GB2301698A - Nuclear shielding materials - Google Patents

Abstract

A neutron absorbing fabricated plate comprises a hermetically sealed casing containing a neutron absorbing layer 4 made from a mixture of at least two materials, one being a matrix powder material, e.g., a corrosion resistant material such as stainless steel, and one being a neutron absorbing powder material such as boron nitride. The manufacturing process involves compacting the powder mixture into blocks (4), assembling them into the casing (1,2,3,5), and hot-rolling the assembly after it has been sealed and evacuated. This enables the neutron absorbing layer to retain some ductility after manufacture, so facilitating post-forming of the plates. Compacts formed of only a corrosion resistant powder material can also be assembled into the casing so that compacts (4) containing the radiation absorbing material can be excluded from contact with regions of the casing where potentially disruptive joining processes between panels, such as fusion welds, will later occur.

Description

NUCLEAR SHIELDING MATERIALS The invention relates to nuclear shielding materials constituting radiation absorbing articles such as plates and panels and also to a process for the fabrication of such articles.

The nuclear industry uses materials which are called "nuclear poisons" to absorb neutron radiation.

Materials such as boron, in the form of boron carbide or similar materials, have long been mixed or compounded with other materials such as aluminium, stainless steel or plastics to produce panels for shielding of reactor fuel rods or other neutron emitting materials.

However, boron in quantities sufficient to provide good shielding has a pronounced adverse effect on the strength, formability and corrosion resistance of steels or other metals. Direct additions of boron are limited to around 1.2% because of the detrimental effects upon formability and weldability. Resultant designs for components are constrained by the need to maximise boron in the structure whilst coping with the poor properties of the material. Hence, there exists a demand for high strength shielding materials having corrosion resistance combined with good formability and weldability.

French patent number 2 646 007 discloses a neutron radiation absorbing article comprising hollow stainless steel casings filled with a mixture of stainless steel and boron carbide powder, the casings and their contents then being hot isostatically pressed to final form. The resulting pressings can be used to produce larger structures. This process enables a higher percentage of boron compounds to be incorporated in the pressings than had previously been possible as well as enabling stainless steel to be used to improve their strength and corrosion resistance. However, the process and the items produced still have disadvantages in that after the hot isostatic pressing process is complete the pressings produced are not formable, due to the brittle nature of the boronated stainless steel fillings.They must therefore be made to order by pressing to the exact shapes needed for final assembly into a desired structure. Furthermore, large pressings need large dies and presses, which are expensive items of capital equipment.

The present invention provides a neutron absorbing article having good corrosion resistance combined with post-manufacture formability and weldability, and a process for manufacture of such articles.

According to the present invention, a neutron absorbing article comprises a hermetically sealed casing containing a neutron absorbing layer sandwiched between sides of the casing, the casing being of a corrosion resistant material and the neutron absorbing layer including a composite material comprising a matrix material and a neutron absorbing material, the matrix material being substantially more ductile than the neutron absorbing material and the neutron absorbing material comprising small grains dispersed in the matrix material. Preferably, the composite material is formed by consolidation of a powder mixture during manufacture of the article, and is preferably a cermet material.

The article may include regions of the neutron absorbing layer formed solely of the matrix material to enable exclusion of the radiation absorbing material adjacent to selected areas of the casing, e.g., areas where joining processes liable to penetrate the casing, such as fusion welds, will be situated.

The matrix material may advantageously be corrosion resistant, e.g., stainless steel. The radiation absorbing material may be a boron compound, such as boron carbide. However, other combinations are possible, for example, boron or boron nitride as the radiation absorbing material and titanium, mild steel, aluminium or plastics as the matrix material.

The invention also provides a process for manufacturing a neutron radiation absorbing article comprising the steps of: mixing at least two materials together, one material being a powder of a final matrix material and one being a powder of a radiation absorbing material; compacting the mixture into compacts; forming a casing from a corrosion resistant material; assembling the compacts into the casing; sealing the casing; evacuating the casing; and hot-rolling the assembly to a desired thickness to produce a neutron absorbing layer sandwiched between sides of the casing, the neutron absorbing layer comprising small grains of the neutron absorbing material dispersed in the final matrix material.

Advantageously, compacts formed of only a corrosion resistant powder material can also be assembled into the casing. Such compacts can be arranged so that compacts containing the radiation absorbing material are excluded from contact with at least one selected region of the casing where potentially disruptive joining processes, such as fusion welds, will later occur.

A further aspect of the invention provides a process for manufacture of a neutron radiation absorbing article, comprising the steps of: mixing a matrix powder material and a neutron radiation absorbing powder material together to constitute a first type of powder material; compacting the first type of powder material to form a first type of shaped compact; compacting a second type of powder material to form a second type of shaped compact, said second type of powder material being a corrosion resistant powder material; fabricating a casing from a corrosion resistant material; assembling the first and second types of compacts into the casing such that the first type of compact is excluded from contact with at least one selected region of the casing; hermetically sealing the casing; evacuating the interior of the assembly; and hot-forming the assembly to a desired thickness.

Mixing of the matrix material powder and the radiation absorbing material powder is preferably done with a suitable liquid binder material, which should also serve to facilitate the mixing process. The mixture can then readily be compacted into a required shape.

Preferably the compacts are compacted to shape in a suitable press, e.g., an hydraulic press.

The invention enables strong post-formable articles to be made with boron concentrations several times greater than are achievable by adding boron to stainless steel.

The invention will now be described by way of example only with reference to the drawings of which: Figure 1 shows a casing and blocks during assembly of an article according to the invention, prior to hot-rolling; Figure 2 shows an alternative arrangement of blocks in a casing as shown in Figure 1; and Figure 3 is a micrograph showing a cross section of an article according to the invention, after hot-rolling.

The invention as now exemplified provides a neutron radiation absorbing article and a method of making the same. As shown in Figure 1, a stainless steel casing 1 was assembled from a frame 2 and a base 3 which was TIG welded to the frame 2. Cermet compacts or blocks 4 were formed by mixing boron carbide powder (particle size 106 - 150 microns) with stainless steel powder (particle size 53 - 150 microns) using decanol/isobutyl alcohol as a lubricant and binder. Mixing was carried out for 5 minutes dry plus 10 minutes with lubricant.

The cermet blocks 4 of size 80 mm long, 46 mm wide and 8 mm thick were then produced by pressing on an hydraulic press at 8 tons/square cm. The blocks 4 were then arranged in the casing 1 and a top plate 5 was TIG (tungsten-inert gas) welded to the casing so that all exposed seams were hermetically sealed. The sealed assembly la was then evacuated through an evacuation tube 6 by pumping down to 5 x 10 4 Torr before sealing of the evacuation tube 6 by simultaneous resistance welding and cutting.The assembly la was then hot-rolled according to the following sequence: Initial gauge 20 mm Pre-heat 10500C 40 minutes Rolling Pass Rolled Gauge 1 19 mm 2 16 mm 3 14 mm 4 12 mm 5 10 mm 6 8 mm 7 7 mm 8 6 mm 9 5 mm Re-heat between each pass for 20 minutes to 10500C In this example, the final plate thickness was 5 mm. However, plates of various thicknesses can be manufactured depending on requirements.

The shapes and thicknesses of the boron carbide loaded areas can be varied to suit the requirements of any particular application by replacing blocks 4 of the boron carbide and stainless steel powder mixture with blocks of plain stainless steel powder 7 in the casing 1 (Figure 2).

Area loading, with respect to boron content, of up to 4 times that of 1.2 % boron stainless steel are possible, depending upon the gauge of the plate and what mechanical properties are required from the finished article.

Figure 3 shows a cross section of a plate after the hot-rolling sequence is completed. The cermet blocks 4 of Figs. 1 and 2 have been consolidated to form a neutron absorbing layer 4a comprising a cermet filler section sandwiched between the top and bottom sides 5a and 3a of a casing formed from the base plate 3 and top plate 5 of the prior assembly.

The hot rolling process has consolidated the cermet blocks to the extent necessary to form a light-coloured stainless steel matrix material M by elimination of the original stainless steel powder grain boundaries.

However, the hot rolling process has not eliminated the grains of boron carbide, these remaining as discrete dark-coloured particles C dispersed throughout the matrix material.

Being a cermet, the neutron absorbing layer 4a is classed as a composite material. The stainless steel matrix material M is substantially more ductile than the grains of boron carbide C.

The cermet filler section 4a is not as corrosion resistant as the stainless steel casing 5a,3a, since a reaction between the boron carbide and the stainless steel somewhat depletes the steel in chromium. For this reason, the cladding formed from the frame 2, the base plate 3 and the top plate 5 is preferably maintained during subsequent component manufacture.

Tensile testing has been carried out on a sample taken transversely from a plate as shown in Figure 3 and it has been established that the cermet material 4a has a proof stress of 60 % that of plain stainless steel.

Since the cermet occupied one third of the thickness, the overall reduction in proof stress was only around 12%.

Products made according to the invention are weldable due to the fact that the boron carbide can be accurately positioned and can therefore be excluded from areas destined to be adjacent to welds by replacing the blocks 4 of the mixture of powders with blocks 7 of stainless steel only in those areas.

Plates produced according to the invention are also formable. As an example, a 5 mm thick plate with a 1.67 mm filler layer 4a, 1.67 mm claddings 3a, 5a, and a boron loading equivalent to that of 5 mm thick 1.4 % boron steel, was bent around an 80 mm former at room temperature with no sign of failure. This can be achieved because the hot rolling operations performed upon the cermet blocks and casing consolidate the cermet blocks and achieve the following results: (A) The original stainless steel powder grain boundaries disappear due to solid state diffusion of the metal molecules across them, so forming a dense stainless steel matrix without porosity.

(B) The boron carbide powder grains react with the chromium in the stainless steel to form a boron chromium carbide complex C, remaining as distinct grains distributed within the stainless steel matrix M.

The above aids ductility of the plates because as the reduced-chromium stainless steel matrix deforms, the small carbide grains can move relative to each other.

This should be compared with plates produced according to the previously mentioned French patent number 2646007, where it is said that after hot isostatic pressing, powder grains are no longer apparent in the filler material.

The neutron absorbing layer is satisfactorily bonded to the casing by virtue of solid state diffusion of the stainless steel powder grains with each other and the stainless steel casing during the hot rolling process. However, such inter-diffusion, at the rolling temperatures and pressures specified, is insufficient to contaminate the stainless steel casing with boron to any significant extent.

The article according to the invention and its manufacturing process allow boron concentrations to be achieved that are several times greater than are possible simply by adding boron to stainless steel.

Despite this, however, a degree of ductility is maintained in the material because the boron compound is distributed in the final matrix material as a powder, whereas larger bodies of boron compound would have a detrimental effect upon mechanical properties.

Though after manufacture the plates can be post-formed to modify their shape, the plates can also be custom-made to a specific application with respect to inclusion of blocks of stainless steel powder in the radiation absorbing layer, so that no boron containing material is liable to be exposed to the environment in the finished component due to welds or other penetrations of the casing adjacent the stainless steel blocks.

Claims (23)

1. A neutron absorbing article comprising a hermetically sealed casing containing a neutron absorbing layer sandwiched between sides of the casing, the casing being of a corrosion resistant material and the neutron absorbing layer including a composite material comprising a matrix material and a neutron absorbing material, the matrix material being substantially more ductile than the neutron absorbing material and the neutron absorbing material comprising small grains dispersed in the matrix material.

2. A neutron absorbing article according to claim 1, in which at least one region of the neutron absorbing layer, adjacent at least one selected area of the casing, consists only of the matrix material, thereby excluding the neutron absorbing material from adjacent said selected area.

3. A neutron absorbing article comprising a hermetically sealed casing containing a neutron absorbing layer sandwiched between sides of the casing, the casing being of a corrosion resistant material and the neutron absorbing layer comprising: a composite material comprising a matrix material and a neutron absorbing material, and at least one region which consists only of the matrix material, said region being adjacent at least one selected area of the casing, thereby excluding the neutron absorbing material from adjacent said selected area.

4. A neutron absorbing article according to any preceding claim, in which the composite material comprises a consolidated mixture of a powder of the matrix material and a powder of the neutron absorbing material.

5. A neutron absorbing article according to any preceding claim, in which the composite material is a cermet.

6. A neutron absorbing article according to any preceding claim, in which the matrix material is corrosion resistant.

7. A neutron absorbing article according to claim 6, in which the matrix material is stainless steel.

8. A neutron absorbing article according to any preceding claim, in which the radiation absorbing material is a boron compound.

9. A neutron absorbing article according to any preceding claim, in which the casing comprises stainless steel.

10. A process for manufacturing a neutron radiation absorbing article comprising the steps of: mixing at least two materials together, one material being a powder of a final matrix material and one being a powder of a radiation absorbing material; forming the mixture into compacts; forming a casing from a corrosion resistant material; assembling the compacts into the casing; hermetically sealing the casing; evacuating the interior of the assembly; and hot-rolling the assembly to a desired thickness to produce a neutron absorbing layer sandwiched between sides of the casing, the neutron absorbing layer comprising small grains of the neutron absorbing material dispersed in the final matrix material.

11. A process according to claim 10, in which compacts formed only of a corrosion resistant powder material are also included in the assembly.

12. A process according to claim 11, in which the compacts formed only of the corrosion resistant powder material are arranged such that compacts containing the radiation absorbing material are excluded from contact with at least one selected region of the casing.

13. A process for manufacture of a neutron radiation absorbing article, comprising the steps of: mixing a matrix powder material and a neutron radiation absorbing powder material together to constitute a first type of powder material; compacting the first type of powder material to form a first type of shaped compact; compacting a second type of powder material to form a second type of shaped compact, said second type of powder material being a corrosion resistant powder material; fabricating a casing from a corrosion resistant material; assembling the first and second types of compacts into the casing such that the first type of compact is excluded from contact with at least one selected region of the casing; hermetically sealing the casing; evacuating the interior of the assembly; and hot-forming the assembly to a desired thickness.

14. A process according to any one of claims 10 to 13, the corrosion resistant materials comprising stainless steel.

15. A process according to any one of claims 10 to 14, the matrix powder material being the same as the corrosion resistant powder material.

16. A process according to any one of claims 10 to 15, the radiation absorbing material comprising a boron compound.

17. A process according to any one of claims 10 to 16, in which mixing of the matrix powder material and the radiation absorbing powder material is done in the presence of a binder material.

18. A process according to claim 17, in which the binder material also serves as a lubricant to facilitate the mixing process.

19. A process according to claim 17 or claim 18, in which decanol or isobutyl alcohol is used as a lubricant and binder for the powders.

20. A process according to any one of claims 10 to 19, in which the compacts are formed to shape in a press machine.

21. A process according to any one of claims 10 to 20, in which before assembly into the casing the porous blocks have a thickness of less than one fifth of their major dimension.

22. A process according to any one of claims 10 to 21, in which the assembly is hot-rolled down to about one quarter of its initial gauge.

23. A neutron radiation absorbing article manufactured by a process according to any one of claims 10 to 22.