**1. Introduction**

University research and teaching reactors are fundamentally intended to help prepare nuclear engineering and other students for entry into the nuclear workforce. They introduce students to the disciplined, structured environment of operating a reactor licensed by the Nuclear Regulatory Commission (NRC). They also offer students hands-on experience, provide opportunities to demonstrate the operation of reactors and a variety of the traditional applications of reactors such as neutron activation, and introduce them to the application of nuclear instrumentation, applied principles of health physics, and more. They can be useful to a wide range of people beyond university nuclear engineering students, including those from National Laboratories, utilities, regulators, and others.

The Idaho State University Aerojet General Nucleonics (AGN) model 201 nuclear reactor is an example of a very safe, low-power, solid-core reactor designed with students and teaching in mind. It was developed in the late 1950's by AGN to satisfy the need of university nuclear engineering departments for a relatively inexpensive, safe, flexible and available reactor with a long design life. The AGN-201's safety results from, *inter alia*, its 'thermal fuse' that terminates excessive sustained operation at high power, a large negative temperature coefficient (−0.035%Δk/k °C−1), and low available excess reactivity (nominally 0.18% Δk/k (\$0.24) at 20°C) [1]. These safety features, and other design features, make it an ideal teaching reactor in an environment with rapid turnover of student operators and other personnel.

The small teaching reactors generally preceded the higher-power reactors at universities. As the university's interest moved to the higher-power reactors, reactors like the AGN-201 s fell into disuse and most were decommissioned. Recently however, there has been a renewed interest in the utility of AGN-201 nuclear reactors [2]. In addition to discussing the potential uses for the AGN-201, this chapter includes discussions of the challenges of replacing obsolete components to facilitate continued operation. To address this issue, Idaho State University teamed with members of the community whose expertise in project management, instrumentation and control, licensing and other subjects complemented the university's expertise and resources.

A nuclear reactor is an example of the integrated operation of many systems to support the operation of a nuclear core. Simple reactors can be excellent examples of the integrated operation of the core, nuclear instrument systems, the reactor operator or 'human in the loop' and control rods and their controls. Although the AGN-201 is a simple reactor, it can be used to measure the operation of the core and understand and gain insight into its operation. It is intended to support teaching, training and research in a wide variety of subjects. For example, human/machine interface studies could even be conducted with the operator to test novel display concepts.

The AGN-201 has a variety of attractive design features. The reactor has direct access to the core via the so-called 'glory hole' that runs horizontally through the reactor center. It also has a graphite thermal neutron column at the top of the reactor and beam ports in the radial portion of the graphite reflector. The core is enriched to a nominal 19.5% and given the reactor's low power, the core should essentially never require replacement [1]. The reactor has extremely low background neutron and gamma flux levels that along with the reactor's unusually sensitive nuclear instrument systems, facilitate a wide range of measurements including some that might not be possible in other reactors. For example, it is possible to observe individual chains of fissions when the core is just barely subcritical and flux has been allowed to decay to very low levels thus allowing measurement of the prompt neutron decay constant using Rossi's-α method [3]. Neutron flux near the allowed maximum power level is high enough to usefully activate foils and illustrate reactor physics principles but too low to result in the accumulation of large amounts of fission products.

The AGN-201 provides and supports a number of potential opportunities for demonstrations and tests that complement the theory from the classroom, research and problem solving. A wide range of demonstrations and tests can introduce students to the instrumentation and activities that are conducted by reactor engineers and reactor operators at higher-powered test and research reactors and commercial power reactors. This knowledge can help an instrumentation and control designer or engineer to produce circuits that are more forgiving of noise and to help a technician to differentiate between electronic noise and normal operation of the detector channel and be more successful in reducing noise.
