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 Table of Contents 
REVIEW ARTICLE
Year : 2012  |  Volume : 35  |  Issue : 3  |  Page : 145-155  

Radiological safety aspects of an accelerator driven system


Accelerator Radiation Safety Section, Health Physics Division, Bhabha Atomic Research Centre, Mumbai, Maharashtra, India

Date of Web Publication5-Sep-2013

Correspondence Address:
C Sunil
Accelerator Radiation Safety Section, Health Physics Division, Bhabha Atomic Research Centre, Mumbai - 400 085, Maharashtra
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/0972-0464.117672

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  Abstract 

Accelerator driven systems (ADS) have the potential to generate nuclear power by coupling a high energy high current proton accelerator to a sub-critical nuclear reactor. The system has several advantages such as inherent safety from a runaway chain reaction, independence of the reactivity from delayed neutrons, resistance to nuclear proliferation, ability to transmute long lived minor actinides and fission products and, to convert thorium to 233 U. The radiological safety aspects of a typical ADS are discussed in which lead bismuth eutectic (LBE) is the spallation target as well as the coolant. In the irradiated target, residual activity is a major concern, particularly the amount of 210 Po. The time evolution of the major isotopes present in LBE is studied along with the prevailing residual gamma dose rates. Few issues that may surface while coupling a reactor to an accelerator are also discussed.

Keywords: Accelerator driven system, lead bismuth eutectic, residual gamma dose rates


How to cite this article:
Sunil C. Radiological safety aspects of an accelerator driven system. Radiat Prot Environ 2012;35:145-55

How to cite this URL:
Sunil C. Radiological safety aspects of an accelerator driven system. Radiat Prot Environ [serial online] 2012 [cited 2020 Jun 2];35:145-55. Available from: http://www.rpe.org.in/text.asp?2012/35/3/145/117672


  Introduction Top


Of the many indexes that are used to measure the overall standing of a country in the present day world, the human development index (HDI) [1] and the energy development index (EDI) [2] are relevant. HDI is a measure of the life expectancy, level of education and the gross domestic product while EDI is based on factors such as per capita energy consumption, access to clean cooking facilities and electricity. Undoubtedly, both these factors articulate the energy needs of a country to achieve a comfortable standard of living for its citizens. It is estimated that India will need about 5000 kWh/capita to attain a respectable HDI. [3] For the second most populous country, this translates into a very large energy production requirement. With fast depleting world energy resources and a commitment to cut down on CO 2 and greenhouse gas emissions, such large sustained demand for energy puts enormous pressure on the economy, technology, natural resources and the environment. Under these circumstances, nuclear energy is an option that India can safely rely on, literally and figuratively. India's nuclear power program has thorium as the focal point given the large deposit of it in the country. Since thorium is neither fissile nor readily fissionable, the last stage of the three stage Indian nuclear program is expected to have reactors using 233 U as fuel obtained from the conversion of thorium in a fast breeder reactor. Nuclear energy option has been put on the backburner by many countries due to the environmental concerns arising from the long-term geological storage of the nuclear waste. Of particular concern are the long-lived transuranic (TRU) elements such as Pu, Np, Am, Cm etc., and fission products (FP) such as 129 I, 135 Cs, 99 Tc, 93 Zr and 107 Pd. While the TRU constitute about 1% of the nuclear waste mass, its radiotoxicity even after 1000 years is about 20,000 times that of the FP. The path of nuclear energy by using 233 U obtained from 232 Th helps minimize the production of long-lived actinide waste.

In recent years, the concept of coupling an accelerator to a reactor operating in sub-critical modes (keff < 1) has gained popularity among the technologists due to its ability to produce power with lower risks of nuclear proliferation and to incinerate or transmute long lived minor actinides. In such an accelerator driven systems (ADS), external neutrons produced from spallation reactions propel the neutron multiplication. The fact that the system is inherently sub-critical and that the neutron multiplication can be stopped quickly by switching off the projectiles makes such a system safer. While the concept was mooted by early researchers several decades ago, [4],[5],[6] the interest in this subject was renewed by the works of Bowman [7],[8] and Rubbia et al.[9],[10],[11],[12] The commercial setup proposed by the former and the energy amplifier concept of the latter require fluxes of the order of 10 14 -10 16 n/s. This is considered possible today because of the advances in the accelerator technology where it is possible to obtain protons with GeV energy and a few tens of mA (10−3 A) beam current. The concept has also been explored to generate power using thorium as fuel. [13],[14] The external neutrons from the spallation reaction gives greater freedom to the reactor physicist and engineers to design systems which can burn thorium fuel. The reactor power is proportional to the accelerator beam current and it is possible to operate a neutron multiplying core at a significant level of sub-criticality enabling flexibility in design and simplifying the reactor control systems. Furthermore, reactivity margin can be chosen to be sufficiently wide while the prompt criticality does not depend upon the delayed neutrons.

ADS have the potential to provide efficient and economic nuclear power with the available uranium and thorium resources. The radiation safety aspects pertaining to the ADS are discussed here.


  Fluka Monte Code Top


The discussion will be based on the results obtained using the FLUKA Monte Carlo code [15],[16] unless specified otherwise. The code is distributed under royalty-free license through the official website. [17] It is a general purpose tool for simulating the interaction of about 60 particles with matter and is used for a variety of uses such as proton and electron accelerator shielding, target design, calorimetry, activation, dosimetry, detector design, ADS, cosmic rays, neutrino physics, radiotherapy etc. The combinatorial geometry package is used to define regions and for particle tracking. The calculations here are performed considering a cylindrical stopping target with 55% Bi and 45% Pb mass fraction, which is the popular target for such applications. The length of the cylinder is taken to be just longer than the range of protons, which has been calculated using the Stopping and Range of Ions in Matter (SRIM) [18] code. The radius of the target is kept equal to the length to maximize the neutron yield (per proton) and also to retain similar attenuation lengths while scoring residual gamma rays in forward and lateral directions. The time evolution of the residual nuclei inventory and the gamma dose rate are calculated for a cooling period up to 10 years after the target has been irradiated with 1 mA, 1 GeV protons for 30 days.


  Concept of an Ads Top


In typical ADS, a proton accelerator delivers 1 GeV kinetic energy per projectile at a rate of about 10 15 per second (1-10 mA) onto a high Z target thereby initiating a spallation reaction. The ensuing hadronic cascade also produces heat, which in turn is used to produce energy. The amount of heat produced is in excess compared to what is consumed to operate an accelerator and hence the name "energy amplifier." The spallation reactions give rise to several secondary particles including neutrons. The secondary neutrons sustain a chain reaction in the subcritical core consisting of fissile and fissionable materials. Therefore, the number of neutrons produced per incident proton will influence the neutron economy and the neutronics in the system. [Figure 1] shows a schematic of the energy amplifier proposed by Rubbia et al.[9] The core, the spallation target and its circulation for heat removal are shown in [Figure 2]. In these Figures, the accelerator injects the proton beam from the top. Bottom injections where the accelerator is situated underneath are also being explored due to favorable seismic stability factors.
Figure 1: A general lay-out of the energy amplifier complex as envisaged by Carlo Rubbia

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Figure 2: The lead bismuth eutectic loop with the subcritical core of the energy amplifier shown in Figure 1

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While for energy production, the secondary neutron yield has to be a maximum per unit beam power; there are many ADS projects that are meant as precursors to a full scale system. They are used for applications such as technological demonstrations, study of the static and dynamic neutronic properties, thermal hydraulics, safety features, target lifetime, window characteristics etc. For example, GUINEVERE (Generator of Uninterrupted Intense NEutrons at the lead VEnus Reactor) [19] and MYRRHA (Multi-purpose hybrid research reactor for high-tech applications [20] are intended as a technology demonstration projects while YALINA [21] and the zero power sub critical assembly in Bhabha Atomic Research Centre (BARC) coupled to a neutron generator were conceived for neutronic study. Megawatt Pilot Target Experiment (MEGAPIE) [22] is a successful project that has been used to study the various aspects of Pb-Bi liquid metal target under intense proton irradiation.

In addition to energy amplification, ADS can also be used to transmute radionuclides by exposing them to the large resonance absorption cross sections, which usually occur above the thermal energy. The neutrons produced in a spallation reaction must reduce its energy slowly and hence spends more time in the vicinity for it to be absorbed by the target nucleus in the right energy domain. With a light element (say light water) as a moderator, the energy reduction is quick and thus is not suitable for such a purpose. However, interactions with a heavy nucleus reduce the energy of the neutron in a slower manner. This results in a higher probability of finding a neutron in the energy region of interest. Lead as a coolant has such an advantage also as has been demonstrated in the transmutation by adiabatic resonance crossing (TARC) experiment [23] at European Organization for Nuclear Research (CERN). In fact, the high efficiency of such a system has prompted researchers to even consider it for medical isotope production.

Spallation reaction

Spallation, according to the Nuclear Physics Academic press, is a type of nuclear reaction in which the high energy of incident particle causes the nucleus to eject more than three particles; thus, changing both its mass number and its atomic number. The reaction proceeds in stages. In the first stage, the incident (projectile) proton interacts with the nucleus triggering an intra-nuclear cascade (INC), inside the nucleus. In the time scale of this interaction (~10−22 s), several energetic secondary particles are produced such as pions, neutrons, protons etc. They have sufficient energy to cause further reactions with the surrounding nuclei, resulting in an inter-nuclear cascade. Of the pions that are emitted, the π° decays to 2 photons while the π+/− decays to μ+/− and . Together they can initiate an electromagnetic cascade resulting in a shower of electrons/positrons and photons, which propagates independently. The inter-nuclear cascade is followed by a pre-equilibrium emission process where energetic particles such as protons, neutron and alpha particles are emitted. Finally, the remaining nucleus will de-excite through evaporation or by a competing fission process. The evaporation process gives rise to a Maxwellian type spectrum characterized by a nuclear temperature. Depending upon the excitation energy of the nucleus and the particle emission threshold, nucleons can be emitted initially while after a substantial reduction in the excitation energy and angular momentum when it is no more possible energetically to eject nucleons, photons are emitted. Another mode of decay that competes with fission is the fragmentation process wherein complex nuclei with mass ≥ 6 is produced, or even multi-fragmentation where the nucleus breaks up into at least three massive fragments. The envelope of all these reactions constitutes what is termed as spallation.


  Radiation Safety Aspects of the Proton Injector Top


The radiation environment in any accelerator can be classified as prompt and residual. [24] The former is what is produced when the projectiles interact with the structural materials either accidently as a part of the beam transport and diagnostics processes or in its final utilization. Accidental beam loss could occur due to missteering of the beam caused by human factors, errors in control software and read-back or feedback systems, malfunctioning of equipment such as a magnet power supply, errors in calibration resulting in wrong voltage or current being applied etc. Any of these factors can shift the beam from its intended trajectory and put it on a collision path with structural materials. Loss of projectiles also happens at slits, collimators, magnets, targets beam dumps etc., during the course of beam focusing, tuning and transport. All these processes result in generating prompt radiation as the projectiles interact with the atoms and nuclei. In the case of a nuclear interaction, the residual nucleus may be radioactive and emit gamma rays constituting the remnant radiation. The prompt radiation requires bulk shielding while local shielding may suffice to bring the remnant radiation to acceptable levels.

Radiation shielding of high energy accelerators is dealt in several reports. [25],[26],[27],[28] Apart from the sophisticated Monte Carlo codes such as FLUKA, GEANT4, [29] Monte Carlo N-Particle eXtended (MCNPX) [30],[31] etc., simple empirical techniques such as the Moyer model [24],[32],[33] are also very popular among the accelerator radiation shielding community. Shielding the prompt radiation requires knowledge about the type, fluence and the energy distribution of the secondary particles. In proton accelerators, the major component of the prompt radiation is neutrons. Other secondary particles such as muons, pions, etc., can also be produced in the interaction of a 1 GeV proton as seen before. The energy distribution and fluence of the particles emitted from such a reaction is highly directional thus requiring additional information on the angular distribution also. Using the fluence to dose conversion coefficients, [34],[35] the dose equivalent at a given direction for a given particle can be estimated. In [Figure 3], the neutron yields from a thick Cu target bombarded by 50-1000 MeV protons are shown. Also shown are the results obtained by Sullivan's technique. [26] In [Figure 4], the neutron spectra at a few angles of emission from a copper target irradiated by 1 GeV proton are shown. The statistical fluctuations appear below 10 keV as the neutrons are transported inside the thick target. Also shown in the figure is the angular distribution of the un-collided energy integrated neutron yield (normalized at 0°) with respect to the beam direction. Nearly, 30% reduction in the yield can be seen while the angle of emission increases from 0° to 30°, indicating the forward peaking of the neutrons emitted.
Figure 3: The integral neutron yield from a thick Cu target bombarded by protons compared with similar results obtained from Sullivan's technique

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Figure 4: Neutron spectra from a thick Cu target bombarded by 1 GeV protons at different angles with respect to the beam direction. Also shown is the angular distribution of the energy integrated neutron yield (closed circles, top abscissa and right ordinate)

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Concrete is invariably chosen as the transverse bulk shield from the structural integrity and civil engineering point of view. The attenuation length of the concrete varies with the neutron energy and a complete shielding calculation if done analytically should take into account all these factors. The beam loss scenario during the transport is another important, but often contentious issue, since it determines the starting point for the shield evaluation. In general, maximum beam losses are expected at low energies where space charge effects play an important role particularly at high beam currents. As the energy of the projectiles increases, the losses (beam power in watts) are expected to reduce. Care must be taken to identify locations where complete beam loss can also happen in the high energy transport region where a full beam loss condition may have to be assumed.

The following analytical expressions can be used to estimate the shield thickness for neutron spectra spread over a large energy domain. It is worthwhile to mention that the secondary radiations while interacting with the shield material may produce tertiary particles, which of course is not taken into account by such calculations. For such elaborate calculations, Monte Carlo codes are better suited. With a shield wall of thickness "d," the dose rate at a location that is a distance of 'r' [Figure 5] is given by
Figure 5: Schematic showing the shield wall with respect to the beam direction

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H0 (E) is the unshielded fractional dose rate at unit distance for a neutron energy E within dE, which can be obtained from the unit neutron spectrum folded with the fluence to dose conversion coefficient λ (E) is the attenuation length [Figure 6] for neutron energy E, [25] Q is the number of neutrons emitted per proton and "I" is the beam intensity (protons per second). The final shield thickness can be calculated taking into account the regulatory requirement and the occupancy factor.
Figure 6: The attenuation length of concrete of density of 2.35 gcm^-3

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Beam dump

Beam dumps are important part of the transport system as the beam often needs to be diverted to a suitable location while the various diagnostics are performed and the transport parameters are fine tuned. The beam dump should therefore absorb a very large amount of beam power. For example, 1 GeV protons delivered at the rate of 1 mA beam current deposits 1 MW power. This is usually confined to a small focal point thus giving rise to a much higher power density. The dump therefore has to be designed with special emphasis on the heat removal and reducing the radiation dose rates. Usually, the composition of a dump is chosen for its high thermal conductivity and low neutron and gamma emission rates. Water activation too needs to be considered since it will be circulating in close proximity to the incident spot for efficient heat removal.


  Radiation Safety of the Spallation Target Assembly Top


The radiological safety aspects of the spallation target assembly of an ADS are in many ways similar to that of spallation neutron sources (SNS), such as the SNS [36] at Oak Ridge National Laboratory and the European spallation source (ESS). [37] In both the cases, high energy high current protons bombard a suitable target giving rise to secondary neutrons. In the case of SNS, the target is mercury while in the ESS it is tungsten. However, in the case of ADS, the choice is usually lead or eutectic of lead and bismuth (LBE) because of it is good coolant properties. It has good neutronic properties [38] such as high neutron yield and low capture cross section, high volumetric heat deposition rate [39] and a low probability of boiling. Because of its importance in ADS, it is considered as the spallation target for all further discussions here.

Neutron yield from LBE

As noted before, the ADS setup is also used with proton of energy lower than what is required for a full scale energy amplifier. Quite often they are the stand-alone type such as the MEGAPIE test facility. It then becomes important to know the neutron yield for radiation shielding purposes. The data also serves as important input for further calculations such as induced activity, air activity etc. In [Figure 7], the neutron yield from LBE is compared with similar results from a Cu target [Figure 3] and to those obtained by Sullivan's technique. Clearly, the secondary neutron production is higher in LBE compared to a copper target, mainly because of the higher order (pn, xpn) reactions. For example, Bi is known to have cross sections for (n, xn) reactions for x ranging from 2 to 12 [40],[41] for neutrons within 200 MeV. The various (p, xn) cross section for the 3 major isotopes of Pb (206, 207 and 208) and for 209 Bi obtained from the experimental nuclear reaction data (EXFOR database) [42] are shown in [Figure 8] and [Figure 9] respectively. From the two plots, it can be seen that there is appreciable cross section for (p, xn) reactions on both Pb and Bi, well within 1 GeV of proton energy.
Figure 7: The integral neutron yield from Cu and lead bismuth eutectic targets and results obtained by Sullivan's technique

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Figure 8: The (p, xn) cross sections for 206,208Pb

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Figure 9: The (p, xn) cross sections for 209Bi

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Typical neutron energy spectra emitted from a LBE target at 0° and 90° with respect to the beam direction when bombarded with protons of 1 GeV are shown in [Figure 10]. It can be seen that the spectrum extends up to almost 1 GeV in both directions and is characterized by an evaporation peak centered around 1-5 MeV and a second smaller peak at about 100-200 MeV. The neutron spectrum so obtained can be folded with the appropriate dose conversion coefficients. In [Figure 11], the fluence to ambient dose equivalent conversion coefficients for neutrons are shown from thermal energy to 10 6 MeV. For neutron energies up to 200 MeV these values have been reported by International Commission on Radiological Protection [34] while Pelliccioni [35] have calculated such coefficients for higher energies.
Figure 10: The neutron spectra in the forward and the lateral directions from lead bismuth eutectic target bombarded by 1 GeV protons

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Figure 11: Fluence to neutron ambient dose equivalent conversion coefficients

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Induced activity in LBE

The irradiated LBE pose severe handling problems due to the residual activity, residual gamma dose rate and the presence of polonium isotopes. Further, liquid LBE has a higher corrosive action on SS compared to liquid lead. [43] Since the LBE is contained in SS vessels, the enhanced corrosion rate will result in quicker structural degradation thereby requiring frequent replacement and consequently higher exposure to the maintenance staff. A typical contour plot of the residual activity produced in LBE immediately after 30 days of irradiation is shown in [Figure 12]. The activity is also shown as a function of the mass number of the residual nuclei in [Figure 13]. It can be seen that the highest yield is obtained for the target-like radionuclides produced by the (p, xnp) and (n, xnp) reactions on Pb and Bi. The production of the radionuclides far from the target elements are due to mechanisms such as fragmentation and fission in addition to (pn, xnp) reactions.
Figure 12: A contour plot (A/Z) of the residual activity

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Figure 13: The residual activity in LBE as a function of the mass of the residual nuclei

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Time evolution of the induced activity

The total activity generated in the LBE is of interest for safe handling of the irradiated target. [44] The total activity, shown in [Figure 14] has a maximum value of about 10 16 Bq. Similar results were obtained for 575 MeV protons incident on LBE. [45] The activity reduces by about four orders of magnitude as the cooling time increases from 1 h to 10 years. The major radionuclides formed within the initial 10 years decay mainly by electron capture, except for 3 H, 204 Tl and 210 Bi which decay by beta particle emission and 208 Po and 210 Po, which decay by alpha particle emission. [Figure 15] shows the time evolution of the activity of a few major radionuclides as the LBE cools down.
Figure 14: Residual activity at different cooling times in lead bismuth eutectic target irradiated with 1 GeV proton of 1 mA beam current for a period of 1 month

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Figure 15: The amount of major radionuclides in LBE expressed as percentage in the total radionuclide inventory as a function of cooling time

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From a detailed analysis of the radionuclides, the following conclusions can be drawn. During the 1 st day of cooling, 203 Pb, 206 Bi, 201 Tl, 200 Tl, 205 Bi, 210 Bi and 210 Pb are found to be the important radionuclides. Since the half-lives of the radionuclides range from hours to days, they are important from the radiological safety point of view only during the initial few hours.

After a day and up to about a month of cooling, 206 Bi, 205 Bi, 203 Pb, 210 Bi, 210 Po, 201 Tl and 195 Au are present in substantial amounts. During the period of a month and up to about a year of cooling, 210 Po, 195 Au and 3 H are the major radionuclides while 207 Bi, 204 Tl, 185 Os and 181 W have lesser contribution. After a year of cooling, mostly 3 H, 207 Bi, 204 Tl, and 195 Au are present. While 208 Po increases during this time, 210 Po, due to it relatively short half-life starts to reduce rapidly. The absolute tritium activity remains more or less constant due to its long half-life, but becomes important after long cooling periods when the short lived radionuclides would have decayed.

A few rare earth elements such as Sm, Dy and Gd and isotopes of Hg and Au [46],[47] have also been identified from a long-term perspective, such as the disposal of the targets as radioactive waste. For example, Konobeyev et al. [45] have reported the presence of 194 Hg and 194 Au after about 200 years of cooling of the LBE.

Polonium production

An important concern in using LBE as a spallation target is the polonium production, particularly 210 Po. It is expected that about 2 g of polonium will be produced in LBE under constant irradiation with 1 mA protons of 650 MeV. Experiments at the MEGAPIE have shown that measurable amounts of polonium are evaporated within an hour at temperatures above 973 K while in the long-term, slow evaporation of polonium occurs at temperatures at about 873 K. This results in a loss of polonium at a rate of 1% per day. [48] It is noteworthy that LBE melts at 398K. Gaseous phase of 210 Po is known [49],[50] to cause contamination in stainless steel (SS) filters. [51]

Under irradiation by 1 GeV protons, several isotopes of polonium are formed with masses ranging from 207 to 211. Among these, 200-207 Po decay either by isomeric transition or by electron capture except for 200 Po, which has about 11% probability of alpha emission. This probability increases with the mass number and 208-211 Po all decay by emitting α-particles while 210 Po and 211 Po decay only through alpha emission. 210 Po has a half-life of 138 days while 211 Po decays with a half-life of 0.5 s. Thus, 210 Po is the most important isotope of polonium that needs to be quantified from the radio-toxicity point of view. 210 Po is formed by the beta decay of 210 Bi, which in turn is formed through the capture of a neutron by 209 Bi. The cross section for 209 Bi (n, γ) is about one barn for thermal neutrons while several resonances [Figure 16] give similar values between 1 keV and 100 keV. [52] At about 1 MeV, the value falls to about 1-10 mb. Therefore, the amount of 210 Po produced in a LBE surrounded by a sub critical core will be much larger than what is produced under proton irradiation since in such a system, the thermal neutron fluence will be comparatively higher. [53]
Figure 16: The capture cross section of 209Bi

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The time evolution of the total polonium activity and that of 210 Po has been studied extensively in the literature. [44],[54] It is observed that 210 Po is not the major constituent among all polonium isotopes at lower incident proton energies and also in the initial cooling period. In fact, at lower proton energies (<500 MeV) 206 Po and 208 Po are the major polonium isotopes formed in LBE. The time evolution of the activity of 210 Po and the sum of the activity of all the polonium isotopes are shown in [Figure 17]. In the figure, the activity of all the polonium isotopes (left ordinate) falls by 5 orders of magnitude while the amount of 210 Po falls by 10 orders (right ordinate) in the same time span. As the cooling time increases, 210 Po becomes important and after about 100 days, the percentage of 210 Po in the total radioactive inventory is close to 50% as seen in [Figure 18]. Evidently, 210 Po is one of the major radionuclides produced in LBE after a cooling period of about 30 days.
Figure 17: The time evolution of the activity all the polonium isotopes (left ordinate) and 210Po (right ordinate) in lead bismuth eutectic as a function of cooling time

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Figure 18: The time evolution of 210Po in lead bismuth eutectic expressed as a percentage of the total radionuclide inventory

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Residual gamma dose rate

The residual gamma dose rate from LBE at a distance of 1 m at 0°, 90° and 180° with respect to the beam direction are shown in [Figure 19] as a function of cooling time. The dose rates are seen to higher at the backward direction and may seem counter intuitive. However, both lead and bismuth are very good shield materials for gamma photons and at 1 GeV, protons have a range of about 60 cm in LBE. Thus, the target dimensions are equivalent to several tenth value layers, which attenuates the gamma photons. Since gamma emission also happens deep inside the target the self-shielding effect is variable. Most of the residual nuclei production is concentrated in the initial region of the target and the gamma rays are emitted isotropicaly and are therefore less self-shielded toward 180° with respect to the beam direction. From the figure, it can be seen that the major reduction in dose rate occurs between about 10 days and 100 days of decay time. The results are also compared with the values predicted by Sullivan's technique. It can be seen the results from the Sullivan's technique approximately agree with the FLUKA results during the initial days of cooling but over predicts afterwards.
Figure 19: The residual dose rate from a thick lead bismuth eutectic bombarded by 1 GeV protons at a rate of 1 mA for a period of 1 month. The FLUKA results are compared with the results obtained by Sullivan's technique

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  Accelerator-Reactor Coupling Top


While the radiation safety issues of a reactor or an accelerator in a standalone mode can be addressed separately, several other issues also come to light when the two have to be coupled. For example, if the reactor is also planned as a general purpose one, access to the reactor top will have to be allowed almost immediately after the beam shutdown for its optimum use. The residual gamma doses rate from the LBE will then have to be estimated in its constant motion. The movement of LBE in the loop is usually achieved with the assistance of forced N 2 circulation while water columns surround the LBE for cooling purposes. All these leave annular gaps, which enables streaming of radiation. In the case of a bottom injection design, the prompt neutron dose rates prevailing above the reactor pool may require additional shielding. Both neutron and residual gamma dose rates, direct and streaming, will depend on the amount of LBE and the water present in the system. The radial shielding of the reactor will have to be determined taking into account the transport of the spallation neutrons in the core with D 2 O and Be reflectors. The amount of 210 Po production will also be higher in a coupled system due to the enhanced thermal neutron fluence. A window usually separates the LBE loop from the high vacuum region of the accelerator and its failure will result in the LBE spilling inside a room or in the beam transport tube. The evaporation rate of 210 Po under various circumstances will then have to be considered keeping in mind the temperature of the LBE.


  Summary Top


The radiological safety aspects of ADS are discussed here from the radiation shielding point of view of the injection accelerator and the LBE spallation target. The extent of residual activity including the polonium production and the residual gamma dose rates from an irradiated LBE are also discussed. Some of the important points that may require attention when the accelerator is coupled to a reactor are also briefly mentioned.

 
  References Top

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