Comparison of Shutdown Dose Rate Results using MCNP6 Activation Capability and MCR2S

Transcription

1 APPLIED RADIATION PHYSICS GROUP TECHNICAL NOTE ARP-097 July 2014 Comparison of Shutdown Dose Rate Results using MCNP6 Activation Capability and MCR2S A. Turner 1, Z. Ghani 1, J. Shimwell 2 1: CCFE, Culham Science Centre, Abingdon, Oxon, OX14 3DB, UK 2: Department of Physics and Astronomy, University of Sheffield, Hicks Building, Hounsfield Road, Sheffield, S3 7RH, UK

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3 Document Change Control Issue No. Date Changes Table of Contents 1 Introduction Methodology Model Tallies Normalisation Results Conclusions and Recommendations... 14

4 1 INTRODUCTION The activation of materials in fusion devices is an important quantity, particularly since the decay of such materials produces a gamma radiation source that persists after plasma operation has ceased. It is important to minimise this shutdown dose rate (SDR) in areas where maintenance access will be needed. Traditionally, the rigorous two step (R2S) method has been used to produce activation gamma sources to determine the SDR, implemented at CCFE in the form of MCR2S as well as independently at several other fusion institutions. R2S is a well-established method of coupling the particle transport capabilities of a transport code (such as MCNP) with an activation code (such as FISPACT). A neutron transport run is performed, to obtain neutron flux and spectra in a mesh tally superimposed over the geometry. An activation calculation is then performed on each voxel of the mesh tally, in turn producing an activation gamma source to be used in a second (photon) MCNP calculation to determine the SDR. Advantages of R2S approach: Relatively simple to implement. Production of gamma source permits flexibility in gamma calculation e.g. modification of geometry, or source placed in different model. Disadvantages of R2S approach: Mesh resolution affects SDR results. Neutron flux and material compositions are averaged over mesh voxels. High mesh resolutions required for accuracy and corresponding high memory consumption. Due to use of a mesh source, decay photons can start in materials/regions where they were not originally produced. Multiple calculation steps can lead to book-keeping/quality control issues. No uncertainty propagation all uncertainty in the neutron flux information is lost (only average gamma source is used rather than a sampled one). These are the issues considered native to the R2S methodology, and CCFE and other fusion institutions are working to develop solutions to these issues. In the latest version of MCNP (MCNP6), the developers have added the capability to perform activation on-the-fly within the neutron transport run using the ACT card, hereby referred to the MCNP6-ACT capability. This feature utilises the CINDER libraries and was previously available in MCNPX. Clearly this will have the advantage of a more realistic treatment, with photons being started where they were created rather than a mesh-averaged photon source. In addition, the photon SDR results would then have a statistical uncertainty that accounted for the neutron transport as well as the photon transport. This report makes a comparison between previously obtained results using MCR2S, and results obtained using the new MCNP6-ACT capability, performed using the DEMO shutdown dose rate computational benchmark model. This model was previously used as part of an exercise to compare the various implementations of the R2S method. 2 METHODOLOGY 2.1 Model The model used in this study is the 2008 DEMO-HCLL model by KIT, as used in the DEMO computational benchmark studies [1]. It consist of a o poloidal sector of a tokamak containing a detailed representation of the HCLL blanket modules and approximate

5 representations of the divertor, in-vessel shield, vacuum vessel (including port extensions), toroidal field coils and central solenoid. Reflecting boundary conditions are used to represent symmetry. Poloidal and toroidal sections are illustrated in Figure 1. (a) Figure 1: MCNP model of 2008 DEMO-HCLL reactor showing mesh division for R2S calculations; (a) poloidal section at Y=2, (b) toroidal section at Z=2. The above figure shows the activation mesh tallies specified for the R2S codes, not applicable for. Materials were assigned as per the original task specification document. The specification requires enriched 6 Li in the lithium-lead material, which at the time of the original study could not be supported in MCR2S and was performed at natural elemental abundances in those calculations. The calculation was repeated here using the new version of MCR2S (version 2), which integrates with FISPACT-II and has the capability to use isotopic material descriptions. (b) 2.2 Sources In the case of the MCR2S based DEMO neutron calculation, the standard parametric plasma source was used. Fusion power is taken as 2.4 GW, corresponding to a DT neutron emission of 8.52x10 20 n/s. In addition, there is no need to account for the sector angular extent when defining the normalisation as the parametric source accounts for the sector angle via a reduced source particle weight - thus this full sector normalisation should be used. The irradiation time is accounted for in MCR2S, but needs to be included as a timedependant source in. The irradiation time to be assumed was 8 years at 50% power, followed by 60 days at 100% power. For the divertor, a different irradiation scenario was to be assumed, of 4 years at 50% power followed by 60 days at full power. This added complexity was ignored for the MCNP6-ACT run, since it was not clear how to simulate such a system, and in any case, the effect on the result was not expected to be significant for short cooling times. For MCNP6 and the time dependent source, the parametric plasma source was used but has been modified to allow the user to input a time-dependent source (a series of source

6 times and probabilities) via the SI and SP cards, along with source biasing parameters (SB cards). The 8 year neutron source time was split into time bins and biased towards later times, which would be more important for gamma production at the shorter decay times of interest. MCNP6 supports both line and multigroup secondary gamma production, and due to the difficulty in obtaining reliable tally statistics (see section 2.3), a multigroup treatment was used, having been found to run over 100 times faster than line data. Since it was not feasible to obtain results using line data, the effect of utilising multigroup gamma data on the results has not been assessed in this study. In addition, the delayed neutron and delayed photon biasing functions were used to increase the sampling of these events, and the SPABI card used in an attempt to increase the population of high energy photons - though the effectiveness of these approaches was not clear. idum j 1 rdum j 5000 si E E E E E E E E E+16 sp E E E E E E E E E-02 sb C ACT DG=MG DN=BOTH nonfiss=p dnbias=5 dgbias=10 spabi:p np $AT - applied 10x splitting to high energy photons > 0.1 MeV Figure 2: MCNP time-dependent parametric plasma source and ACT cards 2.3 Tallies Results were obtained for absorbed and biological dose rates at four different in-vessel locations and at decay times of 1 hour and 10 days. 10 cm diameter spherical cells were used, centred at the following in-vessel locations D1, D2, D3 and D4: D1 centre of vessel: (X,Y,Z) = (750, 60, 0) cm; D2 outboard equatorial midplane: (X,Y,Z) = (990, 60, 0) cm; D3 inboard equatorial midplane: (X,Y,Z) = (500, 60, 0) cm; D4 divertor region: (X,Y,Z) = (570, 60, -480) cm. The original specification called for absorbed dose rates to be obtained for steel-filled tally spheres (F6 tallies, i.e. with self-shielding), and photon flux spectra and biological dose rates for voided spheres (no self-shielding). As such, this requires two calculations, and it was decided to only run one calculation using due to high computational demands. Results and comparisons for photon flux and biological dose rate are therefore provided in the results section, absorbed dose is not calculated using. The gamma-todose coefficients to be used in the biological dose calculations are the ITER-recommended ICRP-74 in both and MCR2S. Tallies for the calculation required the inclusion of tally time bins. MCNP generates decay gamma photons for decay times between 0 and seconds by default, and tally time bins are required to obtain result in the time intervals of interest. Due to this methodology, it is not possible to produce a result for a specific decay time, but only a time bin of finite width ( t). Initially, calculations were performed assuming a 1% t window about the decay times of interest, for example for 1 hour, the time bin would be 3582 to 3618 seconds after shutdown. However, it was found that it was extremely challenging to obtain

7 acceptable statistics in such narrow time bins, even after several days of running with 128 cores. In order to obtain statistically reliable results, it was decided to tally in time bin widths of 20% centred on the decay time of interest. The use of wide time bins will tend to artificially increase the results due to averaging over an exponential decay, though examining the shape of the decay curves this effect was predicted to be small (< 1%). Even with large time bin widths, the calculation was still computationally challenging, and time-dependent weight windows were attempted but not found to be effective. Multigroup decay gamma libraries were utilised for speed, and biased delayed particle production was attempted. The DGBIAS card is not mentioned in the manual but appears to be a valid input keyword and was used in the hope this would increase gamma production. The SPABI card (Secondary PArticle BIasing) was also used to split photons if they were born in a required energy range. These methods did not seem particularly effective, and rather than splitting secondary photons when they happen to occur, the ideal solution would be to have MCNP6 support biasing of the secondary photon production in user-specified energy and time ranges. This does not appear to be a feature that is currently available, though discussions with the developers suggest DPEB (delayed photon energy bias) and DPTB (delayed photon time bias) cards are under development. Biasing production to time ranges of interest would be particularly useful to reduce computational load for fusion problems. Table 1: Irradiation and decay time steps Bin Description Step, sec Bin upper bound, sec Bin upper bound, shakes ==Irradiation== 1 8 years at 50% Irr E E E day at 100% Irr E E E+16 ==Cooling== 3 T_zero to 1 hr less t/2 Cool E E E hr +/- t/2 Cool E E E hr + DT to 10 days - t/2 Cool E E E days +/- t/2 Cool E E E+16 Tally time bins were entered as shown in Figure 3. T E E E E+16 Figure 3: Example tally time card 2.4 Normalisation Using the MCR2S source, each of the four activation meshes was produced with an associated photon source rate. This source rate is therefore the normalisation factor for the flux and dose rare tallies. Dose to steel tallies also carried an additional normalisation factor to obtain units of Gy/hr. In order to obtain the total result from all activation sources, gamma runs are needed for each activation source mesh (each with a separate normalisation), the results of which are then summed.

8 In the case of the calculation, a time integrated source and tally result is being used (photons/cm 2 over time bin, per source neutron). The source normalisation factor in neutrons per second, is therefore obtained by multiplying by the total neutron source over the entire irradiation time and dividing by the time bin width used for the tally of interest (in seconds). 1.0 t = 8 years t = 60 days t = 10 days 1.0 = 8.52x10 20 n/s 0.5 t = 1 hour Tally A Tally B t 0 t d1 t d2 t d3 t d4 t i1 t i2 T A = t d2 t d1 (sec) T B = t d4 t d3 (sec) tot_src = (8*365.25* *1.0)*3600*24*8.52x10 20 = 1.12x10 29 neutrons Tally A: Photon Flux (ph/cm 2 /s) = Tally A (F4) x tot_src / T A Tally B: Photon Flux (ph/cm 2 /s) = Tally B (F4) x tot_src / T B Figure 4: Tally and normalisation method 2.5 Materials Material definitions were provided in the DEMO MCNP model used for the previous benchmarking exercise task, using FENDL-2.1 libraries for neutron transport and MCPLIB04 (update.84p ) for photons. The materials included in the MCNP input file did not contain impurities. The material descriptions were suitable for neutron and gamma transport in the MCR2S workflow, however a detailed material description was used in the MCR2S activation calculations - these detailed material descriptions were provided in the specification document [1]. These same material definitions were used for the calculation. Table 2: Material compositions element M1 (Tungsten), %wt M2 (Eurofer), %wt M9 (LiPb), %wt Li (90% Li-6). Be B C N O Al Si P S Ti

9 V Cr Mn Fe Co Ni Cu Zr Nb Mo Sn Ta W Pb Density g/cm 3 variable 9.54 g/cm 3 Material definitions were provided as FENDL-2.1 format (ZAID.21c). The MCNP program ran successfully with the ACT card, although the following warning message was produced: warning. no cinder match for some delay_library nuclides - see output file. beta delay_library_v2.dat nuclide not found in cinder.dat zaid & state delay ID CINDER id (list continues)... Figure 5: MCNP output showing CINDER library warning message Having contacted a developer at LANL, this message relates to the fact that beta decay data is present in the delay library for some nuclides, but the CINDER database does not include these nuclides and so beta emission from these nuclides will be ignored in any decay chains. Due to time constraints, the potential impact of these missing nuclides has not been investigated, but the importance of these could be studied using FISPACT-II. 3 RESULTS Results for MCR2S have been recalculated using MCR2S V2 which uses FISPACT-II. results are not presented for absorbed dose, which would require a second calculation using steel tally spheres (this was not attempted due to excessive calculation time).

10 Table 3: Photon flux, 1 hour decay time Flux, MCR2Sbased Flux, based relative error Relative to MCR2S-based D1 2.53E E+12 14% 1.14 D2 2.71E E+12 19% 1.25 D3 2.79E E+12 18% 1.28 D4 2.51E E+12 18% 1.56 Table 4: Photon flux, 10 days decay time Flux, MCR2Sbased Flux, based relative error Relative to MCR2S-based D1 3.71E E+11 12% 1.14 D2 3.84E E+11 16% 1.43 D3 3.99E E+11 14% 1.35 D4 3.99E E+11 12% 1.26 Table 5: Photon biological dose rate, 1 hour decay time Dose rate, MCR2S-based Dose rate, based relative error Relative to MCR2S-based D1 3.43E E+04 17% 1.10 D2 3.70E E+04 22% 1.25 D3 3.77E E+04 18% 1.10 D4 3.02E E+04 20% 1.37 Table 6: Photon biological dose rate, 10 days decay time Dose rate, MCR2S-based Dose rate, based relative error Relative to MCR2S-based D1 4.26E E+03 16% 1.16 D2 4.44E E+03 18% 1.31 D3 4.57E E+03 14% 1.17 D4 4.10E E+03 15% Computation time comparison The MCNP-MCR2S runs required, approximately, 500 core-hours of computation and took approximately 10 days. Being in the vacuum vessel region close to the source the calculations were not particularly computationally challenging compared to a deep shielding problem, however the need to produce PTRAC files, run MCR2S on multiple meshes, and

11 run different gamma source runs for each decay time led to a relatively complex process to manage. In comparison, the calculation ran for approximately 6,000 core-hours, in 3 days, thus this method is more computationally intensive but much less demanding in terms of human effort, complexity and overall time (where sufficient computing power is available). The use of line data and narrower time bins is desirable, but this would significantly increase the computational demands hence the ability for MCNP6 to permit the user to bias gamma production to specific time bins is essential going forward. 3.2 Comparisons with other R2S codes Comparisons are made in the following figures including the results for other codes participating in the original computational benchmarking exercise. results are shown for biological dose rate and photon flux only (Figure 8 to Figure 11). 2.5E+04 1h absorbed dose (Gy/hr) 2.0E E E E+03 UNED:R2S-UNED CCFE:MCR2S v2 KIT:R2SMesh 0.0E+00 D1 D2 D3 D4 tally Figure 6: Comparison of absorbed dose rates, 1 hour decay time

12 3.0E+03 10d absorbed dose (Gy/hr) 2.5E E E E E+02 UNED:R2S-UNED CCFE:MCR2S v2 KIT:R2SMesh ENEA:D1S 0.0E+00 D1 D2 D3 D4 tally Figure 7: Comparison of absorbed dose rates, 10 day decay time 8.0E+04 1h biological dose (Sv/hr) 6.0E E E+04 UNED:R2S-UNED CCFE:MCR2S v2 KIT:R2SMesh CCFE: 0.0E+00 D1 D2 tally D3 D4 Figure 8: Comparison of biological dose rates, 1 hour decay time

13 8.0E+03 10d biological dose (Sv/hr) 6.0E E E+03 UNED:R2S-UNED CCFE:MCR2S v2 KIT:R2SMesh ENEA:D1S CCFE: 0.0E+00 D1 D2 D3 D4 tally Figure 9: Comparison of biological dose rates, 10 day decay time 6.0E+12 1h decay photon flux (p/cm 2 s) 5.0E E E E E+12 UNED:R2S-UNED CCFE:MCR2S v2 KIT:R2SMesh CCFE: 0.0E+00 D1 D2 D3 D4 tally Figure 10: Comparison of photon flux, 1 hour decay time

14 8.0E+11 10d decay photon flux (p/cm 2 s) 6.0E E E+11 UNED:R2S-UNED CCFE:MCR2S v2 KIT:R2SMesh ENEA:D1S CCFE: 0.0E+00 D1 D2 D3 D4 tally Figure 11: Comparison of photon flux, 10 day decay time Statistical errors for the R2S calculations are low (< 2%), though error bars are not shown since error propagation from the neutron transport to the gamma source is not conducted and hence values could be misleading. Error bars due to stochastic transport sampling are shown for the results at 2σ. It is clear that results from show reasonable agreement with the other codes in this test case. Whilst generally higher than MCR2S by 20-50%, the degree of variability is similar to that observed between MCR2S and the other R2S and D1S codes. 4 CONCLUSIONS AND RECOMMENDATIONS The use of MCNP6 on-the-fly activation capabilities presents the following advantages over the R2S method: - No effects of mesh resolution: photons are produced in correct source locations. - Lower memory requirements - Single step - less chance of user error regarding multiple activation source and normalisations for decay times. - Ability to obtain results for all decay times in a single calculation via time dependent tallies. - The capability to use line data for decay gamma production (existing R2S codes utilise multigroup gamma spectra). - Much less human effort required. - Uncertainty propagation from the neutron field to the decay gamma dose rate result is a natural consequence of the method. Currently, the following disadvantages are noted: - Significantly increased computational demand over R2S method (accounting for all steps, approximately a factor of 10). However, activation source time biasing may alleviate this. - Results statistically difficult to obtain in narrow time bins, and the bins used here were wider than was desirable.

15 - The use of gamma line data was found to significantly increase computational requirements (by over a factor of 100). The use of a multigroup secondary gamma treatment was necessary to obtain meaningful tally statistics (< 20%) in a reasonable time. The effect of this multigroup structure on the results was not assessed. - Decay gammas are produced from time 0 to seconds, which is thought to have significant computational cost. It may be desirable to limit, or at least bias, the gamma production to the range of decay times of interest. - There is no ability to enable different activation schedules for different components. For example, an option to activate divertor cells for only the last 4 years would be useful. - No ability to extract the gamma source for secondary calculations in different geometry models. - Not all isotopes exist in the CINDER library, nor is the library validated for fusion problems. A fusion-specific library or integration with a more modern activation code such as FISPACT-II would be desirable. MCNP6 activation appears to be a useful feature, and is in many ways in line with the meshbased geometry approach, that is, reducing human effort in exchange for an increase in computational time, as well as addressing long standing issues of uncertainty propagation. There is, however, a need to bias or limit the decay photon production to times of interest in order to improve the efficiency of the calculation, particularly when very narrow decay time windows are desired. According to developers, the SPEB card (secondary photon energy bias) card will be available in MCNP6v1.1, and there are plans for a similar SPTB (secondary photon time bias) card in the future, which will permit biasing of secondary photon production in time bins. Further development to address other limitations is desirable, such as the ability to define different activation schedules for different cells, and a method to extract the decay gamma source for a subsequent calculation. Additionally the CINDER library and activation methodology used should be documented and validated for application to fusion problems. 5 REFERENCES 1 Specification of a DEMO-relevant comparison exercise for evaluation of European shutdown dose rate tools, R. Pampin (for PPPT subtask WP12-DTM04-T13-D15a, July 2012).