**3. Capabilities**

The nuclear instrument systems, convert the neutron flux at the detectors adjacent to the core into instrument readings that the operator interprets to control the core. Each part of the loop can be tested. The neutron flux at neutron detectors

is often assumed by designers to be essentially constant at a given power level, whereas from very low to moderate levels of neutron flux, such as found during very low power and shutdown operation, the neutron flux can vary considerably in amplitude in a random manner. The random manner results from the characteristic random nature of the decay of neutron sources that supply the reactor with neutrons at low power and shutdown.

One consequence of the variation in neutron flux is that it appears as an unwanted variation in the display of a channel and might result in inadvertent period trips. Nuclear instrument channel 'noise' is generally considered an unwanted (and often misunderstood) variation in a signal. It can be electronic noise that is externally introduced to the circuit and must be minimized so it does not distort the true readings or the ability of the reactor operator to identify the average signal. It can also be due to the normal random decay of a neutron source, where it is a valid part of the signal. It can be very difficult to visually identify if the noise is due to the valid operation of the core, or if it is due to electrical interference.

A statistical test called the 'Chi-Squared' test can be applied to data from pulsetype channels such as the startup channel. A Chi-Squared test is often used at power reactors to verify the startup channels and any temporary startup-range neutron detectors used for loading fuel are displaying counts from neutrons rather than noise. The Chi-Squared test will identify if the noise is electronic interference or valid and due to a neutron source, although it will not identify the source of the electric noise.

The AGN-201 offers the opportunity for training and evaluating the nuclear instrument channels with a very low neutron signal, lower than typically encountered at commercial power reactors. The neutron flux at a detector must be low enough that the channels will display changes in signal (jumps) from individual neutron interactions, and the channels must support attaching a scaler-timer. The test is useful when the AGN-201 is shut down and a neutron source is supplying neutrons. If neutron flux is low enough, as it is when the neutron source is inserted, even the channel 2 and 3 ion chambers might be evaluated with the Chi-Squared test. In both cases, a scaler-timer is required to total the counts in a given time interval.

The AGN-201 offers a unique opportunity to explore the variations the current signal of a current neutron instrument channel without the time pressure and limitations on connecting test instruments at a power plant. A properly designed test can demonstrate that current signals from a neutron detector consist of a number of pulses of very small electrical charges, each resulting from the individual disassociation of B10 upon absorbing a neutron.

Teaching-reactors such as the AGN-201 provide the opportunity to measure a wide range of characteristics, and to gain experience and practice in conducting the same measurements that are performed at power reactors during low power physics testing following the loading of the first core, following refueling, and even during power operation to characterize the stability of the reactor. The tests generally involve changing a parameter such as reactor temperature or control rod position, and observing the corresponding change in rate of change in reactor neutron flux. Commercial reactors use a so-called 'Reactivity Computer' to infer the change in reactivity from a change in a parameter. The AGN-201 allows students to build, operate and evaluate the operation of analog and digital reactivity computers themselves [5, 6].

The AGN-201 could be used to evaluate and improve test procedures that would be used on future first-of-a-kind reactors, and to train future reactor engineers and other operating staff. In addition to gaining experience and practice in conducting the measurements, students can develop the skills required to write test procedures

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

and to conduct high-quality test programs in a low-risk environment. The AGN-201 can also potentially offer realistic simulations of conditions at other reactors so newly written test procedures can be conducted and improved prior to being used at the reactor. Test engineers, reactor operators and others, including regulators, from other facilities can benefit from the training available at the AGN-201. The AGN-201 can be useful in observing the principles and some of the parameters being tested at other reactors, thereby allowing the test procedures to be validated and problems discovered.

The AGN-201 operates at very low power levels (microwatt range), often termed 'zero-power' where its operation closely resembles most other reactors when they are operated at low power levels, below the point of adding sensible nuclear heat. Even at power reactors, many of the core physics measurements that are made following refueling or core alterations occur with the core subcritical or with the reactor just critical on delayed neutrons at low power level. They include monitoring the core during shutdown operation, while core alterations during refueling are being made, during the approach to criticality, and reactor state point measurements and core physics parameter measurements in a suite of 'low power physics tests.' Some measurements are made at both low power and at-power, and only a few are restricted to high power operation. The AGN-201 is therefore capable of providing conditions for most of the core physics measurements found at power reactors [7].

The explanations and demonstrations of the theory and measurement techniques of subcritical core physics can be of interest to reactor physicists, instrumentation and control technicians and engineers, operators and managers of nuclear facilities, health physicists and criticality safety personnel. The phenomena of subcritical multiplication of source neutrons requires a 'multiplying medium,' neutron source and neutron detector. The common technique that is used at reactors is an 'inverse multiplication ratio' or '1/M' plot. The increase in count rate as control rods are moved in steps, and corresponding decrease in '1/M' plot are readily apparent. The plot is typically used to infer the point of criticality, in this case the position of the control rods. The reactivity of the AGN-201's control rods have been characterized well enough to illustrate the increases in count rate as positive reactivity is added. The demonstration can be relevant for power reactors to illustrate monitoring techniques during core alterations such as fuel loading and about establishing boron dilution warning setpoints. At pressurized water reactors with a soluble boron shim, the source range channels include the ability to establish a setpoint whose warning will help operators stop a dilution that could lead to an inadvertent reactivity change. The count rate at typical alarm setpoints can be low enough that the random variations in neutron production by the source becomes apparent. The resulting variation in source range channel readings, coupled with the requirement for a response time, can make it difficult to establish a setpoint that provides for enough warning but does not have false alarms.

Subcritical measurements to measure the values of parameters that formerly were measured during low power physics testing can save utilities considerable time and money. One is measuring the reactivity worth of control rods by raising and then dropping control rods, which can also be demonstrated in the AGN-201. Control rod drop times are also measured following refueling and other core alterations. The techniques and difficulties in measuring the positions of the controls during the drop, and the response of the nuclear instrumentation can be demonstrated in the AGN-201.

The state-point measurements of a reactor are measurements of parameters whose values define the operating condition, or 'state' of the reactor. Examples of parameters include reactor temperature and control rod positions are made to evaluate the reactivity of the reactor, and for comparison with core physics

code predictions. Accurate state-point measurements are crucial in assessing the operation of the core and are made when the reactor is first brought critical after a refueling outage, and periodically throughout core life. The technique is simple and involves adjusting parameters such as control rod position, temperature and boron concentration in reactors with soluble neutron poison so the reactor is just critical at a given power level. A careful measurement, where reactor power is essentially constant, provides the best data. The AGN-201 allows operators and reactor engineers to explore their ability to establish just critical conditions, and to compare the measurements of parameters with calculations.

Low power physics measurements are conducted with a critical reactor whose power level is below the point of adding observable sensible nuclear heat, also known as 'reactors without feedback.' The measurements include the state-point measurement mentioned earlier, control rod reactivity worth, moderator temperature measurements, core stability measurements using a 'core oscillator' with variable, regular changes in reactivity, delayed neutron lifetime, irradiation of metallic foils to determine reactor power and more.

The operation of the AGN-201 is licensed and regulated by the Nuclear Regulatory Commission. The reactor and its conduct of operations are periodically inspected, particularly its documentation, and orderly documentation requires timely, accurate, truthful completion of forms, operating logs and more. Operating a nuclear reactor requires developing the valuable skills of discipline, focus and attention to detail, communication and more. The AGN-201 requires the same attitudes and abilities as higher-power test reactors. The full force of regulations is applied to the AGN-201. The opportunity to operate a nuclear reactor, regardless of size, is a unique experience that can benefit people who choose to put forth the time and effort. Students have opportunities to participate in a disciplined, regulated environment that is required of operators of a nuclear reactor that can shape their outlook on life and work ethic at a pivotal point in their lives. Students and other potential operators are invited to study, pass exams, and be responsible for the operation of a nuclear reactor providing a valuable and unique experience for those considering entering the field of nuclear power.
