Radiant energy – Radiation controlling means – Shields
Reexamination Certificate
2000-02-25
2004-11-02
Lee, John R. (Department: 2881)
Radiant energy
Radiation controlling means
Shields
C250S517100, C250S526000
Reexamination Certificate
active
06812476
ABSTRACT:
TECHNICAL FIELD
The present invention relates to an electronic system operating under irradiation, particularly X or gamma radiation, a process for designing such a system intended to make said system function under irradiation whilst incorporating “vulnerable” components, i.e. intrinsically unfit to operate under said irradiation, and the application of said process to the control of a mobile robot.
Although the legal unit of measurement for radiation doses integrated by components is the Gray (Gy), the experts and most reference documents express this magnitude in the old unit, i.e. the Rad. Hereinafter use will consequently be made of said second unit. It is pointed out that 100 Rad=1 Gy.
The term “vulnerable” circuits is understood to mean electronic circuits only able to understand one or a few hundred kRad, typically in the form of gamma or neutron radiation, such as are encountered in nuclear engineering. In general, such circuits have a very large scale integration (VLSI) and are based on CMOS technology, although these features are not limitative.
These “vulnerable” circuits are the only ones able to implement very complex functions. Examples are microcontrollers, digital signal processors, application specific integrated circuits (ASICs) or bulk memories. They are particularly appropriate for the implementation of control systems carried by high technology remote manipulators or on mobile robots.
Thus, the invention is mainly directed at the design of control systems for a nuclear environment, which are at present the highest performance electronic systems working in such an environment. Consequently they are used as the example for the description of a preferred embodiment. However, it does not pass beyond the scope of the invention to apply same to any other electronic system, provided that its complexity makes it advantageous to use components which are “vulnerable” to ambient irradiation.
The term “control system” is not considered here in the very restricted sense often used in nuclear engineering, notably due to the very rudimentary performance characteristics authorized in the prior art and whereof a few examples will be given hereinafter. Subsequently the term control system is used in a broader sense in that its function is to collect informations on the system to be controlled, process them if necessary (e.g. by digital filtering
on-linearity correction), apply thereto one or more digital control laws which can include autonomous operating modes able to take decisions, manage the controls of power amplifiers associated with actuators, ensure safety functions and, in the case of a partial failure, manage degraded operating modes. Such a control may also be able to communicate with an information transmission device, as a function of the possibilities of various working configurations (multiplexing, microwave transmission or any other means).
PRIOR ART
In the prior art, electronic systems operating under such an irradiation and incorporating such vulnerable components are essentially control systems for mobile robots or teleoperated equipment or machines. They can be subdivided into two categories, as a function of the radiation dose which they are able to withstand.
A first category includes control systems which can correspond to the above definition, but which in practice are unable to withstand more than a few kRad and in exceptional cases a few dozen kRad.
Reference can e.g. be made to the Andros mobile intervention robot designed by Remotech, USA. The electronics carried consist of a standard controller constituted by a microcontroller card and variators available in commerce. The controls are transmitted by an umbilical cord. The control system is conventional, close to industrial-type controls, but its radiation resistance or hardness does not exceed 1 to 10 kRad.
A second category covers very simplified control systems unable to comply with the above definition, but able to withstand several dozen kRad or even several hundred kRad when containing virtually no electronics and when their control is displaced to the end of a wire-to-wire link.
An example is constituted by the Oscar mobile intervention robot for which all the control signals are transmitted wire-to-wire by an umbilical cord having a large diameter compared with the robot dimensions and whose length is necessarily limited.
Another example is constituted by the assisted RD 500 remote manipulator used on the La Hague site in the 1990's. All the control signals are transmitted wire-to-wire by an umbilical cord and the actual control is displaced into a non-irradiated area.
More generally, this second category of highly simplified control systems is solely directed at:
acquiring one or several measurements,
optionally processing them in rudimentary manner by simple analog filtering (of the first order) or in the best possible case by digitization under 8 bits with long conversion times (exceeding 10 &mgr;s),
transmitting said measurement or measurements in accordance with a fixed sequential protocol, when not directly wire-to-wire, which then raises the problem of an umbilical cord which is prejudicial as a result of its diameter, weight or its very existence (it makes it impossible to pass through an air lock),
supply directives to power amplifiers which do not really belong to the control system.
The systems of said second category cannot implement high performance, evolved functions and cannot be autonomous and reliable, reliability assuming the existence of degraded modes or redundancies, as well as the capacity to operate autonomously.
At present there is no solution making it possible for such a control system to operate under irradiation. The proof is supplied by document [3], which mentions the project concerning a mobile robot intended to intervene on the Chernobyl site. The specification required a resistance or hardness to 1 MRad and the solution adopted corresponds to said second above category in its most rudimentary form, i.e. complete absence of carried electronics, all the electronics being effected wire-to-wire by an umbilical cord.
The response of the expert is described in document [4], chapter 4, devoted to the design strategy. It only offers four solutions:
A—the shielding of components or equipment,
B—the choice of the location of the equipment,
C—the use of equipment able to withstand radiation and already commercially available,
D—the development of equipment able to withstand radiation.
In practice, it is usually limited to shielding or an offset location of the electronics. Each of the solutions will be examined below.
Solution A, regarding shielding:
Certain components have a box or case designed to resist ionizing radiation. However, it is a lightweight shield against single event upsets (SEUs) encountered by satellites, which are accidental collisions with extremely high energy particles, which can give rise to the local destruction of a microcomponent. They only have a very limited effectiveness against gamma radiation even when reinforced by a supplementary metal screen, because the cumulative gamma radiation dose which can be withstood by a satellite is relatively low (approximately 100 kRad for its entire life) and does not constitute an aim for the designer. The expert knows that despite the common reference made to “ionizing radiation”, it is in fact a different problem to those encountered in the nuclear industry.
In the nuclear industry, the shield against ionizing radiation is constituted by a thick, heavy metal covering (e.g. of lead or Dénal for gamma radiation), because the attenuation provided by said shield is dependent on the atomic mass of the material. Attenuation curves show that the thickness must exceed several centimeters to ensure that the shield has any significance and this thickness increases very rapidly on increasing the admissible radiation dose. Thus, for lead shielding the controls of a gantry operating on a French industrial site (managed by an industrial controller), it was necessary to use an appro
Commissariat a l'Energie Atomique
Pearne & Gordon LLP
Quash Anthony
LandOfFree
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