Design development and implementation of an irradiation station at the neutron time-of-flight facility at CERN

A new parasitic, mixed-field, neutron-dominated irradiation station has been recently commissioned at the European Laboratory for Particle Physics (CERN). The station is installed within the neutron time-of-flight (n_TOF) facility, taking advantage of the secondary radiation produced by the neutron spallation target, with neutrons ranging from 0.025 eV to several hundreds of MeV. The new station allows radiation damage studies to be performed in irradiation conditions that are closer to the ones encountered during the operation of particle accelerators; the irradiation tests carried out in the station will be complementary to the standard tests on materials, usually performed with gamma sources. Samples will be exposed to neutron-dominated doses in the MGy range per year, with minimal impact on the n_TOF facility operation. The station has 24 irradiation positions, each hosting up to $100{\mathrm{cm}}^3$ of sample material. In view of its proximity to the n_TOF target, inside protective shielding, the irradiation station and its operating procedures have been carefully developed taking into account the safety of personnel and to avoid any unwanted impact on the operation of the n_TOF facility and experiments. Due to the residual radioactivity of the whole area around the n_TOF target and of the irradiated samples, access to the irradiation station is forbidden to human operators even when the n_TOF facility is not in operation. Robots are used for the remote installation and retrieval of the samples, and other optimizations of the handling procedures were developed in compliance with radiation protection regulations and the aim of minimizing doses to personnel. The sample containers were designed to be radiation tolerant, compatible with remote handling, and subject to detailed risk analysis and testing during their development. The whole life cycle of the irradiated materials, including their post-irradiation examinations and final disposal, was considered and optimized.

Figure 1

Overview of the NEAR area within n_TOF. The upper part of the figure shows a general scheme of the n_TOF facility. The proton beamline, the position of the neutron spallation target, and the NEAR area (in the brown square) are shown. The lower part of the figure shows different views of the NEAR area. The position of the irradiation station is indicated by the red rectangle. The mobile shielding, the position of which is indicated by the arrow, separates the internal i-NEAR (the pink volume), where the irradiation station is located, from the outer a-NEAR (the green volume).

Figure 2

(a) The New Target Mobile Shielding in the closed position. (b) The shielding in the open position, revealing i-NEAR and the target pool, close to the n_TOF target as it was before the installation of the irradiation station. (c) 3D model of NEAR, with the shielding open, showing the position of the target pool, the future collimator (in yellow), the openings for rabbit tubes, and the rails allowing the shielding movement.

Figure 3

New configuration of the mobile shielding in the open position, with parts A, B, and C, as explained in the text. The different parts and the materials used are indicated. The outermost layer of marble is only present for part A and part B (for the latter case, only in the area facing the spallation target).

Figure 4

(a) Steel part of the collimator photographed from a-NEAR before installation of the borated polyethylene part. The hole in the marble part of the shielding is visible in the foreground of the image. (b) Borated polyethylene part of the collimator installed in the shielding. (c) Section view of the NEAR area showing the target, shielding (steel in red, concrete in gray, and marble in light gray), and access passage. The two parts of the collimator are visible inside the shielding: the steel component (in yellow) and the borated polyethylene components (in white).

Figure 5

Picture of the irradiation station, filled with samples, installed on the original target pool at n_TOF NEAR. The top and the bottom shelves are visible. Four intermediate aluminum supports (two per shelf), with their handle for remote handling, accommodate up to six containers each (CERN-PHOTO-202107-085-11) [39].

Figure 6

Position of the dosimeters in the irradiation station, along with their labels.

Figure 7

Sample container design. Left: picture of the aluminum inner container and its perforated lid. Right: picture of the aluminum outer container configuration with its components.

Figure 8

Extraction of an inner container from an outer container under a fume hood, by use of the reachers.

Figure 9

Three of the main tasks carried out by robots: (a) CERNBot2.0 with double arm configuration cutting cables; (b) Teodor installing the shelf structure on the n_TOF’s spallation original target pool; (c) Telemax robot installing an intermediate support with six sample containers in the NEAR irradiation station (CERN-PHOTO-202107-085-7) [39].

Figure 10

Telemax holding a support structure containing six samples of materials ready for installation (CERN-PHOTO-202107-085-1) [39].

Figure 11

Mock-up trials of recovery scenarios: In the left picture, Telemax is recovering some dropped containers; an intermediate support is on the floor with a sample having fallen out of it. Right side, simulation of three possible scenarios (A, B, C) of a dropped intermediate support. The yellow container was used to replicate the retention vessel in i-NEAR.

Figure 12

fluka Monte Carlo geometry of the area surrounding the n_TOF third generation spallation target, focusing on the shelves of the irradiation station.

Figure 13

Total absorbed dose in the samples on the top shelf (a) and on the bottom shelf (b), averaged over each sample volume. Dose values are expressed in $\mathrm{Gy}/\mathrm{pulse}$, for a reference value of $2.5\times {10}^{19}$ protons on target (POT), expected to be delivered during a normal physics production year of 200 working days. The numbering of the samples corresponds to the different groups of six samples handled by the robots. The target, the position of which is not shown in the figure, is further down in the $x$ direction, meaning that samples 1-2-7-8 and 13-14-19-20 are the closest to the target (on the top shelf and bottom shelf, respectively).

Figure 14

Total dose in a selection of samples on the top shelf. (a) Horizontal section of samples 1 to 12. (b) Vertical section of samples 1, 2, 7, and 8 (from left to right) having the same distance from the target. Dose values are reported in $\mathrm{Gy}/\mathrm{pulse}$.

Figure 15

Particle spectra in a reference position on the top shelf. For comparison with the dominating neutrons, the proton and electron spectra are multiplied by a factor of 100. By comparison, the gray curve represents the neutron fluence in a-NEAR (multiplied by 10).

Figure 16

1-D distribution along the fluka $z$ axis of the residual dose rate for various cooldown times (from 1 day up to 10 years).

Figure 17

The figure shows the ratio between the simulated dose absorbed by the dosimeters and the experimental dose measurement, showing an overall good agreement within the experimental uncertainties.