SINGLE USE SAFETY SYRINGE WITH RETRACTABLE NEEDLE Technical Field This invention relates to a single-use safety syringe. In particular it relates to a retractable needle for use in a single-use safety syringe, whereby the user of the syringe can be protected from damage by the needle after use. Background The danger of harm and possible infection by HIV or hepatitis B virus to medical practitioners using needles in the normal course of their work is well documented. In addition, people who have the habit of administering drugs themselves, run a severe risk of contracting any of the specified viruses or effectively make contact with other viruses if the needle, once used, is re-used in a non-sterilized form . There have been many proposals aimed at reducing the number of so-called needle-bite injuries and various attempts have been made to provide a safe system for the removal of such needles once used, but such previous proposals have had deficiencies. OBJECT OF THE INVENTION It is therefore an object of the present invention to provide a single-use safety syringe employing a retractable surgical needle and to permanently store that surgical needle, once used, in a substantially secure manner, in order to to reduce the injuries by sting with needle as well as to avoid the re-use of the syringe avoiding by this the crossed infections of diseases. It is a particular object of the invention to improve some or all of the known disadvantages of the prior art syringes and to provide a simple safety syringe having a retractable needle which makes the syringe unable to be reused. Finally, the invention provides an alternate means for accommodating the needle of a syringe used to protect against accidental injury arising from unwanted contact with the exposed needle once the syringe has been used. SUMMARY OF THE INVENTION The invention in broad form provides a single-use safety syringe having a retractable needle, including the syringe a hollow barrel, a plunger that slidably fits in the barrel, a piston that fits over the part of the plunger and an actuator mechanism, which is located in the front part of the barrel and which is able to fit therein through the open end of the front part of the hollow barrel, the actuator mechanism having a rod to support the head of the barrel. the needle and means located on the end of the plunger to engage the actuator and have it retract with the needle at the end of the injection. When the actuator is located in the barrel, an inner projection is provided on the barrel and a corresponding projection on the actuator mechanism, for adjustment of the actuator from that open end of the barrel. The projections are preferably formed, for example having bevelled surfaces on one side and straight angled surfaces on the other, so that when the plunger is depressed (ie, after the actuator has been adjusted in this way), the protrusion The actuator mechanism is secured against the corresponding inner protrusion of the barrel, for example by virtue of the opposite straight angled surfaces coming into contact, so that in normal use the actuator will not be released. Preferably it can withstand a force of at least 98 N. In this form, the actuator can not be released from the barrel when the piston is depressed during an injection. On the other hand the projections only need to withstand a force of less than 98 N in order to prevent the actuator from being drawn into the barrel when the needle of the syringe is inserted into the patient. Such force is preferably 49 N. However, truly and in order that the actuator can be drawn into the barrel at the end of the injection, it is preferable that these opposing protruding surfaces be bevelled so that only a smaller force is required, ie, a force preferably between 49 N and 98 N. At the contact point between the actuator and the barrel is preferably provided at least one Piston seal type "0" sufficient so that there is no filtration between the actuator and the barrel during use. It will be appreciated that the actuator will be hollow so that the fluid contained in the barrel can pass through it and therefore through the needle it supports. The end of the actuator rod is preferably provided with at least one flexible finger member. The piston is preferably provided with a solid rod having a head shaped so that when the injection is completed, the head of the piston rod enters the hollow rod of the actuator and passes through the end thereof and is thereby clutched with the actuator, retaining the one or more fingers of the piston rod head beyond the actuator rod. When the plunger retracts, preferably exerting a force between 49 and 98 N of pulling force, the one or more fingers can not be broken and the actuator will be successfully attracted to the barrel. In order to avoid premature clutch of the piston rod head with the fingers of the actuator rod, a depth stop is preferably provided to prevent the piston from clutching with the actuator before retraction of the needle is required . Preferably such depth stop is provided in the form of a pin located in the hole located transverse to the axis of the plunger and located thereon in order to prevent the plunger from coming into contact with the actuator until the pin is depressed, avoiding this the unjustified early destruction of the syringe. Therefore, the present invention is a single-use safety syringe that includes the following components: a hollow barrel, a plunger that fits the barrel and a piston that fits the front of the plunger. There is an actuator that fits the front of the barrel. A standard 6: 100 luer taper is located on the actuator. This taper is designed to receive the head of a needle using the tightening fit. Alternatively, in order to provide a more positive interconnection between the actuator and the head of the needle, a screw thread including the so-called luer safety thread can be advantageously employed. Between the barrel and the outer surface of the actuator, at least one hermetic seal is adapted. At the end of the barrel at least one means is located to prevent the plunger from being fully depressed until the injection is completed and the syringe becomes unusable. In the anterior part of the plunger is located a retractable latch, preferably in the form of a head element for engaging with the anterior part of the internal hole of the actuator which is provided with flexible fingers, hooks or the like, so that when the plunger moves towards the actuator, the retractable hook in the front part will easily enter the clutch means of the actuator. When the plunger is removed, the actuator is attracted to the barrel, to perform the safe destruction of the same. The safety syringe according to the preferred embodiments of the invention has the following advantages when compared to those of the existing market: first, it is provided with corresponding protrusions on the barrel and the actuator to ensure that the actuator does not come loose and move backward, an "O" type piston is provided between the actuator and the barrel, to ensure that no liquid leaks and after injection, when the plunger is pulled out of the actuator can be easily drawn into the barrel, it can not be reused., achieving a truly safe syringe that is effectively destroyed; second, only a minimum force is needed for the piston rod to enter the flexible fingers of the actuator stem, thereby reducing the patient's pain that has been caused by injection on more recent occasions; third, the depth stop at the trailing end of the plunger prevents the syringe from being destroyed before it is used to give an injection, through a malfunction causing premature destruction of the syringe. Preferably, the plunger is provided with a breaking point. The abrupt rupture of the plunger also renders the syringe both ineffective for subsequent use and provides additional safety since there are no means by which the needle may be caused to re-emerge or extend from the syringe body. In addition, it is preferable to provide means by which the plunger may in fact lock in its position before abruptly breaking. This can be achieved by using a sleeve or collar inserted at the end of the barrel, for example either by tightening or screwing it into place. A profusion or projection may be provided on the plunger adjacent to the point of rupture and a passage for example similar to a thread may be provided in the sleeve or collar so that the plunger may be wound through the collar or sleeve and secured by suitable means, eg , protruding or retaining means so that when the plunger breaks at the point of rupture there is no means by which it can be made to re-enter the barrel since it is retained in the collar or sleeve. BRIEF DESCRIPTION OF THE DRAWINGS The invention can be better understood from the following non-limiting description of the preferred embodiments, in which: Figure 1 is an exploded view of a single-use safety syringe according to one embodiment of the invention; Figure 2 is a cross-sectional view of the assembled syringe of Figure 1, showing the syringe before being used with a cap that covers and protects the needle; Figure 3 is a cross-sectional view of the syringe of Figure 2, showing the retracted plunger, the barrel being filled with an injectable solution; Figure 4 is a cross-sectional view of the syringe of the previous figures at the end of the injection, the piston having the clutch engaged with the actuator; Figure 5 is a cross-sectional view of the syringe of the previous figures, the plunger being withdrawn so that the actuator and needle are safely withdrawn towards the barrel of the syringe; and Figure 6 is a cross-sectional view of the syringe of the previous figures, the plunger being secured in the sleeve at the end of the barrel and broken at the point of rupture destroying the syringe securely. Detailed Description of the Drawings Referring to Figure 1 there is shown an exploded view of a syringe, generally referred to 10, in accordance with a preferred embodiment of the invention. The syringe includes a barrel 11, plunger 12, piston 13, actuator 14, needle head 15 that supports a needle 20, "O" type piston 16 and a depth stop 17. A cover 29 is provided to protect the needle before to be used. An inner projection 21 is located in the main part of the barrel 11. A corresponding projection 22 is located on the actuator 14, for engaging the actuator 14 when it is inserted in the barrel 11 from the open end of that main part of the barrel 11. advantages of inserting the actuator 14 from that end of the barrel 11 are in the facility to assemble and decrease the damage in situations where the head 15 of the needle 20 is already fitted in the actuator 11; What would happen if the actuator were assembled when inserted from the other end (i.e., plunger) of the barrel 11 of the syringe. The flexible spring fingers 23 are located at the end of the hollow rod 18 of the actuator 14. The solid rod 19 is provided at the end of the plunger 13 with a securing head 24. An orifice 25 is located in the plunger 13 to accommodate the pin 7 which functions as a depth stop. The break point 26 is provided on the plunger 13. The plunger also has projections 28 which allow the plunger 13 to be secured by rotation in the turn securing sleeve 29 located at the end of the barrel 11. Referring generally to the Figures 2 to 6, a syringe 10 is shown having various parts illustrated as in Figure 1, the same reference numerals being used everywhere. The safety syringe 10 according to the invention has the following characteristics: At the junction between the actuator 14 and the barrel 11 a sealing member 16 is located in the form of an "O-ring" or the like. Also corresponding projections 22, 21 are located on the actuator 14 and on the inner surface of the barrel 11 respectively. At each edge of the projection 22 of the actuator a chamfer and a right angle are formed respectively, the chamfer and the right angle corresponding with a bevel and a right angle located on the projection 21 inside the barrel 11. In essence the orientation of the shapes respective of the two projections 21, 22 it is understood that in use, the normal pressure as exerted during an injection will not cause the actuator to move outwardly or effectively inwardly. Actually the use of the depth stop 17, before it is pressed, will ensure that this can not happen accidentally either. However, the use of the securing head 24 which engages the projections 23 is in fact sufficient to easily remove the actuator 14, once the securing head 24 of the piston 13 has engaged the actuator 14 when pressing the stop 17. it provides a breaking point 26 on the plunger 13, so that the plunger 13 can be easily broken along the breaking point 26. At the rear end of the plunger 13 is located a depth stop 17 that can be pressed, but which contact with the end of the barrel 11, until such time as it is pressed in order to prevent the plunger 13 from being fully depressed and consequently clutch with the actuator 14. In Figure 2, the needle is protected by a cover 27 before be used. The plunger 13 is prevented from clipping with the actuator 14 by virtue of the depth stop 17 which is in the outer position. When used, the lid 27 is removed to reveal the needle 20 and the needle 20 is inserted into the fluid to be used for injection (such as in a container - not shown). The plunger 13 is then removed as in Figure 3 to take the fluid in the barrel 11. The injection is then carried out after the depth stop 17 has been pressed so that the plunger 13 is now clutched with the actuator. 14 as in Figure 4. At the end of the injection, the securing head 24 will enter the hollow rod 18 of the actuator 14 and further into the flexible fingers 23 so that it is retained therein. By removing the plunger 13 once again, the actuator
14 and the needle head / needle assembly 15, 20 is made to be withdrawn towards the barrel 11 of the syringe 10 as shown in Figure 5. As shown in Figure 6, the projections 28 located on the plunger 13 are twisted through the sleeve
29 assurance by adapted turn at the end of the barrel. The breaking of the plunger 13 at the breaking point
26 makes the syringe 10 completely safe and useless. It will be appreciated by those skilled in the art that many modifications and variations may be made to the embodiments described herein without departing from the spirit or scope of the invention. Throughout the specification the word
"comprises" and its derivatives are proposed to have an inclusive rather than exclusive meaning unless the context otherwise requires.