CA2097412C

CA2097412C - Fuel bundle for use in heavy water cooled reactors

Info

Publication number: CA2097412C
Application number: CA002097412A
Authority: CA
Inventors: Adi R. Dastur
Original assignee: Atomic Energy of Canada Ltd AECL
Current assignee: Atomic Energy of Canada Ltd AECL
Priority date: 1993-05-31
Filing date: 1993-05-31
Publication date: 2005-08-23
Anticipated expiration: 2013-05-31
Also published as: CA2097412A1

Abstract

A fuel bundle for use in a heavy water cooled pressure tube reactor, having a plurality of elongated fuel rods or elements disposed therein, and in which a positive reactivity component is produced by a change in neutron spectrum and flux across the bundle, upon a decrease in coolant density, the improvement consists of placing fissile material for producing an increase in neutron multiplication, in a first region of the fuel bundle wherein the thermal neutron flux tends to decrease upon a decrease in coolant density, placing fertile material, in a second region of the fuel bundle wherein the thermal neutron flux tends to increase upon a decrease in coolant density, placing neutron absorber material mixed with the fertile material to absorb thermal neutrons in the second region, having neutron scattering material, disposed in a third region of the fuel bundle between the first and second regions, such that the mixture of the fertile material and the absorber in the second region redistributes the neutron flux across the bundle and thereby produces a negative reactivity component, when the coolant density decreases.

Description

FUEL BUNDLE FOR USE IN
HEAVY WATER COOLED REACTORS
FIELD OF THE INVENTION
The present invention relates to a fuel bundle for a pressure tube type heavy water cooled reactor, and in particular a fuel bundle for providing control of the reactivity upon coolant voiding, in a reactor such as the CANDU'°° reactor.
BACKGROUND OF THE INVENTION
An example of a heavy water cooled reactor is the Candu'°' reactor which contains a plurality of fuel channels each of which contains a plurality of fuel bundles, generally arranged end to end. Each fuel bundle in turn contains a set of solid fuel rods or elements, which contain fissionable material (e.g. Uranium dioxide), which are mechanically assembled together. The fuel bundles are placed inside the fuel channel and coolant flows over the fuel bundles to cool the fuel and remove the heat from the fission process. This heat is transferred by the coolant to a heat exchanger which produces steam that drives a turbine to produce electrical energy. Heavy water coolant flows through water gaps in each fuel bundle and, in particular through the gaps between the fuel rods. The coolant water is continuously heated by the fuel as it flows through the fuel bundles. The water flowing in the water gaps is pressurised and does not boil significantly.
When the reactor is operating normally the volume of coolant water in the fuel channels remains constant and the temperature of the water entering and exiting the fuel channel also remains approximately constant.
During an abnormal operating condition such as a sudden increase in power level or a breach in the coolant circuit the water starts to boil. This boiling reduces the number of molecules of coolant inside the reactor core and actually within most of the fuel channels. This phenomenon may be termed coolant voiding.

Reduction in the number of water molecules, in other words coolant voiding, has an adverse affect on the neutronic behaviour of the reactor. The rate of neutron multiplication increases rapidly and thereby the heat generation in the fuel increases rapidly and the coolant volume and flow rate over the fuel is no longer sufficient to carry the heat out from the fuel aggregate which would permit the safe operation of the fuel.
The reactor has to be shut down by actuating one of the two shut-down systems designed for this purpose.
Although the operation and the integrity of the shut-down systems provided for this purpose have been accepted by licensing authorities to be sufficient to ensure safe operation of the reactor; there is still a need for this inherently unstable behaviour of the reactor to be controlled. It is not good practice to have a possible situation where an increase in power can lead to positive reactivity.
One of the main reasons for this positive reactivity on coolant voiding results through a change in the neutron spectrum or neutron energy distribution in the fuel bundle, which leads to a change in neutron multiplication and therefore to a change in the neutron flux and power pxoduction rates. The role of neutron absorption in heavy water coolant is negligible.
One possible approach to reducing or eliminating this positive reactivity is to reduce the amount of heavy-water moderator associated with a fuel channel. Tl~is enhances the moderating role of the coolant and consequently when the coolant is lost, the moderation of neutrons is reduced which in turn reduces the rate of nuclear fission. This can be achieved by reducing fuel channel spacing in the reactor vessel (reducing lattice pitch), or by reducing the moderator volume by displacing with empty tubes, or by increasing the size of the calandria tubes which surround the fuel channel.
These approaches have disadvantages .in that they increase the unproductive loss of neutrons by leakage from the reactor or by absorption in the extra tubes required for moderator displacement. This loss of neutrons increases fuel consumption and thus fuel cost. Packing the fuel channels closely produces problems in the fuel handling system due to lack of space between the channel end fittings needed to accommodate the fuelling machine head.
The inventor and D.B. Buss describe, in a paper entitled "The Influence of Lattice Structure and Composition on the Coolant '~loid Reactivity in Candu''"", presented in June 1990 at the 11th Annual Meeting of the Canadian Nuclear Society, a method for producing negative reactivity response upon coolant voiding. This paper provides a background theory on the mechanism of void reactivity and methods of void reactivity reduction and more specifically a method for creating negative reactivity. In this method , a negative reactivity component is created by placing neutron absorbing material in a central region of the fuel bundle, where the thermal neutron flux increases on voiding. This negative reactivity component can be made sufficiently high to completely offset the positive reactivity component that is produced on voiding. This type of negative reactivity response upon a decrease in coolant density is desirable in that it imparts an inherently safe characteristic to the Candu'm reactor. The reactor will therefore tend to shut down (without the action of a shutdown system) following an increase in power.
A disadvantage of the above method is that in order to override and provide additional neutron multiplication to compensate for the absorber, it is necessary to increase the U~ concentration or enrichment in the fuel bundle. Consequently, the fuelling cost in Candutm reactors using low void reactivity fuels depends on the design discharge fuel buxnup rate, as well as the targeted void reactivity. The increase in U~
enrichment can be viewed as a reduction in the resource utilisation advantage that Candu''" reactors have over light water reactors (LWR's).
There is therefore a need for a fuel bundle that will result in a reduction in the reactive power following a decrease in coolant density, either during normal operation or an accidental situation, while reducing the cost penalty of the additional fissile material required to achieve a required design discharge fuel brirnup. It is also desirable that such a fuel bundle be useable in presently operating Candu~"' heavy water reactors.
SUMI'Y OF THE IN'VEI~1TI01~1 The present invention seeks to provide a fuel bundle that produces negative reactivity to counteract positive reactivity components in response to a loss of coolant or essentially of voiding of the coolant and which exhibits little or no accompanying penalty in the utilization of natural uranium and further which may be used in presently operating heavy water reactors and also in future heavy water reactors of similar design.
In accordance with this invention there is provided in a fuel bundle for use in a heavy water cooled pressure tube reactor, having a plurality of elongated fuel rods or elements disposed therein, and in which a change in reactivity is produced by a change in neutron spectrum and flux across the bundle, upon a decrease in coolant density, the improvement comprising:
fissile material for producing an increase i.n neutron multiplication, disposed in a first region of the fuel bundle wherein the thermal neutron flux tends to decrease upon a decrease in coolant density;
fertile material, disposed in a second region of the fuel bundle wherein the thermal neutron flux tends to increase upon a decrease in coolant density, neutron absorber material mixed with the fertile material to absorb thermal neutrons in the second region;
neutron scattering material, disposed in a third region of the fuel bundle between the first and second regions;
the mixture of the fertile material and the absorber in the second region redistributing the neutron flux across the bundle and thereby producing a negative reactivity component, upon the decrease in coolant density.

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This negative reactivity is produced by redistributing material within the pins and amongst the pins of the fuel bundle. The materials chosen are such that when they are placed in the central pins of the fuel bundle a loss of coolant or voiding of the coolant results in an activation of these materials with the result that they absorb parasitically an increased number of neutrons. The materials placed in the outer pins of the fuel bundle are such that on a loss of coolant they result in a decrease in the production of neutrons. Therefore, in both cases, there is a response in a direction in which the overall neutron multiplication rate decreases.
Also, during the early part of the fuel life they prevent the transmission of low energy neutrons from the moderator to the fuel. This raises the energy level of the neutrons in the fertile region and increases the rate of formation of fissile material (plutonium). The additional fissile material produced compensates for the penalty in uranium utilization that is incurred by the addition of L1~ in the fissile region.
1n addition to these requirements the materials have to be restricted to those that are chemically and physically compatible with the fuel in which they are mixed and also with the reactor environment.
Furthermore, their behaviour should be such that as the fuel moves through the reactor from its initial insertion to its final exit the material will remain in such concentrations that they function adequately to maintain the type of response for which the bundle was designed.
This fuel bundle design makes use of the control that can be provided by the judicious placement of certain materials in the fuel bundle mixed with the fuel that will modify the neutronic behaviour of the reactor core.
The reduction in power is achieved by the creation of negative feedback reactivity upon coolant voiding or decrease in coolant density.
This phenomenon is due to the redistribution of the neutron energy spectrum over the volume of the fuel bundle.

BRIEF DESCRIPTION OF THE DRAWINGS
These and other features of the invention will become more apparent from the following description in which reference is made to the appended drawings wherein:
FIGURE 1 shows a cut-away isometric view of a nuclear reactor core;
FIGURE 2 is a cross-sectional view of a 37 element fuel bundle;
FIGURES 3(a)-3(d) show cross-sectional view of fuel bundles according to the present invention;
FIGURE 4 is a graph showing thermal neutron flux distribution;
and FIGURE 5 is a graph showing a change in neutron flux distn'bution on coolant voiding.
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT
Referring to FIGURE 1 a typical Candu'm heavy water reactor assembly is shown generally by numeral 1. A shield tank 2 surrounds the reactor core or calandria 3, which in turn contains a plurality of calandria tubes for fuel channels 4 each of which contains a plurality of fuel bundles 6, generally arranged end to end and extending along the length of the fuel channel 4. Each fuel bundle 6 in turn contains a set of fuel rods, elements or pencils 22 which are mechanically assembled together, as shown in FIGURE Z. The fuel bundles 6 are placed inside the fuel channel and coolant flows over the fuel bundles to cool the fuel and remove the heat from the fission process. This heat is transferred by the coolant via suitable pipes 7 to steam generators (not shown) which in turn produce steam which run the steam turbines (not shown) to produce electrical energy. Heavy water coolant 10 flows through water gaps in each fuel bundle 6 and, in particular through the gaps between the fuel rods 22. The coolant water 10 is continuously heated as it flows through the fuel _~_ ~'~~~
bundles 6. A moderator is also introduced into the reactor via suitable inlet 11 and outlet 12 pipes.
Referring to FIGUI~ 2, a fuel bundle for use in a heavy water pressure tube reactor is shown generally by numeral 20. The fuel bundle 20 consists of a set of thirty seven pins 22 which have been assembled together such that they are mechanically stable both when they are outside the reactor and especially during their operation inside the reactor. The group of thirty seven fuel pins are encased by a pressure tube 24. A
calandria tube 26 is spaced from and encases the pressure tube 24.
Neutron absorbing and fertile material is placed in pins in a central region 2g of the bundle 20. Fissile material is placed in pins in an outer region 30 and neutron scattering material such as heavy water is disposed between the pins of the central region 2g and outer region 30. The pins in the central region 28, being seven in number (a central pin surrounded by six pins), each contain uranium dioxide fuel which has been depleted in the fissionable isotope U~ to behave as fertile material mixed with the absorber material.
An effective absorber material whiclh will respond suitably to the neutron spectrum in the above fuel bundle has been found to be isotopes of dysprosium in the form of dysprosium dioxide, dysprosium oxide or dysprosia. Other suitable absorbers are dysprosium oxide in a matrix of depleted uranium dioxide or cobalt or any suitable long lived reactive isotope resulting from nuclear fission. The use of suitable radioactive waste products as an absorber in the present invention also serves as a fission product disposal strategy.
The outer region 30, which consists of the outer ring of pins, as shown in FIGUI~ 2, consists of U~ and U~~ both as uranium dioxide, in which the U~ concentration has been increased. This increase in U~
concentration or enrichment in the outer region 30 of the fuel bundle, serves to overnde or provide the additional neutron multiplication that is required to compensate for the dysprosium oxide that is placed in the central pins 30. The outer region is chosen for the fissile material since this is where the neutron population decreases on voiding, and therefore a decrease in neutron multiplication can only be accomplished in this region 30 by the addition of the enriched uranium thereto. The addition of the neutron absorber in the central fuel pins and enriched fuel in the outer fuel pins creates negative reactivity. It is this negative reactivity that neutralizes the positive reactivity produced normally in an unmodified fuel bundle on coolant voiding.
It may be seen therefore that the distribution of the neutron population across an unmodified fuel bundle determines where the absorber material and fissile material is placed in order to produce the requisite negative reactivity component.
The increasing neutron absorption rate can only be accomplished, in this case, by adding neutron absorber at the center of the bundle because that is where the neutron population rises on the loss of coolant or coolant voiding.
The efficiency of the use of the redistribution of the material to achieve negative void reactivity can be improved by a modification of the geometry of the fuel bundle, as may be seen in FIGURES 3(a)-3(d). The volume of fuel that contains the absorber if increased provides more efficient use of the change in neutron flux. Placing the enriched fuel elements closer to the pressure tube 24 increases the magnitude of the negative reactivity component that is produced on loss of coolant. The negative reactivity is produced as a result of the flux decrease in the outer ring 30 of the bundle when the coolant voids. This flux increase depends on the location of the fuel ring. The closer it is to the pressure tube the bigger the flux decrease.
One way in which enriched fuel can be placed closer to the pressure tube is by reducing the size of the enriched fuel pins 32 and increasing the number of the enriched fuel pins 32 as in FIGURE 3(d). This fuel bundle design provides more void reactivity reduction for the same increase in _9_ cost compared with the fuel bundle design of FI~Ut~ 2 which has a uniform fuel pin size. The fuel bundle of FIGUItIF 3(d) is a result of the requirement that the bundle performance in terms of power output should be the same as an unmodified bundle.
S The power output can be increased by having more pins 3~ with fuel enrichment as shown in FIGURE 3(b). This bundle design has the dual advantage of providing reduced or zero or negative void reactivity as required and in addition providing more power than the bundle design that is presently used in heavy water reactors.
In the above bundles use has been made of material that is neutron multiplying of fissile material and neutron absorbing of parasitic material.
In addition, neutron scattering material is used to replace some of the volume occupied by the fuel. This is essentially achieved by decreasing the volume of the fuel and increasing the volume of heavy water 3b inside the fuel channel, as is shown by FI~'tTI~E 3(e).
It is convenient to e~cpress the cost of void reduction as the increase in U~ enrichment required to achieve the targeted void reactivity while maintaining the design discharge fuel burnup.
The increase in U~ enrichment requirement can be viewed as a reduction in the resource utilization advantage that heavy water reactors have over the Light Water Reactors (LWRs). Therefore, there is considerable incentive to improve the low void reactivity fuel (LVRF) designs by reducing the cost of void reduction. The improvement can be achieved by:
2S a. decreasing the dysprosium requirement for a given void reactivity reduction, b. decreasing the incremental U~ enrichment required for a given amaunt of dysprosium.
The dysprosium requirement can be decreased by replacing some of it by another absorber, I1~, in the form of depleted uranium from the tailings, i.e. waste product, of fuel enrichment plants for LWRs. Another ~~~~'~~~~

advantage of using depleted uranium is the expected high conversion ratio because of the relatively hard neutron spectrum in the inner fuel pins.
Increase in fissile plutonium formation in the depleted uranium would offset the lack of U~ in the inner fuel pins and contribute significantly to the overall energy produced by the fuel bundle.
The use of depleted uranium can therefore reduce the dysprosium requirement for a required void reduction. It also reduces the U~
requirement for a given fuel discharge burnup since the energy produced in the depleted uranium is essentially free.
The efficacy of the dysprosium to reduce void reactivity should also be increased in the presence of depleted uranium because of the lack of fissile material in the inner pins. The flux rise in the inner pins on voiding will not result in a significant increase in fission rate due to the lack of U''~. This should reduce the dysprosium requirement for a given void reactivity reduction.
The following fuel bundle designs were used in a WIMS simulations to evaluate the U~ requirements for specific targets of discharge fuel burnup and coolant void reactivity:
a. standard 37-element fuel bundle design, shown in FIGTJItE 2 b. standard CAIVFIrEXtm 43-element fuel bundle design, (not shown) and c. 43-element fuel bundle design with a large central pin, shown in FIGI~ 3(a).
Turning back to Figure 2, depleted uranium and dysprosium were used in the inner two rings, i.e. region 2S, being the innermost seven fuel pins of the standard 37-element design. The amount of dysprosium in the inner seven pins and the U~ enrichment in the outer 30 pins were adjusted to give the desired void reactivity and discharge fuel burnup.
Referring to 1FIG1<JItE 3(a) a 43-element fuel design is shown generally by numeral 40, which is similar to the standard CANJFLI;Xtm design except fox the large central fuel pin 42. Depleted uranium is used in the inner eight fuel pins. However, the size of the central pin 42 allows the dysprosium to be located only in the central pin, where the void reduction effect is maximum and the parasitic load under nominal conditions is minimum. This fuel design is therefore expected to be a relatively more cost-effective low void reactivity fuel design.
Effects of Depleted Uraniueee The effects of using depleted uranium in the LVRF designs were investigated using the 43-elements fuel design with a large central pin of Figure 3(a). Table 1, below, gives the U~ contents in the four fuel rings 1-4, numbered 44, 46, 48 and 50, respectively, for two cases:
a. depleted bundle, where depleted uranium (0.25 wt% U~) is used in the inner two fuel rings and Slightly Enriched Uranium (SEU) is used in the outer two fuel rings, and b. regular bundle, where SEU is used in all fuel rings.
Eoth fuel bundles give the same discharge fuel burnup of 21,000 Mwd/teU. However, the bundle-averaged fuel enrichment for the depleted bundle is 1.12 wt% U~, which is lower than the 1.20 wt% U~
required for a regular unmodified bundle.
FIGURE 4 shows the thermal neutron flux distribution with the two fuel bundles. The low flux level in the central pin, i.e., fuel 1, suggests that placing an absorber in the central pin will give the least U2~ enrichment penalty under nominal operating conditions.
FIGURE 5 shows the change in thermal neutron flux distribution within the fuel bundles due to coolant voiding. It is clear that the thermal flux increases in all the fuel rings upon voiding, However, the increase is smallest, i.e., less than 1%, in the outermost ring. The largest increase, about 12%, occurs in the central pin for both bundles. This suggests that the maximum void reactivity reduction effect will be achieved by putting the absorber in the central pin.

Ring Ring Ring Ring Bundle Average Depleted0.25 0.25 1.57 1.57 1.12 Regular 1.20 1.20 1.20 1.20 1.20 Table U~ (wt%) in depleted and regular fuel bundle 21,000 MWd/te Burnup While the invention has been described in cannection with a specific embodiment thereof and in a specific use, various modifications thereof will occur to those skilled in the art without departing from the spirit and scope of the invention as set forth in the appended claims.
The terms and expressions which have been employed in the specification are used as terms of description and not of limitations, and there is no intention in the use of such terms and expressions to exclude any equivalents of the features shown and described or portions thereof, but it is recognized that various modifications are possible within the scope 1S of the claims to the invention.

Claims

1. In a fuel bundle for use in a heavy water cooled pressure tube reactor, having a plurality of elongated fuel rods or elements disposed therein, and in which a positive reactivity component is produced by a change in neutron spectrum and flux across said bundle, upon a decrease in coolant density, the improvement comprising:
fissile material for producing an increase in neutron multiplication, disposed in a first region of said fuel bundle wherein said thermal neutron flux tends to decrease upon a decrease in coolant density;
fertile material, disposed in a second region of said fuel bundle wherein said thermal neutron flux tends to increase upon a decrease in coolant density;
neutron absorber material mixed with said fertile material to absorb thermal neutrons in said second region;
neutron scattering material, disposed in a third region of said fuel bundle between said first and second regions;
said mixture of said fertile material and said absorber in said second region redistributing said neutron flux across said bundle and thereby producing a negative reactivity component, upon said decrease in coolant density.

2. A fuel bundle as defined in claim 1, said fertile material being depleted uranium.

3. A fuel bundle as defined in claim 1, said first region being an outer region of said fuel bundle.

4. A fuel bundle as defined in claim 1, said second region being a central region of said fuel bundle.

5. A fuel bundle as defined in claim 1, wherein said neutron absorber material has a geometry similar to that of said fuel rods and said neutron absorber material is positioned to replace selected fuel rods to achieve said neutron flux redistribution across said bundle.

6. A fuel bundle as defined in claim 1, wherein said absorber material is comprised of materials selected from dysprosium, dysprosium oxide in a matrix of depleted Uranium dioxide or Cobalt.

7. A fuel bundle as defined in claim 1, said fissile material being selected from enriched uranium or plutonium.

8. A fuel bundle as defined in claim 1, said scattering material being heavy water.

9. A method for creating negative reactivity in a fuel bundle for use in a heavy water cooled pressure tube reactor, having a plurality of elongated fuel rods or elements disposed therein, and in which a positive reactivity component is produced by a change in neutron spectrum and flux across said bundle, upon a decrease in coolant density said method comprising the steps of:
placing fissile material for producing an increase in neutron multiplication, disposed in a first region of said fuel bundle wherein said thermal neutron flux tends to decrease upon a decrease in coolant density;
placing a mixture of fertile and neutron absorber material, in a second region of said fuel bundle wherein said thermal neutron flux tends to increase upon a decrease in coolant density, said absorber being selected to give rise to negative reactivity in response to said change in neutron spectrum and decrease coolant density; and placing neutron scattering material, in a third region of said fuel bundle between said first and second regions, said mixture of said fertile material and said absorber in said second region redistributing said neutron flux across said bundle and thereby producing a negative reactivity component, upon said decrease in coolant density.

10. A method as defined in claim 9, said fertile material being depleted uranium.

11. A method as defined in claim 9, said first region being an outer region of said fuel bundle.

12. A method as defined in claim 9, said second region being a central region of said fuel bundle.

13. A method as defined in claim 9, wherein said neutron absorber material has a geometry similar to that of said fuel rods and said neutron absorber material is positioned to replace selected fuel rods to achieve said neutron flux redistribution across said bundle.

14. A method as defined in claim 9, wherein said absorber material is comprised of materials selected from dysprosium, dysprosium oxide in a matrix of depleted Uranium dioxide or Cobalt.

15. A method as defined in claim 9, said fissile material being selected from enriched uranium or plutonium.

16. A method as defined in claim 9, said scattering material being heavy water.