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GSI
FRS

Isotopic production in spallation reactions

Since some years, spallation reactions have gained a renewed interest for several reasons. On the one hand, they are planned to be used in the so-called Accelerator Driven System as an intense neutron source. On the other hand, spallation reactions lead to the production of unstable nuclei. This reaction is actually exploited in ISOL-type facilities.

Fission reactors
What is the origin of radioactive waste?

How can it be treated?

Hybrid reactor

How does our research contribute to solve this problem?

Fission reactors

Conventional nuclear power plants are based on the energy release in nuclear fission. This energy has been stored in the heaviest nuclei during the process of the natural synthesis of heavy elements in some preceding supernova, long time ago. These heavy elements are part of the matter, our earth is constituted of. A controlled 'burning' of the reactor is induced by a balanced flux of thermal neutrons. When a neutron is captured by a fissile nucleus, this nucleus is going to fission and to emit a few neutrons. In our power plants, this fissile nucleus is 235U. These neutrons are produced by the fission process with energies of a few MeV (about 10 billion degrees Kelvin). In collisions with the moderator material of the reactor, these neutrons are thermalized. That means they are slowed down to energies corresponding to the temperature of the surrounding medium, a few hundred degrees Kelvin. One takes care that exactly one of these neutrons is captured by another 235U nucleus. Thus, a controlled chain reaction is maintained. In other words, the reactor is just 'critical'.
The fuel of our nuclear power plants is 235U, which can fission after the capture of thermal neutrons. In natural uranium it appears with 0.7 %, while 238U, which does not fission after capture of thermal neutrons, is the most abundant isotope. The rods consist of enriched material: the portion of 235U is increased to a few percent.
 

Additional information:

Nuclear energy (in French)

Nuclear energy - a solution for the future

Nuclear physics and reactors

Nuclear power

Oklo - Natural nuclear reactor

What is the origin of radioactive waste?
Two kinds of nuclear waste are produced in nuclear power plants (fission reactors):
1. By successive neutron capture of 238U and consecutive beta decay, 239Pu and some even heavier nuclei (called 'minor actinides', because these belong to the chemical class of actinides and because only small amounts are produced) are breeded. Some have very long half-lives, e.g. 239Pu has a half-life of 20000 years.
2. The residuals of the fission process, named fission products, are almost all radioactive. Most of them decay into stable nuclides in a time span of some minutes to some years. Therefore, the radioactivity reduces considerably by just depositing the used rods for a few years. However, others have very long half-lives, e.g. 129I with 16 million years.

Additional information:

Radioactive waste

Waste from the nuclear power cycle

How can it be treated?

For the treatment of these two classes of radioactive waste, there exist two different strategies: 239Pu and the minor actinides can be brought to fission by the capture of neutrons. In this way, they can even be considered as fuel. However, for some of them one needs neutrons of appreciably higher energies than thermal. This process is called 'incineration', because the fission process is an energy-producing process. Fission products are formed as the 'ash' of this process. They are similar as the fission products of the primary fuel 235U.
Also the fission fragments can capture neutrons. By this process, they are 'transmuted' into other isotopes, and eventually by some consecutive beta decay into other elements. Again, for different nuclides the optimum energy range of neutrons to be captured is different.
The task of incineration and transmutation of nuclear waste is to put those constituents having long half-lives into a neutron flux of suitable energy. The products of such a treatment should have shorter half lives, such that the radioactivity of the waste falls below the natural level in a reasonable time.
The only affordable way for the production of the necessary neutron flux is the operation of an adapted fission reactor. Normal fission reactors (ordinary nuclear power plants) are not well suited for this purpose. The first problem is that thermal neutrons are not optimum for most of the problematic nuclides to be incinerated or transmuted. The second problem is that the neutrons captured by the waste influence the balance of the chain reaction in the reactor, they modify (mostly reduce) the criticality of the reactor.
 

Additional information:

ANDRA - National radiactive waste managment agency, France

Radioactive waste management research

Hybrid reactor

For overcoming these difficulties, two approaches are actually discussed. A more conventional option is the use of 'fast' reactors. They are operated with neutrons of higher energies. (The fast breeder was one of those early types. A new generation should be much safer.) These can solve the first problem mentioned. A more innovative option, proposed by Rubbia in Europe and Bowman in the US, is the construction of a sub-critical reactor. This sub-critical reactor would not run by itself. There are not enough neutrons for a stable chain reaction. The missing amount of neutron flux for the operation of the reactor is provided by a 'spallation neutron source' inside the reactor, which is fed by a beam of 1 GeV protons, inserted into the reactor. This sub-critical reactor, also named accelerator-driven system (ADS), has several advantages:
a) it is intrinsically safer, because it stops when the accelerator is switched off.
b) it can better adapt to some variation of the criticality (due to a variation of the constitution of different nuclides in the reactor) during the operation of the reactor than a conventional reactor by tuning the intensity of the proton beam,
c) it can be operated by natural thorium, which is much more abundant than the actually used 235U (this could solve the energy problem of mankind for several ten-thousands of years),
d) when operated with thorium, no 239Pu which constitutes a problem for proliferation, is breeded,.

Our research at GSI intends to provide valuable information to better understand the complex processes in the spallation neutron source inside the reactor. This understanding is important for the design of an ADS.

Two first small-scale prototypes of an ADS will be built in Europe in the next years: Trade in Italy and Mhyrra in Belgium..

Additional information:

Transmutation tools

Transmutation of radioactive waste

How does our research contribute to solve this problem?

While the nuclear reactions occurring in a conventional fission reactor are limited to the energy range of fission neutrons below a few MeV, the nuclear reactions occurring in an accelerator-driven system, consisting of a sub-critical reactor and a neutron source driven by 1 GeV protons extend to energies up to the primary proton energy. In addition to the detailed understanding of the neutronics and the complex transport phenomena of light particles, the production of heavy residues by proton- and neutron-induced fragmentation and fission reactions needs to be known for the design of such a system, because it has decisive consequences for the shielding and the activation of the installation, the radiation damages of construction materials and the chemical properties of the spallation target. In contrast to the situation in conventional fission reactors, where all relevant nuclear data could be measured, the large range of energy and the variety of target materials involved in an accelerator-driven system demands for a different strategy. Only a limited number of selected key reactions can be studied in full detail and serve to benchmark, improve and develop nuclear-reaction codes, which are then used to calculate the reactions occurring in the accelerator-driven system in their full variety.

The measurements of evaporation and fission residues have started since the proton accelerators became available in the 50’s. For 40 years, production of residues was measured using chemical and/or spectroscopic methods. In the 90’s, the GSI gave birth to a new generation of machines, coupling an intense and powerful heavy-ion accelerator and a precise recoil spectrometer (the FRS). The installation of a cryogenic hydrogen targets (¹H and ²H) [1] permitted to start the campaign of measurement of spallation-residue cross sections in inverse kinematics. We could detect, identify unambiguously and analyse several hundreds of isotopes per system before radioactive disintegration with an accuracy in the order of 10% to 15% in most cases. Moreover, thanks to the high-precision measurements of the velocity of final residues, we could determine by which mechanism (spallation-fission or spallation-evaporation) they were produced. All these strongly contrasts with the scarce and usually cumulative cross sections obtained with other techniques. The high efficiency of the spectrometer coupled to the very short time-of-flight (about 300 ns) contributes strongly to the quality of our results.

In the frame of S184 collaboration the following systems have been measured:

These data made a part of several PhD thesis and numerous publications. Data for several measured systems are shown in figure below as the charts of nuclide. Note, that the system ²³⁸U+²H is still under analysis.

     Click on the picture to enlarge.

Using the measured production cross sections, combined with the known decay properties, the short- and long-term radioactivities in the target material can be calculated. The number of atomic displacements being the reason for radiation damages in the structural materials can now be estimated from the measured kinetic-energy distributions. The data also allow estimating the admixtures of specific chemical elements in the liquid target, accumulated in long-term operation of the reactor, which enhance the corrosion of the walls or any material in the container.

References:

[1] P. Chesny et al., "Liquid hydrogen target for cross section measurements relevant for nuclear waste incineration", GSI Scientific Rep. 1996, GSI 97-1, 190.