H. Iida

Evaluation of Shutdown Gamma-ray Dose Rates around the Duct Penetration by Three-Dimensional Monte Carlo Decay Gamma-ray Transport Calculation with Variance Reduction Method

For the evaluation of gamma-ray dose rates around the duct penetrations after shutdown of nuclear fusion reactor, the calculation method is proposed with an application of the Monte Carlo neutron and decay gamma-ray transport calculation. For the radioisotope production rates during operation, the Monte Carlo calculation is conducted by the modification of the nuclear data library replacing a prompt gamma-ray spectrum with a decay gamma-ray spectrum. By multiplying each correction factor, which is ratio of the actual activation level after shutdown to the production rate during operation, with each decay gamma-ray flux due to each radioisotope, the decay gamma-ray dose rate is evaluated. In order to improve the statistical error, a variance reduction method is proposed by the application of the weight window importance technique and the specification of the decay gamma-ray generation location. We identify the cell producing the decay gamma-ray which can contribute the decay gamma-ray flux in evaluation locations, and forcibly terminate the gamma-ray transport calculation in the cells except for the identified cells. In order to validate the effectiveness of the method, shielding calculation for actual ITER (International Thermonuclear Experimental Reactor) configuration is performed, and small statistical errors below criteria are obtained. The effectiveness of the proposed method for ITER design analysis is demonstrated.

Radiation Shielding for ITER to Allow for Hands-on Maintenance inside the Cryostat (Methodology for Estimating Shutdown Dose Rate in a Complex Geometry)

In the shielding design of the ITER machine, which is a tokamak fusion experimental reactor and has a very complex geometry, it is important to have a reliable estimation of the dose rate levels after reactor shutdown for realising hands-on maintenance around the torus. The ITER project position is that dose rates inside the cryostat be kept low enough to allow for human access shortly after shutdown for limited periods to provide rescue and/or maintenance activities. The methodology of estimation for dose rates after shutdown in such a complex geometry machine is discussed. The Monte Carlo method is preferable to conduct neutron transport calculations in the ITER geometry. The Conversion Factor method, which was used for dose rate estimation in the 1998 ITER shielding design, is described with an example of dose rate estimation around penetrations in the vacuum vessel. A Full Monte Carlo method is proposed showing the possibility of eliminating uncertainty accompanied with conversion factors from neutron flux or isotope production rate to dose rate after shutdown.

Evaluation of biological dose rates around the ITER NBI ports by 2-D Sn/activation and 3-D Monte Carlo analyses

Abstract Shielding analyses for the International Thermonuclear Experimental Reactor (ITER) neutral beam injector (NBI) ports have been performed using two-dimensional discrete ordinates S n method with activation analyses and three-dimensional Monte Carlo method. From the two-dimensional S n /activation analyses, it was found that a conversion ratio relating fast neutron flux to the biological dose rates at 10 6 s after reactor shutdown is 1.5–2.0×10 −5 μSv/h per (cm −2 s −1 ) for the fast neutron flux with energies above 0.1 MeV. From the results of three-dimensional Monte Carlo analyses, the fast neutron flux above 0.1 MeV at the cryostat is about 3.6×10 6 cm −2 s −1 for the case of a 50–60 cm thick NBI port wall (thin case), and 5.2×10 5 cm −2 s −1 for a 60–65 cm thick port wall (thick case). The corresponding biological dose-rates at the cryostat are about 7 and 2×10 1 μSv/h for the thin and thick cases, respectively. These dose rate levels satisfy the tentative design target of 100 μSv/h specified for the ITER Engineering Design Activity (EDA).

ITER radiation shielding and neutronics analysis

Abstract Two dimensional radiation transport calculations have been carried out to determine radiation streaming through the ITER equatorial ports. The NBI port has been identified as the most critical. Shielding requirements were estimated to minimize the nuclear heating rates in the toroidal field (TF) coils and the activation of the cryostat, where hands-on maintenance is anticipated. The shielding efficiency of steel/water port walls was investigated as a function of port and wall dimensions and steel volume fraction. For open ports, i.e. the neutral beam injector ducts, 40–50 cm thick port walls composed of 40% SS-60% H 2 O–75% SS-25% H 2 O will reduce the TF coil heating to acceptable levels, while 60–65 cm thick walls are necessary for reducing the dose rates at ∼2 weeks after shutdown to levels of 750 μSv h −1 at the cryostat.

A Handy Method for Estimating Radiation Streaming through Holes in Shield Assemblies

Abstract A ‘handy method’ has been developed for performing quick estimates of radiation streaming through cylindrical and rectangular holes in shield assemblies. Quick estimates are often needed during conceptual studies or detailed design of nuclear plants. For example, ITER (International Thermonuclear Engineering Reactor) has a large number of penetrations in the blanket and vacuum vessel and biological shield. This method estimates the levels of radiation that leaks through these holes as well as the magnitude of the fluxes along the hole wall. Results obtained using the ‘handy method’ are in good agreement with 3-D Monte Carlo calculations suggesting that the method provides a practical tool for streaming analysis.

Dose Rate Analyses around the Equatorial and Divertor Ports during ITER In-Vessel Components Maintenance

The shutdown dose rates around the equatorial and divertor maintenance ports of ITER were evaluated with the 2-D/3-D combined approach, using the three-dimensional continuous-energy Monte Carlo code, MCNP-4B and the two-dimensional discrete ordinate code, DOT3.5. The neutron flux-to-shutdown dose rate conversion factor is derived with the two-dimensional geometry using the THIDA code system and the assumed operation scenario, i.e. the neutron fluence of 0.3 MWa/m2 in ten years of operation. The dose rate around the equatorial and divertor ports after 106 seconds (11.6 days) after reactor shutdown ranges from 100 to 200 μ Sv/h. Attempts to further reduce the dose rate by improving the shield design were made to follow the principle of ALARA.

Streaming analysis for radiation through ITER mid-plane port

Abstract Two-dimensional shielding analyses have been performed on the maintenance and test module mid-plane ports in the International Thermonuclear Experimental Reactor (ITER). Nuclear responses in the toroidal field (TF) and poloidal field (PF) coils around these ports have been calculated. Shield plugs with 20-mm-wide gaps between the blanket modules exist in both ports. In the case of the test module port, they are installed at the back of a 500-mm-thick test module. In order to satisfy design limits, 540- and 150-mm-thick shield plugs are required for the maintenance and the test module port, respectively. In addition, nuclear responses as a function of the gap width have also been estimated. In the case of the 50-mm-wide gap, it is found that 750 and 390-mm-thick shield plugs are required for the maintenance and test module ports, respectively.

Evaluation of the Environmental Gamma-ray Dose Rate by Skyshine Analysis During the Maintenance of an Activated TFC in ITER

Gamma-ray exposure dose rates at the ITER site boundary were estimated for the cases of removal of a failed activated Toroidal Field (TF) coil from the torus and removal of a failed activated TF coil together with a sector of the activated Vacuum Vessel (VV). Skyshine analyses were performed using the two-dimensional SN radiation transport code, DOT3.5. The exposure gamma-ray dose rates on the ground at the site boundary (presently assumed to be 1 km from the ITER building), were calculated to be 1.1 and 84 μSv/year for removal of the TF coil without and with a VV sector, respectively. The dose rate level for the latter case is close to the tentative radiation limit of 100 μSv/year so an additional ∼14 cm of concrete is required in the ITER building roof to satisfy the criterion for a safety factor often for the site boundary dose rate.