Patent Application
20080308754
The invention relates to a multi-layered radiation protection wall for shielding against the gamma and/or particle radiation, particularly for shielding against radiation of a reaction site on a high energy accelerator facility, and a radiation protection chamber with the radiation protection wall.
High energy accelerators for particle beams are used more and more throughout the world. In doing so, intensity and energy are increased permanently. For instance, currently proton accelerators with energies up to the range of tera-electron volt (TeV) are planned and proton accelerators with energies up to some giga-electron volt (GeV) and intensities up to 1016 protons/sec are planned, e.g. for spallation sources.
The latter accelerators are not only planned as neutron sources for fundamental research, but are also discussed as nuclear facilities for energy production, by which subcritical systems can be brought into a critical state by an additional neutron flow. Furthermore, those facilities can be used for the so-called incineration, during which long-lived radioactive substances are changed into short-lived ones.
When running high energy accelerators, one problem is the production of high-energy secondary radiation in the target areas (Target of the particle beam, in which it is deposited) or in case of beam losses during the transport on the path of the beam guidings of the high energy or primary beam to the target.
While the charged particles generated in nuclear reactions are often stopped in the structure of the accelerator, the generated neutron and gamma radiation has a high capability for permeating, even through shieldings with a thickness of some meters. Furthermore, at very high energies inter alia pions are generated, which decay into myons. Latter have also a very high coverage and have therefore to be stopped in special beam annihilators.
In case of heavy ion accelerators the situation is yet more difficult, because already at lower intensities comparable production rates for secondary radiation arise, similar to proton accelerators. So far, the production of radiation at such accelerator facilities caused the installation of mostly very massive shieldings at the places of beam losses. Often iron or concrete was used as shielding material like in nuclear technology. Such concrete shieldings consist of hard-casted walls and ceilings, but also single shielding modules assembled from single parts can form an overall shielding.
For special shielding requirements heavy varieties of concrete with appropriate additives like magnetite, limonite or barite, concrete with densities up to 3.6 g/cm3 can be used besides normal concrete with a density in the range of 2.3 g/cm3 (see also DIN25413). But in practice, normal concrete is mostly used in the sense of optimizing cost and attained shielding result.
Producing the radiation depends on the kind of radiation, the energy, the intensity and the loss rate. Furthermore, the shielding thickness depends on limit values to be met according to the national legislations. The limit values are defined as annual dose limit values or referred to the dose rate in μSv/h.
Recently, using shielding arrangements with bulk material was proposed. For instance, gypsum or iron ore were proposed as bulk material. Though being naturally founded material was heaped up around these facilities as soil up to now, but not incorporated directly into the shielding. On the other hand, the problem of activation arises, when natural material is used in the shielding arrangement, because this material is relatively close to the sources.
From the patent applications DE 103 27 466 (Forster) and DE 103 12 271 A1 (Brüchle et al.) gypsum is known as alternate material for parts of a radiation protection structure and the shieldings of high energy accelerators respectively. This material proved to be well suited shielding material, too.
Using such shieldings, which have bulk material as shielding substance, implicates some enhancements, but the previous developments and proposals to construct shieldings for accelerator facilities have mostly been planned in particular consideration of the shielding properties.
A further effect addressed by the present invention, being important and due to the inventor's findings not being sufficiently considered is the activation of radiation protection material, particularly the generation of radioactivity by secondary radiation, which causes nuclear reactions in the shieldings. In these unwanted side-effects the generation of radionuclides is particularly caused in spallation sources by protons and neutrons in the shielding layers. A plurality of radionuclides can be generated by evaporation of nucleons and clusters. This problem is yet deteriorated by the fact that the heavier the target nucleus of the used shielding material is the greater the variability of the generated radionuclids becomes.
If natural material, which should be recirculated to a natural utilization after termination of using the facility, is used for shielding purposes, the level of the generated radioactivity has to go below certain limits in order to comply with the specifications of the national legislation. So, for example, one has to go below under a nuclide-specific approval value in Bq/g for the unlimited release according to German radiation protection law. In case of several radionuclides the total exhaustion after applying the sum rule has to be less than one. The total exhaustion is defined as:
Where Fi is the real activity per mass and radionuclide and where one has to be sum up over all radionuclides (i).
According to German law there is still a further limit value for the restricted release beside the unlimited release (able for being deposited), but not considering potential legal limit values an activity is desirable, which is as low as possible.
Calculations by the inventors, however, showed that, when operating a high energy accelerator facility at very high intensities over several decades, the used shielding material is activated so highly that it is not able for being cleared after switching off the facility and in the reconstruction time, not even for restricted release as the case may be, and it has to be stored for years or decades before it can be released. This applies also for natural filler material (soil, sand, water etc.), which is used just for the reason to be recirculated to a natural utilization as soon as possible after terminating the using of the facility. But if its exhaustion is above the legal limits, this object cannot be met, because the material would have to be stored intermediately or would have to be disposed with enormous costs as radioactive waste.
From the patent application DE 103 27 466 A1 a structure with a sandwich construction method for a radiation protection building is known. This structure, however, comes from a room for medical proton treatment, whose requirements are not comparable, because of the essentially lower energies.
Summarizing, especially multi-layered radiation protection arrangements or walls for high energy accelerator facilities have to be further improved with respect to the radioactive activation of the material and its deactivation properties, in consideration of operating over several years or decades with high beam energies and intensities and the disposal thereafter. Particularly, this aspect is of special importance, if natural shielding material is used, which on the one hand is radioactively activated after having operated the facility and on the other hand there is few experience in handling higher quantities of such material.
Therefore, it is the object of the invention to provide a multi-layered radiation protection wall, particularly for shielding against high energy gamma and/or particle radiation from high energy and/or nuclear reactions for a radiation protection chamber, which offers an easy-to-handle radioactive activation with respect to the future disposal of the used material also after a long time of operation and high beam energies and intensities, and whose parts can be reused at least partially.
It is a further object to provide such a radiation protection wall for a high energy accelerator facility, with which at the time of deconstruction as few as possible material incurs, which has to be disposed as activated, and as much as possible material is below under the predefined limits and can be reused.
Particularly it is an object of the invention to provide such a radiation protection wall and a radiation protection chamber, which can be produced, assembled, disassembled and disposed cost-efficiently and with little work.
It is a further object to provide such a radiation protection wall and a radiation protection chamber, which avoid or at least lower the disadvantages of known shieldings.
The object is already solved in a surprisingly simple manner by the matter of the patent claims 1 and 19. Advantageous further developments of the invention are derived from the subordinate claims.
According to the invention a multi-layered radiation protection wall is provided for shielding against high energy gamma and/or particle radiation, particularly from high energy or nuclear reactions, generated by primary radiation in the range above 1 GeV, particularly above 10 GeV or even higher. Preferably, the radiation of a reaction site on a high energy particle accelerator facility is shielded or attenuated herewith. In the most applications, the radiation to be shielded is secondary radiation generated by a reaction of the primary radiation with a target, but it can also be a residual or a part of the primary beam itself.
The radiation protection wall has a sandwich-like structure with at least a first and a second layer arrangement, wherein the first layer arrangement comprises at least a primary shielding layer and the secondary layer arrangement comprises at least a secondary shielding layer, particularly consisting of different material and being functionally different.
In order to be able to shield the high energy radiation efficiently, the primary shielding layer is preferably constructed as spallation layer and the secondary shielding layer preferably as moderation layer.
According to the invention, the first or the second layer arrangement, particularly preferred both, are multi-layered or divided into a plurality of adjacent and already during assembling predefined separable partial sections, so that a simple and separated disassembling and a separated and selected reuse or disposal of the partial sections are made possible. Dividing into partial sections can be implemented by dividing into several adjacent separated moderation layers and/or spallation layers and/or by separating the moderation layer(s) and/or the spallation layer(s) laterally (across the plane defined by the layer).
This offers the enormous advantage that already when planning the radiation protection wall and the radiation protection chamber respectively, a so-called “cave”, which is made at least partially from such radiation protection walls, one can differentiate between partial sections with predictably high exposure doses and partial sections with predictably low exposure doses, and that these partial sections can be assembled dividably or separably, in order to be able at disassembling to dispose the more and the less exposed partial sections separately and/or to reuse them. By doing so the costs of disposal can be reduced considerably.
With other words: According to the invention the partial sections, which are highly activated by the operation, can be separated from the partial sections, which have shielding properties and are less activated, i.e. their activity level is lower. Soon after terminating the usage, these layers, which can contain natural material and are only lowly activated, are ready for release for unlimited use or at Least for disposal and are ready for a natural usage again. It is obvious that the invention is not restricted to comply with any national limit value regulations.
After close-down, the higher activated partial sections are either stored intermediately or used in other comparable nuclear facilities further.
Preferably, the first and/or the second layer arrangement are constructed separably multi-layered on their part. With other words: The first layer arrangement comprises a plurality of 2, 3 or more spallation layers and/or the second layer arrangement comprises a plurality of 2, 3 or more moderation layers to achieve a separability along the normal of the layer additionally to the lateral separability. Herewith, concerning the concept development in two dimensions—in polar coordinates azimuthal and radial—planning the disassembling can be adjusted to the expected exposure dose, so that a two-dimensionally modular or differentiated disassembling is possible.
These advantages have special effects, if the moderation layer(s) and/or the spallation layer(s) are made from bulk material layers, because in this case a separated disassembling can be done especially simple.
In order to border the bulk material layers, the radiation protection wall has a solid statics-giving concrete base layer. Furthermore, (thin) dividing walls, for instance made from concrete, are provided between the spallation and the moderation layers to ensure the separated disposal. Frontally, laterally adjacent sections of bulk material layers are separated from each other by dividing elements. With other words: The dividing layers and the dividing elements form boxes adjacent to each other or areas to be filled, into which the spallation material and the moderation material respectively are filled, in order to form the two-dimensionally sub-divided radiation protection wall that way inter alia with spallation material and moderation material reparated from each other.
According to a particularly preferred embodiment of the invention, at least one lateral position, particularly in a central area, the radiation protection wall provides downbeam at least the following layer structure in the following order:
Preferably, several or all moderation layers or sections contain mainly (more than 50%) elements with an atomic number lower than 30 or consist of such elements. These elements are especially suited to moderate light nuclear fragments and nucleons. For moderation, particularly of neutrons, moderation layers made from gypsum or material with bounded water have proven to be particularly suited. But also fluid sections or layers are imaginable, e.g. made from water. Furthermore, it has appeared that simple soil, sand, flint, feldspar, lime feldspar, potassic feldspar or similar natural raw material can be used as moderation layer(s).
But the spallation layer(s) placed downbeam of the moderation layers contain mainly (greater than 50%) elements with an atomic number above 20 or 25 or consists of such elements. for example, an iron containing material has particularly proven its worth as spallation material. This material can be obtained at low costs and can preferably be disposed or reused as the case may be.
Preferably, the moderation layer(s) have a density less than or equal to 3.5 g/cm3 and the spallation layer(s) have a density greater than or equal to 3.0 g/cm3.
Particularly, the radiation protection wall according to the invention is formed by the downbeam positioned wall of the radiation protection chamber, into which a primary high energy beam from a particle accelerator is directed onto a reaction site or a target.
Therefore, the radiation protection chamber has at least the following components:
Thereby, thus the first radiation protection wall provides a central area to attenuate the radiation escaping from the reaction site in a predefined solid angle around the forward direction of the high energy beam and a peripheral area around the central area and is constructed from separated partial sections such that during disassembling partial sections from the central area and partial sections from the peripheral area are able to be disassembled or deconstructed separately from each other and are able to be reused or disposed.
The lateral radiation protection walls may have a layer structure different thereof.
At especially high beam energies it can be advantageous, if an additional beam annihilator, so-called “Beamdump”, is placed in forward direction of the primary high energy beam or downbeam of the reaction site. The beam annihilator is preferably joint downbeam to the first radiation protection wall outside the radiation protection chamber or is at least partially integrated into the radiation protection wall.
In the following the invention is described in more detail by means of embodiments and with reference to the drawings, wherein same and similar elements are partially provided with same references and the characteristics of the different embodiments may be combined with each other.
The irradiation chamber for nuclear collisions, which is currently planned at the applicant's in the context of the project FAIR (=Facility for Antiproton and Ion Research), is used as an example for the radiation protection wall according to the invention.
The high energy primary beam 70 enters the chamber 1 through a beam entry area 70 and hits the target 50. Though the primary beam 70, in this example 1012 protons/sec with an energy of 30 GeV, generates secondary radiation 90, which is emitted in all directions, but nevertheless has a maximum in the forward direction. Particularly, this secondary radiation 90 shall be shielded effectively.
Each of the radiation protection walls 110, 210, 310, 410 has an inner solid base layer or supporting concrete layer 140, 240, 340, 440 and an outer solid base layer or a supporting concrete layer 150, 250, 350, 450. The front and lateral outer concrete layers 150, 350 and 450 are on their part two-layered in layers 152, 154; 352, 354 and 452, 454 respectively.
Furthermore, each of the radiation protection walls 110, 210, 310, 410 has an inner layer structure 120, 220, 320, 420 made from a spallation material like iron, iron granulate or iron ore. The front spallation layer arrangement 120 is on its part two-layered in spallation layers 122, 124. The lateral spallation layer arrangements 320, 420 have only one spallation layer 322, 422 each.
Externally adjacent to each of the spallation layer arrangements 120, 220, 320, 420 there are moderations layer arrangements 130, 230, 330, 430 made from soil. The front moderation layer arrangement 120 is on its part three-layered in moderation layers 132, 134, 136. Each of the lateral moderation layer arrangements 330, 430 has two moderation layers 332, 334 and 432, 434 respectively.
The concrete layers 140, 152 serve as inner and outer base wall for filling with iron ore bulk material for the spallation layers and bulk soil for the moderation layers. The soil has a composition as it is usual at the location of the research establishment. Intermediate layers and a tension anchor (not shown in
The spallation layers consist of material with an atomic number higher than the atomic number of the material of the moderation layers. In the spallation layers mainly spallation reactions are caused by high energy neutrons, which lead inter alia to the production of volatility neutrons. The volatility neutrons have lower energies than the neutrons of the secondary radiation, generation of further radionuclides take place with a lower probability. If the thickness of the layer is large enough, a bigger part of the neutrons of the secondary radiation is converted into neutrons of the volatility nuclei. If this thickness of the layer is fitted to the primary beam (kind of ion, energy, intensity) and to the target (element, thickness) in such a manner that the secondary radiation generated in the target is strongly scattered and attenuated, the layers following downbeam are only lowly activated, the level of generated radioactivity is low.
Particularly, the front radiation protection wall 110 and its layers respectively are on the one hand laterally, i.e. perpendicular to the respective plane of layer, and on the other hand subdivided into partial sections by dividing the layer arrangements 120, 130 into further separated layers 122, 124 and 132, 134, 136 respectively. The Sub-dividing is made in this example outwards from the inner as follows:
Also the lateral radiation protection walls 310 and 410 are subdivided into partial sections as follows:
Furthermore, concerning the rear radiation protection wall 210 the following applies:
Dividing walls (not shown in
Particularly, the front radiation protection wall is fitted to the anisotropy of the secondary radiation 90 by the sectional sub-dividing according to the invention.
The inner, i.e. the central, layer sections 21, 15, 16, which are oriented to the target have to provide the highest shielding properties and have therefore the highest activation. The other sections are less activated due to their peripheral position or their position being more outwards. Therefore, most of the remaining partial sections are ready to be released unlimitedly immediately after using the facility or after a short waiting time. Advantageously, on the one hand one can build in as few material with the necessary layer thickness and the unavoidably increased activation as necessary and on the other hand one can build in as much natural material as necessary, in order to achieve the dose rate to be below a certain value outside the chamber 1 or outside the facility.
Therefore, the invention described herein optimizes two values:
Particularly, concerning the front radiation protection wall 110 according to the invention the following applies:
The various layers can be provided as solid layers (base concrete layers) or as bulk material layers (spallation layers, moderation layers) or even as fluid layers (moderation layers). More precisely, the moderation layers contain bulk material as shielding material, e.g. natural material like gypsum, soil, sand etc. and the inner and outer base layers 140, 152, 154 are ferroconcrete layers, which serve for structuring the chamber statically.
The radiation chamber was optimized in two respects:
In
The calculations have been done by using the radiation transport program FLUKA (A. Fasso, A. Ferrari, J. Ranft, P. R. Sala: New developments in FLUKA, modelling hadronic and EM interactions Proc. 3rd Workshop on Simulating Accelerator Radiation Environments, KEK, Tsukuba (Japan) 7-9 May 1997. Ed. H. Hirayama, KEK proceedings 97-5 (1997), p. 32-43).
In table 1 the activation in the various partial sections 1 to 24 is calculated for a beam time of 30 years and an average intensity of 1.00E+12 protons/sec at 30 GeV. The target causes a proton reaction rate of about 1%. Thereby, an intensive high energy secondary radiation is generated (neutrons, protons, pions, myons). The secondary radiation in turn generates radioactivity in the shielding layers as follows.
Hereby, the sections 1 to 12 consist of soil, the sections 13 to 19 of iron ore and the sections 20 to 24 of concrete. The activation is given in units of the total exhaustion for the unlimited release for three different decay times, namely 5 years, 1 year and 1 month. Therein, values less than 1 mean unlimited release.
It is apparent that almost all sections, which contain soil, are already able to be released unlimitedly after a decay time of one month. Only the section 10 is, after one month with an exhaustion of 3.83, clearly above the release value. Waiting for five years brings this layer down to a value of about 1.
Alternatively, also the thickness of the iron ore layer of sections 15 and/or 16 can be increased to bring the exhaustion of soil activation down to a value below 1 after a one-month decay time.
Partially, the concrete and the iron ore layer sections are highly activated. Thus, in forward direction the iron ore sections 15 and 16 have the highest activation with an exhaustion value of the release activity of 275 (section 15) after an one-month decay time. Accordingly, the concrete layer placed before is also highly activated (section 21 with a value of 142. As well a five-year waiting time is not sufficient to bring the exhaustion rate below one. This material is not able to be released unlimitedly, i.e. it can be used as shielding material in other facilities again or disposed according to the respective national radiation protection law.
The most important generated radionuclides are indicated. The exhaustion rate of the release value (unlimited release) according to the German radiation protection regulation is illustrated for a 30-year operation with 1012 protons/sec and an one-month decay time.
Here the radionuclide Na-22 (half-life time 2.6 years) has the highest relative exhaustion. Further radionuclides, which arise, are H-3, Be-7, Mn-52, 54, Sc-46, V-48, Cr-51, Fe-55, 59 and the cobalt isotopes Co-56, 58, 60.
Summarizing, taking into account the radioactivity, which arises in the different partial sections, during the construction of the shielding facility entails the following advantages:
The invention, however, cannot only be used for high energy accelerator facilities, but can also be transferred to facilities, in which neutrons with lower energies or thermalized neutrons are released, like e.g. nuclear reactors for power generation or research reactors (Activation by capturing neutrons with n,γ-reactions) or spallation neutron sources. Totally, the invention is to be used for kinds of radiation, which cause an activation of substances and material in the radioactive sense.
It is apparent for the person skilled in the art that the foregoing described embodiments are to be understood as illustrative and that the invention is not restricted to these embodiments, but can be changed variously without departing from the scope and the spirit of the invention.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2004 063 732.6 | Dec 2004 | DE | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/EP05/12404 | 11/19/2005 | WO | 00 | 4/30/2008 |
