EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCH

CERN/INTC 2001-025 INTC/P-145 08 Aug 2001

Measurements of Fission Cross Sections for the Isotopes relevant to the Thorium Fuel Cycle

The n_TOF Collaboration

ABBONDANNO U.20, ANDRIAMONJE S.10, ANDRZEJEWSKI J.24, ANGELOPOULOS A.12, ASSIMAKOPOULOS P.15, BACRI C-O.8, BADUREK G.1, BAUMANN P.9, BEER H.11, BENLLIURE J.32, BERTHIER B.8, BONDARENKO I.26, BOS A.J.J.36, BUSTREO N.19, CALVINO F.33, CANO-OTT D.28, CAPOTE R.31, CARLSON P.34, CHARPAK G.5, CHAUVIN N.8, CENNINI P.5, CHEPEL V.25, COLONNA N.20, CORTES G.33, CORTINA D.32, CORVI F.23, DAMIANOGLOU D.16, DAVID S.8, DIMOVASILI E.12, DOMINGO C.29, DOROSHENKO A.27, DURAN ESCRIBANO I.32, ELEFTHERIADIS C.16, EMBID M.28, FERRANT L.8, FERRARI A.5, FERREIRA–MARQUES R.25, FRAIS–KOELBL H.3, FURMAN W.26, FURSOV B.27, GARZON J.A.32, GIOMATARIS I.10, GLEDENOV Y.26, GONZALEZ–ROMERO E.28, GOVERDOVSKI A.27, GRAMEGNA F.19, GRIESMAYER E.3, GUNSING F.10, HAIGHT R.37, HEIL M.11, HOLLANDER P.36, IOANNIDES K.G.15, IOANNOU P.12, ISAEV S.27, JERICHA E.1, KADI Y.5, KAEPPELER F.11, KARADIMOS D.17, KARAMANIS D.15, KAYUKOV A.26, KAZAKOV L.27, KELIC A.9, KETLEROV V.27, KITIS G.16, KOEHLER P.E.38., KOPACH Y.26, KOSSIONIDES E.14, KROSHKINA I.27, LACOSTE V.5, LAMBOUDIS C.16, LEEB H.1, LEPRETRE A.10, LOPES M.25, LOZANO M.31, MARRONE S.20, MARTINEZ-VAL J. M.30, MASTINU P.19, MENGONI A.18, MEUNIER R.7, MEZENTSEVA J.26, MILAZZO P.21, MINGUEZ E.30, MITROFANOV V.27, NICOLIS N.15, NIKOLENKO V.26, OBERHUMMER H.1, PAKOU A.15, PANCIN J.6, PAPADOPOULOS K.13, PAPAEVANGELOU T.16, PARADELA C.32, PARADELIS T.14, PAVLIK A.2, PAVLOPOULOS P.15,35,*, PEREZ-PARRA A.28, PERRIALE L.19, PERLADO J. M.30, PESKOV V.34, PIKSAIKIN V.27, PLAG R.11, PLOMPEN A.23, POCH A.33, POLICARPO A.25, POPOV A.26, POPOV Y.P.26, PRETEL C.33, QUESADA J.M.31, RADERMACHER E.5, RAUSCHER T.35, REIFARTH R.11, REJMUND F.8, RUBBIA C.22, RUDOLF G.9, RULLHUSEN P.23, SAKELLIOU L.12, SALDANA F.5, SAMYLIN B.27, SAVVIDIS I.16, SAVVIDIS S.17, SEDYSHEV P.26, STEPHAN C.8, SZALANSKI P.24, TAGLIENTE G.20, TAIN J.L.29, TAPIA C.33, TASSAN–GOT L.8, TERCHYCHNYI R.27, TSABARIS C.13, TSANGAS N.17, van EIJK C.W.E.36, VANNINI G.20, VENTURA A.18, VILLAMARIN A.28, VLACHOUDIS V.5, VLASTOU R.13, VOINOV A.26, VOSS F.11, WENDLER H.4,**, WIESCHER M.39, WISSHAK K.11, ZEINALOV S.27, ZHURAVLEV B.27 ______* Spokesperson ** Technical Coordinator 1. Atominstitut der Österreichischen Universitäten, Technische Universität Wien, Austria 2. Institut für Isotopenforschung und Kernphysik, Universität Wien, Austria 3. Fachhochschule Wr. Neustadt, Wien, Austria 4. CERN, EP Division, Geneva, Switzerland 5. CERN, SL Division, Geneva, Switzerland 6. Centre National de la Recherche Scientifique/IN2P3, CENBG, Bordeaux, France 7. Centre National de la Recherche Scientifique/IN2P3, CSNSM, Orsay, France 8. Centre National de la Recherche Scientifique/IN2P3, IPN, Orsay, France 9. Centre National de la Recherche Scientifique/IN2P3, IreS, Strasbourg, France 10. Commissariat à l’Energie Atomique/DSM, Gif-sur-Yvette, France 11. Forschungszentrum Karlsruhe GmbH (FZK), Institut für Kernphysik, Germany 12. Astro-Particle Consortium, Nuclear Physics Lab., University of Athens, Greece 13. Astro-Particle Consortium, Nuclear Physics Dep., Technical University of Athens, Greece 14. Astro-Particle Consortium, Nuclear Physics Instit., NCSR "Demokritos", Athens, Greece 15. Astro-Particle Consortium, Nuclear Physics Lab., University of Ioannina, Greece 16. Astro-Particle Consortium, Nuclear Physics Lab., University of Thessaloniki, Greece 17. Astro-Particle Consortium, Nuclear Physics Dep., University of Thrace, Greece 18. ENEA, Applied Physics Division, Bologna, Italy 19. Laboratori Nazionali di Legnaro, Italy 20. Instituto Nazionale di Fisica Nuclear-Bari, Italy 21. Instituto Nazionale di Fisica Nuclear-Trieste, Italy 22. Universita degli Studi Pavia, Pavia, Italy 23. JRC-EC, IRMM, Geel, Belgium 24. University of Lodz, Lodz, Poland 25. Laboratorio de Instrumentacao e Phisica Experimental de Particulas,LIP-Coimbra & Departamento de Fisica da Universidade de Coimbra, Portugal 26. Joint Institute for Nuclear Research, Frank Laboratory of Neutron Physics, Dubna, Russia 27. Institute of Physics and Power Engineering, Kaluga region, Obninsk, Russia 28. Centro de Investigaciones Energeticas Medioambientales y Technologicas, Madrid, Spain 29. Consejo Superior de Investigaciones Cientificas – University of Valencia, Spain 30. Universidad Politecnica de Madrid, Spain 31. Universidad de Sevilla, Spain 32. Universidade de Santiago de Compostela, Spain 33. Universitat Politecnica de Catalunya, Barcelona, Spain 34. Kungliga Tekniska Hogskolan, Physics Department, Stockholm, Sweden 35. Department of Physics and Astronomy, University of Basel, Basel, Switzerland 36. Delft University of Technology, Interfaculty Reactor Institute, Delft, The Netherlands 37. Los Alamos National Laboratory, New Mexico, USA 38. Oak Ridge National Laboratory, Physics Division, Oak Ridge, USA 39. University of Notre Dame, Notre Dame, USA CONTENTS

1. INTRODUCTION...... 1

2. STATUS OF NUCLEAR DATA FOR THE THORIUM CYCLE ISOTOPES...... 5

3. THE ISOTOPIC SAMPLES...... 12

4. THE EXPERIMENTAL SET-UP...... 13

5. THE ENERGY RESOLUTION...... 15

6. COUNTING RATES...... 16

7. BEAM TIME REQUEST...... 17

8. REFERENCES ...... 18

–

1. INTRODUCTION. The present concern about a sustainable energy supply [1] is characterised by a considerable uncertainty: the green house effect and foreseeable limits in fossil fuel resources on the one hand, the concern about the environmental impact of nuclear fission energy and the long term fusion research on the other hand, have led to the consideration of a variety of advanced strategies for the nuclear fuel cycle and related nuclear energy systems. The present research directories concern such strategies as the extension of the life span of presently operating reactors, the increase of the fuel burn-up, the plutonium recycling, and in particular the incineration of actinides and long-lived fission products, the accelerator driven systems (ADS), like the “Energy Amplifier” (EA) concept [2,3] of C. Rubbia, and the possible use of the Thorium fuel cycle [4]. The detailed feasibility study and safety assessment of these strategies requires the accurate knowledge of neutron nuclear reaction data [5,6]. Both, higher fuel burn- up and especially waste incineration options require improved and partly new data on actinides. It is well known that only two fertile elements, i.e. 232Th and 238U, exist in nature, with the first showing major radiotoxicity advantages over the second. In the thorium cycle shown in Figure 1-1, the fissile element 233U is regenerated by the 232Th(n,γ)233Pa(β–)233U process. The basic 232Th(n,γ) and 233U(n,f) reaction cross sections have to be known very precisely. The other fundamental parameter of this cycle is the cross section of the 233Pa(n,γ) reaction, which due to the high specific activity of 233Pa can only be indirectly determined. In the uranium cycle the main fissile element is 239Pu, that is generated via 238U(n,γ)239Np(β–)239Pu. The (n,γ) and (n,f) reactions are the most important in developing waste incinerators, since they allow to transform long-lived radioactive isotopes into short-lived ones. Charged particle producing reactions, such as (n,p) and (n,α), reactions have generally much lower cross sections, usually below 100 mb, and are therefore of lesser importance. However, the (n,n’) and (n,xn) reactions with cross sections attaining several barns are of great interest in the conception of ADS projects, since they govern the neutron flux and a large part of the energy density. The LLHEa component of the radioactive waste comes from parasitic reactions on the different constituents of the fuel. For the Th-cycle the parasitic reactions are 233U(n,2n)232U, 232Th(n,2n)231Th(β–)231Pa and 231Pa(n,γ)232Pa(β–)232U leading to the production of 232U, which is responsible for a large part of the short term (few centuries) radiotoxicity, and 231Pa responsible for the long term radiotoxicity. The importance of the thorium fuel cycle is twofold: as an alternative fuel, it is the basis for safe and sustainable energy generation through accelerator driven systems, as well as the design and development of nuclear waste incinerators, based on the Th-U fuel. Indeed studies [7,8] for incinerating the radioactive waste from LWR’sb and for an alternative fuel with limited waste production allowing in addition a long-term sustainability have led to new interest in the accelerator assisted Th fuel cycle. a Long Lived Heavy Element. b Light Water Reactor.

–1–– 6.8 d 237 237U Np

6 y

22 h 2.14 10 236mNp 236U 236Pu 236Np 1.6 105 y 7 y

235 2.34 10 U 2.85 y

8 y

7.04 10 24.1 d 1.2 min 234 234Th 234Pa U y 5

22 min 2.46 10 233Th 233Pa 233U 27 d

232Th 232Pa 232 1.3 d U

25 h 68.9 y 231Th 231Pa

y 4 Fertile Fissile y 5 3.28 10 Radiotoxic 230Th 1.6 10 (n,γ) capture EC β-decay 229Th

α-decay

228Th (n,2n) reaction 208Tl (E : ≥ few MeV) 227Ra n 212Po P.P.Ðslide 190418

Figure 1-1: Schematic view of the “Thorium cycle”.

The summary report of the INDC/IAEA consultant’s meeting [9] concluded that: • There is a growing wide interest for advanced nuclear power technologies of the future involving new types of nuclear fuel, providing inherent safety, being resistant to nuclear proliferation, reduction of weapon grade plutonium and solving the problems of radioactive wastes. • The Th-U cycle is the most prospective one, which meets the above goals. • The nuclear data for nuclides of the Th-U fuel cycle have been evaluated in the early seventies and middle of the eighties and do not fulfil the current accuracy requirements. Similar efforts to the U-Pu cycle need to be undertaken for the Th-U cycle.

–2–– In particular, the concept of the “Energy Amplifier” in Europe [4,6] envisions transmuting the actinides (except uranium) from a light water reactor spent fuel in such subcritical assemblies [8]. In order to simulate with the required accuracy the behaviour of such alternative systems, and to advance towards their design, testing and eventual full deployment of a prototype (XADSc) based on the Th-cycle, basic nuclear data for the eight isotopes relevant in the Th–cycle, i.e. 230Th, 232Th, 231Pa, 233Pa, 232U, 233U, 234U and 236U, in the form of differential neutron cross sections need to be derived in a consistent and accurate way, as well as to be formally evaluated following standard procedures and made available in a way compatible with simulation tools and industry practices in general. In the presently available compilation databases [10] though, nuclear data for the Th-cycle, studied in only few experiments contrary to fuel or weapon nuclei like 235U, 238U and 239Pu, are in a less than adequate level, while the prospect of innovative concepts broadened considerably the range of data requirements. The IAEA recommendations [9] included 232Th, 231,233Pa, and 232,233,234,236U as first priority isotopes. 236U was added because of its build-up in the equilibrium fuel composition. For the fast reactors and ADS systems the neutron spectra are rather similar and the corresponding data are required with the following accuracy [9,11] for both concepts: 232Th(n,f) with 5%, 231Pa with 20%, 233Pa(n,f) with 20%, 232U(n,f) with 20%, 233U(n,f) with 1%, 234U with 3% and 236U with 5%. From the eight isotopes, the 233Pa and 232U are not easily experimentally accessible for measurements due to their high specific activity, which implies problems in their availability with the required purity but also due to the severe limitations for transportation. 230Th: In the fast energy region it has about twice the fission cross section of 232Th. For instance, the fission spectrum averaged neutron induced fission cross section is 163 mb as compared to 78.5 mb for 232Th. This leads to a slight increase of the power production coming from thorium fission and also helps in breeding 233U fuel by its contribution to the neutron balance. 231Pa: This is the main actinide produced in the Th cycle, primarily through the (n, 2n) reaction on the main element 232Th and subsequent beta decay of 231U. In thorium fuelled reactors, as the Energy Amplifier (EA), production of 231Pa takes place in significant quantities and a net stockpile of the order of 5 kg of 231Pa will persist during the whole life time of an EA plant. Therefore, it constitutes a considerable source of radiotoxicity. On a time scale of a few 100'000 years or larger, it could present significant hazards from surface or near surface dissolution. The neutron fission cross section is expected to be measured with a precision of 20% at least. 232Th: In an ADS or equivalent fast reactor it accounts for 3% of the fissions, and at least for 8% in a gas cooled reactor. Indeed, near the fission threshold, 232Th has an important contribution of 2% to delayed neutrons as compared to 0.25% for 233U. So the knowledge of the neutron induced fission is requested with good accuracy, of the order of 5%. 233U: The cross section of this main fissile element is to be known with an accuracy of 1%.

c Experimental Accelerator Driven System.

–3–– 234U: Formed by neutron capture in 233U, it decays by alpha emission with a half-life of 2.5x105 years. In thorium reactors, the formation of 235U by neutron capture in 234U helps to reduce the burn-up swing in long lived cores. Since 234U also gets formed by (n,2n) reactions in 235U, the 234U isotope is common to both uranium and thorium fuel cycles. The 234U isotope plays a role in thorium fuel cycle similar to the 240Pu isotope in the Pu fuel cycle. 236U: In the Th fuel cycle this isotope plays a similar role as 242Pu in the U/Pu fuel cycle. It is of interest because of its build-up in the equilibrium fuel composition, so its fission cross section is expected to be measured with 5% precision. The 232Th isotope is considered in the Th-fuel to be naturally present and assumed to be 100% abundant. In the literature has been stated that the 230Th isotopic composition of ores varies between 0% to 11%, depending on the amount of associated U and its effect in producing an admixture of 230Th daughter. However, the natural Thorium is presently considered in ADS studies as mono-isotopic 232Th element. Based on the above motivation and the importance for each isotope for the design and the full deployment of the XADS [6], we plan to determine the neutron induced fission cross sections of the five isotopes 232Th, 231Pa, 233U, 234U and 236U in the energy range from 1 eV to over 20 MeV, i.e. up to the maximum energy, where the (n,f) reaction starts to be dominated in ADS applications by other reactions, diminishing thus its importance. The energy range available for these measurements will be, however, extended up to neutron energies above 250 MeV. The measurements of the above fission cross sections will be performed by using a set of PPAC detectors, achieving a precision of better than 5%. The cross sections will be determined relative to the fission cross sections of 235U, 238U and 209Bi, which are used as "standards" up to 200 MeV. The fission cross section of these isotopes, in addition to their interest to the present nuclear technology, bears important information on Nuclear Structure due to their specific nucleonic composition, giving very complicated and pronounced fine structure of the potential surface energy around the fission barrier. The neutron induced fission process can be understood [12] by introducing a double-humped fission barrier (DHB), in good agreement between experimental data and theoretical predictions for the light and heavy actinides. However, the fission of the thorium nuclei departs from this general behaviour, giving rise to the notion of the “Th-anomaly”. In particular, experimental results on the nuclei 230Th and 232Th indicate a dominant additional barrier, resulting in the creation of a shallow third well [13] roughly 1 MeV deep, just deep enough to accommodate some very deformed metastable states. This effect suggests the existence of the so-called triple-humped fission barrier (THB), predicted by A. Bohr already in 1955 [14,15]. The designed features of the CERN-PS neutron source are very attractive for the study of such quantum effects in fission due to the high resolution. These features permit to reveal structures of vibration resonances, directly confirming the existence of the THB for these anomalous elements in the vicinity of and above the barrier. The first stage of this program can start with the measurement of the Nuclear Data for the Th-cycle isotopes and in particular of the 232Th fission cross section. Simultaneous measurement of both the excitation function and the fission fragment angular distribution over the whole energy range could provide through “channel analysis” a full set of quantum numbers. The determination of the spectroscopic properties of the vibration

–4–– resonances may throw some light on the question of the “thorium anomaly”, not resolved yet since the first half of the seventies. The experimental and theoretical effort in this field opened up even more fundamental questions on the mechanism and the time feature of the fission process, the role of the shell and pairing effects, the fine structure of the surface potential energy at high and super-high deformations in a wide region of nuclear temperature. After the completion of the first phase presented in this proposal and with the acquired experience with the CERN-PS neutron beam, we plan to propose the expansion of these measurements towards the determination of the differential cross-section as function of the fission fragment mass, charge, kinetic energy and angular distribution. These second stage measurements will allow to study • the so-called bifurcation point, where the mass-asymmetric fission valley is splitting into two independent parts, • the damping of shells through the analysis of fission fragment shapes at scission when practically all total kinetic energy of the fission fragment is influenced by their coulomb interaction, corresponding to neutron energies above 20 MeV up to 150 MeV, • the viscosity of nuclear matter and • the rare fission processes.

2. STATUS OF NUCLEAR DATA FOR THE THORIUM CYCLE ISOTOPES. Several Nuclear Reaction Data Centres co-operate [16] in the creation and the maintenance of international databases of experimental and evaluated nuclear reaction data for various incident particles such as neutrons, charged particles and photons. These nuclear data files have been under evolution in the last 40 years to satisfy the nuclear data needs in various fields of science and technology. Outstanding efforts by the nuclear data community and by the nuclear data centres in the last four decades with their extremely limited funding and resources have helped provide valuable nuclear data to the users all over the world. However, except for selected standard reference data, there is not yet one single universal nuclear data file with recommended best neutron cross section values. However all these libraries are in a single internationally recognised ENDF format [17]. The Evaluated Nuclear Data File version B (ENDF/B) format is being used throughout the world by the nuclear data evaluators to define evaluated data. Despite the huge and well-organised amount of neutron induced reaction data, the onset of new applications broadened the range of needed data beyond the canonical reactor-driven sets. The nuclear data of the Th-cycle isotopes (230Th, 232Th, 231Pa, 233Pa, 232U, 233U, 234U and 236U) did not receive sufficient attention in the past. Most of the available experimental data are those generated two to three decades ago. In addition, generally, these experimental data are of poor quality and very limited in comparison to the comprehensive nuclear data generated for the uranium-plutonium cycle, like 235U, 238U and 239Pu. As result, the evaluated neutron data files [10], JENDL-3.2 and ENDF/B-VI/R5 recently released by the IAEA/NDS, show large discrepancies. One of the striking discrepancies is that the resolved and unresolved resonance regions are not the same in most of the cases (Table 2-1). Moreover, the cross

–5–– section shapes in the epithermal and resolved resonance regions are significantly discrepant for all these isotopes. The n_TOF Collaboration performed recently the evaluation [18] of the 233Pa(n,γ) cross section using existing experimental results. Upper limit of the resolved Upper limit of the unresolved ISOTOPE resonance range resonance range ENDF/B-VI/R5 JENDL-3.2 ENDF/B-VI/R5 JENDL-3.2 232Th 4 keV 3.5 keV 50 keV 50 keV 232 No unresolved 53 eV 200 eV 1 keV U parameters 233U 60 eV 150 eV 10 keV 30 keV 234U 1.5 keV 1.5 keV 100 keV 50 keV 231Pa 14.3 eV 115 eV 1 keV 40 keV 233Pa 38.5 eV 16.5 eV 10 keV 40 keV Table 2-1: Energy ranges of resolved and unresolved resonance regions.

For all these isotopes Nuclear Data are available in the three evaluated data files [10]: JENDL-3.25, ENDF/B-VI/R5 5+6 and JEF-2.27. The data of two isotopes 233U and 232Th are also contained in the Russian file BROND. JEF-2.2, for these isotopes has been essentially a carry-over from ENDF/B data, though some differences in the data of 232Th and 233U are seen in comparison to ENDF/B-VI/R5. A critical analysis of the two basic nuclear data files (JENDL-3.2 and ENDF/B-VI/R5-Rev.5) indicates that experimental data are incomplete or even missing in many cases as well as the strong dependence of the evaluated cross sections upon theoretical models and nuclear systematics. Most of the experimental data were obtained two decades ago and are concentrated on the comprehensive nuclear data generated for the uranium-plutonium cycle. 231Pa: There are significant differences in the 231Pa(n,f) in the two evaluated data files. An interesting measurement of the 231Pa fission cross section between 0.1 eV and 10 keV has been reported recently by Kobayashi et al [19] using a lead slowing down spectrometer at Kyoto University, Japan. The 2200 m/s value is reported to be 25.0 ± 0.82 mb as compared to 19.7 mb in the JENDL-3.2 file and 10.4 mb in the ENDF/B-VI/R5 file. The values of 231Pa(n,f) in ENDF/B-VI/R5 in the 1 keV to 10 keV energy region are a factor of 20 to 60 larger than in JENDL-3.2. These new experimental results [19] are only about three times larger than the values in JENDL-3.2. Figure 2-1 presents an inter-comparison of fission cross sections of 231Pa. 232Th: The main fertile isotope 232Th has an extensive experimental database of cross sections. However, the discrepancies between the evaluated files are much larger as compared to 238U, the equivalent and major fertile isotope in the uranium-plutonium fuel cycle. Apart from the subthreshold region, the data fall in a band of 5-10% (Figure 2-2). 233U: Except in the thermal region, the discrepancies seen in the case of 233U are very large. The uncertainty in the fission cross section data affects directly the uncertainty in the calculated fission power, and influences criticality dimensions, etc. Details of the discrepancies as a function of neutron energy are illustrated in Figure 2-3. The most striking differences are found between 60 and 150 eV.

–6–– 231Pa, T = 293K, Fission

1

-1 10 JENDL-3.2 ENDF/B-VI -2 10

-3 10 Cross Section (barn) Cross -4 10

-5 -4 -3 -2 -1 2 3 4 5 6 7 10 10 10 10 10 1 10 10 10 10 10 10 10 Neutron Energy (eV)

1 JEF-2.2

-1 10

-2 10 Cross Section (barn) Cross -3 10

-5 -4 -3 -2 -1 2 3 4 5 6 7 10 10 10 10 10 1 10 10 10 10 10 10 10 Neutron Energy (eV)

10

1

-1 10 Ratio (JENDL/ENDF)

-2 10 S.Yayo, P.Pavlopoulos S.Yayo,

-5 -4 -3 -2 -1 2 3 4 5 6 7 10 10 10 10 10 1 10 10 10 10 10 10 10 Neutron Energy (eV)

Figure 2-1: Intercomparison of fission cross sections of 231Pa as function of the neutron energy.

–7–– 232Th, T = 293K, Fission

1

-1 10 JENDL-3.2 ENDF/B-VI

-2 10 Cross Section (barn) Cross

5 6 7 10 10 10 Neutron Energy (eV)

1

-1 10 BROND-2 JEF-2.2

-2 10 Cross Section (barn) Cross

5 6 7 10 10 10 Neutron Energy (eV)

10 4

10 3

10 2

10 Ratio (JENDL/ENDF) 1

-1 P.Pavlopoulos S.Yayo, 10 5 6 7 10 10 10 Neutron Energy (eV)

Figure 2-2: Intercomparison of fission cross sections of 232Th as function of the neutron energy.

–8–– 233U, T = 293K, Fission

10 4

10 3

10 2 ENDF/B-VI JENDL-3.2

Cross Section (barn) Cross 10

1 -5 -4 -3 -2 -1 2 3 4 5 6 7 10 10 10 10 10 1 10 10 10 10 10 10 10 Neutron Energy (eV)

10 4

10 3

10 2 BROND-2 JEF-2.2

Cross Section (barn) Cross 10

1 -5 -4 -3 -2 -1 2 3 4 5 6 7 10 10 10 10 10 1 10 10 10 10 10 10 10 Neutron Energy (eV)

6

4

2 Ratio (JENDL/ENDF)

0 P.Pavlopoulos S.Yayo, -5 -4 -3 -2 -1 2 3 4 5 6 7 10 10 10 10 10 1 10 10 10 10 10 10 10 Neutron Energy (eV)

Figure 2-3: Intercomparison of fission cross sections of 233U as function of the neutron energy.

–9–– 234U, T = 293K, Fission

10 ENDF/B-VI JENDL-3.2 1

-1 10

-2 10

-3 10

Cross Section (barn) Cross -4 10

-5 10 -5 -4 -3 -2 -1 2 3 4 5 6 7 10 10 10 10 10 1 10 10 10 10 10 10 10 Neutron Energy (eV)

10 JEF-2.2 1

-1 10

-2 10

-3 10

Cross Section (barn) Cross -4 10

-5 10 -5 -4 -3 -2 -1 2 3 4 5 6 7 10 10 10 10 10 1 10 10 10 10 10 10 10 Neutron Energy (eV)

10 3

10 2

10

1

Ratio (JENDL/ENDF) -1 10

-2

10 P.Pavlopoulos S.Yayo, -5 -4 -3 -2 -1 2 3 4 5 6 7 10 10 10 10 10 1 10 10 10 10 10 10 10 Neutron Energy (eV)

Figure 2-4: Intercomparison of fission cross sections of 234U as function of the neutron energy.

–10–– 236U, T = 293K, Fission

10 2 ENDF/B-VI JENDL-3.2 10

1

-1 10

-2 10

-3 Cross Section (barn) Cross 10

-4 10

-5 -4 -3 -2 -1 2 3 4 5 6 7 10 10 10 10 10 1 10 10 10 10 10 10 10 Neutron Energy (eV)

10 2 BROND-2 JEF-2.2 10

1

-1 10

-2 10

Cross Section (barn) Cross -3 10

-4 10 -5 -4 -3 -2 -1 2 3 4 5 6 7 10 10 10 10 10 1 10 10 10 10 10 10 10 Neutron Energy (eV)

10 3

10 2

10

1

-1 10

Ratio (JENDL/ENDF) -2 10

-3

10 P.Pavlopoulos S.Yayo, -5 -4 -3 -2 -1 2 3 4 5 6 7 10 10 10 10 10 1 10 10 10 10 10 10 10 Neutron Energy (eV)

Figure 2-5: Intercomparison of fission cross sections of 236U as function of the neutron energy.

–11–– The unresolved resonance region, for which average resonance parameters are provided, is defined from 60 eV to10 keV in the ENDF/B-VI/R5 and between150 eV to 30 keV in JENDL-3.2. From the point of view of reactor physics, this means that resonance self- shielding effects can be only accounted up to an energy of 30 keV. In this case the excellent energy resolution of n_TOF facility is clearly an advantage for improving the positions and shapes of the resonances and for extending the resolved resonance region to higher energies. 234U: In the data files, the resolved resonance region extends up to 1.5 keV but the unresolved resonance region extends up to 100 keV in ENDF/B-VI/R5 as compared to 50 keV in JENDL-3.2. In the 1-10 MeV region, the fission cross sections are within a band of ~5% as illustrated in Figure 2-4. 236U: The isotope 236U is relevant and plays a role similar to 242Pu in the plutonium fuel cycle. There are significant discrepancies between a few keV and 100 keV (Figure 2-5). The discrepancies in several neutron cross section data are illustrated with all details for each neutron energy region in references [20] and [21] and with a number of graphical inter- comparisons of JENDL-3.2 and ENDF/B-VI (Rev. 5). The quality assurance in design and safety studies in nuclear energy in the next few decades require new and improved data with high accuracy and energy resolution. The European project n_TOF-ND-ADS [22] with the objective to measure neutron cross sections in the interval from 1 eV to 250 MeV is inspired by the international co-ordination efforts that successfully created the FENDLd file. There is a need to create a "reactor FENDL" or a "RENDL project" that would achieve convergence of the various evaluated nuclear data files for reactor applications. As a starter, the nuclear data of the set of isotopes of thorium fuel cycle discussed in this proposal is a challenging sample for consideration.

3. THE ISOTOPIC SAMPLES. The measurements of neutron induced fission cross sections require the availability of high purity samples. Samples with considerable purity can be achieved by mass spectroscopy available at the CSNSM collaborating Institute having experience in the isotopic separation and the production of monoisotopic targets through slowing-down of the ion beam and ion implantation. However, this production method is limited to stable or low activity isotopes, i.e. 232Th and possibly 235U, within the present radioprotection regulations of CSNSM. The two Russian Institutes JINR/FLNP, IPPE & IAT (Obninsk) of the n_TOF Collaboration will provide the high purity isotopic samples shown in Table 4-1. The Protactinium and Uranium samples will be prepared by vacuum evaporation on a thin (1 µm) carbon substrate in Russia and will be rented by the n_TOF Collaboration for a maximum period of two years. The preparation time and the transport formalities are already planned and they will not exceed a period of three months.

d Fusion Evaluated Nuclear Data.

–12–– 4. THE EXPERIMENTAL SET-UP. The samples will be placed perpendicular to the neutron beam in between two 20×20 cm2 Parallel Plate Avalanche Counters (PPAC) as indicated in Figure 4-1 and Figure 4-2. These counters measure the position of the two fission fragments with practically 100% efficiency. The final assembly consists of 12 PPACs and 11 samples mounted in an aluminium stack. Samples with "standard" fission cross sections, i.e. 235U, 238U and 209Bi, will serve both for flux normalisation and for the determination of the respective fission cross section ratios. The sample substrates are sufficiently thin to detect the two fission fragments in coincidence.

Figure 4-1: The position of the fission detectors in the n_TOF beam line.

Since the PPACs operate at very low gas pressure (7 mbar), the samples and the PPACs are placed inside an aluminium gas vessel. The distance between the PPACs and the samples is 5 mm. The solid angle covered by a PPAC pair is almost 4π. However, when the fission fragments are emitted near 90o with respect to the beam, self-absorption effects in the sample and in the PPAC windows are limiting the overall efficiency (Table 4-2). We concluded that 300 µg/cm2 thick samples constitute an optimum compromise with a simulated overall efficiency of 80%.

Neutron beam

Figure 4-2: The PPAC detectors set-up.

–13–– The fast anode signals (10 ns total width, less than 1 ns timing resolution) are recorded with 1 GHz Flash ADCs of 8 bits resolution, thus providing the neutron arrival time. In addition very good spatial resolution can be obtained by using the cathode signals as well. The cathodes are segmented by aluminium strips of two millimetres deposited on each side of the 1.5 µm Mylar cathode foils. These strips are connected to delay lines (5 ns time delay per cell), allowing thus a position determination from the delayed signals collected at each end of the delay lines.

Thickness Radius Surface Sample Mass Activity Total Masse Isotope Purity [%] [µg/cm2] [cm] [cm2] [mg] [µCi] [mg] 230Th 99.99 293 4 50.2 23.0 138 232Th 100 293 4 50.2 23.0 0.01 138 231Pa >99 307 2 12.6 4 1.1 24 233U >99 284 4 50.2 14.3 826 86 99.84±03 with 234U 284 4 50.2 14.3 533 86 0.078±001 235U 235U 100 284 4 50.2 14.3 0.19 86 236U >99.8 284 4 50.2 14.3 6 86 238U >99.99 284 4 50.2 14.3 0.01 86 Table 4-1: The quality and quantity of the Th-cycle isotopes produced in form of disc samples. Six samples of the same isotope will be used for each measurement with the assembly of the PPAC detectors.

The PPAC detectors have a number of important advantages. They are the thinnest available detectors for timing and position sensitive measurements. These detectors are transparent to neutrons and γ-rays and are not sensitive to radiation damage. As there is not much matter in the beam, the set-up is compatible with other measurements.

Proportion of Fission Angular limitation Fragments detected with more (Angle between fragment and than 10 MeV residual energy target surface) inside the PPAC Target Facing the Through Facing the Through thickness target backing target backing 100 µg/cm2 93% 83% 5° 9° 300 µg/cm2 92% 82% 7° 11° 500 µg/cm2 90% 80% 8° 12° 800 µg/cm2 87% 77% 11° 15° 1000 µg/cm2 79% 66% 16° 22° Table 4-2: The efficiency for coincident detection of two fission fragments as a function of sample thickness.

e Corresponding to six samples for the set of PPAC detectors.

–14–– Since we detect and identify both fragments, the corresponding signal amplitudes and their coincident observation provide a powerful tool for discriminating between fission and processes such as α-emission, or recoil nuclei emitted from the mylar backings. The latter events have indeed been observed in our measurements of May 2001 [18] and have been attributed to carbon recoils. The precise timing and the position determination allow the unambiguous assignment of observed events to the sample where they occurred, as well as the interaction point on the sample and the emission angle of the fragment. The following informations are essential for obtaining accurate results: • A very fast response signal is mandatory for energetic reactions (above 50 MeV) • The spatial distribution of the neutron flux is not uniform, especially for high-energy neutrons. Knowledge of the interaction point on the sample allows to determine the flux profile and to correct for inhomogeneities of the samples. • From the emission angles the detection probability of a given event can be evaluated. Preliminary fission cross sections between 1 eV and 1 GeV have been calculated [18] using the neutron flux from reference [23], which are shown in Figure 4-3 in comparison to previous evaluations.

10 3 235 PS213 U ENDF-B.6(0 to 20 Mev) INDC(NDS)368(20 to 250 MeV) 235 U(n,f) n_TOF Exp. Data

10 2 The TOF Collaboration 209 Bi n_TOF Exp. Data 209 Bi INDC(NDS)368 10 1

10 0

238 U n_TOF Exp. Data 10 1 238 U INDC(NDS) 368 Fission Cross Section (barn)

10 2

0 1 2 3 4 5 6 7 8 9 10 10 10 10 10 10 10 10 10 10 Neutron Energy (eV)

Figure 4-3: Comparison between experimental data for 235U, 238U and 209Bi with evaluated cross sections.

5. THE ENERGY RESOLUTION. The neutron energy is determined by the time-of-flight between the PS proton pulse and the neutron arrival at the sample, i.e. the signal from the PPAC anodes. The time taken by the fragment to travel from the sample to the PPAC anode is of the order of 2 ns, in the worst case, resulting in a time resolution of 250 ps. Consequently, the time resolution at high energies is completely determined by the 6 ns pulse width of the PS beam.

–15–– 6. COUNTING RATES. Figure 6-1 shows the evaluated neutron flux per bunch of 7×1012 protons obtained from the first PPAC measurements [19]. This flux fits quite well with the predictions [23] at least up to 1 MeV neutron energies. The fission reaction rates from the samples will be measured relative to the simultaneously recorded fission rates of 235U, 238U, 209Bi. The ratio of the number of counts provides the fission cross section of the sample relative to the cross section of the "standard" isotopes, 235U, 238U, 209Bi, diminishing thus systematic errors due to the different beam profiles as function of the neutron energy. However, the homogeneity of the sample thickness should be better than one percent.

5 10 PS213 Preliminary Results

The TOF Collaboration protons)

12 4 10 / 7x10

2 Threshold 209 Threshold Bi 238U

3 10

Flux (dn / dlog(E) cm Exp. Data 235U(n,f) Exp. Data 238U(n,f) 209 2 Exp. Data Bi(n,f) 10 0 1 2 3 4 5 6 7 8 10 10 10 10 10 10 10 10 10 Neutron Energy (eV)

Figure 6-1: Neutron flux from 1 eV to 400 MeV based on fission data of the 235U, 238U and 209Bi isotopes.

The expected rates are plotted in figures 6-2 to 6-4, considering that each isotope is measured with 6 samples according to the characteristics described in Table 4-1.

10 5 Fission on U-235 Fission on Pa-231 18 5.6 1017 protons-on-target 3.0 10 protons-on-target Resolution= 1% or 230 bins per decade 10 3 Resolution= 1% or 230 bins per decade

10 4

3 10 10 2

10 2 Reactions Rate (counts/bin) Reactions Rate (counts/bin) 10

10

1 1 -1 2 3 4 5 6 7 -1 2 3 4 5 6 7 10 1 10 10 10 10 10 10 10 10 1 10 10 10 10 10 10 10 Energy (eV) Energy (eV) Figure 6-2: The expected rates with six PPAC detectors for 235U and 231Pa, corresponding to 5.6×1017 and 3×1018 protons-on-target respectively for a resolution of 1%.

–16–– Fission on Th-232 Fission on U-233 18 5 4.0 10 protons-on-target 10 4.4 1017 protons-on-target Resolution= 1% or 230 bins per decade Resolution= 1% or 230 bins per decade 10 3

10 4

2 10 10 3

10 2 Reactions Rate (counts/bin) Reactions Rate (counts/bin)

10

10

1 1 -1 2 3 4 5 6 7 -1 2 3 4 5 6 7 10 1 10 10 10 10 10 10 10 10 1 10 10 10 10 10 10 10 Energy (eV) Energy (eV) Figure 6-3: The expected rates with six PPAC detectors for 232Th and 233U, corresponding to 4×1018 and 4.4×1017 protons-on-target respectively for a resolution of 1%.

Resolution= 1% or 230 bins per decade Fission on U-236 1018 protons-on-target Resolution= 1% or 230 bins per decade

10 4

10 3

10 3

10 2

10 2 Reactions Rate (counts/bin) Reactions Rate (counts/bin)

10 10

Fission on U-234 1.0 1018 protons-on-target 1 1 -1 2 3 4 5 6 7 -1 2 3 4 5 6 7 10 1 10 10 10 10 10 10 10 10 1 10 10 10 10 10 10 10 Energy (eV) Energy (eV) Figure 6-4: The expected rates with six PPAC detectors for 234U and 236U, corresponding to 1018 protons-on-target for a resolution of 1%.

7. BEAM TIME REQUEST. In conclusion, we propose to determine the neutron induced fission cross sections of the five experimentally accessible isotopes 232Th, 231Pa, 233U, 234U and 236U, relevant to the thorium fuel cycle. We intend to perform these measurements with a set of PPAC detectors recording fission reactions simultaneously from six isotopically pure samples. We ask for 1019 protons-on-target corresponding to 1.5×106 dedicated PSB bunches. Under the assumption of two bunches per supercycle dedicated to n_TOF, the proposed measurements will cover a period of more than 20 weeks. Our programme will thus clearly benefit from the possibility of four bunches per supercycle even for short periods.

–17–– 8. REFERENCES 1 The recent European Commission’s GREEN PAPER: “Towards a European strategy for the security of energy supply”, December 2000, Brussels. 2 C. Rubbia et al., “An Energy Amplifier for Cleaner and Inexhaustible Nuclear Energy Production Driven by a particle Accelerator”, CERN/AT/93-47(ET) 1993. 3 C. Rubbia, et al., "Conceptual Design of a fast Neutron Operated High Power Energy Amplifier", CERN/AT/95-44 (ET); see also C. Rubbia, "A High Gain Energy Amplifier Operated with fast Neutrons", AIP Conference Proceedings 346, International Conference on Accelerator-Driven Transmutation Technologies and Applications, Las Vegas, 1994. 4 C. Rubbia et al., "A Realistic Plutonium Elimination Scheme with Fast Energy Amplifiers and Thorium-Plutonium Fuel", CERN/AT/95-53 (ET); C. Rubbia et al., "Fast Neutron Incineration in the Energy Amplifier as Alternative to Geological Storage: the Case of Spain", CERN/LHC/97-01 (EET). 5 "The NEA High Priority Nuclear Data Request List", Status in May 1998, OECD-NEA Nuclear Science Committee. 6 Members of the Technical Working Group under the chairmanship of Prof. C. Rubbia, “A EUROPEAN ROADMAP FOR DEVELOPING ACCELERATOR DRIVEN SYSTEMS (ADS) FOR NUCLEAR WASTE INCINERATION”, April 2001, ENEA, Roma, Italy, ISBN 88-8286-008-6. 7 C: Rubbia, "Resonance Enhanced Neutron Captures for Element Activation and Waste Transmutation", CERN/LHC/97-04 (EET). 8 H. Arnould et al., "Experimental Verification of Neutron Phenomenology in Lead and Transmutation by Adiabatic Resonance Crossing in Accelerator Driven Systems", submitted to Nucl. Instr. and Methods. 9 Summary Report of the Consultans' Meeting on "Assessment of Nuclear Data Needs for Thorium and other Advanced Cycles", INDC(NDS)–408, IAEA, Vienna, August 1999. 10 JENDL-3.2, OECD NEA Data bank (1996); JEF-2.2, OECD NEA Data bank(1996); ENDF/B-VI, OECD NEA Data bank (1996); BROND-2, OECD NEA Data bank (1996); See also J. Cobo et al, "Notes on the Study of the Most Reliable Neutron Cross Section Data", CERN/AT/95-035 (ET). "JENDL-3.2, The Japanese Evaluated Nuclear data Library by the JAERI Nuclear data Centre and the Japanese Nuclear Data Committee," Summary of Contents, H. D. Lemmel (Editor), IAEA-NDS-110, IAEA Nuclear Data Section, Vienna, Austria, Europe. Data obtained on CDROM from the IAEA Nuclear Data Section, 1999 P.F. Rose (Editor), "ENDF/B-VI Summary Documentation," Report BNL-NCS-17541 (ENDF-201), Brookhaven National laboratory 1991, Data Library ENDF/B-VI, Rev. 5, Update 1998, Data obtained on CDROM from the IAEA Nuclear Data Section, 1999 JEF2.2, CDROM IAEA-NDS-CD-04, data obtained on CDROM from the IAEA Nuclear Data Section, 1999

–18–– 11 Y. Kadi et al., "Sensitivity Analysis of Neutron Cross Sections Relevant for Accelerator Driven Systems", International Conference on Nuclear Data for Science and Technology, October 7–12 2001, Tsukuba, Japan. 12 V.M. Strutinsky, Nucl. Phys. A95 (1967) 420. 13 P. Moeller and J.R. Nix, Proc. Int. Conf. on physics and chemistry of fission, Rochester, 1973 (IAEA, Vienna, 1974) p. 103. 14 A. Bohr, Proc. Int. Conf. on peaceful uses of atomic energy, Geneva, 1955 (United Nations, New York, 1956) vol. 2 p. 220. 15 Blons et al, Nucl. Phys. A414 (1984) 1. 16 For more details, visit the web-site of the IAEA-NDS at: http://www-nds.iaea.org. 17 ENDF-102 "Data Formats and Procedures for the Evaluated Nuclear Data File ENDF-6," Cross Section Evaluation Working Group, February 1997, BNL-NCS-44945, Rev.2/97, Brookhaven National Laboratory. 18 U. Abbondanno et al.,The n_TOF Collaboration, "Status Report [July 2001]", CERN/ INTC 2001–021, 2 August 2001. 19 K. Kobayashi, "Measurements of Neutron-Induced Fission Cross Section of Protactinium- 231 from 0.1 eV to 10 keV with Lead Slowing-down Spectrometer and at 0.0253 eV with Thermal Neutron facility," J. Nucl. Sci. Tech. 36, 549 (1999). 20 S. Ganesan and P. K. MacLaughlin, "Status of Thorium Cycle Nuclear Data Evaluations: Comparison of Cross Section Line Shapes of JENDL-3 and ENDF/B-VI Files for 230Th, 232Th, 231Pa, 233Pa, 232U, 233U and 234U," Report INDC (NDS)-256, (1992) Nuclear Data Section, International Atomic Energy Agency, Vienna, Austria. 21 S. Ganesan, "A review of the current status of nuclear data for major and minor isotopes of thorium fuel cycle", presented at the "International Topical Meeting on Advances in Reactor Physics and Mathematics and Computation into the Next Millennium", Pittsburgh, Pennsylvania, USA, May 7-11, 2000. 22 The "n_TOF-ND-ADS project" of the 5th FWP of the European Commission under contract number "FIKW-CT-2000-00107". 23U. Abbondanno et al.,The n_TOF Collaboration, "Technical Report", CERN/ INTC 2000–018, 3 November 2000.

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