The experimental efforts described above have resulted in a wealth of new information. In this section, we summarize our view of the present situation. It should already at the onset be stressed that in ADS, the data needs develop over time. Initially, some basic data were needed for the assessment whether ADS was worth continued attention. At present, the nuclear data research is on a level where data needed for the design of a demonstration facility should be the target. If plans for a full-scale plant or a close-to-full-scale prototype mature, the nuclear data needs evolve even further, and if industry deployment becomes a reality, a massive nuclear data campaign will be motivated. For each step, the target accuracies increase. At present, we have experimental data with uncertainties in the 5 % range on about five nuclei for each of the most important nuclear reactions. For successful design of a prototype, half that uncertainty and good data on at least 20 nuclei should be a minimum goal. For industrial-scale employment, uncertainties as low as 1 % for some selected key materials should be reached, and 2-5 % uncertainties would be desirable for a wider range of types of reactions than before. The number of nuclei to be studied would however still stay around 20 or so, because the number of elements where the high- energy neutron flux is sizeable in such a device is fairly limited. Increased attention to cross sections of relevance to shielding and radiation protection could be anticipated though.
It is a fairly limited class of reactions that are of interest for the further development of ADS in the near future. These are elastic scattering, inelastic neutron emission, light ion production, heavy ion production and fission.
Table 1. Summary of the nuclear data situation for the most important neutron-induced nuclear reactions above 20 MeV. See the text for details.
Reaction Status Uncertainty
(n,n) Done up to 100 MeV 5 %
Can be done up to 200 MeV
(n,xn’) Underway at 100 MeV 10 % Can be done up to 200 MeV
(n,light ion) Done up to 100 MeV 5 % Up to 200 MeV underway
(n,f) Cross sections up to 200 MeV 15 % Possible up to 5 GeV
Absolute scale problem dσ/dΩ, yields, etc. remaining
Overall limitation: normalization 5 %
Elastic scattering has been studied on a range of nuclei up to 96 MeV. At present, results on seven nuclei have been published [28]-[30], and an overall uncertainty of about 5 % has been achieved. A novel normalization method has been established that allows
elastic scattering data to be normalized absolutely to about 3 % uncertainty. This method, however, works only for elastic scattering. Feasibility studies have shown that the technique as such works up to about 200 MeV, and preparations for extended studies up to this energy is underway.
An experimental programme on inelastic neutron emission, i.e., (n,xn’) reactions, is in progress [31]. Data have been taken on lead and iron, and the method as such seems to work. It is too early to quote a final uncertainty in the results, but 10 % seems feasible.
Data on light ion production has been acquired on about ten nuclei at 96 MeV [14, 32, 33]. Normalization has been obtained by simultaneous detection of np scattering at
an angle where the cross section uncertainty can be estimated to about 5 %, which is the dominating uncertatinty in the final light ion production cross sections. These studies are presently being extended to 180 MeV [34].
Fission cross sections have been studied at many facilities up to about 200 MeV energy. The energy dependencies of the cross sections agree fairly well in shape, but the absolute scale differs by up to 15 %. It is at present not clear what causes this. One possibility is the normalizations used. Another possible cause is that the sensitivity to low-energy neutrons is not under control for some of the experiments. Dedicated experiments to remedy this situation are underway.
In principle, fission cross sections can be measured up to several GeV using white beams with a very high initial proton energy, like at the CERN-nTOF facility [35]. The neutron beam intensity is very low, but the cross sections are large and it is possible to detect a major fraction of the fission fragments, resulting in reasonable statistical precision. A major problem, however, is normalization, since the beam intensity is very difficult to monitor at these very high energies.
There are only a few examples of other fission data than cross sections. This means that important fission parameters, like angular distributions, yields, etc., essentially remain to be investigated at high neutron energies.
9.
Towards Application: From Cross Sections to Nuclear Data
The quality and reliability of the computational simulation of a macroscopic nuclear device is directly related to the quality of the underlying basic nuclear data. The argument that mi- croscopic nuclear experiments and theory development are imperative to enhance the status of applied nuclear research is generally accepted and in fact used by several institutes to construct their scientific program. Maybe less transparent is how this microscopic informa- tion can actually be applied. This brings us directly to the important task of a nuclear data evaluator, namely the provision of the quantitative link between two huge fields of research: fundamental nuclear physics and nuclear applications. This is done by means of a so-called evaluated data file. Often, a data file consists of a mixture of experimental data and cal- culated results, computed from nuclear reaction models or even simple systematics. As long as the contents of the data file are in correspondence with reality (which we assume is represented by the available experimental data) any combination of these inputs is allowed. This is to serve the common goal of nuclear data evaluation: to store all possible reaction channels on a data file, with maximum quality of the cross sections and other quantities like resonance and fission parameters.
An advantage of this way of working is the modularity from the point of view of human expertise. An evaluator or team of evaluators combines all experimental data and results of relevant nuclear models into one data file per isotope which, after it has been processed by a code like NJOY, is the evaluators’ end product. Then, the applied physicist, e.g. somebody involved in reactor analysis, can take this data file as the starting point, and in principle does not need to worry about the trajectory that resulted in this data file. Of course, undesired results from applied calculations frequently leads to feedback to the evaluator, leading to an iterative process that eventually should lead to a nuclear data file that embodies the best compromise between microscopic data and requirements from integral benchmarks. The only, very improbable, alternative approach to the data evaluation method would be one nuclear model supercode that (a) produces better results than all data files combined for all energies and isotopes, and (b) is so fast that it can be hardwired directly in a transport code. Although we all hope that somebody can produce such a miracle, it is very likely that data libraries will remain the key ingredients of applied nuclear simulations in the future. We should mention that much depends on the required accuracy of the nuclear data, i.e. how precise do the basic data need to be for a reasonably accurate determination of the macroscopic quantity (such askeff or the number of neutrons produced per incident proton in an accelerator-driven system). Sensitivity studies should guide the data evaluator at this point. In sum, since one is in the position to use the best available code or experimental data for each partial nuclear reaction channel, as long as certain sum rules are obeyed, the method of storing nuclear data in evaluated files arguably allows the closest possible connection between nuclear reaction physics and practical applications. After processing, the data libraries can serve as input for deterministic or Monte-Carlo nuclear transport codes and activation codes. For a few decades now, evaluated data files are serving as crucial starting points of various applications.
The starting point of serious computational modeling of a realistic accelerator-driven system is basic nuclear reaction information. Feasibility calculations of such systems, which include items such as neutron and energy balance, the radiotoxicity of spallation products, damage and activation, rely critically on well-tested nuclear data. In this contri- bution, we aim at classifying the basic nuclear research that is required to fulfill these needs and to demonstrate how such basic nuclear data is connected with an accelerator-driven ap- plication. The emphasis will be on nuclear model calculations for thin targets that will have direct influence on more macroscopic components.
Required nuclear data for transmutation (i.e. more general than ADS alone) consists, first and foremost, of low-energy nuclear reaction information: neutron-induced reactions for major and minor actinides in both the thermal and fast energy range, with extra emphasis on minor actinides as compared to conventional reactors. Similarly, nuclear data for fission products require additional effort. The emergence of ADS as a possible solution to the waste problem comes with a whole new class of nuclear data requirements. This consists certainly not only of nuclear data beyond 20 MeV that will be discussed hereafter. Traditionally, most effort for actinides has been put on energies up to several MeV, i.e. including the thermal and fast energy range. Arguably, several (especially minor) actinides of the current data libraries may not be in good shape for energies between 5 and 20 MeV, simply because that energy range has never been application-relevant. For shielding materials, this situation is generally better because of the fusion-related effort of nuclear data up to 14 MeV. ADS also
requires nuclear data for materials that have been somewhat neglected, notably Pb and Bi, because they have, so far, never fitted into any serious applied research program.
For ADS systems, there is an obvious need for nuclear data beyond 20 MeV. The ac- tual provision of these intermediate energy nuclear data to the users is now also developing rapidly. There are basically two ways to provide a link between thin target nuclear reaction physics and applied analyses. The first method, which is traditionally linked with high en- ergies, is to perform the calculation of both microscopic nuclear reactions and macroscopic transport processes by the same computer code. LAHET [36], preferably combined with modern intranuclear cascade codes such as BRIC [37] and INCL4 [38] is a well-known examples of intranuclear cascade codes that work according to this principle. Quality state- ments about such codes can be obtained by comparison of the results with available thin- target experimental data. Then, after choosing geometry specifications for the accelerator, target and/or the reactor, the code could be set in “production mode” to predict processes that take place in a realistic device. Additional validation is possible using integral ex- periments, in which the neutron flux and other macroscopic quantities of interest can be measured for a thick target.
The alternative method is by means of evaluated nuclear data files, as outlined in the be- ginning of the introduction. At present, the intranuclear cascade and data evaluation meth- ods are regarded as complementary valuable approaches for analyses of accelerator-driven systems. The aforementioned effort in fission and fusion reactor studies has resulted in data files that cover nuclear reactions up to 20 MeV. However, the matching energy between the two methods should be around 200 MeV. There are several reasons for this particular en- ergy. First of all, below 200 MeV the predictive power of several pre-equilibrium/statistical model codes (such as the TALYS code described in this contribution) seems to be superior to intranuclear cascade codes for continuum reactions. Also, individual reaction mechanisms (giant resonances, direct collective reactions, etc.) constitute a relatively larger fraction of the reaction spectrum and require an individual, more sophisticated treatment. When results from such detailed reaction mechanisms are collected and included in a data file (and even- tually fed to a transport code), they form a data source that in quality can never be matched by one single computer code.
An obvious follow-up question after this enumeration of advantages is: Why should nuclear data files stop at 200 MeV and not go all the way up to the incident energy of the proton beam of an accelerator-driven system. At this point, there are several reasons in favor of intranuclear cascade codes at energies above 200 MeV. Above 150-200 MeV, pion production becomes important and the present low-energy codes (and the ENDF6-format) do not yet cover such reactions. Also, above about 200 MeV we need reliable optical model parameterizations that have predictive power in regions where there is no experimental data. For the moment, intranuclear cascade codes that handle both the cross section generation and the transport part for various types of particles are preferable above 200 MeV.
Summarizing, we argue that a global calculation scheme for accelerator-driven systems should consist of a combination of intranuclear cascade codes above 200 MeV and evalu- ated data libraries for energies up to 200 MeV. An important development is the integration of the two nuclear data methods (intranuclear cascade + data files/transport) into one code system (MCNPX) [39]. This allows maximal flexibility concerning the choice of intranu- clear cascade models and data files.
It is evident that the total nuclear data task is quite sizeable: one has to keep track of nuclear processes starting from 1-1.5 GeV incident protons down to the final reaction stages with thermalizing neutrons and gamma-ray cascades for a large number of residual nuclides. Along this trajectory, a whole panoply of different appropriate nuclear models applies, each with their own validity range. In addition, the required physical methods and modeling may differ from nuclide to nuclide. With regard to the tractability of the work, it is important to observe that certain parts of the accelerator-driven system may depend more critically on the quality of nuclear data, and therefore deserve more attention, than other parts [40]. In addition, the data requirements may differ per material. It can easily be imagined that for a target material like lead, the number of produced neutrons per incident proton is a key quantity, whereas for a shielding material like iron, the secondary particle energy-angle distribution is more relevant (for economical aspects of the shielding) as well as activation cross sections. These simple considerations show that sensitivity studies for the different parts of the whole device are indispensable as a parallel area of research in the field of nuclear data, both at low and intermediate energy. They provide a valuable guideline of isotopes and reactions to be measured and evaluated.
In this contribution, we focus on nuclear data up to 200 MeV. The main emphasis is on the TALYS code, which was developed to simulate nuclear reactions and to generate nuclear data for evaluated libraries.
10.
Theory and Nuclear Model Software
Nuclear theory and the associated nuclear model software has become indispensible in mod- ern nuclear data evaluation. To perform adequate inter- and extrapolation on the energy and angular grids per reaction channel, transport and reactor codes rely on a complete descrip- tion of a nuclear reaction in a data file, and not only on the data that happen to be available through measurements. The preferred method is to let a nuclear model code generate an evaluation that is entirely complete in its description of reaction channels: with incident and outgoing particle energies and angles on a sufficiently dense grid, and all possible re- action yields. The adjustable parameters of the nuclear model code should be fitted to reproduce the experimental data available for the nucleus under study. The starting point is then a complete data file of reasonable, and often good, quality. Next, this evaluation needs to be updated with the crucial experimental data points with a precision that can not be accomplished by the model code. This concerns always the resolved resonance range, and sometimes the unresolved resonance and fast neutron range (especially for light and fissile nuclides). Thus, the model code ensures the completeness of the data file, and the model code+ experimental data the highest possible quality, i.e. realistic values, of the data file at
a given moment in time. The process needs to be repeated whenever better versions of the model code and more precise experimental data become available. If the methodology is well-automated and properly quality assured, such revisions will take much less time than the first evaluation.
* Exciton model * Spherical OM * DWBA * Rotational CC * Vibrational CC * Giant resonances * p−h LD phenom. * γ−ray emission − angular distribution − cluster emission * Kalbach systematics * Width fluctuations − Moldauer − GOE triple integr. − HRTW * Hauser−Feshbach * Fission competition − isotopic yields * −ray emissionγ * GC+ Ignatyuk * Fission competition * Hauser−Feshbach * Exciton (any order)
* Exclusive channels
− isotopic yields
* −ray cascadeγ * All flux depleted * Recoils * Discrete levels
* Abundancies * Deformations * Masses
* Level density par. * Resonance par. * Fission barrier par. * Thermal XS * Microscopic LD * Prescission shapes *File ’output’ spectra, ... files with *Dedicated keywords defined by * transport libs * activation libs * Weak−coupling element fe mass 56 projectile n energy 14. * Keywords, eg: local / global * Phenomenology Input: Optical Model: Nucl. Structure:
Direct reaction: Preequilibrium: Output:
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