**2. Reactor description**

The AGN-201 nuclear reactor is a solid-core reactor with no active cooling system. The core is constructed of nine 25.6-cm diameter fuel disks (see **Figure 1**). Four of the disks are 4-cm thick, three of the disks are 2-cm thick and two of the

### *Idaho State University AGN-201 Low Power Teaching Reactor: An Overlooked Gem DOI: http://dx.doi.org/10.5772/intechopen.105799*

disks are 1-cm thick. Each 4-cm thick fuel disk contains 96 g of 235U, each 2-cm thick fuel disk contains 58 g of 235U, and each 1-ck thick fuel disk contains 29 g of 235U. The overall core height is 24 cm. A graphite reflector surrounds the core both radially and axially. The graphite reflector is 20-cm thick and has a density of 1.75 g/ cm3 . The reactor fuel consists of slightly less than 20 wt. % enriched uranium. The uranium is in the form of 15-micron diameter particles of UO2. The UO2 particles are pressed with 100-micron diameter polyethylene particles. The density of 235U in the UO2-polyethelyene fuel is 61 mg/cm3 and the overall uranium density in the fuel is 305 mg/cm3 . The mass ratio of uranium to polyethylene is 1:3.16. The approximate critical mass of the AGN-201 reactor is 665 g 235U [1].

The reactor uses a total of four control rods; two safety rods, one adjustable coarse rod, and one adjustable fine rod. The control rods are made from the same UO2 polyethyelene fuel as the core. To ensure safety, the fueled control rods enter the core from the bottom (see **Figure 1**) so that gravity, along with compressed springs, ensure rapid removal upon reactor SCRAM. The bottom four fuel disks as well as the lower reflector have holes drilled through them to accommodate the control rods.

In addition to the control rods, the AGN-201 reactor is equipped with a thermal fuse as an ultimate reactor safety shutdown device. The thermal fuse is located just below the core center line (see **Figure 1**). The fuse is similar in construction to the

**Figure 1.**

*AGN-201 reactor core, reflector, and control rods [1].*

reactor fuel with two key differences. First, rather than polyethylene, the fuse uses polystyrene. Second, the density of uranium in the fuse material is double the value used in the fuel. The two differences coupled with the location of the fuse results in maximizing the fission rate in the fuse compared to all other locations in the core. In the event of a runaway power transient, heat will be generated in the thermal fuse at a greater rate than any other location in the core. As the fuse temperature rises, it will tend to soften when it reaches 100°C and will melt before any the reactor fuel reaches its melting point of 200°C. The AGN-201 reactor design has the lower portion of the core and reflector held in place by the thermal fuse. In the event of the thermal fuse melting, the lower portion of the core and reflector will move downward approximately 5 cm compared to the upper portion of the core which is stationary since it is supported separate from the thermal fuse. The net result will be a dramatic increase in neutron leakage which will terminate the transient. It must be noted that the thermal fuse is a single use safety device.

The reactor core and a portion of the reflector are contained within a gas tight core tank. The core tank is then located within the remainder of the graphite reflector (see **Figure 2**). Surrounding the graphite reflector is a 10-cm thick lead shield. The lead shield is primarily used for gamma ray shielding. The reactor core, reflector, and lead shielding are located within the reactor tank. A graphite thermal column is located on the top lead shielding to support experiments and measurements involving thermalized neutrons. The reactor tank is then located within a 200-cm diameter water filled tank. The radial thickness of the water is approximately 55 cm. The water filled tank is used to absorb neutrons that escape from the core.

To provide access for experiments, a 2.54-cm diameter hole traverses the reactor tank, lead shielding, graphite reflector, and reactor fuel. The hole through the center of the reactor core is commonly referred to as the "glory hole". The glory hole aluminum pipe ensures the core, reflector, lead shield, and water remain properly

**Figure 2.** *Reactor tank assembly [1].*

#### *Idaho State University AGN-201 Low Power Teaching Reactor: An Overlooked Gem DOI: http://dx.doi.org/10.5772/intechopen.105799*

sealed. When not in use, the glory hole is typically open to the air atmosphere. When starting the reactor, a neutron source is placed in the glory hole and when the reactor is shut down and not in use, a cadmium neutron absorber is placed in the glory hole to ensure reactor startup cannot occur. In addition to the glory hole, there are four access ports located in the graphite reflector (see **Figure 2**). The access ports are 10.16 cm in diameter and penetrate through the reactor tank, lead shielding and graphite reflector. When not in use, the access ports are typically filled with graphite, lead, and wood (to simulate water).

The reactor radial thermal flux profile is provided in **Figure 3**. The flux profile shows a general Bessel function trend in the core region followed by an exponential drop in the reflector, lead, and water regions. It should be noted that, unlike water reflected thermal reactors, the AGN-201 reactor does not experience an increase in the thermal neutron flux as neutrons enter the reflector. This is primarily due to the difference in neutron scattering properties of water compared to graphite. The flux profile plot demonstrates the effectiveness of the neutron shielding associated with the water shielding tank. The thermal neutron flux at the outer edge of the shielding tank is four orders of magnitude lower than at the center of the reactor.

Reactivity control is carried out using four control rods. Two safety rods, each with a reactivity worth of 1.25% Δk/k (\$1.68), are operated in a binary fashion. When starting the reactor, the safety rods are driven fully into the core. No intermediate stopping locations are used for the safety rods. When the reactor is SCRAMed, the safety rods are completely removed form the core. The removal mechanism relies on both gravity as well as compressed springs. A single coarse control rod with

**Figure 3.** *Reactor radial flux profile [1].*

a reactivity worth identical to the safety rods (1.25% Δk/k (\$1.68)) is raised into the core region during reactor startup. Typically, the coarse control rod is driven to its maximum insertion location, although there are scenarios where the coarse control rod is stopped short of the maximum insertion location. Similar to the safety rods, upon reactor SCRAM, the adjustable coarse control rod is rapidly ejected from the core by relying upon gravity and compressed springs. Finally, the fine control rod has a reactivity worth of 0.31% Δk/k (\$0.42). The fine control rod is typically driven into the reactor until criticality occurs. Adjustments in the coarse control rod and the fine control rod can then be made to adjust the desired reactor power. Unlike the safety rods and the coarse control rod, the fine control rod is not rapidly ejected from the core when the reactor is SCRAMed. Rather, the fine control rod is driven out of the core at the same rate that it can be driven into the core.

Three monitoring channels are used in the ISU AGN-201 reactor. The three monitoring channel detectors are located within the water filled reactor tank as shown in **Figure 4**. The AGN-201's nuclear instrumentation consists of three different nuclear instrument channels and offer students the opportunity to understand the functions performed by separate portions of the circuit as the incoming signal is processed. Students can study the nuclear instrument channels in a laboratory and then observe them at the reactor.

The three channels are comprised of commercial-grade components. They are more accessible than power plant channels to students and others who wish to study them and their operation over a wide range of neutron flux at an actual reactor. Students can study the instrument systems and their theory and design, and then observe the systems in operation at a wide range of neutron flux. Analog and digital designs of nuclear instrument systems, with a variety of neutron detectors, can be evaluated by using the AGN-201. The AGN-201's nuclear instrumentation consist of the three commonly-found types of nuclear instrument channels that follow the same operating approaches and perform the same functions as the nuclear instrument channels typically found in most reactors. Each channel has a unique but complementary principle of operation. Together, they provide the reactor operators and others with indications of reactor power and the rate of change in power over the entire operating range. Of course they also supply signals for reactor trips.

**Figure 4.** *Reactor assembly plan view [1].*

Channel 1 is the startup, source range, channel and uses a BF3 filled proportional counter. The source range channel illustrates a standard approach that allows the source range channel to display a very low neutron flux in the presence of significant gamma radiation. A proportional-type BF3 neutron detector produces pulses when gamma radiation and neutrons interact with the BF3 that fills the detector. The pulses are amplified and shaped, the lower-amplitude pulses due to gamma interactions within the detector are rejected while the remaining higher-amplitude pulses from neutron interactions are further amplified and displayed. The channel displays count rates from the reactor without a source to well above critical. Channel 1 is designed to initiate a SCRAM signal for low power situations when the count rate falls below the setpoint.

Channel 2 is used to monitor the reactor power using a log scale as well as for indication of the reactor period. The channel 2 detector is a BF3 filled ionization chamber. Channel 2 generates a SCRAM signal when the reactor power falls below 3 x 10−13 W or when the reactor power exceeds 5 W. Additionally channel 2 generates a scram signal if the reactor period is less than 5 seconds. The wide range logarithmic neutron instrument channel (channel 2) illustrates a standard approach that allows the channel to detect and display a current signal that is proportional to power over 7 decades. Channels 1 and 2 rely on different applications of wide-range logarithmic amplifiers. The source range nuclear instrument channel's wide-range logarithmic amplifier converts the frequency of incoming pulses from neutron interactions to voltage. The wide range logarithmic current channel's amplifier converts a direct current to a voltage. In both cases, variations in count rate or current level that are due to the normal and expected variations in neutron flux are often misinterpreted as 'noise' that can lead to the period meters having too much variation to be useful indicators to the reactor operators, and the period circuits spuriously tripping. Circuit designers frequently assume the neutron signal is relatively constant and do not anticipate the large noise component that is inherent due to sources. The AGN-201 provides the actual variations in neutron flux that drive oscillations in period meters and indications of reactor power and can be used to evaluate the effect of circuit modifications to reduce the amplitudes of the oscillations.

Channel 3 is used to monitor reactor power using a linear scale. The channel 3 detector is a BF3 filled ionization chamber. Channel 3 generates a SCRAM signal when the reactor power exceeds 5 W or whenever the linear rotating switch indicator is less than 5% or greater than 95% of full scale.

**Figure 5** shows the SCRAM circuit arrangement for the three monitoring channels. It is important to recognize that the SCRAM circuit arrangement is a single signal SCRAM [4]. If any one of the channels identifies a situation that triggers a SCRAM, the reactor will be SCRAMed. That is, the AGN-201 SCRAM circuit is not a two-out-of-three arrangement.

In addition to the monitoring channels, a series of additional interlock circuits are used to prevent reactor startup or to SCRAM the reactor in the event of undesired situations (see **Figure 6**) [4]. The reactor shielding tank water temperature is monitored to ensure that the maximum allowed excess reactivity is not exceeded. If the reactor water temperature falls below 15°C the reactor excess reactivity is unacceptably large and reactor operation is prevented or discontinued. The reactor shielding tank water level is monitored to ensure sufficient shielding is present. Finally, a seismically activated switch is used to prevent reactor operation or discontinue reactor operation in the case of a seismic event. Similar to the reactor monitoring channels, the interlocks follow a series approach so that if any one of the interlocks is triggered, the reactor will not be allowed to operate.

The reactor is operated from a relatively simple console located in the same room as the reactor. The original console was used for approximately fifty years. In

**Figure 5.** *SCRAM circuit arrangement [1].*

**Figure 6.** *Interlock circuit [1].*

2020, the original console was replaced with an upgraded console (see **Figure 7**). The primary motivation for upgrading the console centered on the use of vacuum tubes for the SCRAM circuits in the old console. Obtaining replacement vacuum tubes became very difficult since these items are no longer manufactured in large quantities. The upgraded console uses solid state relays rather than vacuum tubes. In addition to the use of solid-state relays, the upgraded console has all new wiring, instrumentation, switches, and knobs.

While the AGN-201's core will essentially never be exhausted, support systems such as the instrument systems and their neutron detectors, reactor controls and control rod drives require periodic upgrading. The current financial state of universities and the perceived difficulty in conforming to regulatory requirements tends to encourage using the original 60-year-old tube-based control systems and other equipment until their failure rates leave no choice but to modernize. The cost of the engineering and manufacturing of upgraded instrumentation and equipment by outside firms can be too great for universities. Idaho State University recruited community volunteers with experience in project management and expertise in the design, construction, operation and startup of instrumentation and control, licensing of reactors and other relevant subjects for the university's second attempt to replace the original tube-based control system. The first attempt involved the design and construction of a complex, multiple-level printed circuit board that could not easily be modified. The second and successful attempt used a breadboard approach of circuit boards with holes that could be used to mount components. The *Idaho State University AGN-201 Low Power Teaching Reactor: An Overlooked Gem DOI: http://dx.doi.org/10.5772/intechopen.105799*

**Figure 7.** *Upgraded console installed in 2020 [1].*

second attempt had very few changes to the design, a likely result of the lifetimes of experience of the community members in designing, repairing and maintaining analog systems. One of the main considerations was if the replacement system was to be analog or digital. The advantages and disadvantages of replacing the existing analog control system with a functionally equivalent analog system or attempting to replace it with a digital system were weighted. A replacement analog functional replacement appeared to be simpler and easier from a regulatory standpoint.

From a lifecycle cost standpoint, the analog system's lifetime was envisioned to be decades, whereas digital technology is rapidly advancing, and the lifetime of a digital system was envisioned to be a few years. Analog enjoys far superior cyber security than digital, and maintaining cyber security appeared to be an unnecessarily potential burden to the university. It was decided to replace the system under a 10 CFR 50.59 evaluation. The community expert in licensing helped write the 10 CFR 50.59 document and helped ensure applicable codes and standards had been followed. The community member also purchased and donated some of the components. Another community member and two graduate students worked with the community members to document the project in their thesis. One of the community members became the Project Manager and kept the project moving even during the height of the COVID-19 pandemic shutdown. He also reviewed the design and construction and assisted with troubleshooting. The collaboration of community and university personnel worked well to produce and complete the replacement instrumentation and had the time to transfer knowledge. It is anticipated that the same model will be applied to other modernization efforts going forward.
