Plan de orientación y mentoría desarrollado en la experiencia Presentación:

The graphite subcritical reactor facility (GSR) was constructed at Penn State in 1958 as part of a graduate student project. The pile was intended to expand upon the research reactor facility’s capabilities to educate students in the burgeoning field of nuclear engineering. Since then, it has been used continuously for 55 years as part of the reactor physics curriculum. Currently the GSR is used as the basis for teaching subcritical physics to 100 undergraduate students each year. Additionally, the facility is used by researchers who need a well-thermalized neutron field for their experiments. Recently, the facility has been used to develop sensitive neutron detectors for nuclear safeguards purposes. The inherent simplicity and flexibility of the GSR ensures that it will be useful for many years to come.

VI–1. FACILITY DESCRIPTION

The GSR is constructed of several hundred blocks of reactor-grade graphite to form an array that measures 266 cm x 161.5 cm x 178 cm. The entire facility is clad in cadmium sheeting covered in aluminium as shown in Figs VI–1 and VI–2. The cadmium cover shields the users from the neutron flux from the pile. The facility can be used as a sigma pile by inserting from one to five 37 GBq (1 Ci) Pu-Be neutron sources at various points. It can also become a subcritical reactor by replacing some of the graphite with natural uranium rods. The fuel was donated by the Atomic Energy Commission in 1958. The fuel can be configured in four analysed loadings for different experiments, see Fig. VI–3. The maximum flux in the pile is approximately 10⁴ n/cm²s. The maximum keff is approximately 0.7. By removing graphite, various measurements can be performed with neutron detectors and activation foils.

VI–2. EDUCATIONAL UTILIZATION

The students perform three experiments on the GSR facility: neutron moderation in graphite, neutron diffusion in graphite and criticality estimation. These experiments are enhanced using modern computer simulation tools. For each experiment, the students measure the neutron flux at various positions using BF3 detectors or indium foils. They compare these measurements to simulations using MCNP and a simple Monte Carlo simulation written in the programming language of their choice. Typically, the students use MATLAB or C++. These results are compared to literature values corrected for differences in material characteristics. Because of the relative homogeneity of the GSR, the measurements and simulations agree well with published data.

The pile is initially configured as shown in Fig. VI–4, with no fuel and a single Pu-Be neutron source in the centre of the pile. The students measure neutron flux at various distances from the source using either cadmium-covered indium foils or a cadmium-covered BF3 proportional-mode detector. From the data, the students can calculate the slowing down length of the Pu-Be neutrons in graphite. Following this experiment, the source is removed from the centre of the pile and replaced with four sources at the far end, configured as shown in Fig. VI–5. The students can now measure thermal neutron flux (bare indium foils or BF3 detectors along the major axis of the pile to determine the extent of neutron diffusion in the pile. The GSR is then loaded with fuel (loading 1.3, Fig. VI–3) and keff is measured using critical buckling calculations. Each exercise is also accompanied by simulations in MCNP in order to train the

students on combining simulation and measurement techniques. This prepares the students for the final laboratories at the TRIGA research reactor.

FIG. VI–1. Penn State Graphite Subcritical Reactor (cadmium cover partially removed).

FIG. VI–2. Diagram of Graphite Subcritical Reactor showing structural components and source locations for laboratory experiments.

FIG. VI–3. Various possible fuel loading configurations.

FIG. VI–4. Graphite pile configured for neutron moderation measurements.

FIG. VI–5. Graphite pile configured for neutron diffusion and critical buckling (fuel loaded) measurements.

VI–3. RESEARCH UTILIZATION

The graphite assembly is used as a neutron source for a diverse field of researchers. The pile can be configured to provide a volumetric source of thermal neutrons. The spectrum can also be tuned by placing varying layers of graphite between the sources and the detectors. Using this technique, the spectrum can change from pure Pu-Be or ²⁵²Cf to a fully thermalized spectrum.

Four Pu-Be sources can be loaded into one end of the pile for diffusion measurements over the length of the pile. See Fig. VI–5 for one configuration. The shorter end of the pile (towards the right) presents a thermal neutron field for researchers if the cadmium cover is removed.

Measurements taken close to the pile will show variations in the field caused by the physical location of the sources. Since the flux is low at this point, the fixture can be used in this configuration for detector development and testing. Additionally, a diverse array of fields can be created using a combination of neutron sources, natural uranium fuel and neutron and gamma shielding and moderating material. The laboratory is equipped with activation foils and gamma spectroscopy equipment as well as BF3, boron-lined and ³He neutron detectors.

VI–4. CONCLUSION

The Penn State GSR is a valuable tool for education and research in the 21st century. The simplicity of the system is an advantage, not to be underestimated. Today the majority of work involves computer simulation of complex systems; from reactor design to nuclear safeguards to space research. The GSR represents a simple and easily simulated physical system with which to train students and benchmark simulations.

REFERENCES TO ANNEX VI

[VI–1] REMICK, F. J., A Graphite Moderated Subcritical Reactor – Sigma Pile, Master’s Thesis, The Pennsylvania State Univ., PA (1958).

[VI–2] HEIDRICH, B.J., Experiments in reactor physics – Laboratory manual, Course:

Nuclear Engineering 451, The Pennsylvania State Univ., PA (2013)

ANNEX VII. THE SUBCRITICAL ASSEMBLY OF POLYTECHNIQUE MONTRÉAL