Beam instability correction

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- 1015 Fast Pb-Bi UO2/UN

U/MA/Zr Pavlopoulos et al. (2003) CSMSR (Russia) 10 / 800 (1 GeV, 10 mA) 0.95 5x1014 Intermediate Pb-Bi Np/Pu/MA molten salt Degtyarev et al. (2005, 2006)

From US projects mentioned in the Table 3 only the Spallation Neutron Source (SNS) was finished up to now. ATW project was postponed due to its inutility. Japan projects are further developed in the JAERI under the TEF facilities. HYPER project in South Korea was closed in 2006. Italian and French XADS (eXperimental Accelerator Driven System) stayed up to now only in the planning phase. Belgium XADS project developed into European project called MYRRHA with start of construction in 2015. All three Russian projects were stopped in the planning phase because of the lack of money.

1.8. Experiments focused on nuclear data measurements

Cross-sections of various reactions are of fundamental importance for future ADS. Many construction materials, which are nowadays commonly used in nuclear reactors, will be exposed to extreme neutron fluxes of high energies. At this region of energies, only very few cross-sections are known. Precision of cross-section knowledge is even more important for materials of transmutation interest. Bad knowledge of cross- sections and properties of nuclear reactions in general can lead in production of even longer-lived isotopes than is the transmuted one or at least to low transmutation rates. On the other hand, with good knowledge of cross-sections and thus proper choice of neutron energy and time of irradiation, negative effects can be minimized or eliminated.

A few international initiatives were established to gain nuclear data for future ADS. Within the Fifth Framework Programme (FP5) of the European Union following researches were done [25]:

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Thorium Cycle project

Thorium cycle project coordinated by Nuclear Research and Consultancy Group from Netherlands was focused on the measurement of key data for thorium fuel cycle in reactors and ADS systems. Various mixtures of Th/Pu fuel were studied under high burn-up in order to decrease its long-term radioactivity and thus demands on geological repositories.

CONFIRM

CONFIRM project was collaboration on Nitride Fuel Irradiation and Modeling. Research was oriented on the oxide and nitride ADS fuels without uranium. Special design of fuel pellets was developed in order to reach extremely high burn–up. Special attention was paid to safety parameters of the fuel, which must be fulfilled through whole fuel irradiation. Coordinator of this project was Royal Institute of Technology from Stockholm, Sweden.

HINDAS

High and intermediate energy nuclear data for accelerator-driven system (HINDAS) was European project focused directly on nuclear data. Experimental data were measured on various accelerators throughout Europe. Nuclear models were improved according to experimental data. Energy scope of the HINDAS project was on energies from 20 to 2000 MeV. Libraries of nuclear data were extended up to 200 MeV (format ENDF was used).

n-TOF

The neutron time-of-flight facility (n-TOF) has been developed in the European Organization for Nuclear Research (CERN) since 2001. Time-of-flight method with fly path 200 meters is used to determine energy of the neutrons. The main goal of the project is to produce, evaluate and disseminate high precision cross sections for the majority of the isotopes relevant to the waste incineration and the ADS design.

The Sixth Framework Programme [26] followed in the support of various research activities related to ADS and transmutation. Direct relation to the ADS has following four sub-programmes:

EUROTRANS (EUROpean Research Programme for the TRANSmutation of High Level Nuclear Waste in a Accelerator Driven System).

EUROPART (EUROpean Research Programme for the Partitioning of Minor Actinides).

EFNUDAT (European Facilites for Nuclear Data Measurements). We used this programme to get access to the quasi-monoenergetic neutron source in The Svedberg laboratory in Uppsala, Sweden. More details about this programme are described in the chapter 7 Section 3.

1.8. Experiments focused on nuclear data measurements

17 RED IMPACT (Impact of Partitioning, Transmutation and Waste Reduction Technologies on the Final Waste Disposal Project).

1.9. Summary of ADS research goals

Research of various ADS aspects continues nowadays both on simple setups and experiments, and on more complicated assemblies. Simple setups are used to measure the cross-sections of GeV down to MeV neutrons, and to study the spallation reaction and high energy neutron transport in more detail. More complex systems verify neutron multiplication, transmutation rates, heat production, long-term stability and overall suitable concepts for future XADS. Special attention starts to be paid to the engineering problems in construction of future ADS systems.

There is also increasing motivation towards improving the precision of predictions of the codes used to simulate production and transport of high-energy spallation products in material. More realistic simulations will help to design more effective spallation neutron sources, subcritical blankets or better radiation shielding. Good codes can also spare budget in all stages of ADS life. But for codes development and improvements, a lot of real experimental data for comparisons and benchmark tests is needed.

My research in the field of accelerator driven systems involves both the simple and complex experiments. The simple experiments are represented by the neutron cross- section measurements of the (n,xn) threshold reactions. Spallation experiments on the Energy plus Transmutation (E+T) setup belong to the complex experiments. Series of experiments of both types are described and compared with simulations in the following chapters.

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Chapter 2

Energy and Transmutation of Radioactive Waste project

There is a long tradition of spallation and high energy neutron studies in the Joint Institute for Nuclear Research (JINR Dubna, Russia). During the 1980s and 1990s, wide range of spallation targets was irradiated and the neutron production was studied with the respect to the target shape, dimensions, material and to the surrounding volumes. This aim culminated at the end of 1990s in the Energy plus Transmutation (E+T) project. The leader of this project was for almost last two decades M. I. Krivopustov, who established a big international team with interest in transmutation studies. Target systems Gamma-2, Energy plus Transmutation and Gamma-MD were developed and irradiated with protons and deuterons from the Nuclotron accelerator.

Since 2009, M. Kadykov has been a new leader of the collaboration. The collaboration was renamed to Energy and Transmutation of Radioactive Waste (E&T RAW) and got a better position in the JINR structure, so a further development is foreseen. Collaboration is still growing and has nowadays approximately 85 members from 15 countries (Armenia, Australia, Bulgaria, Czech Republic, Poland, Germany, Russian federation, Belarus, Ukraine, Mongolia, Serbia, Kazakhstan, Greece, India, and Moldova). Two new target systems are developed, the first setup called Kvinta was already tested in experiment, the second setup called Ezhik is in the phase of technical design.

Focus of our group from Řež is on high energy neutron measurement and beam diagnostics. We use so called reversed activation neutron detectors – we put foil of a known isotope into unknown neutron field. Energy range of studied neutrons is from 5 up to about 80 MeV. Other groups from the collaboration use activation analysis on different isotopes, solid state nuclear track detectors (SSNTD), He-3 counters and nuclear emulsions to study other parts of the neutron spectrum.

E&T RAW targets will be described shortly in the following sections. Main physical purpose of all targets is to study spallation reactions caused by GeV protons and deuterons, transport of high energy neutrons and transmutation. Use of various target and blanket materials, geometries and surrounding moderators enables to study their influence on neutron field. Systems have a big advantage in possibility of measuring integral data – transmutation rates of actinides in real spallation field. GAMMA-2, E+T and GAMMA-3 setups were introduced into a Coordinated Research Project of IAEA and these targets are now acknowledged as “IAEA benchmark targets”.

2.2. Gamma-2

Gamma-2 setup consists of a lead target 8 cm in diameter and 20 cm long. Later the target was prolonged to 50 cm. It is surrounded with paraffin moderator of 6 cm thickness. Gamma-2 setup was irradiated with protons in the energy range 0.5 –

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4.15 GeV [27], respectively 1 – 2 GeV at the prolonged version [28]. Main experimental task of this setup was a study of spallation reactions and transport of high energy neutrons. First measurements with radioactive samples and their transmutation in the field of moderated neutrons were done. Scientific program on this target was more or less closed, but the target will be still ready for new irradiations if there is a need.

Figure 7: Gamma-2 setup consisting of lead target (discs) and paraffin moderator.

2.3. E+T setup

Further step in the transmutation studies was a more complex target system called “Energy plus Transmutation” setup (E+T setup). Setup was irradiated with 0.7, 1, 1.5, and 2 GeV protons, results of 1.6, 2.52, and partly 4 GeV deuteron irradiations are the main topic of this PhD thesis, that is why I will describe the target in more detail. The 0.7 GeV proton experiment was a subject of my diploma thesis [29], results of all proton experiments were the main subject of successfully defended PhD thesis of A. Krása [30]. Results concerning the proton experiments were also published as JINR Preprints [31], [32], [33], [34]; and presented on many conferences and workshops.

The E+T setup consists of a cylindrical lead target (diameter 84 mm, total length 480 mm) and a surrounding subcritical uranium blanket (206.4 kg of natural uranium). Target and blanket are divided into four sections. Between the sections there are 8 mm gaps for user‟s samples, detectors and emulsions. Each section contains target cylinder 114 mm long and 30 identical natural uranium rods, which are secured in a hexagonal steel container with a wall thickness of 4 mm. The front and back of each section are covered with hexagonal aluminum plate 6 mm thick. The four target-blanket sections are mounted along the target axis on a wooden plate of 68 mm thickness, which is moreover covered with 4 mm thick steel sheet. Uranium rods are hermetically encapsulated in aluminum coverage of thickness 1 mm, respectively 2 mm at the bases. Each rod has an outside diameter of 36 mm, a length of 104 mm, and a weight of 1.72 kg. Density of the uranium is considered to be 19.05 g·cm-3.

Around the blanket, there is a radiation shielding consisting of a wooden box, cadmium plates and polyethylene ((CH2)n) in the box walls. Cadmium plates have

2.3. E+T setup

21 density of 0.8 g·cm-3 and is granulated. On the floor inside the shielding box a 38 mm thick textolite2 plate is placed. Shielding moderates and absorbs only a part of the high energy neutrons emerging from the setup, so there is a dosimetry limit on the beam flux.

Figure 8: Cross-sectional side view (left) and front view (right) of the "Energy plus Transmutation" setup. All dimensions are in millimeters.

Figure 9: Photo of the Energy plus Transmutation setup with the biological shielding (left). Detail of the natural uranium blanket (right).

2

Textolite (Latin textus – a cloth, and Greek lithos – stone) is a material consisting of several layers of fabric (filler); it is soaked by a synthetic resin.

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More detailed information about the setup can be found for example in [31]. The detailed analysis of the influence of different setup parts and uncertainties in their geometrical and physical definitions on the neutron flux and possible sources of systematic uncertainties of obtained experimental data are analyzed using MCNPX simulation code in [31].

2.4. Gamma-3

Gamma-3 setup, sometimes also called Gamma M-D3, is a setup consisting of cylindrical lead target and big graphite moderator. Target has a diameter of 8 cm and length of 60 cm. Graphite moderator consists of blocks 25x25x60 cm3 and 20x20x60 cm3 big; total volume is 110x110x60 cm3. In the moderator there are four cylinders, that can be pulled out and contain holes for sensors. Besides this, there are a few small plain holes through the moderator. Whole setup is placed on rails in F3 hall for easier manipulation.

Figure 10: Photo of Gamma-3 setup in F3 experimental hall (left) and graphite cylinder with holes for samples.

Up to now there was only one experiment on Gamma-3 setup with 2.33 GeV deuterons. Main experimental task was a study of radioactive sample transmutation;

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I, 237Np, 238Pu, 239Pu, and 241Am were used. Next experiments are planned in the first half of the year 2011.

2.5. Kvinta setup

A new “ready to use” target has been available for the E&T RAW collaboration since the end of the year 2009. It is a setup of massive uranium target and lead shielding. Target has three sections of the same shape as E+T blanket – a hexagon, but filled completely with uranium rods (weight 315 kg). Target is surrounded with massive

2.5. Kvinta setup

23 lead shielding of total weight 1780 kg. Target is permanently placed in the shielding and the inner volume of the target is accessible only through four thin slots. Plastic holders are used to place samples inside the target [35]. There were two pilot measurements done during the 2009 winter run of the Nuclotron, setup was shortly irradiated with deuterons of 1 and 4 GeV energy.

d

Figure 11: Schema of Kvinta target. On the left there is a cut-view on the uranium target with supporting structures and plastics used for sample placement, on the right there is a view on the lead shielding enfolding the target [35].

2.6. EZHIK

Completely new target complex called EZHIK is nowadays projected and it should be ready to use by the end of 2012. Then it will be the main experimental device of the E&T RAW collaboration, although all previous targets will still exist and will be available for users.

Name EZHIK means hedgehog in Russian, the parallel with hedgehog is because of the vertical channels sticking from the target. The target complex EZHIK is a quasi-infinite target from metallic uranium with wide range of measurement channels and positions. Basic scheme can be seen in Figure 12. The original technical solution of asymmetric beam input into a quasi-infinite target (first applied in [36]) is implemented in somewhat modified form. It provides results equivalent to those that could be obtained with 8 t uranium target in the case of conventional axial beam input into a cylindrical symmetric target, but with just about 3 t of target material from natural uranium [37].

Scientific program of the target EZHIK will be developed in three main fields. First direction will be focused on gathering of integral data, mainly in the direction of fission rates and transmutation cross-sections of actinide fission fragments. For this, wide range of support data will be measured – particle fluxes, energy and heat distribution, isotopes equilibrium, neutron multiplication and dosimetry quantities. Second direction will be devoted to simulations. It is expected that all differences between the models and experiments, which were observed in the past, will be more

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pronounced at quasi-infinite target and thus it will be easier to find the reason and correct it. Third direction will be focused on structural and fuel materials irradiated with large doses of relativistic beams and high energy neutrons. Radiation damage and gas production will be studied.

Uranium

Graphite

Lead Measurement channels

Figure 12: Scheme of the new target EZHIK [35].

Besides the basic version with uranium marked EZHIK-U will be developed also a version EZHIK-Pb, which will be geometrically identical, but whole inner volume will be filled by natural lead. EZHIK-Pb will be used for verification and adjustment of basic measurement systems and methods as well as background measurements with proton and deuteron beams in the projected energy range before main experiments with uranium target EZHIK-U will be made.

2.7. Placement of the E&T RAW targets

For the E&T RAW collaboration is now allocated whole F3 experimental hall at the Nuclotron accelerator, see Figure 13. Targets stand in the hall on rails, so they can be quickly moved in/out of the beam. There is a crane in the hall to manipulate with heavier parts of the targets and equipment.

2.7. Placement of the E&T RAW targets

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0 1m 5m 10m

Scale:

Beam

Room for E&T RAO personal

A B C D Нейтронная защита 1 2 3 4 4 5 6 7 8 9 10 Experimental setups A– «Energy+Тransmutation»; B– «Еzhik»; C– «Gamma-3»; D– «Кvinta».

Hall diagnostic system: 1– Ionization chamber

2– Activation foils

3 - Profile meter 4– Scintillator telescope

5 – Pneumatic transport system

6 - B F3 detector

7– Neutron spectrometer

8– Stilben detector; 9 -Detector «Isомеr»(3_He);

10 - Detector LaBr3(Ce).

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Chapter 3

Experimental background

My work is focused on the studies of high energy neutrons produced in spallation reactions and their transport in the setup. High energy neutrons are in this case neutrons with energies from approximately 5 MeV up to 100 MeV. These neutrons can be measured by multiple methods (time of flight, nuclear emulsions, proton recoil detectors etc.) but specific conditions in the E+T setup makes these methods hard to use or unsuitable.

Main limitations of high energy neutron measurements are the following: - lack of space – a need to measure the neutrons inside the setup, - neutron field is changing on centimeter scale,

- presence of thermal, epithermal and resonance neutrons, - presence of protons, deuterons and heavier charged particles, - huge gamma background,

- specific conditions in JINR Dubna - problems with transport of electronics and with its operation due to highly intensive short bunches from the accelerator. Method of neutron activation detectors solves most of these problems. Samples can be small, thin, are insensitive to gamma and they do not need any power or maintenance during irradiation. Unirradiated samples can be easily transported, are simple to handle and relatively cheap (compared to electronic equipment). Last but not least there is a long tradition in using neutron activation detectors for high energy neutrons measurement at NPI.

Following chapter will discuss the equipment, methods, and corrections used in experiments. My PhD work is focused mainly on the experimental part of the E+T experiment, so this description will go into detail on some places. I was the first one in our group who routinely applied some of the corrections into the experimental data and studied their effect. Results of these studies are also presented.

3.1. Activation detectors

Neutron activation analysis method is mostly used for detecting the small amount of some isotope in compound. It is a very sensible method with sensitivity level up to 10-13 gram per gram [38]. It can measure qualitative as well as quantitative content of tens of isotopes in one measurement. It uses known fields of neutrons or a system of