Note: Descriptions are shown in the official language in which they were submitted.
CA 02097412 2004-05-28
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.
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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
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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,
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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.
~'~~v~~
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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.
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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
CA 02097412 2004-05-28
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
~~~~'~~~~
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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
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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.
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Ring Ring Ring Ring Bundle
1 2 3 4
Average
Depleted0.25 0.25 1.57 1.57 1.12
Regular 1.20 1.20 1.20 1.20 1.20
Table
1
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.
