This benchmark experiment was conducted as a joint venture between the US Department of Energy (DOE) and the French Commissariat à l'Energie Atomique (CEA). Staff at the Oak Ridge National Laboratory (ORNL) in the US and the Centre de Valduc in France planned this experiment. The experiment was conducted on October 13, 2010 in the SILENE critical assembly facility at Valduc. Several other organizations contributed to this experiment and the subsequent evaluation, including CEA Saclay, Lawrence Livermore National Laboratory (LLNL), the Y-12 National Security Complex (NSC), Babcock International Group in the United Kingdom, and Los Alamos National Laboratory (LANL).
The goal of this experiment was to measure neutron activation and thermoluminescent dosimeter (TLD) doses from a source similar to a fissile solution critical excursion. The resulting benchmark can be used for validation of computer codes and nuclear data libraries as required when performing analysis of criticality accident alarm systems (CAASs). A secondary goal of this experiment was to qualitatively test performance of two CAAS detectors similar to those currently and formerly in use in some US DOE facilities. The detectors tested were the CIDAS MkX and the Rocky Flats NCD-91. The CIDAS detects gammas with a Geiger-Muller tube, and the Rocky Flats detects neutrons via charged particles produced in a thin 6LiF disc, depositing energy in a Si solid-state detector. These detectors were being evaluated to determine whether they would alarm, so they were not expected to generate benchmark quality data.
The neutron activation measurements were performed using several different types of activation foils. These foils included Au, Ni, In, Ti, Fe, Mg, and Co. The neutron activation foils were placed at several locations around the lead reflected SILENE critical assembly, some with additional concrete shielding and some unshielded. Within a few hours after SILENE was pulsed, these foils were collected, and their activation was measured by CEA Valduc staff. Note that the Ti foil activation data were not included as part of the experimental data reported by Valduc, but they were part of the experimental setup. This foil was included because the Valduc staff had experience irradiating Ti with fast systems, and they wanted to evaluate using Ti with a thermal system. Several Ti neutron activation reactions have a threshold energy above 5 MeV, but 47Ti(n,p)48Sc has a threshold around 2 MeV. This reaction did not produce good experimental results because the natural abundance of 47Ti is only about 7.5%. Furthermore, the half-life is about 3.5 days, which was logistically a problem because there were five other activation products with shorter half-lives that needed to be measured first.
There were also three types of TLDs. The TLDs provided by CEA Valduc were Al2O3 powder (TLD-500), and LiF TLDs were provided by ORNL (TLD-700 crystals and powder). The TLDs were placed around the lead-reflected SILENE critical assembly in close proximity to the neutron activation foils. The TLDs were collected at the same time as the neutron activation foils after the experiment was conducted, but the TLD doses were not read until several weeks or months later at Valduc and ORNL. Similar to this experiment, measurements of neutron activation and TLD doses were performed at the SILENE critical assembly facility using no shield/reflector (ALARM-TRAN-AIR-SHIELD-001) and a polyethylene shield (ALARM-TRAN-CH2-SHIELD-001).
The neutron activation and TLD dose data submitted here are for a pulse of the SILENE critical assembly when the critical assembly is surrounded by a lead shield/reflector. Whether the lead surrounding SILENE is referred to as a reflector or a shield depends on perspective. If the focus is on system criticality and reactivity, then the lead is referred to as a reflector. If the focus is on neutron activation foil and TLD responses, the lead is referred to as a shield. In this report, the lead surrounding the SILENE is referred to as a reflector, shield, and reflector/shield, all of which are discussing the same piece of equipment. In some instances, additional concrete shielding was used between the lead-reflected SILENE critical assembly and the neutron activation foils and TLDs. In total, 41 neutron activation products were measured from 33 activation foils, and 17 TLDs measured photon doses at 7 locations. Based on the analysis of the measured data, the uncertainties of the measurements, and the benchmark uncertainties, 37 measurements of neutron activation products and 4 TLD dose measurements from 4 locations were found to be acceptable for use as benchmark data.