CA1097444A

CA1097444A - Low reactivity penalty burnable poison rods

Info

Publication number: CA1097444A
Application number: CA296,725A
Authority: CA
Inventors: William L. Orr; Pratap K. Doshi
Original assignee: Westinghouse Electric Corp
Current assignee: CBS Corp
Priority date: 1977-03-15
Filing date: 1978-02-10
Publication date: 1981-03-10
Also published as: NL7802756A; IT1093753B; EG13142A; IT7821008A0; JPS53113991A; FR2384323B1; GB1554997A; ES467726A1; FR2384323A1; IL53983A; JPS5816712B2; CH626739A5; DE2808907C2; ATA160278A; AT364039B; DE2808907A1; SE7802947L; BE864931A; ZA78703B

Abstract

ABSTRACT OF THE DISCLOSURE
A burnable poison rod wherein the displacement of the moderator coolant is minimized thereby increasing the core moderation and burnable poison depletion. This is accomplished by arranging annular pellets of burnable poison in the rod and permitting reactor coolant to flow up through the annular inner bore.

Description

BACKGROUND OF THE INVENTION
. ~
mi~ invention relates to burnable poison rods of nuclear reactors and more particularly to burnable poison rods having an increased moderator to poison ratio.
In many nuclear reactors designs, a coolant which also serves as the moderator is clrculated through the - reactor core in order to remove the heat produced by the fi~sion process within the coreO The core consists of a specified number of fuel rod~ which are held in bundles by spacer grids and top and bottom fittings. me fuel rods are constructed of cylindrical tubes containing a nuclear fuel such as low enrichment uranlum dioxide fuel pellets~ me bundles, known as fuel assemblies, are arranged in a pattern which approximates a right circular cylinder.
Durlng reactor operation, the fis~ionable isotopes within the nuclear fuel pellets absorb neutron~ and sub- -sequently fission generating heat. In addition to the depletion of fis3ionable material, the fission process results in the formation of ~ission products, some of which readily absorb neutrons. mese effects, depletion and the ~ission products, are partially offset by the buildup of ~issionable i~otopes such as plutonium which occ~rs in the non-fussio~ absorption of neutrons in ~ertile materials such .. . . .. .

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46,5&2 as U-238. Therefore, in order to compensate for the decrease in the reactivity of the core that occurs with the depletion of the fissionable fuel and the buildup of fission products, excess reactivity is built into the reactor at the start of each cycle. This excess reactivity is controlled by neutron absorbing material in the form of boron dissolved in the primary coolant and burnable poison rods.
The concentration of the boron dissolved in the primary coolant is varied to provide control and to com-10 pensate for the long-term reactivity requirements of fuel ~ `
depletion, fission product poisoning, burnable poison depletion, and the cold-to-operating moderator temperature change. However, as the boron concentration is increased, the moderator temperature coefficient becomes less negative.
The use of a soluble poison alone would result in a positive moderator coefficient at the beginning-of-life for the first cycle and can result in a positive coefficient in subsequent cycles depending upon the fuel loading for that cycle.
Therefore, burnable poison rods are used to reduce the 20 soluble boron concentration sufficiently to insure that the ;
moderator temperature coefficient is negative for power operating conditions.
A burnable poison absorbs neutrons without pro-ducing new or additional neutrons and without being trans- ~;
formed into new poisons as a result of neutron absorption.
A typical burnable poison that exhibits these traits is boron-10 which is approximately 20% of the naturally occur-ring boron. Boron-10 on being irradiated by thermal neu-trons undergoes the reaction B10 ~ nl~ Li7 ~ He4 which 30 results in the absorption of a neutron and the depletion of -

-2 r ' ' ~6,582 boron-10 without the production of additional poison.
Burnable poisons are conventionally used in rods installed at vacant rod cluster control (RCC) locations within the fuel assembly. The burnable poison concentration in the rods and the number o~ burnable poison rods inserted in the core are specified so that the soluble boron con-centration is reduced sufficiently to insure that the moderator temperature coefficient is negative for power operating conditions. During operation the poison content in these rods is depleted, thus adding positive reactivity to offset some of the negative reactivity from fuel deple-tion and fission product buildup. At the end-of-life conditions some residual poison may remain resulting in a net decrease in the core lifetime. In addition, the burn-able poison rods displace moderator and the parasitic structural materials of the burnable poison rod absorb neutrons further decreasin~ the avallable reactivity life-time of the core. In addition to reactivity control, the burnable poison rods are strategically located to provide a ~0 favorable radial power distribution.
In the U.S. Patent No. 3~51~3~ to P. M. Wood, issued May 5, 1970 a burnable poison rod is described wherein a borosilicate glass tube is disposed in the annular space between two concentric metal tubes. An internal axial void is provided within the inner metal tube which provides a gas plenum to receive the gaseous reaction products such as helium gas that result when the boron absorbs neutrons.
The burnable poison rods are appropriately placed within the fuel assembly in vacant RCC location. While the patent to Wood does describe a particular type of burnable poison rod-

-3- r ` I ~ g ~ 4 4 46,58 it would be advantageous to minimize the amount of burnable poison remaining at the end of life in order to increase the core lifetime.
A fuel assembly shroud that employs burnable poison is described in U.S. Patent No. 3,663,366 to T. O.
Sauar, issued May 16, 1972. The shroud comprises compound plates of stainless steel and zirconium wherein enriched boron-10 is dispersed in the sheets of stainless steel.
However, the Sauar patent does not describe the use of burnable poison in poison rods capable of being arranged with fuel elements in the fuel assembly.
The use of boric acid or borosilicate glass in control rods of educational nuclear reactors is described in U.S. Patent No. 3,110~656 to M. M. Mills, issued November 12, 1963. Of course, the use of a neutron absorber in control rods is fundamental but suf~ers from the common disadvantages of control rods in general in that they are ;-not designed to deplete with irradiation. In addition~
burnable poisons have been added to the nuclear fuel mat-erial and has been employed in the composition of the fuel element cladding.
While there exists in the prior art, numerous uses for burnable poison in nuclear reactors, the problem of pro-viding sufficient burnable poison at the beginning of the life of the reactor core to compensate for excess reactivity ;
while minimizing this ne~ative reactivity contribution near the end of the core life so as to prolong the core life, nevertheless, has not been solved.
SUMMARY OF THE INVENTION

A burnable poison rod wherein the displacement of -7 4 ~ ~
46,582 moderator coolant by the poison rod is minimized thereby increasing core moderation and burnable poison depletion.
The burnable poison rod wherein displacement of coolant is minimized compensates for the excess reactivity that exists at the beginning of core life while increasing the depletion of the burnable poison throughout the life of the core.
Increased depletion of the burnable poison minimizes the reactivity penalty of the burnable poison rod at the end of the core life thereby prolonging the life of the core.
It is an object of this invention to provide a burnable poison rod wherein the displacement of reactor coolant is minimized so as to increase depletion of the burnable poison.
It is a particular object of this invention to provide a burnable poison rod with an inner channel open to .
the reactor coolant that mlnimizes displacement of the coolant and thereby increases depletion of the burnable poison.
It is another particular object of this invention to provide a burnable poison rod having an internal compart-ment containing a mixture of coolant and burnable poison that minimizes displacement of the coolant and thereby increases depletion of the burnable poison.
It is another particular object of this invention to provide a burnable poison rod comprising a hollow metal tube open to the coolant having a coating of burnable poison thereon tha~ minimizes displacement of the coolant and thereby increases depletion of the burnable poison.
BRIEF DESCRIPTION OF THE DRAWINGS
While the specification concludes with claims -5~

~L~997~44 Ll 6, 5 8 2 specifically pointing out and distinctly claiming the sub~ect matter of the invention, it is believed the inven-tion will be better understood from the following descrip-tion taken in conjunction wlth the accompanying drawings, wherein:
Figure 1 is a partial cross-sectional view ln elevation of a fuel assembly; ;
Figure 2 is a cross-sectional view taken along line II-II of Figure l;
Figure 3 is a cross-sectional view in elevation of a burnable poison rod;
Figure 4 is a cross-sectional view of a burnable poison rod; and Figure 5 is a cross-sectional vlew in elevation of an alternate burnable poison rod.
DESCRIPTION OF THE PREFERRED EMBODIMENT
In order to compensate for the decrease in reacti-vity of the core that occurs with the depletion of the fissionable fuel and the accumulation of flssion products, excess reactivity is built into the reactor at the start of each cycle. The invention described herein provides a means to control the initial excess reac'ivity without creating a reactivity penalty near the end of the core life and without adversely effecting the moderator temperature coefficient.
Referring to Figures 1 and 2, a fuel assembly ~;
referred to generally as 10 comprises an upper end support 12, a lower end support 14, guide tubes 16, positioning grids 18, fuel elements 207 and poison rods 22. Upper end support 12 and lower end support 14 support guide tube 16 and fuel element 20 while positioning grids 18 maintain ~ ~ 9 ~ 4 ~ ~ 46,582 proper alignment among guide tubes 16 and fuel element 20.
Guide tubes 16 may be hollow cylindrical tubes of zireonium or other low neutron absorbing material that are capable of supporting poison rods 22 therein. Fuel assem-blies 10 are arranged vertically within a reactor vessel (not shown) to form a core (not shown) so as to produee heat by nuclear fission in a manner well understood by those skilled in the art. The reactor coolant which may be water flows through the reactor vessel upwardly through and in heat transfer relationship with fuel assemblies 10. In sueh a manner, heat is transferred from fuel assemblies 10 to the reactor coolant. In sueh a configuration, poison rods 22 are capable of absorbing neutrons thus controlling the reaetivity level of the core so that excess reaetivity may be added to the eore by inereasing the enriehment of the nuclear fuel in each fuel assembly 10. ~oading the core with initial excess reactivity increases the length o~ time that the core is capable o~ producing heat without being reloaded with fresh fuel assemblies 10. In this eoneept it is, howeverJ important that the poison rods 22 become depleted near the end of the core's life so that the poison rods 22 will not create a reactivity penalty by absorbing neutrons.
Referring now to Figures 3 and 4, poison rod 22 has a cylindrical metallic outer sheath 24 that may be manu-~actured from zircaloy tubing and may have an outside dia-meter of approximately o.38 inehes and an inside diameter of ;~
approximately 0.33 inches. A lower end plug 26 having a center bore 28 therein is attached to the lower end of outer sheath 24 by suitable means such as welding. A cylindrleal 46,582 metallic inner sheath 30 is concentrically disposed within outer sheath 24 and is attached at its lower end to lower end plug 26. Inner sheath 30 may be manufactured from zircaloy tubing and may have an outside diameter of approx-imately . 20 inch and an inside diameter of approximately 0.16 inch. An upper end plug 32 having an opening 34 is attached to outer sheath 24 and inner sheath 30 near thelr top ends. Inner sheath 30 and outer sheath 24 define there- ~
between an annulus 36 that is closed at its ends by lower ~-end plug 26 and upper end plug 32. A coil spring 38 is ~ -disposed in annulus 36 and rests therein on lower end plug 26. Pellets 40 which are annular pellets conforming to annulus 36 are disposed in annulus 36 and rest on coil spring 38. Pellets 40 may be composed of a burnable poison such as boron carbide-aluminwll oxide (B4C - A1203), other borides such as zirconium di~ride (ZrB2), or oxides such as ; ;~
gadolinium oxide (Gd2O3). Coil spring 38 serves to maintain pellets 40 in approximately the same location relative to outer shea~h 24. As pellets 40 become depleted by neutron absorption they release reaction products such as helium gas. An annular space 42 may be provided near the top of poison rod 22 between the bottom of upper end plug 32 and the top of the stack of pellets 40 so as to provide a cavity for accommodating the reaction products of pellets 40. Of course, these reaction products may also be accommodated in the lower part of annulus 36 in the space around coil spring 3~.
St.ill referring to Figures 3 and 4, on its inner side inner sheath 30 defines an inner bore 44 that extends from lower end plug 26 up into upper end plug 32. Near its ~9744~
46,582 lower end, inner bore 44 is in fluid communication with center bore 28 and at its upper end inner bore 44 is in fluid communication with outlet plenum 46 which is defined within upper end plug 32. Outlet plenum 46 is also in fluid communication with openings 34. Reactor coolant which is normally water and acts as a neutron moderator not only flows around outer sheath 24 but also flows upwardly through an opening in guide tube 16 and upwardly through center bore 28, inner bore 44, outlet plenum 46, and through openings 34. In this manner, inner bore 44 becomes sub-stantially filled with water. The water in inner bore 44 substantially increases neutroll moderation in and around poison rod 22 which, over the :life time of the core, sub-stantially increases the deplekion Or pellets 40 and in-creases the core burnup. The use of such a burnable poison rod, allows the inltial reactivity of the core to be in-creased without incurring an end of life penalty. It is estimated that the use of a poison rod 22 as herein described will increase the first core burnup by about 350 MWD/MTU
20 which decreases the fuel cycle cost for the first core by about 1.3%. Under these conditions the resulting savlngs in yellow cake (U3O8) is about 14,000 lbs. While minor dif-ferences in the size of pellet 40 may not be important, the -~increase in water fraction of poison rod 22 is important.
Therefore, not only should the annular thickness of pellet ~ - `
40 be manufactured as small as possible but the amounk of water displaced by poison rod 22 should be minimized thereby increasing neutron moderation. Therefore, the invention provides a burnable poison rod with an inner channel open to the reactor coolant that minimizes displacement of the _g_ ~7~44 46,582 coolant thereby increasing neutron moderation thus increasing depletion of the burnable poison and increasing nuclear ~uel burnup.
While there is described what is now considered to be the preferred embodiment of the invention it is, of course, understood that various other modifications and variations will occur to those skilled in the art. The claims, therefore, are intended to include all such modifi-cations and variations which fall within the true spirit and scope of the present invention. For example, coil spring 38 may be replaced by another biasing mechanism. Pellets 40 may be replaced by a powder of a higher poison composition. ;~`
Such powder will allow annulus 36 to be made smaller thus increasing the inner space ~or water thereby further mini-mizing displacement of water. In additlon, a side of guide tube 16 may be coated with an electro-deposited layer of a burnable poison which would elimlnate pellets 40 and inner tube 30.
Another alternative is shown in Figure 5 wherein lower end plug 26 and upper end plug 32 are closed and sealed to outer sheath 24. A water soluble burnable poison compound such as boron or cadmium is enclosed within outer sheath 24. In this configuration, when the burnable poison has been depleted only water remains which greatly minimizes the displacement of water. Moreover, in this concept the fluid nature of the solution provides a more even depletion of the poison in the rod.

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A fuel assembly for a nuclear reactor in-cluding fuel elements, guide tubes, and burnable poison rods supported by an upper end support, a lower end support, and intermediate positioning grids, said burnable poison rods comprising:
an elongated tubular outer sheath manufactured from a low neutron absorbing material;
an elongated tubular inner sheath manufactured from a low neutron absorbing material disposed concentrically with-in said outer sheath and defining an inner bore on its inner side, said outer sheath and said inner sheath defining an annulus therebetween;
a lower end plug having a center bore therein in fluid communication with said inner bore and the reactor coolant and being attached to said outer and inner sheaths;
an upper end plug having an outlet plenum therein in fluid communication with said inner bore and said reactor coolant; and annular pellets of a burnable poison disposed in said annulus and supported by a coil spring, whereby said center bore being in fluid communication with the reactor coolant allows reactor coolant to flow through said inner bore and through said outlet plenum thereby increasing the coolant fraction of said burnable poison rod thus increasing neutron moderation which substantially increases the deple-tion of said annular pellets and increases the core burnup.

2. The fuel assembly according to claim 1 wherein said fuel assembly further comprises biasing means disposed in said annulus and resting on said lower end plug for supporting said annular pellets.

3. The fuel assembly according to claim 2 wherein said biasing means is a coil spring disposed in said annulus.

4. The fuel assembly according to claim 1 wherein said inner sheath and said outer sheath are manufactured from a low neutron absorbing material.

5. The fuel assembly according to claim 1 wherein said annular pellets are composed of boron carbide-aluminum oxide.

6. The fuel assembly according to claim 1 wherein said annular pellets are composed of zirconium diboride.

7. The fuel assembly rod according to claim 1 wherein said annular pellets are composed of gadolinium oxide.