Materials issues in high power accelerators

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Nuclear Instruments and Methods in Physics Research A 562 (2006) 666–675 www.elsevier.com/locate/nima

Materials issues in high power accelerators Louis K. Mansur Oak Ridge National Laboratory, Oak Ridge, TN 37831-6138, USA Available online 10 March 2006

Abstract High power accelerators present a broad array of materials issues for scientists and design engineers. Some materials considerations are unique to accelerators per se, or to a particular accelerator. For example, high intensity stress waves and the resulting cavitation erosion in heavy liquid metal targets may occur where the sharpness of the beam pulse produces shock by thermal expansion. Other irradiation environment properties and materials response are similar to those for fission or fusion reactors. For example, the high displacement doses in the target structures of high power accelerators, produced by the impinging beam and the spallation neutrons, are of similar magnitude to those in high flux reactor cores and first walls of future fusion reactors. In addition to the central issue of radiation effects in metallic structural alloys, designers will be concerned with radiation effects in polymers and ceramics, which may be employed as seals, sensors, or insulators. Other applications place demands on materials that are not related to radiation but to other aspects of aggressive environments, such as compatibility of materials with special purpose fluids. Examples are discussed of problems, materials research and development solutions and recent experimental and calculational results. r 2006 Elsevier B.V. All rights reserved. PACS: 29.20.Lq; 61.72.Ji; 61.80.Az; 61.80.Ed; 61.80.Hg; 61.80.Jh Keywords: Accelerator materials; Radiation effects; Embrittlement; Corrosion; Cavitation erosion; Fatigue

1. Introduction High intensity proton-beam accelerators are either in operation, in design and construction, or in the planning stages worldwide. Over the next two to three decades it is expected that the number and operating power levels of such accelerators will increase substantially. The purposes of these facilities cover a range of basic and applied research and technology and applications [1]:







Neutron spallation sources for research in the physical, chemical, and life sciences, for materials irradiation, and for isotope production. Accelerator-driven sub-critical devices (ADS) for transmutation of nuclear wastes and for energy amplification. Radioactive beam facilities for fundamental nuclear physics research. Facilities that use muons or neutrinos for particle physics research. Corresponding author. Tel.: +1 865 574 4797; fax: +1 865 241 3650.

E-mail address: [email protected]. 0168-9002/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2006.02.145

For the spallation neutron sources, the power levels range from 1 to 5 MW, and for the transmutation facilities the power levels would be 50–100 MW and higher. Although the power of the latter is higher than the former, the power densities will be similar to or lower than the spallation sources. To our knowledge there are no operating facilities for the transmutation of nuclear waste. At present the highest power spallation neutron source for neutron scattering is the Schweizer Institut fu¨r Nuklearforschung Quelle (SINQ) at the Paul Scherrer Institute (PSI) [2], which operates at about 1 MW with a 0.6 GeV proton beam. The beamstop experimental area of the Los Alamos Neutron Science Center (LANSCE) at Los Alamos National Laboratory (LANL) operates at a similar power level with a 0.8 GeV proton beam [3]. The neutron scattering targets and future upgrades at the LANSCE and at the ISIS, located at the Rutherford Appleton Laboratory (RAL), operate in the range of hundreds of kW. The Japanese Proton Accelerator Research Complex (JPARC) [4] is a multipurpose facility meant to serve

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several research fields itemized above. The facility is a partnership between the Japan Atomic Energy Research Institute (JAERI) and the High Energy Accelerator Research Organization (KEK) of Japan. It consists of three accelerators—a 200 MV Linac, which will later be upgraded to 400–600 MV; a 3 GeV rapid cycling synchrotron; and a 50 GeV synchrotron. The Spallation Neutron Source (SNS) project under construction at Oak Ridge National Laboratory (ORNL) in the United States is a partnership among Argonne National Laboratory (ANL), Brookhaven National Laboratory (BNL), Jefferson Laboratory (JLab), Lawrence Berkeley National Laboratory (LBNL), Los Alamos National Laboratory (LANL), and ORNL. The main purpose of the facility is the production of neutron beams for neutron scattering research [5]. The SNS project will be complete in June 2006. Conceptual design of the SNS was completed in 1997 [6]. The radioactive ion beam facilities include the Isotope Separator On Line (ISOL) type, where a proton beam produces rare isotopes through collisions with high atomic number targets, as well as the Fragmentation type [7]. In the latter concept, heavy-ion beams in the range of hundreds of MeV per nucleon collide with low atomic mass targets to produce rare isotopes. High power accelerators present numerous challenges for the community of scientists and engineers. In many cases, materials lifetime and performance are the key enabling features that will determine the viability of these facilities. Materials scientists and engineers must contribute solutions to difficult issues associated with radiation fields and other non-conventional problems. 2. Materials issues In large accelerator complexes there are seemingly limitless questions related to materials. The choices and design decisions require deliberate analysis of available data and experience. In some cases, however, new concepts and/or existing accelerators that have been upgraded in power or otherwise entail new conditions will subject materials to environments that are more aggressive than in any experience. Intensive efforts may then be required. In particular, materials R&D is needed where risk to the feasibility or success of the project must be reduced. Examples of such issues that have been addressed recently for spallation neutron sources are discussed. It is clearly important to carry out materials R&D to:







Establish the viability of a new concept that makes exceptional demands on materials. Demonstrate that materials are in hand or could be developed to fulfill a key function. Obtain sufficient experimental data and modeling analysis to support estimates of materials and component lifetimes. Qualify materials for their planned applications.

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The latter activity is usually required for new fission power reactors and research reactors to permit construction and licensing. However, this phase may not be completely achievable before operation of new accelerators. It requires more effort than the other activities listed above. Prototypical facilities and conditions are needed to fully qualify materials for their intended service. Such facilities may not be available for first-of-kind high power accelerators. To make the discussion specific we focus on the SNS. There protons are accelerated to energy of 1 GeV and impinge on the target, producing neutrons. The spallation neutrons initially are of high energies, with the shape of the energy spectrum resembling a skewed fission spectrum with a tail up to 1 GeV. The neutron scattering applications of this facility require that the neutrons be slowed down and thermalized in moderators adjacent to the target. In broad brush, the accelerator comprises an ion source, a linear accelerator, an accumulator ring, and a target station. Three sections comprise the linear accelerator, a drift tube linac (DTL), a coupled cavity linac (CCL) and a superconducting linac (SCL). At the end of the SCL the H energy reaches 1 GeV. A high energy beam transfer line guides the beam to the accumulator ring, where it is converted from a 1 ms long H pulse to a less than 1 ms proton pulse. The beam is then guided via a ring-to-target line to the target station. The target station includes the mercury target and circulation loop, neutron moderators and refrigeration systems, neutron beam lines and scattering instruments, as well as a service cell for installing and removing targets and performing some sectioning operations on radioactive components [8]. The target is positioned within an iron and concrete shielding monolith. Fig. 1 shows the overall layout of the SNS. Superimposed are labels indicating some key locations for radiation effects to materials. There are radiation effects issues throughout the SNS. They vary from location to location and range in severity from key concern for materials lifetime, to an issue to consider in design along with other factors. These latter areas include components in the tunnels of the accelerators and beam transfer lines, for example, such as magnets, electrical power supply components and electronic instruments. In addition there are three beam dumps, the Linac dump, the ring extraction dump and the ring injection dump. The latter is expected to operate continuously, absorbing up to 150 kW, and accumulating up to nearly 4 dpa per year of SNS operation (5000 h) at the front of the beam stop vessel [9]. Active cooling by means of a water loop is supplied because of the high power level. This beam dump approaches the threshold where radiation effects in the materials of construction are a key concern. The other two dumps will operate at less than 5% of the power of the ring injection dump, and accumulate proportionally lower radiation damage. The mercury target container, however, will experience a much higher level of radiation damage, about 21 dpa per

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Collimator straight section of accumulator ring Ring-to-targetAccumulator beam transport ring (RTBT)

Locations where radiation damage Is a key concern or an issue to consider General Radiation Levels In Tunnels

Ion DTL source

CCL

Ring Injection Dump, Stripper Mechanisms and Magnets

SCL

Magnets

Spallation

Spare section

Linear accelerator system (linac)

Target, High-energy Target beam transport station (HEBT)

Beamline Window Moderators

Fig. 1. Layout of the Spallation Neutron Source with some areas of radiation exposure to materials differentiated as to whether they are a key concern or an issue to consider, incident proton energy 1 GeV, beam current 2 mA. Table 1 Calculated radiation dose rates and doses, for full power years, for key SNS locations Spallation target peak displ. dose rate Ring injection dump peak displ. dose rate RTBT second doublet coil peak ionizing dose rate Ring injection magnet peak ionizing dose rate Coupler peak displ. dose rate Coupler peak ionizing dose rate

106 dpa/s (102 in pulse) (2/3 n, 1/3 p) 2  107 dpa/s (3/5 p, 2/5 n) 4  102 Gy/s 1.5  102 Gy/s (1/5 n, 4/5 p) 2  1016 dpa/s (3/4 n, 1/4 p) 3  104 Gy/s

Spallation target peak displ. dose (1 y)

36 dpa

Ring injection dump peak displ. dose (1 y)

7 dpa

RTBT second doublet coil peak ionizing dose (10 y) Ring injection magnet peak ionizing dose (10 y)

10 MGy 5 MGy

Coupler peak displ. dose (10 y)

6.3  108 dpa

Coupler peak ionizing dose (10 y)

95 kGy

Second column also shows the approximate proportions contributed by protons and neutrons. Results in top row after [10,11]; results in second row [9]; results in other rows [12].

SNS year [10,11]. In addition, it is subject to stress loads by the pulsed beam, contact with rapidly flowing mercury, and most likely to cavitation erosion. For these reasons it is deemed the most critical component with respect to materials-limited lifetime. Table 1 gives radiation dose rates and doses for a number of key locations at the SNS. The doses are expressed in units of displacements per atom (dpa) and/or in Gy, describing energy deposition by nuclear and electronic processes, respectively. It is clear from the above discussion that radiation effects present important and pervasive materials issues related to the design and operation of high power accelerators. How radiation will affect properties and performance must be considered for materials and components throughout. Certain irradiation environment properties and materials response are similar to those encountered in fission reactors or to those that will be experienced in planned fusion reactors [13]. For example, the high displacement doses in the target structures of high power accelerators, produced by the impinging beam and by the resulting nuclear reaction products such as spallation neutrons, are of similar magnitude to the displacement doses in high flux

reactor cores and in the first walls of planned fusion reactors. Added to the central issue of radiation effects in metallic structural alloys, accelerator designers will be concerned with radiation effects in polymers and ceramics, which may be employed as seals, sensors, or insulators. In addition to radiation effects, which are relatively independent of design in their universality, there are key issues that depend upon the particular type of system and its specific design approach. Accordingly, in the sections below we give a brief introduction to radiation effects in materials, followed by applications of these concepts that have been employed for the SNS. To balance the discussion we also give several examples of design-specific materials issues that are not primarily driven by radiation effects. 3. Radiation effects in materials Most properties of materials can be changed by radiation. Mechanical properties such as tensile strength and ductility, physical properties such as electrical, optical and thermal properties, and even the dimensions of components can be altered dramatically by irradiation. Some materials require relatively high doses for significant

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changes while others, notably polymers, require only low doses. Underlying these property changes are more fundamental processes such as the production, diffusion and aggregation of atomic defects; evolution of microstructural features such as dislocations, cavities and precipitates; compositional changes; and ionization and excitation of the electronic structure of component atoms. Page space does not permit a review of radiation effects in materials here. A number of reviews have been published previously. In particular, the reader is referred to comprehensive reviews for metallic alloys [14,15] and ceramics [16,17]. Below we summarize key points relevant to the present context Radiation effects originate in the displacement of atoms (nuclear stopping) and in ionization and excitation (electronic stopping). Displacement damage is the dominant process for metallic alloys and is important for both ceramics and semiconductors. It can also be significant for polymers, although it usually has been neglected. The unit of dose for displacement damage is the displacement per atom, dpa, where 1 dpa is the dose at which on average every atom has been displaced once. Ionization and excitation can generally be neglected for metals, except for ultra high energy deposition rates above about 10 keV/ nm. The neglect is possible because of the electronic structure of metallic crystals, which consists in part of a Fermi electron sea that is not localized to a particular atom. Electronic stopping can cause significant effects in polymers, ceramics and semiconductors. The unit of dose for ionization and excitation is the Gray or Gy, which is defined as the absorption of 1 J/kg. However, it is important to note that in general usage the energy deposition in Gy describes total energy deposition, including the usually smaller fraction that goes to nuclear stopping. In addition, transmutation reactions can be significant where they create atoms not part of the original composition. Hydrogen and helium are the most prolific transmutants in high energy proton and neutron spectra. These can exacerbate the radiation damage produced by energy deposition. The customary measure of transmutations is in reference to dpa, for example as appm He/dpa. The typical highest time averaged damage rates in fast neutron spectrum reactor cores, future fusion reactor first walls, and high power accelerator targets are all 106 dpa/s, 4103 Gy/s. (For pulsed spallation neutron sources such as the SNS, the instantaneous damage rate during the o1 ms proton pulse is of order 102 dpa/s.) Transmutation rates vary considerably depending upon the particle energy spectrum. For example, high power spallation accelerator targets produce on the order of 50–100 appm He/dpa, whereas fast reactor cores and fusion reactor first walls give about 0.2 and 15, respectively, in the same units. The time and energy scales for radiation effects by displacement damage cover a large span [14]. The initial displacement of atoms is complete in about 0.1 ps, whereas the evolution of microstructure that takes place as a result

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of the diffusion and clustering of displacement produced defects causes accumulating changes in physical and mechanical properties in times as long as 1–100 Ms, a span of more than 20 orders of magnitude. The characteristic energies for important processes range over more than 10 orders of magnitude. The initial proton and spallation neutrons may have energies up about a GeV or more. Through processes of energy transfer to primary knock-on atoms, secondary knock-ons, and creation of thermalenergy-rich local regions, the energy reaches ambient thermal values, which are on the order of several hundredths eV. Metallic alloys. In metallic alloys, these hierarchies of displacement-related processes at different physical scales ultimately produce substantial changes in properties. The most important property changes with respect to applications in high energy accelerators and nuclear reactors are termed radiation-induced-swelling, -creep, and -embrittlement. Swelling. Swelling is a macroscopic volume increase, accounted for by a distribution of nanoscale cavities at high number densities. Vacancies produced by displacements accumulate in the cavities; the corresponding interstitials are absorbed at sinks such as pre-existing dislocations and dislocation loops that are formed by the clustering of interstitials. Swelling can reach tens of percent at several tens of dpa. Theory and experiments designed to investigate mechanisms have led to understanding of swelling, and to the capability to design swelling resistant alloys [14]. Fig. 2 shows microstructures of two austenitic stainless steel alloys irradiated in a fast spectrum nuclear reactor [18]. On the left is a solution annealed type 316 stainless steel showing high swelling at about 40 dpa. The white features are cavities that account for about 10% volume

Fig. 2. Transmission electron micrographs of irradiated type 316 austenitic stainless steels. On the left is a solution annealed alloy. On the right is a compositionally modified variant that was tailored for swelling resistance. After Ref. [18].

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L.K. Mansur / Nuclear Instruments and Methods in Physics Research A 562 (2006) 666–675

swelling. The dark features are large blocky precipitates that increase swelling by enhancing the collection of point defects in the cavity [19]. On the right is an alloy that has been modified with P and Si additions and irradiated to about 35 dpa. These alloying additions produced a high density of fine scale precipitates that eliminated swelling by trapping gases at the interfaces and by increasing the overall point defect sink strength of the material. Swelling is a significant concern in structural alloys irradiated between approximately 0.3 and 0.6 Tm, where Tm is the melting point. It can cause significant overall dimensional changes in components. It is sensitive to temperature, dose and dose rate, so that the unavoidable gradients in these quantities in irradiated components can lead to significant distortions beyond simple dimensional enlargements. In addition, cavity distributions are possible easy paths for fracture [20]. Swelling may place limits on component lifetimes by producing changes in thermal hydraulics or by changing dimensional tolerances beyond allowable limits. Fortunately, swelling is not anticipated to be a problem in components for accelerator targets operating at o0.3 Tm such as in the SNS. It should be noted, however, that certain ADS designs would put Fe–Ni–Cr based alloys squarely in the temperature regime for swelling. In those cases, alloys that are inherently swelling-resistant, such as ferritic/martensitic steels, or alloys designed for swelling resistance, such as P-, Ti-, and Si-modified austenitic stainless steels [21] must be employed. Creep. Radiation-induced creep can be manifested in two related ways. If the material is subject to an applied stress and is free to move, then a shape change will occur during irradiation; if the material is subject to an applied stress while constrained, then the stress will be relaxed during irradiation. The latter occurs by the conversion of the initial elastic strain to plastic strain. Radiation-induced creep is driven by several types of asymmetrical partitioning of point defects [14]: unequal partitioning of interstitials to dislocations that are differently oriented to the stress direction; unequal partitioning of vacancies and interstitials between dislocations and other sinks; and transient unequal partitioning of vacancies and interstitials to dislocations because of several distinct time dependent processes. The steady state processes are cumulative, where a longterm net flux of one defect type over the other occurs at all dislocations or at particular dislocations oriented preferentially with respect to applied stress. These processes drive steady climb of dislocations. The climb itself can produce creep, where there is a net transfer of atoms from planes more parallel to the stress direction to planes more perpendicular to the stress direction. This process also drives dislocation glide, which produces an additional contribution to irradiation creep. Radiation-induced swelling entails climb of all dislocations by net interstitial absorption, regardless of orientation. The resulting unpinning from obstacles produces an additional component of

steady irradiation creep by dislocation glide. See Refs. [21,22] and references contained therein for descriptions and theoretical models of these processes. Observed temperature, stress, dose rate and other dependencies are reviewed in reference [20]. Irradiation creep can occur at all temperatures of interest. Above about 0.55 Tm or so, however, it is dwarfed by thermal creep, a well known process. The importance of irradiation creep in applications is that it can relax engineered stress distributions and lead to dimensional instability in component shapes and sizes. Irradiation creep can be a significant problem for tight tolerance geometries, such as in fast spectrum reactor cores [23], where it could affect thermal hydraulics, for example. On the other hand it also could be beneficial in relaxing stresses induced by non-uniform swelling. Although its occurrence is inevitable, irradiation creep is not expected to be a problem in liquid metal spallation targets because of their open structure. Embrittlement. Radiation-induced embrittlement refers to the hardening and concomitant loss of ductility seen after irradiation of structural alloys. It is caused by the formation and growth of vacancy and interstitial clusters, precipitates and cavities during irradiation. These features restrict the movement of deformation dislocations and therefore increase the yield stress of the material. Simultaneously, grain boundaries can be weakened by irradiation. Weakening occurs by multiple processes including radiation-induced solute segregation, and by the accumulation of transmutation products, especially helium, at or near grain boundaries. Fig. 3 is an illustration of the fact that embrittlement can lead to crack formation when an irradiated material is subjected to stress loading. In this work tungsten specimens were irradiated with 800 MeV protons and tested in compression to 20% strain at room temperature [24]. The figure shows results for tests on an unirradiated specimen and on an irradiated specimen at a dose of about 3 dpa.

Fig. 3. Tungsten compression specimens irradiated with 800 MeV protons at LANSCE and tested in compression to 20% strain: (a) before irradiation, (b) after a dose of 3.2 dpa. Other specimens irradiated to the higher doses of 14.9 dpa, and 23.3 dpa also showed similar failure cracks. The diameter of the specimens is approximately 3 mm. After Ref. [24].

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(a)

316L

50

(b) SA, RT

1200

CW EBW

1000

SA, 250°C CW

800

EBW Fission [9] SA, 250°C

40

30

CW, 250°C 600

20

Strain-to-necking (%)

Yield strength (MPa)

1400

671

400 10 200

0 0

2

4

6

8

10 12 0 2 Displacement dose (dpa)

4

6

8

10

0 12

Fig. 4. Yield stress and strain-to-necking vs displacement dose for AISI 316L in solution annealed, 20% cold-worked and electron-beam welded conditions irradiated in SINQ. Filled and empty symbols represent test temperatures of 25 and 250 1C, respectively. Data from fission reactor irradiations (T test ¼ T irrad ¼ 250 1C) are included for comparison. After Ref. [25].

100

DATABASE* EC316LN(p)** HTUPS316(p)** EC316LN(n)** HTUPS316(n)** 316L(p)***

80 Uniform elongation, %

Without irradiation the test did not produce failure, but the irradiated specimen shown, as well as others irradiated to higher doses and similarly tested, failed by the formation of cracks. Embrittlement during irradiation is generally characterized by progressive changes in yield stress and in measures of ductility. Fig. 4 shows this behavior for type 316L stainless steel irradiated with 600 MeV protons and spallation neutrons at the SINQ. The specimens were tested at temperatures of 25 and 250 1C in several conditions, including solution-annealed (SA), cold-worked (CW), and electron beam welded (EBW) [25]. For comparison, results for material irradiated in fission reactor are included. When a high enough stress is applied, structural failure of engineering components can be caused by embrittlement. Additional types of failures may occur such as the formation of cracks that lead to loss of vacuum or coolant integrity. Irradiation embrittlement may necessitate the early replacement of components to avoid such failures. Irradiation embrittlement is considered the primary radiation effects issue for liquid metal target containers operating at low temperatures, such as those for the targets of the SNS and JPARC. Fig. 5 shows some of the embrittlement data that was analyzed as part of the SNS materials R&D program. The points denoted as ‘‘database’’ in the legend were obtained from data measured in fission reactor irradiations for the purpose of acquiring measurements relevant to fusion reactor first wall materials [26]. Only the data from that compilation for irradiations at temperatureso200 1C are shown because they are considered most relevant to low temperature liquid metal target containers. The other

*Fusion program database for 316 SS, irradiated and tested at 0 ~ 200°C **ORNL data for LANSCE irradiation ***LANL data for LANSCE irradiation

Type 316 LN stainless steel recommended for SNS target module

60

40

One SNS year 20

0

0

0.01

1

100

dpa Fig. 5. Measured reduction of uniform elongation with displacement dose. The lighter diamond symbols indicated in the legend as ‘‘database’’ are from fission reactor irradiations below 200 1C. Dashed lines indicate band within which these points fall. Other points are from irradiations in a spallation environment at the LANSCE facility as part of the SNS program. The points labeled 316L were obtained by LANL in research for the APT program. After Ref. [27].

points were obtained from specimens irradiated in a spallation environment at the LANSCE facility [27]. As indicated, the data cover three variants of type 316 stainless steel: 316LN, 316L and HTUPS316. Based on these results

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and on a substantially larger base of experimental measurements, type 316LN shows better performance under irradiation with respect to low temperature uniform elongation and fracture toughness [28]. Type 316LN stainless steel has been chosen for the target container module of the SNS [29]. Although well more than 20 dpa could possibly be achieved, there are no prototypical irradiations in mercury available. Thus, we have recommended the first target be removed at 5 dpa. Ceramics. Similar to metallic alloys, ceramics properties are affected by displacement damage and by transmutation production. In addition, some ceramics can suffer displacement production by ionization-induced radiolysis, where chemical bonds are broken and atoms may leave their normal lattice positions. This process is known to occur in SiO2 and in alkali halides, but not in AlO, BeO, or AlN, for example. Many physical and mechanical property changes of ceramics have been investigated, and some of these will be of relevance for accelerator applications. Reviews are available for radiation effects in ceramics. See for example [16,17]. One topic of interest in this connection is the phenomenon of radiation-induced conductivity (RIC). When insulating ceramics are exposed to ionizing radiation, their conductivity increases. This is an instantaneous effect, which disappears when the irradiation ceases. It occurs by the transient excitation of valence electrons into the conduction band. The phenomenon has been investigated because of scientific interest in the basic processes, and because it is clearly important to measure the magnitude of the effect to determine whether it could be significant for applications. In particular, the question of whether the performance of electrical insulators could be comprised by typical radiation fields needs to be answered. Fig. 6 shows the measured electrical conductivity of fine grained 99.99% pure alumina cable [30]. The experiments were carried out in the HFIR reactor at Oak Ridge National Laboratory. Electrical conductivity was measured as a function of temperature without a radiation field and with radiation fields as high as 42 Gy/s. At the highest radiation dose rate the electrical conductivity was increased with respect to the non-irradiated control at all temperatures. However, the absolute values of the conductivity remain relatively low under all the conditions measured. Referring to Table 1 it can be seen that typical dose rates in various accelerator locations are well below the 42 Gy/s of the above experiment. Therefore, we may conclude that loss of electrical insulation in alumina (the most prominent electrical insulating ceramic in use) in typical high power accelerator applications is not of concern Polymers. Irradiation can change the mechanical and physical properties of polymers. The predominant changes are molecular chain scission and chain crosslinking. For a given polymer type, irradiation environment and temperature, chain scission or crosslinking will usually dominate. Although polymers are often classified qualitatively as scissioning type or crosslinking type, we have found in our

Temperature (°C) 750 600

450

300

100

10-7

10-8 Electrical Conductivity (S/m)

672

10-9

10-10

42 Gy/s 7 Gy/s

10-11

0.7 Gy/s

10-12 Thermal (without ionizing radiation) 10-13 0.5

1

1.5 2 1000/T (K-1)

2.5

3

Fig. 6. Electrical conductivity measured in pure alumina cable (CR 125) with and without ionizing irradiation in the HFIR, after Ref. [30].

research that when some ‘‘scissioning type’’ polymers in gamma or electron irradiation are irradiated with heavy ions they become crosslinking type [31]. The physical reason for this is that heavy ions produce intense ionization along their tracks, orders of magnitude more dense in terms of energy deposition in eV/nm than gamma photons or electrons, for example. In turn, the high density of broken bonds increases the probability of crosslinking compared to the isolated chain breaks produced by gamma photons or electrons [31]. Polymers are sensitive to the irradiation environment. Irradiation in vacuum can improve the dose endurance by an order of magnitude over that in air [32]. The presence of oxygen acting together with the irradiation induced broken bonds and radiolytic molecular fragments can lead to more rapid property alterations. Some molecular level changes that occur by radiolysis are the freeing of hydrogen and small molecules. As a result of subsequent diffusion of these small molecules, gases can be released by irradiation. The consequent changes in the physical and mechanical properties, therefore, can be quite large because the bulk chemical composition of the polymer then has been altered by radiation, in addition to the changes in the types of bonding among the atoms that remain. Compared to metals and ceramics, radiation effects in polymers can occur at low doses. Primarily this is because polymers consist of relatively few molecular units of very high molecular weight. These molecules give the polymer its characteristics. The molecules are so large that typically tens of percent of the molecules can suffer at least one event that produces a scission or crosslink in doses as low as

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10 kGy. Oversimplifying, the material would then have several smaller molecules in place of a larger one, or one three-dimensional network in place of several original molecules, respectively, imparting a different set of macroscopic properties. Polymers exhibit a wide range of sensitivity to radiation, depending upon their chemical composition and structure. Sensitive polymers show degradation at doses of order kGy, whereas the most radiation resistant polymers can have useful lifetimes well in excess of MGy. Compilations are available that rank polymeric materials in terms of resistance to radiation or maximum usable dose [32–34]. Among the least resistant polymers are PTFE (Teflon), acetals and polypropylene. These should be avoided in applications where there are significant radiation fields. Among the most resistant are polyimide (Kapton), phenolic resins and polystyrene. A key to high radiation resistance is the presence of aromatic or phenolic rings in the polymer structure, either on the molecular backbone or as pendant groups. The presence of these rings provides greater opportunity for crosslinking when bonds are broken, while decreasing the possibility of scission of the parent molecule by a single bond break. 4. Design-specific materials issues Although radiation damage can affect many sectors of the accelerator complex there are numerous other more specific materials issues. Liquid metal targets exposed to pulsed beams in high power accelerators are critical components in spallation neutron sources like the SNS and JPARC. In particular, there are two important topics for the structural alloy containers of such targets that are associated with the pulsed beam structure—cavitation erosion and fatigue. Cavitation erosion. Cavitation erosion is caused by a sudden energy deposition when a beam pulse is absorbed in the liquid. Thermal expansion then follows because of the rapid resulting heating. The thermal expansion in turn gives rise to pressure waves. The pressure waves travel to the vessel walls and are followed by rarefactions. In these regions of tensile stress in the mercury, fracture of the liquid occurs. Cavitation bubbles form throughout the mercury. They next collapse in the hydrostatic compressive stress field of a subsequent pressure wave. The collapse is violent, giving rise to microscopic high speed liquid jets and shock waves, which produce localized erosion pits in the container wall. There has been intensive research directed at mitigating pitting in liquid mercury targets, because it is considered likely that this degradation mechanism will be the lifetime limiting process for this type of target. Because a review of this area is covered elsewhere in this conference [35], a development of this topic is not included here. Background, earlier results, and status on cavitation erosion have been described in recent reviews [36,37]. Fatigue. Fatigue loading is another issue of interest in pulsed targets. The energy absorption and liquid metal

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expansion described in the previous paragraph also are responsible for imposing high cyclic stresses on the target container. These fatigue loads on materials in contact with mercury naturally lead to the question of whether the materials will be prone to early failure by fatigue. For the SNS a large effort was devoted to fatigue testing. Fatigue curves were measured for stress amplitude versus cycles to failure for wide ranges of key variables. Tests in mercury and air were compared. The fatigue behavior of 316LN stainless steel was characterized by a family of bilinear fatigue curves, which were dependent on frequency, environment, mean stress and thermomechanical treatment. Generally, fatigue life increased with decreasing stress and leveled off in the high cycle region to an endurance limit below which the material did not fail in mercury or air. For fully reversed loading as well as for tensile mean stress loading conditions, mercury had no effect on the endurance limit. However, at higher stresses above the yield stress, an effect of mercury was observed at low frequencies. Fatigue lifetime decreased with decreasing frequency; the response was more pronounced in mercury than in air. Fracture surfaces of specimens tested in mercury showed widespread brittle intergranular cracking as opposed to typical transgranular cracking for specimens tested in air. The work is fully documented in [38,39] and references contained therein. Irradiation-assisted Stress Corrosion Cracking (IASCC). IASCC is an important degradation phenomenon that can occur in water-cooled structures [40,41]. When water, displacement damage and stress are present, there is potential for the problem. This phenomenon was first reported for components in or near the cores of Boiling Water Reactors (BWRs) in 1962. It has been observed in 300 series stainless steels and high nickel alloys. In earlier reports, the parts affected were either relatively small, such as bolts and springs, or designed for replacement, such as control blades and instrumentation tubes. Recently, more structurally significant components of reactor cores such as core shrouds have also been degraded. It is an intergranular cracking phenomenon, whose threshold in BWRs is 0.5 dpa. For PWRs the threshold is several dpa. Most available data are in the range 270–370 1C. It is generally accepted that decreasing the temperature below these values may decrease prevalence. It can be eliminated by controlling O too10 ppb, and/or H4200 ppb. This explains the difference in observed thresholds of dose in BWRs and PWRs. In the latter it is possible to reduce the level of radiolytic oxygen in the water by adding hydrogen, leading to less oxidizing conditions. Broad recommendations for design of accelerator systems and components in light of the possibility of IASCC can be given. Avoid designs and fabrication methods that increase stress—e.g., unrelieved residual stresses, sharp corners, and other stress raisers. Avoid high strength alloys—e.g., for example, cold-worked or high strength austenitic stainless steels would be more prone to cracking than solution-annealed alloys. Consider control of

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water chemistry—addition of hydrogen and removal of radiolytic oxygen and removal of impurities, such as chlorides and sulfates. With water chemistry control one should restrict very long term water-cooled structures under significant stress to less than a few dpa. If there is no water chemistry control applied, then restrict displacement doses to less than 0.5 dpa. Because IASCC is not yet thoroughly characterized or fully understood, design for non-routine replacement of ‘‘permanent’’ structures. Forty years, roughly the planned lifetime of current high power accelerator facilities, is too long for irrevocable decisions in a complex irradiation environment. Beam Charge Stripper Foils. Because of the better spatial stability and beam handling for negative ions, the LINAC portion in a proton accelerator complex is used to accelerate H ions. Before the ions are injected into the storage ring, however, the two electrons are stripped and the beam is circulated as protons [42,43]. Inevitably, the foil is subject to radiation damage and high heating by the injected beam. Further, because of the location of the foil, the circulating beam in the ring also interacts with it. In the SNS, for example, the average proton in the ring will interact with the foil 6 or 7 times in the course of about 1000 circuits before it is extracted to the target. The foil must be thick enough to strip electrons efficiently. However, the thicker the foil the higher will be the parasitic scattering of the circulating beam, and the higher will be the energy deposition and temperature of the foil. It can be shown that the probability of scattering per foil traversal is proportional to the square of the atomic number, to the mass density and to the thickness of the foil. Therefore, foils of low atomic number and physical density that have a low product of thickness and physical density are preferred. Carbon will be employed for the SNS—it satisfies the above criteria, and in both its graphitic and diamond-like forms is a very high temperature material. The primary stripper foil has been specified to have a thickness of 300 mg/cm2. At this thickness the foil will be 97% efficient in stripping H to protons, and the foil temperature will remain below 2500 1C. It has been determined empirically that above this temperature the foil would deteriorate quickly, with a lifetime of a most a few hours. Below this temperature each foil can be expected to last on the order of 100 h [44]. A secondary foil, thicker by nearly two orders of magnitude, of carbon also will be used to strip the H and H0 that miss or avoid stripping in the first foil. Ions that are not injected into the storage ring are sent to the ring injection dump. It will be recalled from the discussion surrounding Fig. 1 and Table 1 that the ring injection dump sustains a fairly high level of radiation damage. Stripper foils and beam dump are tied together by materials lifetime considerations. For a thin foil, more power is sent to the beam dump, so that the dump requires more cooling and sustains more radiation damage, leading to a lower lifetime of beam dump components. At the same time the foil operates at lower temperature and sustains less radiation damage,

leading to a longer foil lifetime and less parasitic scattering of the circulating beam in the ring. Conversely, thicker foils reduce the power to the dump and reduce radiation damage there, leading to a longer lifetime to beam dump components. In that case, however, the temperature and radiation damage of the foil would be higher, leading to a shorter foil lifetime, more foil hits by the stored beam and more large amplitude protons injected into the ring. This is one example of regions in high power accelerators that present interesting lifetime and cost optimization issues between materials science/engineering and accelerator physics/engineering. 5. Summary A pervasive issue in accelerators is radiation damage to materials. In particular, targets of these accelerators present unusual conditions relative to the extensive body of information that has been accumulated on radiation damage to materials. That experimental data is largely centered on fission reactor neutron irradiations, which are in some ways qualitatively similar but in other ways quite different than high energy charged particle irradiations. In light of needs for feasibility, lifetime and performance information, the requirement for radiation damage data has resulted in worldwide near-term project related R&D. The general topic has also presented important new opportunities for materials scientists to study fundamental behavior under previously unexplored irradiation conditions. In addition to the above common issues for all high power accelerators, there are numerous design-specific challenges that are being met related to particular target types and other components. All the examples summarized above relate directly or indirectly to the high energy charged particle beam, although some of these are not in the realm of radiation damage. In particular, for short pulse liquid metal targets the problem of cavitation erosion in the target container walls stands out. Substantial progress has been made to understand and mitigate erosion, but the expected severity of the phenomenon demands more work to achieve substantially more reduction. There are numerous other design specific issues that can be handled successfully, including irradiation-assisted stress-corrosion cracking, which can afflict water-cooled systems; fatigue problems induced directly by beam pulses or by startup and shutdown; and materials/accelerator physics optimization related to beam charge stripper foils, for example. Acknowledgements I am grateful to many colleagues for collaborations and consultations. For target materials: T.S. Byun, K. Farrell, M.L. Grossbeck, J.R. Haines, S.J. Pawel, B.W. Riemer, and S.J. Zinkle, ORNL; H. Ullmaier, Forschungszentrum Ju¨lich; S.A. Maloy , LANL; Y. Dai, Paul Scherrer

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