Analysis of Induced Gamma Activation by D-T Neutrons in Selected Fusion Reactor Relevant Materials with EAF-2010

Samples of lanthanum, erbium and titanium which are constituents of structural materials, insulating coatings and tritium breeder for blankets of fusion reactor designs have been irradiated in a fusion peak neutron field. The induced gamma activities were measured and the results were used to check calculations with the European activation system EASY2010. Good agreement for the prediction of major contributors to the contact dose rate of the materials was found, but for minor contributors the calculation deviated up to 50%.


Introduction
Activation of fusion reactor materials is an important issue from the viewpoint of licensing, safety, maintenance and decommissioning.The activation is produced mainly by fusion peak neutrons for which many reaction channels are open and by slow neutrons for which capture cross sections are very large.It is therefore necessary to investigate the induced activities of these materials in 14 MeV neutron fields experimentally and apply the results to validate codes and data libraries used for R&D work.
In the present work, we re-analyse previous experiments on activation of selected fusion reactor relevant materials with inventory calculations with the EASY package based on the inventory code FISPACT [1] and activation data from the EAF-2010 library [2].In particular we consider the activation of lanthanum, erbium and titanium in a fusion peak neutron field.The activation experiments were done at the DT neutron generator laboratory of Technical University of Dresden and had been analysed previously with earlier versions of the EAF file [3, 4].
Lanthanum oxide is a candidate material for the heat shield in the divertor and therefore exposed to an intense fast neutron field.Erbium oxide was suggested as an insulating coating as well as a possible tritium barrier for liquid breeder blankets.Titanium is a constituent of structural materials but most notably present in the candidate breeder material lithium titanate (Li 2 TiO 3 ).

Calculations to Estimate Shut-down Dose Behaviour in a Fusion Power Reactor
Survey calculations with EASY-2010 applying a fusion peak neutron spectrum were conducted for all three materials to determine the main contributions to the contact dose rate which is a means to estimate waiting times before materials can be recycled or handled hands-on.The results of these survey calculations were compared with recycling and hands-on limits based on ICRP recommendations [5].The FISPACT code calculates -ray contact dose rates at the surface of a semi-infinite slab following a scheme devised by Jaeger [6].According to the experience of the authors, the beta dose rate at very short distances can be similar to the gamma dose rate.
Aim of the experimental investigation is to provide data for comparing model inventory calculations.However, there is no neutron source presently available which would permit such irradiation experiments under full reactor-like conditions.Instead one is limited to comparably short irradiation times and less neutron flux density.The irradiation facilities of choice are accelerator based DT neutron generators since their neutron field can be characterized very well.For the survey conditions presented in the following sections, power plant conditions were assumed which means a flux of 14 MeV neutrons corresponding to a neutron wall loading of 1.0 MW/m 2 , for a period of one year.The 175 group activation library eaf_n_gxs_175_fus_20100 together with the eaf_dec_20100 decay library were used for the survey calculations and the calculations for comparison with experiment.

Lanthanum
The only stable isotope of lanthanum is 139 La.After exposure to reactor-like conditions as described above, the contact dose rate, presented in Fig. 1, is dominated by 140 La and 136 Cs until the recycling limit is reached 200 days after shut-down.The remaining contact dose rate is dominated by 134 Cs and 137m Ba up to the hands-on limit which is reached after 50 years.137m Ba is a radionuclide with a short half-life.After rapid decay of the initial population it is still fed by the presence of 137 Cs which is not contributing to the contact dose rate as calculated by EASY and therefore does not show up in Fig. 1.Long-term activity is mostly due to 137 La and 138 La.

Erbium
Natural erbium consist of six isotopes, the composition is presented in Table 1.The evolution of the shut-down dose rate is also shown in Fig. 1.For the first ten days after shut-down, the contact dose rate  is dominated by 161 Er, 165 Er, 168 Tm.The latter contributes mostly to the dose rate for another 500 days.The recycling limit is finally reached after 1500 years with the decay of the metastable 166m Ho (half-life 1200 years).The hands-on limit is reached after approximately 20000 years.

Titanium and Lithium Titanate
The isotopic composition of natural titanium is presented in Table 1.The calculated contact dose rates for pure titanium and also titanium titanate are shown in Fig. 2. Also in case of titanium the contact dose rate is mostly determined by a few radio isotopes.The important radio isotopes are 48 Sc, 46 Sc, 42 K.With these the recycling limit is reached after about 4.4 yr and the hands-on-limit after about 109 yr.For lithium titanate these limits are reached a little earlier which is due to the "dilution" of the radio isotopes, lithium and oxygen produce no long-lasting radio isotopes under these conditions.

Experiment
The samples were foils of La, Er, and Ti with a purity of 99.9%.They were irradiated at the DT neutron generator of Technical University of Dresden.The neutron generator is a deuterium accelerator with a deuteron energy of 320 keV bombarding a titanium-tritium target.Samples were set up approximately 2.0 cm away from the neutron source in case of Er and La, and 7.5 cm in case of Ti.The local neutron fluence at the sample position was determined with niobium foils attached to the front and back of each sample.In addition, Zr foils were used for the Ti sample since the combination of Nb and Zr dosimetry foils can provide information on the neutron spectrum which is important for the 47 Ti production.The production cross section of the latter reaction shows a sharp increase in the energy range of the typical DT fusion neutron peak.The calculated neutron spectrum in front of the tritium target of the neutron generator for the position of the Ti foils is shown in Fig. 3.The cross section of the reaction 93 Nb(n,2n) 92m Nb was considered constant in the energy range 14-15 MeV with a value of 464 mb and an uncertainty of 4.2%.
After neutron irradiation, gamma ray spectra were obtained with a high purity germanium (HPGe) detector at several times after irradiation.The attenuation of gamma-rays, which is appreciable especially at low gamma energies, was estimated by 3D Monte Carlo calculations.

Results and Discussion
For the analysis of the measured induced activities, a second calculation with the EASY system has been performed.The neutron spectrum applied in this calculation was obtained from a detailed MCNP calculation of the target assembly of the DT neutron generator.The results of experiment and calculation are presented in Tables 2-4.
Error estimates are also shown in the tables and these were calculated as follows: C/C is the relative error of the FISPACT calculation which includes the half-life time and cross section uncertainties from the EAF-2010 data.
E/E is the experimental error which takes into consideration the uncertainties of the half-life and the gamma line intensity (strongly depending on isotope), detector efficiency (3%), and counting error (typical <1%).Half-lives and gamma line intensities and their errors were taken from Refs.[2, 3].
The table includes also the uncertainty of the neutron flux measurement which was estimated to be 5% considering cross section uncertainty of the 93 Nb(n,2n) 92m Nb reaction and errors due to the activity measurement.

Lanthanum
The nuclides 136 Cs and 140 La dominate the contact dose rates for the first 200 days after shut-down.These activities were measured.However, as in case of 182 Ta, 140 La is produced in a neutron capture reaction which could be better evaluated in a thermal neutron field rather than the fusion peak field of this experiment. 136Cs and the minor dose rate contributor 139 Ba were predicted well.The long-term activation products 134 Cs and 139 Ce which determine the activity up to the hands-on limit could not be measured due to their low activity in the sample.

Erbium
The activity of 161 Er, the most important nuclide for the first few hours after shut-down, was predicted well within the experimental error using the strongest gamma line.
A huge underestimation of the calculation was found for 168 Tm.It is important to consider this isotope since it dominates the contact dose rate from one day to one year after shut-down.Under reactor conditions, 168 Tm is produced by the reaction chain 170 Er(n,2n) 169 Er followed by beta-decay with a half-life time of 9 days.The resulting 169 Tm is subjected to (n,2n) reaction to 168 Tm.There are some contributions from sequential charged particle reactions on 168 Er as well.However, the only stable isotope of Tm, 169 Tm, may be present in Er as an impurity.An analysis of the samples is underway.The production of 167 Ho is very well predicted by the calculation and even within the experimental uncertainty.The nuclides 165 Er and 166m Ho could not be measured in this experiment.

Titanium
The production of the two scandium isotopes 46 and 47 was predicted quite well.The 47 Sc production was overestimated by 13%.However the FISPACT calculation also has a relatively high uncertainty.
It should be noted that the production cross section of 47 Sc from natural titanium has a contribution from 47 Ti via the (n,p) and 48 Ti via the (n,d) and (n,np) reactions, see table 4. The cross sections for both of these reactions are rising quickly at around 14 MeV neutron energy, so that a slight deviation of the calculated neutron spectrum, which is the base for the FISPACT calculation, may be the cause besides an uncertainty in the production cross section.

Conclusions
The most important contributions to the total -ray dose rate from activities induced in a fusion peak neutron field in the elements La, Er, and Ti were predicted well by the activation system EASY-2010 up to the recycling limit.Despite a higher uncertainty, nuclides originating from neutron capture reactions were predicted within 20% of the experimental value although the experiment was not designed to test such reactions.Nuclides responsible for most of the activity after the recycling limit is reached could not be measured in these experiments due to their low activity in the samples.A large underestimation of the calculation was found for 168 Tm in Er suggesting 169 Tm as an important impurity in Er which is currently under investigation.
Parts of the work, supported by the European Communities under the contract of Association between KIT-Euratom, were carried out within the framework of the European Fusion Development Agreement.The views and opinions expressed herein do not necessarily reflect those of the European Commission.

Figure 3 .
Figure 3. Neutron spectrum in the position of the Ti sample calculated with the DROSG and MCNP codes.

Table 1 .
Isotopic composition of the considered materials.

Table 2 .
Comparison of calculated and experimental radionuclide productions for lanthanum.

Table 3 .
Comparison of calculated and experimental values for erbium.

Table 4 .
Comparison of calculated and experimental values for titanium.