FR2476894A1 - NUCLEAR REACTOR - Google Patents

Abstract

THE NUCLEAR REACTOR ACCORDING TO THE PRESENT INVENTION INCLUDES A CORE INCLUDING A BED OF PRISMATIC ELEMENTS 31 OF FERTILE MATERIAL IN WHICH HOLES ARE FORMED 35. LONG CYLINDERS 52 ARE DEDUCTED IN PRISMATIC ELEMENTS AND BALLS 43 OF FISSILE MATERIAL THROUGHOUT THE TROUSES. PASSAGES 45 FROM TOP TO BOTTOM. EACH BOULET INCLUDES A MASS OF GRAPHITE WITHIN WHICH ARE FLOODED BALLS OF FISSILE MATERIAL. THE VOLUME OF THE BALLS IS LOW IN RELATION TO THE VOLUME OF THE OPENINGS THROUGH THESE BALLS AND THE VOLUME OF THE BALLS IS LOW IN RELATION TO THE VOLUME OF THE MASS IN WHICH THEY ARE FLOODED. CONTROL BARS 45 EXTEND IN TUBES 39 MOUNTED IN HOLES SO AS TO SEPARATE THE CONTROL BARS FROM THE BALLS SO THAT THE CONTROL BARS CAN ENTER THE CORE FREELY. A HEAT CONTAINER, FOR EXAMPLE HELIUM, PASSES THROUGH HOLES BY CIRCULATING THROUGH THE TUBES CONTAINING THE CONTROL BARS AND THROUGH THE FERTILE TUBULAR ELEMENTS 52. IN THE PRESENT INVENTION FERTILE MATERIAL IS SEPARATED FROM FISSILE MATERIAL CONTRARY TO THE PROVISIONS. IN CONVENTIONAL REACTORS OF THIS TYPE. THE HANDLING AND PROCESSING OF SPENT FUEL IS EASY.

Description

Nuclear reactor

  The present invention relates to nuclear reactors.

  res and it relates in particular to b reactors in the

which the coolant or refrigerant of the heart is brought to a high temperature. These reactors will be called in this presentation s high temperature reactors * The heat transfer fluid in a pressurized water reactor is at a temperature between 2600C and 3160C. In high temperature reactors, the heat transfer fluid is found at temperatures between 9820C and 12040Ce

  Although the present invention is applicable only-

  ment to high temperature gas-cooled reactors, it can also be applied to reactors b pressurized ordinary water (PWR), b boiling ordinary water reactors

  (BWR) and b of heavy water reactors. To the extent that the pre-

  sente invention can thus be implemented, any applica-

  tion mentioned above is within the scope of this invention.

  Apart from the advantage that high reactors

  temperature due to their better thermal efficiency

  dynamic, such reactors are of particular interest

in a use to obtain energy under tempera-

  high tures for chemical processes, such as the large amounts of energy required for the synthetic production of methane, menthanol and gasoline as well as the production of large quantities of hydrogen for synthesis

  fuels from hydrogen and carbon. These reactions

  teurs typically include a fissile material such as uranium enriched in U-233 or U-235 or plutonium

  enriched with PU-239 or PU-241 or mixtures of fissile isotopes

uranium and plutonium with fertile matter. Fertile material can be Th-232 or U-238 * When the reactor

works, a certain amount of fertile matter is trans-

  formed of fissile material, Depending on the design of the reactor, a

more or less quantity of "burnt" fissile material

  pletely is obtained from the fertile matter.

  - 2 - The natural uranium available in the nuclear industry exists in limited quantities * It is therefore desirable

  that fuel can be obtained by reprocessing the

bustible exhausted and the reactor has as high a yield as is practically possible (i.e. the energy produced per unit of quantity of fissile material destroyed is maximum) o It is desirable that the fuel of the high temperature reactors discussed here, when depleted, readily lends itself to a

reprocessing.

  It is desirable to make a reactor, particularly

  particularly suitable for the production of energy under temperature-

high res for processes that require very large

quantities of energy, this reactor having to operate, in pre-

feeling good performance, with suitable fuels

  of themselves easily to a reprocessing.

  Typical reactors that provide fluids,

  usually gases, under high temperatures are the reaction

General Atomic HTGR (high temperature gas reactor)

  and the Vestinghouse VHTR reactor (very high temperature reactor

erasure). The HTGR reactor is described in G-A13158, General Atomies - "High Temperature Nuelear Heat Study"

  (December 12, 1974). In this reactor, the fuel is pre-

  feels in the form of prismatic elements inside the

  which are mixed fissile particles and fertile particles. The VUTR reactor is described in W NL 2445-1, "The Very High Temperature Reactor For Process Heat", which is a technical and economic report. In this reactor, the fissile particles and the fertile particles

  are in separate tubes. The reprocessing of the fuel-

  exhausted target of these reactors presents problems of re-

treatment difficult to solve. In addition, these reactors

gent high initial overreactivity. This reactivity must be absorbed by poisons or chemicals of moderation and this absorption leads to a serious decrease.

performance.

In U.S. Patent No. 3,464,883, a reactor has been described in which beads or beads of fissile material and fertile material are passed slowly through the heating zone of the core. It is said in this patent that the balls are 0.2 h 2 mm and preferably 0.2 to 0.7 mm in diameter. The balls pass through conduits located in the heart. It is indicated that the smallest section of the duct is equal to four times the diameter of the balls, that is to say

that this section is between 0.8 and 8 mm.

US Pat. No. 3,470,843 describes a reactor in which the fuel is a paste of a fissile material in an alkali metal or bismuth or even gallium. The pLte is obtained from a slurry which pumps pump out by the tube openings in which the dough is contained. It is difficult and inconvenient to handle the fuel in the reactors of the Nos. 3,464,883 and 3,470,843, both while the fuel is providing energy and after it is depleted In addition, reprocessing of the fuel

  exhausted raises serious chemical and physical problems.

  There is also a so-called AVR ball-bed reactor in which bodies, usually in the form of large spheres or balls, move at low speed through the heart, producing heat as they're moving. Reactors of this type are described in "High Temperature Reactor Por Process Heat Application", Nuelear Engineering and Design, NO 34, 1975 (Special Issue); in an article entitled "HHT Demonstration

  Plant * by E. Armat et al. $ in the report of the Confe-

  ENC-79 of the European Nuclear Society; in US patents. Nos, 3 336 203 and 3 971 444. The spheres are balls

  graphite containing balls of fissile and fertile material.

  The balls have a diameter of about 6 centimeters, that is h-

  say that their dimensions are a little larger than those

  ping pong balls. As described in the U.S. patent pre-

cited NO 3 971 444, each ball can include an outer -4 envelope in graphite around a mass of graphite in

  which are embedded in the fissile particles or beads and fer-

  tiles. Typically, the diameter of the beads is 0.3 to 0.4 mm. The mass of the fissile beads (of U-235 enriched b 93%) is approximately 10% of the mass of the fertile beads

(U-238). It has been reported that reactors of this type are

  make the heat transfer gas up to about 9820C and they have been

  used to produce hydrogen gas in large quantities.

  The spent fuel pellets from the reactor

  the bed of balls can be easily handled during

their use in the heart as well as during retirement-

is lying. However, the presence of transformed fissile material

  in the spent balls introduces complications in the o

  separation period. In addition, the mass of the balls of

bustible is important. In addition, because the balls contain fertile matter, the effectiveness of the bars

order is not maximum.

  An object of the present invention is to remedy the drawbacks described above that present the nuclear reactors of the prior art and to make it possible to obtain a high temperature reactor which requires minimal initial reactivity and whose fuel elements can be easily handled and constitute a small part of the mass of the reactor, these elements, when they are exhausted, can be easily reprocessed "In this reactor, the bars

  have high efficiency.

  According to a preferred embodiment of the present

invention, a nuclear reactor is obtained whose core

  takes a bed of fertile material with holes formed along this bed. The fissile elements are moved through the holes at a low speed. When the fissile material

  found in the holes, these holes for transporting the ma-

  fissile are called "spikesn in the English technique

  Saxon. Each fissile element is in the form of a body or matrix of a moderator made of a material absorbing slow neutrons, such as for example graphite, in which balls or pearls of fissile material enriched in U-235 or U-233 are drowned. The balls are distributed uniformly throughout the body. Typically, the mass of fissile material in each body is about 1 or

  2 grams and the enrichment of the beads is around 8 to 93%.

The bodies are typically in the form of balls having a diameter of about 60 mm and can

  be handled easily. The volume of each ball is low.

ble relative to the volume of the body or ball. The relationship between the mass of the above aggregate of beads inside each body and the volume of that body is such that each body is subcritical from a nuclear point of view. The bed serves as

  matrix inside which are embedded tubes of ma-

  fertile. The size of each body is appreciably smaller than the dimension of cross section of the holes provided

  in the reactor and through which said bodies pass.

  These bodies are grouped tightly in the openings.

The mass of the bead aggregate in the bodies in the reactor is calculated relative to the absorption of neutrons and the leakage of neutrons in such a way that the reactor is nuclear critical. Each hole has an inlet through which the bodies are introduced and an outlet through which the bodies exit from the holes and from there leave the reactor. The outlet opening for each hole has a door for controlling the flow of bodies out of the hole. The bodies move through the reactor in such a way that their fissile material is exhausted at

  a rate set in a single pass through the reactor.

  The control rods extend into tubes located in the openings of the bed. Each bar is separate

combustible bodies through the tube so that it can fall out

  place freely and is not obstructed by combustible bodies.

Reprocessing of spent fuel is easy.

beds compared to the ball bed reactor of the prior art because the mass to be reprocessed in the reactor according to the present invention is significantly lower than in

the case of the ball-bed reactor. The material in this

denying includes not only spent fissile fuel but also fertile and processed matter. This

  is also valid for the reactor of patent 3,464,883.

  For other types of reactors, reprocessing requires

removal of the sheaths with their contents. The nutrients

  those of the reactor bed according to the present invention only need to be reprocessed at intervals of 5 to 10 years. As the

  ball-bed reactor, the reactor according to the present invention

  tion does not require any significant over-reactivity at the start of the function-

  because the fuel is continuously renewed.

Another advantageous aspect of the present invention resides in the fact that the control rods are surrounded by the fissile balls; this increases the effectiveness of these

bars in the reactivity setting.

  The present invention will be described in more detail below with reference to the accompanying drawings, in which s

  - Figure 1 is a perspective view of a reac-

nuclear tor according to a preferred embodiment of the present invention, part of the wall having been torn off to show the internal parts; - Figures 2k and 2B together form a view in

  perspective showing a stack of elements that form a com-

laying of the reactor shown in FIG. 1 as well as the associated parts of the reactor; - Figure 3 is a perspective view of a typical interior member of a stack of the bed; - Figure 4 is a perspective view of an upper element of a stack of the bed; - Figure 5 is a perspective view of a lower element of a pile of the bed g - Figures 6A, 6B, 6C together form a partial view in longitudinal section showing the mutual arrangement of the fissile and fertile elements; - Figure 7 is a sectional street showing a fissile fuel element used in the reactor of Figure 1; - Figure 8 is a sectional view between a fissile fuel ball used in the reactor of Figure

- Figure 9 is a side elevation view

  partly in section showing a control bar used in the reactor of FIG. 1; and

  - Figure 10 is a side elevational view

trant a fertile element used in the reactor of figure l. The apparatus shown in FIG. 1 is a nuclear reaction apparatus. This apparatus comprises a box 21

in reinforced concrete mounted on a footing 23 in reinforced concrete. A lîint6-

There is a nuclear reactor at the bottom of the casing, comprising a pressure enclosure or vessel 25 containing the core 27 as well as the other internal equipment of the nuclear reaction apparatus 20 The core comprises a bed formed by stacks 31 of prisms 33 tightly tightened together. against each other

  typically, each prism 33 is hexagonal.

tion in the shape of a regular hexagon0 Except on the outside of the bed, the sides of the prisms of each stack 31 bear against the sides of six adjacent stacks 31 and have the same dimensions as these dimensions The prisms 33 are therefore mutually blocked inside the bed, the upper and lower rows of prisms 33a (figures 1, 4) and 33b (figures 1 "5) and the prisms 33e (figure 1) of the peripheral cells serve as neutron reflectors and therefore have a construction different from that of the prisms 33d to 33f (figure 2A) which are inside the structure of the heart0 Each prism 33d (figures 1, 2, 3 and 6) - 8is composed of graphite. Its height is approximately 86 cm and the distance between two constituent edges or vertices is approximately 107 cm. The prism has a plurality of holes 35 of large

  diameter and a plurality of holes 37 of small diameter extend

dant parallel to its axis between its bases. It also includes a central hole or axial hole 38. The large diameter holes 35 are dimensioned so as to receive the combustible bodies 43 which, typically, have a diameter of approximately 60 mm. Typically, these holes 35 have a diameter of about 18 or 20 cm. The holes 37 have a diameter slightly larger than 2.5 cm. The hole 38 has a diameter of about 15 cm. A tube 39 is suspended from opposite radial supports 41 coaxially with each hole 35 of the prisms which are mounted in the upper part of the core 27. Typically, the tubes 39 must have an internal diameter of approximately 2 cm as well as a suitable thickness. The supports 41 must be

dimensioned so as not to hinder the passage of bodies 43.

  In a preferred embodiment of the present invention, no tube 39 is provided in the prisms 33 mounted in the lower region of the core unless the control rods enter this region. The tubes 39 suspended in the extreme lower prisms 33f (FIG. 6C) up to which

  enters the control bar 45 are each provided with an element

  closure 48. The closure element 48 has openings 50. It is not essential that the control rods 45 penetrate to the prisms 33f just above the prisms 33b. In fact, since reactor 20 requires a minimum of initial reactivity, it suffices that the control rods

  penetrate only a few prisms from the top.

The tubes 52 are arranged in the openings 37 of the interior prisms 33d to 33f. These tubes 52 (FIG. 10) are

  compounds of graphite uniformly impregnated with fertile matter.

The tubes 52 typically have an outside diameter

  25.4 mm laughter and an internal diameter of 12.9 mm. They are held in place in the holes 37 by washers 54 - 9 -

  (Figures 2B, 6) and extend from the bottom of each hole.

  The prisms 33a in the upper row (Figures 1, 29 4 and 6) are similar to the prisms 33d except that there are

  no fertile tubes 52 in the holes 37th These prisms 33.

serve as neutron reflectors. As in the case of the prism 33d, the larger holes 35 are dimensioned so as to easily accommodate the bodies 43 in which the fissile balls are embedded. The radial supports 41 are dimensioned so as not to obstruct the passage of the bodies 43. The smaller holes 37 convey large quantities of coolant through the prisms. The central hole 38 is used to receive handling devices for handling each prism at the moment when it is removed from the structure or replaced by this structure. The external walls of the tubes 39 serve to pocket the bodies 43 from entering the tubes. 39 and obstruct the passage of the bars

45.

  It is not essential that each lower prism 33b (FIGS. 1, 2, 5 and 6) has the holes 37 but it is mandatory that it has the holes 35. In the practical implementation of the present invention, the prism lower 33b does not include the central tube 39 in the holes 35. In addition, each hole 35 of the lower prism 33b includes a door 51 used to control the outlet of the bodies 43 of the reactor 20f * The hole 35 of the lower prism 33b has an element closure 53 which is inclined inward and which has an axial hole having a diambtre such that the bodies 43 can come out one by one (or at a higher rate if desired) by gravity. The door 51 comprises an element 55 whose sides are

inclined to correspond to the internal inclination of the element

closure 53. The element 55 can be adjusted up or down to increase or decrease the dimensions of the passage 57 between its sides and the closure element 53 so that a larger or smaller number of body 43 per unit of time passes through the opening of the base,

- 10 -

The peripheral prisms 330 (Figure 1) are similar

  res prisms 33d except that they only have holes 37.

  These prisms 33c have neither the holes 35 and their tubes 39

  nor the tubes 52 of fertile material in the holes 37. These taken

  my main ones serve as reflectors. In the construction of the heart, the prisms 33a, b. this d, e and f are calibrated mutually so that the holes 35 and

37 extend from one end of the stack to the other and the elements

fertile elements 52 extend over the same length. The tubes 39 also have the same length. The holes 35 and 37 are, of

  typically spread evenly over all of the

my. Typically, the number of holes 37 can be

  about 400 and that of the holes 35 about 20. Instead of

  separate fertile elements 52, the fertile material can be distributed throughout the matrix 31 although the use of

fertile tubes could facilitate reprocessing.

  Above the core 27, inside the tank

  pressure 25 there is a pressure chamber 61 for the heat

  carrier. Typically, the coolant is of

lium but other gases such as carbon dioxide can also

serve as a coolant.

  Outside the tank 25 but inside the box

sound 21, the nuclear reaction apparatus 20 includes a plurality of

  a set of heat exchangers 63 and 65. A plurality of exchangers

63 and a plurality of heat exchangers 65 may

be distributed along the outer periphery of the

  tank 25. Each heat exchanger 63 and 65 has fittings

primary tubes (not shown) which convey the coolant

  through a tightly closed circuit. The circulating coolant

  culvert in the primary ducts heats. the circulating coolant

lant in the secondary conduits (not shown). The calo-

very hot carrier coming from the secondary conduits is trans-

  to the processing apparatus to which the reactor apparatus 20 supplies energy via the outlet pipes 67. The cold secondary coolant is returned to the conduits

- 11 -

  secondary by inlet pipes 69.

A turbine 71 and a circulator 73 are associated k

each heat exchanger 63. Each turbine 71 is connected

  dedicated to the conduits (not shown) of the associated exchanger 63 and is entranéapar the primary coolant which flows Z through these conduitso The turbine 71 drives the associated circulaetour 73. In a variant, each or a few = one of the turbines 71, can or can be replaced by a motor

electric powered by the electricity generated in the appliance

  reil 20 of nuclear reaction The primary coolant is

  transferred so as to drive each turbine 71, as

presented by arrows 75 (figure 1) p and enters the

  circulator 73. The primary coolant is sent by Io circ-

culprit in the pressure chamber 61, as represented by

arrows 77. From the pressure chamber, the cal-

  primary carrier flows through the holes 35 and 37 and the tubes 39, as shown by the arrows 79 (Figure 4). The primary coolant is heated by the nuclear reactor

  in the holes 35 of the prisms 33 and is returned via

diary of the primary conduits (not shown) to the exchangers

heat.

A turbine 81, an electric generator 85 and a

compressor 83 are associated with each heat exchanger 65.

The primary coolant from the pressure chamber 61 -25 flows through the holes 35 and 37 and the tubes

39 through the heart 27 and is heated. The primary coolant

  very hot mayor is routed through a central duct 82 of the heat exchanger 65, as shown by the arrows

  84, to the turbine 81 and drives this turbine. The tur-

bine 81 drives generator 85 and compressor 83 by

through gears and shafts (not shown).

  Generator 85 supplies electrical power to the processing device powered by the device and also supports

internal parts of the device 20, as shown by the arrows 86.

  The primary coolant from turbine 81 is routed

- 12 -

  down through intermediate conduits (not shown

sensed) of the heat exchanger 65. These intermediate pipes

  diaries (not shown) are in heat exchange relationship with the secondary conduits (not shown) of the heat exchanger 65. The resulting primary coolant, cooled and expanded, flows upward through the annular space

(not shown) between the intermediate conduits and the envelope

outer louver of the exchanger 65 to the compressor 83. The compressor 83 compresses the primary coolant and returns it to the pressure chamber 61 for another circulation, as

represented by arrows 87.

The nuclear reaction apparatus 20 includes a plurality of

reality of control bars 45 which penetrate into the tubes 39. Means 41 for driving the bars 45 extend upwards from the box 21 (FIG. 1) and are driven by means of drive bars 93. The bars 45 of each stack 31 are mutually coupled (FIG. 2) and extend from coupling elements 95 fixed to an associated drive bar 93. In the event of an emergency stop, the control bars 45 enter the tubes 39 under the effect

gravity. The control bars 45 are brought into posi-

by an external drive device (not shown

  you). This drive device attacks the drive bars.

93 and is activated when a stop occurs

emergency.

  A typical control bar 45 is shown in FIG. 9. This bar 45 is constituted by a long hollow cylindrical element over a whole part (the lower part) of its length. The bar has, at its lower end, a tapered end 101 and, at its upper end, a nozzle

103 adapted to be coupled to the coupling device 95.

  The lower end of the hollow space 105 contains steps

pins of a neutron absorbing material 107, for example B4C, these pads being held in place by a spring

109.

- 13 -

A transfer castle 111 intended to transfer the

prisms 33 extends upwards from the box 21. The cht-

transfer plate 111 communicates with a handling machine

  that works to remove and / or replace the prisms 33.

The nuclear reaction apparatus 20 includes a

  duit 121 (FIG. 1) for entering fuel elements, this

  duit entering the pressure vessel 25 at the top of it

  this. This inlet duct 121 is connected to the pressure chamber.

  sion or plenum 123 (figure 2) for fuel elements which

supplies the combustible bodies 43 to the holes 35 by the intern

  diary of the guides 124. The balls arrive from a hopper (not shown) through tubes 121 (Figure 1) and 123 (Figure 4). The tubes 121 and 123 must be slightly inclined downwards so that the fissile balls 43 advance by

  gravity. The plenum 123 and the guides 124 are arranged in a ma-

  niere L be disengaged from the control rods 45. An outlet 125 of fuel elements is connected so as to receive

  the bodies 43 coming from a plenum 127 with which the openings

tures 129 of the prisms 33b communicate. The combustible bodies 43 circulate through the holes 35, the openings 129 and the outlet 125 at a regulated speed. The mass of the fuel in the reactor fuel bodies is calculated in relation to the neutron absorption and the neutron leakage from the reactor

in such a way that the reactor is critical.

Each body 43 (FIG. 7) characteristically comprises

ristique, a sphere of pyrolytic graphite in which are embedded beads or pearls 141 of nuclear fuel fis-

if the. Each ball (figure 8) includes a fissile core 143

  coated with a layer 145 of graphite. The coating 145 is in

closed in an expandable layer 147 of pyrolytic graphite.

On the outside is a layer 149 of pyrolytic graphite.

tick. The diameter of these beads or pearls 141 is included

between 0.3 and 0.4 mm.

  It is understood that the foregoing description has not been

- 14 -

given for purely illustrative and non-limiting purposes and that variants and modifications may be made thereto within the framework of the present invention;

- 15 -

Claims (9)

  1. Nuclear reactor comprising a core (27) comprising   both a plurality of conduits (39) for control rods and control rods (45) which can move axially in said conduits, characterized in that the core comprises a bed (31) containing a fertile nuclear material and   as a plurality of holes (35) which are closed therein said holes extending along the length of said beds, each hole   both an inlet (38) and an outlet (51) 1 the reactor being supplied   fueled by a plurality of separate fissile fuel bodies (43) arranged inside each of said holes, each aforesaid body taking a mass of neutron moderating material having a small section of neutron absorption, in which beads are embedded (141) of fissile material, the volume of each of said balls being small compared to the volume of the body in which this ball is embedded, the ratio between the mass of aggregate of balls found in each body and the volume of said body being such that each said body is soes-critical of the nuclear point of view, said combustible bodies being introduced into each hole by the entry of celuiai for their displacement progressive through said hole and their ultimate evacuation   higher of this hole by the exit of this one the mass of balls of the body aggregate being in said holes of the bed   aforementioned being calculated in such a way with respect to the absorb- tion of neutrons and the neutron leakage from said reactor that this reactor is critical0 2, Reactor according to claim 1, characterized in that the holes are vertical, the inlet being at the top of each hole and the outlet being h the base of each hole, so that the bodies circulate through the reactor under the effect of gravity   3. Reactor according to claims 1 or 2, charac- terrified by the fact that the outlet is formed in a blir the rate of discharge of the bodies out of each hole to a predetermined value, 2476894- _ 16 - 4. Reactor according to any one of the claims 1, 2 or 3, characterized in that a gas is purchased mined through the holes between the inlet and the outlet, said gas absorbing the heat given off by the body when the reaction nuclear is critical. 5. Reactor according to any one of the claims   1, 2, 3 or 49 characterized in that a tube (39) is supported inside each hole (35) at a distance from the wall of said hole and that the control bar (45) is placed through said tube. 6. Reactor according to claim 5, characterized by the fact that a coolant circulates through the tube. 7. Reactor according to any one of the claims   above, characterized in that the bed comprises a matrix (33) of neutron moderating material having a small cross section of neutron absorption in the what. fertile material tubes (52) are embedded, the fertile tubes generally extending along said bed parallel to the holes. 8. Reactor according to claim 7, characterized in that the matrix comprises a stack of prisms formed from the neutron moderating material and that the fertile tubes extend through said stack.   9. Reactor according to any one of the claims   above, characterized in that the fissile material is uranium highly enriched in one or more categories consisting of U-233 or U-235.   10. Reactor according to any one of the claims. 1 to 8, characterized in that the fissile material is Pu-239.