GB2137012A - Burnable absorber coated nuclear fuel - Google Patents

Abstract

A nuclear fuel body (20) which is at least partially covered by a burnable neutron absorber layer (30) is provided with a hydrophobic overcoat (32) generally covering the burnable absorber layer (30) and bonded directly to it. In a method for providing a VO2 fuel pellet with a zirconium diboride burnable poison layer, the fuel body is provided with an intermediate niobium layer.

Description

SPECIFICATION Burnable absorber coated nuclear fuel The present invention relates generally to burnable neutron absorbers (also called burnable poisons) for nuclear reactors and, more particularly, to an improved burnable absorber coating for nuclear fuel.

It is known that nuclear fuel may have various shapes such as plates, columns, and fuel pellets disposed in end-to-end abutment within a tube or cladding made of a zirconium alloy or stainless steel.

The fuel pellets contain fissionable material, such as uranium dioxide, thorium dioxide, plutonium dioxide, or mixtures thereof. The fuel rods are usually grouped together to form a fuel assembly. The fuel assemblies are arranged together to constitute the core of a nuclear reactor.

It is well known that the process of nuclear fission involves the disintegration of the fissionable nuclear fuel material into two or more fission products of lower mass number. Among other things the process also generates neutrons which are the basis for a self-sustaining reaction. When a reactor has operated over a period of time, the fuel assembly with fissionable material must ultimately be replaced due to depletion. Inasmuch as the process of replacement is time consuming and costly, it is desirable to extend the life of a given fuel assembly as long as practically feasible. For that reason, deliberate additions to the reactor fuel of parasitic neutron-capturing elements in calculated small amounts may lead to highly beneficial effects on a thermal reactor.Such neutron-capturing elements are usually designated as "burnable absorbers" since they too are depleted after some time so that there is a compensation with respect to the concomitant reduction in the fissionable material.

The life of a fuel assembly may be extended by combining an initially larger amount of fissionable material as well as a calculated amount of burnable absorber. During the early stages of operation of such a fuel assembly, excessive neutrons are absorbed by the burnable absorber which undergoes transformation to elements of low neutron absorbing cross section which do not substantially affect the reactivity of the fuel assembly in the latter period of its life when the availability of fissionable material is lower. Accordingly, with a fuel assembly containing both fuel and burnable absorber in carefully proportioned quantity, an extended fuel assembly life can be achieved with relatively constant neutron production and reactivity.

Burnable absorbers which may be used include boron, gadolinium, samarium, europium, and the like. Burnable absorbers are used either uniformly mixed with the fuel (i.e., distributed absorber) or are placed discretely as separate elements in the reactor, so arranged that they burn out or are depleted at about the same rate as the fuel. Thus, the net reactivity of the core is maintained relatively constant over the active life of the core.

U.S. Patent 3,427,222 discloses a uranium dioxide fuel pellet coated with a mixture of uranium dioxide and a zirconium diboride burnable poison applied by a plasma spraying technique (see column 4, "Example I"). That patent also disclosed a uranium dioxide fuel pellet coated with the burnable poison boron applied by chemical vapor deposition. It is noted therein that the deposition rate was slow at low temperatures while the coating was not as adherent at high temperatures (see column 5, "Example Ill").

It is known that a nuclear fuel contained in an aluminum can may be coated with a layer of niobium to prevent the fuel from reacting with the can (British Patent 859,206; page 1; lines 12-30).

It is also known that minute nuclear fuel particles, such as uranium dioxide particles, may be coated with a single layer or several layers of the same or different non-absorber materials, including niobium, for such purposes as protecting the fuel from corrosion and helping to retain the products of fission. The coatings may be applied by various techniques, such as depositing from a vapor of the coating material, depositing from a decomposing vapor, and electroplating (British Patent 933,500).

in Dispersion Fuel Elements, an AEC Monograph by A. N. Holden published in 1967 by Gordon and Breach of New York, there is mentioned coating fuel particles in dispersion fuels to prevent interaction of the particles with the matrix and to retain fission products (page 30). Uranium dioxide coated with niobium by vapor-phase reduction is disclosed (page 48). Also disclosed is uranium dioxide coated with chromium, by vapor-phase reduction using chromium dichloride, which was deposited over a niobium undercoat (page 48).

The present inventors are aware of the earlier documented work disclosed in a commonly assigned U.S. Patent Application entitled "Coating a Uranium Dioxide Nuclear Fuel With a Zirconium Diboride Burnable Poison", by Walston Chubb, concomitantly filed with the present application, wherein spalling problems with chemically vapor depositing zirconium diboride on uranium dioxide were overcome by first deposition (by sputtering, chemical vapor deposition, etc.) a thin undercoat layer of niobium (of between about 3 microns and about 6 microns in thickness) on the uranium dioxide and then chemically vapor depositing the zirconium diboride on the niobium layer.

Fuel pellets coated with a boron containing burnable absorber such as elemental boron, boron-1 0 isotope (the isotope of elemental boron having the burnable absorber property), zirconium diboride, boron carbide, boron nitride, and the like suffer from varying degrees of moisture adsorption. For example, uranium dioxide fuel pellets coated with zirconium diboride, after manufacture, must be furnace dried in a time-consuming operation and then loaded into the fuel rods in a low humidity glove box environment. This is required because the zirconium diboride, being hygroscopic, takes on a thin layer of moisture (moisture adsorption) from the air itself.The added lengthy drying step (typically about 1 to 3 hours at temperatures of 200--600"C in a vacuum of less than or equal to 1 torr) and humidity controlled pellet loading environment add to the time, complexity and the cost of the nuclear fuel processing line. Moisture is to be avoided in nuclear fuel because excessive hydrogen in the fuel pellet, appearing mostly as moisture, causes hydriding of the Zircaloy fuel rod cladding which may result in a breach in the fuel rod cladding.

It is therefore the principal object of the present invention to provide fuel elements having pellets coated with a neutron absorbing material which are not subject to moisture absorption so that they can be stored and then inserted into the fuel elements without lengthy and expensive preparations.

With this object in view, the present invention resides in a burnable neutron absorber coated nuclear fuel body comprising a nuclear fuel substrate containing a fissionable material and a layer containing a burnable absorber covering at least a part of said substrate, an overcoat layer containing a reactor compatible, hydrophobic material generally covering, and bonded directly to, said burnable neutron absorber layer.

The invention will become more readily apparent from the following description of a preferred embodiment thereof shown, by way of example only, in the accompanying drawings, in which: Figure 1 is a longitudinal sectional view of a fuel rod containing burnable absorber coated fuel pellets having the non-hygroscopic overcoat layer of the invention.

Figure 2 is a transverse sectional view along the line lI-Il of Figure 1.

Figure 3 adds an undercoat layer to the fuel pellets of the fuel rod of Figure 1.

Figure 4 is a transverse sectional view along the line IV--IV of Figure 3.

Nuclear fuel includes uranium in the form of uranium dioxide (or thorium dioxide, plutonium dioxide, or mixtures thereof) pellets each having a generally cylindrical configuration with an approximately 8 mm diameter and an approximately 12 mm length. Desirable zirconium diboride burnable absorber coating thicknesses on the fuel pellets include a thickness of between about 8 and 1 6 microns (and preferably of between about 9 and 10 microns which corresponds to a target boron-1 0 loading of generally 0.6 mg per lineal cm).

The degree of moisture adsorption depends on the technique used to deposit the zirconium diboride layer. It has been found that sputtering produces a somewhat porous coating which contributes to moisture adsorption, while chemical vapor deposition appears to have less moisture adsorption problems.

As shown in Figures 1 and 2, a fuel rod 10, for use in a nuclear reactor fuel assembly, includes a tube 1 2 having a top end plug 14 and a bottom end plug 1 6 providing a chamber 1 8 in which a plurality of fissionable fuel pellets 20 are placed in end-to-end abutment biased against the bottom end plug 1 6 by the action of a spring 22. The pellet 20 diameter is slightly less than that of the tube 12 so as to form a clearance space 24. Both the spring 22 and clearance space 24 accommodate any thermal expansion of the pellets 20 during operation.

Preferably the fissionable body portion or substrate 26 of the fuel pellet 20 consists essentially of uranium dioxide, although other forms of uranium, as well as plutonium or thorium, may be used. Also, preferably the burnable absorber layer 30 covering at least a part of the substrate 26 consists essentially of elemental boron or zirconium diboride, although other forms of boron, as well as gadolinium, samarium, europium, and the like, may be used.

To make the burnable absorber coated nuclear fuel pellet 20 non-hygroscopic (hydrophobic), the burnable absorber layer 30 is covered by an overcoat layer 32 which is directly bonded to it. The overcoat layer 32 contains a reactor compatible, hydrophobic material. Preferably the overcoat layer 32 has a thickness of between about 2 microns and about 6 microns. The overcoat layer 32 should be applied before the burnable absorber layer 30 has been exposed to air to avoid trapping any moisture (absorbed by the burnable absorber) in the fuel pellet 20. Reactor compatibility factors to be considered for such an overcoat layer include cost, neutron absorption cross section, compatibility with burnable absorbers, compatibility with the tube (cladding) 12, and melting point.Therefore, a reactor compatible, hydrophobic material is deemed to be a material chosen from the following group: niobium, zirconium, magnesium, aluminum, silicon, carbon, titanium, chromium, iron, nickel, copper, yttrium, molybdenum, barium, and cerium.

In a first preferred embodiment, elemental boron is used for the burnable absorber layer 30 and is bonded directly to the substrate 26 which is uranium dioxide, while the overcoat layer 32 consists essentially of niobium. In one example, uranium dioxide fuel pellets were coated by conventional chemical vapor deposition (CVD) techniques first with elemental boron and then with niobium utilizing a vertical pipe surrounding vertically stacked fuel pellets. The boron coating 30 was prepared via the pyrolysis of B2HG, and the niobium coating 32 was prepared via the hydrogen reduction of niobium pentachloride (NbCl5). These gaseous CVD precursors were introduced into the bottom of the pipe end the by-products were exhausted from the top of the pipe. The fuel pellet substrates 26 had been cleaned by light sanding, repeated ultrasonic cleaning in distilled water, and vacuum drying.

Thermocouples were mounted on the walls of the pipe. The pellet substrates 26 were heated to a thermocoupie-measured predetermined wall temperature by an upper furnace while the precursor gases were preheated to a thermocouple-measured preselected wall temperature by a lower furnace.

Satisfactory coatings were obtained under various conditions as summarized in Table 1.

TABLE 1 SUMMARY OF CONDITIONS FOR PREPARING BORON/NIOBIUM COATINGS Temperatures (OC) Flows (Mole Percent) Run Time Totai Flow Run No. Layer (min.) Gas Preheat Pellet Zone B2H6 H2 NbCI5 (cc/min) 1 B 45 230 600 0.015 99.985 - . 17010 Nb 172 650 850 - 99.938 0.062 16510 2 B 60 230 615 0.015 99.985 - 17010 Nb 20 650 850 - 99.983 0.107 16017 3 B 60 230 610 0.015 99.985 - 17010 Nb 35 650 850 - 99.909 0.091 16315 4 B 35 230 610 0.015 99.985 - 17010 Nb 34 650 845 - 99.946 0.054 17169 In a second preferred embodiment, as shown in Figures 3 and 4, zirconium diboride is used for the burnable absorber layer 30 and is bonded by CVD to an undercoat layer 28 of niobium, with the undercoat layer 28 being bonded by CVD to the substrate 26 which is uranium dioxide. The overcoat layer 32 consists essentially of CVD niobium. The necessity for an undercoat layer of niobium (or the like) when depositing zirconium diboride by chemical vapor deposition (CVD) on uranium dioxide has been previously mentioned. Preferably the undercoat layer 28 has a thickness of between about 3 microns and about 6 microns. The technique is similar to that discussed in the first preferred embodiment. The CVD precursor for the zirconium diboride was zirconium tetrachloride and boron trichloride. Gaseous zirconium chloride was prepared by reacting HCI and zirconium and carrying the reaction products in a hydrogen stream. Satisfactory coatings were obtained under various conditions as summarized in Table 2.

TABLE 2 SUMMARY OF CONDITIONS FOR REPARING Nb/ZrB2/Nb COATINGS Temperatures ( C) Flows (Mole Percent) RunTime Total Flow Run No. Layer (min) Gas Preheat Pellet Zone BCl3 HCl H2 Nbcl5 ZrCl4 (cc/min) 15632 1 Nb 45 650 850 --- --- 99.921 0.079 -- ZrB2 60 600 800 0.140 0.053 99.680 --- 0.128 17098 Nb 67 650 850 --- --- 99.946 0.054 --- 15668 2 Nb 59 650 850 --- --- 99.942 0.058 --- 17089 ZrB2 37 600 805 0.279 0.204 99.298 --- 0.220 17196 Nb 69 650 850 --- --- 99.951 0.049 --- 17088 3 Nb 44 643 865 --- --- 99.907 0.093 --- 17136 ZrB2 76 600 800 0.187 0.234 99.498 --- 0.082 17114 Nb 48 650 850 --- --- 99.915 0.085 --- 17155 4 Nb 60 650 840 --- --- 99.942 0.059 --- 17195 ZrB2 30 605 805 0.279 0.204 99.298 --- 0.220 17196 Nb 50 660 840 --- --- 99.932 0.068 --- 17197 5 Nb 65 650 855 --- --- 99.941 0.059 --- 17280 ZrB2 25 600 805 0.279 0.204 99.298 --- 0.220 17196 Nb 55 650 843 --- --- 99.938 0.062 --- 17231 6 Nb 81 650 843 --- --- 99.959 0.041 --- 17192 TABLE 2 (continued) SUMMARY OF CONDITIONS FOR PREP ARING Nb/ZrB2/Nb COATINGS Temperatures ( C) Flows (Mole Percent) Run Time Total Flow Run No. Layer (min) Gas Preheat Pellet Zone BCl3 HCl H2 NbCl5 ZrCl4 (cc/min) 6 ZrB2 27 600 804 0.279 0.204 99.298 --- 0.220 17196 Nb 72 650 844 --- --- 99.945 0.055 --- 17194 7 Nb 27 650 870 --- --- 99.811 0.189 --- 17112 ZrB2 75 600 825 0.140 0.234 99.544 --- 0.082 17106 Nb 33 650 890 --- --- 99.870 0.130 --- 17062 8 Nb 65 650 843 --- --- 99.920 0.080 --- 17199 ZrB2 37 602 803 0.279 0.204 99.298 --- 0.220 17196 Nb 54 650 843 --- --- 99.922 0.078 --- 17153 9 Nb 64 650 860 --- --- 99.936 0.064 --- 17105 ZrB2 55 620 817 0.140 0.234 99.543 --- 0.082 17106 Nb 77 650 853 --- --- 99.946 0.054 --- 17103 10 Nb 71 650 850 --- --- 99.949 0.052 --- 17194 ZrB2 37 600 810 0.279 0.204 99.298 --- 0.220 17196 Nb 52 650 850 --- --- 99.934 0.066 --- 17196 11 Nb 69 650 848 --- --- 99.956 0.044 --- 17228 ZrB2 55 600 809 0.140 0.105 99.640 --- 0.114 17101 Nb 77 650 845 --- --- 99.956 0.044 --- 17206 Typically, the invention is used to circumferentially surround (i.e., coat only the cylindrical wall of) the fuel pellet substrate 26 with a burnable absorber layer 30 and the overcoat layer 32 (and the undercoat layer 28 if needed). However, in some applications it may be desirable to coat the entire fuel pellet substrate 26, including its top and bottom surfaces. In other appiications, it may be advantageous to coat only a part of the nuclear fuel substrate with the burnable absorber layer and then generally cover (or partially cover) the burnable absorber layer with the overcoat layer. Also, where substrates, burnable absorber layers, and overcoats/undercoats may contain uranium dioxide, zirconium diboride, and niobium, respectively, it is preferred that they respectively consist essentially of such uranium dioxide, zirconium diboride, and niobium.

Claims (12)

1. A burnable neutron absorber coated nuclear fuel body (20) comprising a nuclear fuel substrate (26) containing a fissionable material and a layer (30) containing a burnable absorber covering at least a part of said substrate (26), characterized by an overcoat layer (32) containing a reactor compatible, hydrophobic material generally covering, and bonded directly to, said burnable neutron absorber layer (30).

2. A nuclear fuel body as claimed in claim 1, characterized in that said pellet substrate (26) consists essentially of uranium dioxide and said burnable absorber layer (30) comprises a boron containing material.

3. A nuclear fuel body as claimed in claim 1 or 2, characterized in that said burnable absorber layer (30) consists essentially of boron and is bonded directly to said pellet substrate (26).

4. A nuclear fuel body as claimed in claim 1 or 2, characterized by an undercoat layer (28) containing niobium disposed between, and bonded directly to, said pellet substrate (26) and said burnable neutron absorber layer (30), said burnable neutron absorber layer (30) consisting essentially of zirconium diboride.

5. A nuclear fuel body as claimed in claim 4, characterized in that said undercoat layer (28) consists essentially of niobium.

6. A nuclear fuel body as claimed in claim 4 or 5, characterized in that said undercoat layer (28) has a thickness of between 3 microns and 6 microns.

7. A nuclear fuel body as claimed in any of claims 1 to 6, characterized in that said overcoat layer (32) consists essentially of niobium.

8. A nuclear fuel body as claimed in any of claims 1 to 7, characterized in that said pellet substrate (28) is generally cylindrically shaped having a diameter of about 8 mm and a length of about 12 mm, said burnable absorber layer (30) has a thickness of 8 microns to 1 6 microns, and said overcoat layer (32) has a thickness of 2 microns to 6 microns.

9. A nuclear reactor fuel assembly comprising fuel rods containing burnable neutron absorber coated nuclear fuel bodies as claimed in any of claims 1 to 8.

1 0. A method for coating a uranium dioxide containing nuclear fuel body with a zirconium diboride containing burnable poison, characterized in that a layer containing niobium is first bondably deposited on at least a portion of said nuclear fuel body, and a layer of said burnable poison is then bondably deposited, by chemical vapor deposition, on at least a part of said niobium containing layer.

11, A method as claimed in claim 10, characterized in that said niobium containing layer consists essentially of niobium which is deposited by chemical vapor deposition.

12. A method as claimed in claim 10 or 11, characterized in that said burnable poison layer consists essentially of zirconium diboride.