an absorption means. Another desire in connection with the construction of the control rods is that these ought to be as light as possible, in order to make welding seams or joints thereon between the control rod blades and other possible additional components withstand the forces caused by the control rod through its own weight when it is subjected to the influence of exterior forces, for example in connection with earthquakes. Therefore, in order to reduce the total weight of the control rod, it is important to provide the absorption means in such a way that it has a weight as low as possible while retaining the total absorption capacity of the control rod. Therefore, the combination of a high absorption capacity and low weight is of importance for the absorption means. Furthermore, in connection with irradiation, the absorption means of boron carbide is exposed to local expansion forces due to the formation of helium, as described above. By B 4 C these local expansion forces give rise to swelling and undesired crack formation and possibly decomposition of the boron carbide as the subjecting thereof to irradiation goes on. Thereby, the cracking tendency of the boron carbide is closely related to its fracture toughness, and an increase of the latter is a desired object for absorption means of boron carbide. Furthermore, B C generally comprises a surplus of C falling out as platelets of graphite, contributing to the fragility of B C. From the above discussion it is thus evident that a higher fracture toughness and a lower density are desired for absorption means in the shape of boron carbide, but that improvements related to these properties ought to be effected with retained or improved neutron absorption capacity of the absorption means. Today, absorption means comprising B C are present as rods of sintered powder of B 4 C or as B 4 C in the shape of powder or particles obtained by crushing or other treatment of larger pieces of B 4 C produced in a conventional way. Irrespectively of which of these types of boron carbide being used as absorption means in control rods, the above discussion with reference to fracture toughness and low density is still valid , and these improvements of properties are desired. SUMMARY OF THE I NVENTION A main object of the present invention is to provide a method by which the above mentioned problems related to the absorption capacity, weight and fracture toughness are solved in a satisfying way for a neutron absorption means for a control rod in a nuclear plant. Therefore, the invention comprises a method as initially defined , characterized in that a powder, comprising said compound, and a powder, comprising the neutron absorption agent in an essentially pure state, are mixed and subjected to a pressing at an elevated temperature, in such a way that a solid body is formed, the content of the neutron absorbing agent in relation to the additional component being higher in the solid body than in the original compound. Thereby, one obtained purpose is to provide an increased absorption capacity per unit of weight of the absorption means by increasing the quantity of soluted neutron absorbing agent. In the case that an increased amount of soluted neutron absorbing agent also leads to a lower density, double advantages are obtained, namely both higher absorption ability and lower weight by a certain volume of the absorption means. If, however, an increased solute amount would result in a higher density, it is advantageous if the obtained increase of absorption ability is relatively larger than the increase of the density. As the production method comprises pressing at elevated temperature, a body of the neutron absorption means with nearly 100% of the theoretical density is obtained. The nearly complete absence of pores thereby results in a very good fracture toughness of the formed body, as the fracture toughness is closely associated with the content of pores which may act as crack initiating locations in the material in which they exist. According to a preferred embodiment of the method according to the invention, it is characterized in that the compound is B x C y and the relation between the number of boron atoms and carbon atoms (x/y) is greater than 4 in the solid body formed. The method has turned out to be particularly advantageous for boron carbide, among other things for the reason that boron, which is the neutron absorbing agent, has a lower density than carbon , which forms the additional component, and that, consequently, the weight of one mole B x C y decreases as the number of boron atoms increases at the expense of the number of carbon atoms. Furthermore, an additional advantage is obtained as the absorption means obtains a greater neutron absorbing ability thanks to the increased amount of soluted boron. I n addition thereto, the produced body obtains a density of nearly 100% of the theoretical density, thanks to the use of pressing at an elevated temperature, thus supporting the fracture toughness. As an additional advantage of the method according to the invention, when applied to boron carbides, it shall be noted that, at tests, the fracture toughness of the formed bodies of B x C y has proven itself to increase as the x/y-ratio therein increases above 4. The method is advantageous as it makes it possible to increase the x/y-ratio with retained or better fracture toughness. According to another preferred embodiment of the method according to the invention 4 < x/y < 12. Above the ratio of 12 it is not longer possible to homogeneously solve the boron atoms at the expense of the carbon atoms in the boron carbide and the structures arising above this limit are considered to be disadvantageous when using the boron carbide as an absorption means for control rods. According to another preferred embodiment of the method according to the invention, this is characterized in that 6 < x/y < 12. Except that the density of the absorption means decreases with the increased solution of boron in the boron carbide, it has been shown at tests that the fracture toughness of the boron carbide increases to a considerable degree in this interval along with an increasing quantity of soluted boron. According to a particularly preferred embodiment of the method according to the invention 8,5 < x/y < 12. Hereby, the high quantity of soluted boron at the expense of the carbon atoms is supposed to further increase the advantages resulting therefrom and mentioned earlier as to absorption ability, weight and fracture toughness. According to another preferred embodiment, the pressing at an elevated temperature is hot isostatic pressing (HIP). However, other types of pressure sintering are not excluded as an alternative in such cases where they may be possible. However, HIP has turned out to be particularly suitable in the case of boron carbide. Another object of the invention is to provide a neutron absorption means of the type initially mentioned, which has the advantage of having a higher neutron absorption ability than conventional B 4 C, the density simultaneously being lower and the fracture toughness being higher than for said conventional boron carbide. This object is achieved by means of a neutron absorption means as initially defined, which is characterized in that x/y > 4. By providing a neutron absorption means in the shape of boron carbide having an increased amount of solute boron at the expense of the carbon atoms in relation to the conventional case of B C, the invention offers an alternative to increasing the boron carbide absorbing ability by increasing 10 B in relation to 11 B in the boron present, which, as mentioned above, is a technically complicated solution to the problem of increasing the absorption ability while retaining or lowering the density of the compound. Furthermore, by increasing the x/y-ratio, a higher fracture toughness of the neutron absorption means is obtained. According to a preferred embodiment of the neutron absorbing means according to the invention, it comprises a solid body. Thereby, certain advantages related to the handling of the absorption means are obtained, but, above all, there is a possibility to form the body to the geometries that are considered to be advantageous in different cases. In an alternative embodiment of the neutron absorption means according to the invention , it is at least partly present as particles. This promotes, for example, the ability of the absorption means to maximally fill the space in which it is to be stored in the control rod. Preferably, 4 > x/y < 12, more preferably 6 ≤ x/y < 12 and most preferably 8,5 < x/y < 12. The advantages thereof have already been discussed in connection to the method of the invention and will therefore not be further discussed here. Furthermore, the invention relates to the use of a compound formed by B x C y in a neutron absorption means for a control rod in a nuclear plant, wherein x/y > 4. The advantages thereby obtained have already been discussed in connection to the description of the method and the neutron absorption means according to the invention and, therefore, they will not be further discussed here. By reasons mentioned above, it is preferred, by said use, that 4< x/y < 12, more preferably that 6 < x/y ≤ 12 and most preferably that 8,5 < x/y < 12. Further advantages and characteristics of the invention will be evident from the following, detailed description of a preferred embodiment of a method and the remaining dependant claims. BRI EF DESCRIPTION OF THE DRAWI NGS In the attached drawings: Fig 1 is a diagram where mole percent boron and weight % boron respectively in B x C y are against the x/y-ratio, Fig 2 shows material density and quantity of boron in boron carbide as a function of the x/y-ratio, and Fig 3 is a diagram where the fracture toughness of boron carbide is set against the x/y-ratio. DETAI LED DESCRI PTION OF AN EMBODIMENT OF THE METHOD OF THE I NVENTION The method is performed in the following way: to a powder consisting of B 4 C amorphous boron in the shape of a powder is added, these powders subsequently being mixed until a homogenisation thereof is obtained. Thereafter, the mixed powder is form pressed in such a way that a pressed body is obtained, which after that is provided with a covering of glass or metal which is evacuated or closed in vacuum when it is placed in an arrangement for hot isostatic pressing and is subjected for such a pressing and with the object of providing a boron carbide body with an elevated content of boron in relation to carbon. Normally a suitable binder is supplied to the mixture before it is form pressed , in order to obtain a desired mechanical strength of the pressed body. By the quantity of amorphous boron added to the powdered compound, which comprises B 4 C, it is possible to control the x/y-ratio of the finally formed body. Preferably, the quantity of added amorphous boron is chosen in such a way that the final x/y-ratio of the formed body of B x C y is found in the interval 4-12. As the x/y-ratio increases, a density change is obtained in the final body as well as changes of the fracture toughness thereof. These changes are evident in figure 2 and 3. In figure 3 two different values of the fracture toughness are shown for the x/y- ratio of 5. Finally it shall be pointed out that a number of variants and modifications of the preferred embodiment of the method will be evident for a person skilled in the art without deviating from the idea of the invention. Thereby, it shall be understood that the invention is restricted only to what is defined in the attached claims and in the above description of the invention. For example the technique according to the invention may also be applied during the use of boron with an elevated concentration of 10 B, either in the B x C y compound or in the pure boron powder. It shall also be understood that x and y in B x C y only have as their purpose to show a relation between the quantity of B and quantity of C, so that y may constantly be given the value of 1 in these cases.

Claims: Claims 1 . A method of producing a neutron absorption means for a control rod in a nuclear plant, the neutron absorption means comprising a compound of at least one neutron absorbing agent and an additional component, characterized in that a powder, comprising said compound, and a powder, comprising the neutron absorbing agent in an essentially pure state, are mixed and subjected to a pressing at an elevated temperature, in such a way that a solid body is formed, the content of the neutron absorbing agent in relation to the additional component being higher in the solid body than in the original compound. 2. A method according to claim 1 , characterized in that the compound is B x C y and the relation between the number of boron atoms and the carbon atoms is greater than 4 in the formed solid body. 3. A method according to claim 2, characterized in that 4< x/y < 12. 4. A method according to claim 2, characterized in that 6< x/y < 12. 5. A method according to claim 2, characterized in that 8,5< x/y < 12. 6. A method according to anyone of the preceding claims, characterized in that the powder comprising the compound is B 4 C and the additional powder is essentially pure boron. 7. A method according to anyone of the preceding claims, characterized in that the powder is subjected to form pressing before the pressing at the elevated temperature. 8. A method according to anyone of the proceeding claims, characterized in that the pressing at an elevated temperature is hot isostatic pressing. 9. Neutron absorption means for a control rod in a nuclear plant, the absorption means comprising a compound with the composition B x C yt characterized in that x/y is greater than 4. 10. Neutron absorption means according to claim 9, characterized in that it comprises a solid body. 1 1 . Neutron absorption means according to claim 9, characterized in that it is present at least partially as a powder. 12. Neutron absorption means according to claims 9-1 1 , characterized in that it is arranged to be provided in at least one closeable cavity in the control rod. 13. Neutron absorption means according to anyone of the claims 9-12, characterized in that 4< x/y ≤ 12. 14. Neutron absorption means according to anyone of the claims 9-12, characterized in that 6< x/y < 12. 15. Neutron absorption means according to claims 9-12, characterized in that 8,5< x/y < 12. 16. Use of a compound of B x C y in a neutron absorption means for a control rod in a nuclear plant, wherein x/y is greater than 4. 17. Use according to claim l δ, characterized in that 4< x/y < 12. 18. Use according to claim l β, characterized in that 6< x/y < 12. 19. Use according to claiml δ, characterized in that 8,5< x/y < 12.

Inventors: ERIKSSON SVEN, ERIKSSON, SVEN
Applicants: ASEA ATOM AB, ERIKSSON SVEN, ABB ATOM AB, ERIKSSON, SVEN
Application year: 1997
Main classification: G21C7
IPC codes: G21C7, G21C21