Methodology for Management and Physicochemical Control of the Cooling Water of the IPR-R1 Triga Nuclear Research Reactor

The IPR-R1 Triga research reactor, located at CDTN (Nuclear Technology Development Center) in Brazil, is one of the oldest reactors in operation worldwide. The Brazilian Nuclear Energy Commission (Cnen) has granted a Permit for Permanent Operation license for the CDTN Triga reactor. This authorization includes some conditions to be settled within a period of up to two years, that is, until the end of 2019, which has not yet been carried out and is currently being prepared by CDTN. For this purpose, Project 0006.23 – Maintenance of Licensing of the IPR-R1 Triga Reactor was created in CDTN's Multiannual Program. In this project is included the Subproject: 6.23.3 – Preparation of the system to manage the aging of the IPR-R1 Triga reactor. Inserted in this context, the main objective of this paper is the proposal of good practices to be adopted in the future by CDTN, related to the Projects of Licensing and Aging of Triga reactor, for the management and physicochemical control of its cooling water, considering its electrical conductivity and pH, following IAEA recommendations. So, the present work is relevant, as it addresses a proposal for monitoring and controlling pH and electrical conductivity of the cooling water of Triga reactor. As a result of this proposal, the useful life of Triga reactor may be extended and there will be greater reliability and safety in its operation.


INTRODUCTION
Triga IPR-R1 research reactor, located at CDTN (Nuclear Technology Development Center) in Brazil, is one of the oldest reactors in operation worldwide.It is in operation for more than 60 years.
At the start of operations of the IPR-R1 reactor in 1960, its maximum output thermal power was 30 kW.In 1970, fuel elements were added to the core, increasing the power to 100 kW, which is the current maximum operating power.In 2002 modifications were carried out in the core and new fuel elements were added, allowing the power to reach levels of 250 kW, according to experiments carried out by Mesquita et al. (2002) [1].
The Brazilian Nuclear Energy Commission (Cnen) has granted a Permit for Permanent Operation license for the CDTN IPR-R1 Triga research reactor.This authorization includes some conditions to be settled within a period of up to two years, that is, until the end of 2019, which has not yet been carried out and is currently being prepared by CDTN.For this purpose, Project 0006.23 -Maintenance of Licensing of the IPR-R1 Triga Reactor was created in CDTN's Multiannual Program.In this project there is the Subproject: 6.23.3 -Preparation of the system to manage the aging of the IPR-R1 Triga reactor of which this work is part In the aforementioned subproject, a mandatory experiment was included in every research reactor, that is, the performance of tests to verify possible fission product leaks in the reactor's fuel elements [2].Due to the Covid pandemic and the home-work activities, this experiment has not yet been carried out, but it is intended to be performed as soon as the Triga reactor returns to operation.
All conditioning actions required by the regulatory agency are based on recommendations and standards from the International Atomic Energy Agency (IAEA) and the Brazilian Nuclear Energy Commission (Cnen).
The approach to the study of aging in nuclear reactors, in addition to paying attention to the economic factors directly involved with the extension of their operational life, also provides important data on safety issues.The most recent case involving the process of extending the life of a PWR reactor was at Angra 1 nuclear power plant in the last 20 years.For the IPR-R1 reactor, it would be very important to know if it is necessary to carry out any corrective measures for the extension of its life, after the recent granting of the Permit for Permanent Operation.
In the specific case of Triga reactor, most of its fuel elements are in the core where corrosion can occur, which threatens the integrity of fuel coatings.Its coolant must be treated and controlled to maintain its low electrical conductivity and pH close to neutrality, in order to minimize corrosion of the core components, particularly fuel elements.At some point in the future, like other reactors, IPR-R1 will be permanently shut down.
So, the main objectives of this paper are: • Proposal of good practices to be adopted in the future by CDTN, related to the Projects of Licensing and Aging of Triga research reactor, for the management and physicochemical control of its cooling water (considering its electrical conductivity and pH), following IAEA recommendations [3,4].
• Perform, in the future, during the activities related to the Projects of Licensing and Aging of Triga research reactor, gamma spectrometry of the water from the IPR-R1 reactor well, with samples of the coolant with the reactor off and with it in operation, during the experiments to check for possible leaks of fission products in the reactor fuel elements (sipping test) [2].

MATERIALS AND METHODS
Nuclear reactors have some process variables whose operational limits cannot be exceeded.The parameters defined as Operational Limits and Conditions (OLC) must be monitored and their values recorded and archived for later consultation.They must also cause the automatic shutdown of the reactor, if one of the limits is exceeded [5][6][7].These limits are necessary to maintain the integrity of the main physical barrier to protect against the uncontrolled release of radioactive material during the operation of the facility.Continuously monitoring of process variables evolution and maintaining files is important for immediate or subsequent security analyses, showing both short-term and long-term trends.The organization that operates the facility must have the records on file, to allow verification by the regulatory agencies of the non-violation of security limits.
The control of nuclear reactors must have appropriate instrumentation to measure, monitor, record and exercise control over the parameters involved in their operation.IAEA (2008) [5] recommends several parameters that can be used as operational limits for the initial licensing, which depend on the characteristics of each research reactor.IAEA (2008) [5] also recommends periodic assessment of such parameters over installation lifetime in addition to monitoring of technological progress.Establishing which parameters will be used to establish operational limits is the most important item in the Safety Analysis Reports of nuclear reactors.Practical procedures will be elaborated to be adopted by CDTN in the future for the management and physicochemical control of the cooling water (electrical conductivity, pH and temperature) from the IPR-R1 Triga reactor, according to IAEA (2011) [4] recommendations.In the experimental part, the analytical instruments shown in Figure 1 [8] will be installed in the primary circuit of the IPR-R1 reactor.They are: • Two digital meters/controllers with probes for monitoring electrical conductivity and water temperature (George Fischer-Signet, model 8850-2 GF conductivity indicator/transmitter) [9].Each equipment has two cables (conductivity and temperature), which connect the probes to the conductivity meters (indicator and analog output).
• A pH controller meter (with sensor), brand Dosatronic PH 1000 TOP [10].This is a highprecision, fast-response microprocessor instrument for automatic pH analysis and control over its entire range from 0 to 14.
The output signals from pH, electrical conductivity and temperature sensor monitors will be connected to the Data Acquisition System (DAS) developed in the research [11].All parameters will be displayed on a video monitor.The characteristics of DAS are as follows: • Data Acquisition System (DAS) with USB-6211 output, manufactured by National Instruments Co. (2007) [12].This is a multifunctional device, offering analog inputs, digital inputs, digital outputs and two 32-bit counters.
• LabVIEW ® supervisory program (academic license) developed by National Instruments Co.
(2007) [12].This is engineering software created specifically for applications that require test, measurement and control, with fast access to hardware and information obtained from the acquisition system data.Source: [8] Additionally, as schematized in Figure 2 [13], it will be performed a spectrometry based on a model 5019 coaxial HPGe (hyper-pure germanium) detector, with 50% nominal efficiency, model DSA-2000, coupled to a Canberra gamma spectrometer equipped with 8.0 channels.The system will be connected to a computer with a multichannel spectra acquisition board with the Genie 2K program, from the Nuclear Spectrometry Laboratory -LEN, from the Analysis and Environment Service -Seama/CDTN.The assembly will detect and identify fission products that may be released.The presence of isotopes Cs-137, La-140 and I-131 being verified, indicates that some of the sampled fuel elements present leaks, that is, they have a compromised coating, since these elements are known indicators of leaks in nuclear reactors.

EXPECTED RESULTS AND DISCUSSIONS
IAEA (2011) [4] publication recommends periodic verification of the integrity of fuel elements.
With the modifications introduced in IPR-R1 Triga reactor over the years of operation, especially the inclusion in its core of fuel elements coated with stainless steel, for operation at the power level of 250 kW, the probability of occurrence of corrosion due to electric currents has been increased, by the formation of galvanic cells between the different materials in contact.
Monitoring the physical and chemical quality of the coolant in light water nuclear reactors, both in power and research ones, is one of the most important tasks in their safe operation.The responsible for the facilities, in order to minimize personnel exposure to radiation, need to continuously monitor fuel performance, which is responsible for releasing radioactivity, through the gaseous and liquid effluents from the plant's cooling system.
Coolant quality tests commonly applied in water-cooled reactors assess temperature and pressure.However, new studies justify the analysis of other physicochemical factors, such as pH Legend: 1. High purity germanium detector with lead shield ; 2. Dewar vial with cryostat and preamp ; 3. Computer with analog-todigital converter and program to manage a multi-channel analyzer ; 4. Polarization source for the detector ; 5. Electrical Signal Amplifier; 6. Oscilloscope.
and electrical conductivity as a function of temperature, in order to minimize undesirable effects, like erosion and deposition of corrosive products on heat transfer surfaces.The recommendation of IAEA (2011) standards [4] is that the pH of the cooling water of research reactors is between 5.5 and 6.5.In the IPR-R1 values of 5.2 and 7.0 have already been reached, as discussed over the next paragraph.
Among the expected results, depending on the experiments to be carried out described in methodology section, are the values of electrical conductivity and pH for the water from Triga reactor.To exemplify the expected results of this proposal, it will be comment on the next paragraph the values of electrical conductivity and pH from previous similar studies about CDTN Triga.The recommendation of the IAEA standards (2011) [4] is that the pH of the cooling water of research reactors is between 5.5 and 6.5.In the IPR-R1 values of 5.2 and 7.0, measured monthly between Jun/2016 May/2017, have already been reached and showed in Figure 3 [14].Regarding electrical conductivity, the IAEA publication (2011) [4] recommends that it be below 1.0 µS.cm-1.
In Triga IPR-R1 this value was never reached, the lowest value reached was 1.3 µS.cm-1, measured monthly between Jun/2016 May/2017 as can be seen in Figure 4 [14].All these samples for the IPR-R1 were analyzed by the Analytical Chemistry Laboratory of the Analysis and Environment Service (Seama) of CDTN [14].
The pH is one of the most important parameters in water quality control in water-cooled reactors.Because it influences three of the four objectives of chemical control of water, which are: the control of the radiation field, the integrity of the fuel and the integrity of the reactor materials.
pH is an indicator that gives the total concentration of ionic impurities.The increase in the level of ionic impurities adversely influences the following aspects: the corrosion of materials in the reactor cooling system, the increase in the radiation field and fuel performance.Therefore, pH needs to be close to neutrality and the electrical conductivity in water should be kept as low as possible.
Emphasizing that, nowadays, there is no system that monitors these parameters in real time in IPR-R1 reactor.Source: [14] Due to the natural aging of the installation, its fuel elements show signs of wear over the years, such as stains and corrosion pitting.The presence of coated steel fuel elements introduces another factor in the matter of preserving the integrity of fuel elements, which is the contact of components coated with different metals.It is known that the contact of different metals in an electrolyte medium, in this case, Triga's cooling water, can lead to the formation of galvanism.These galvanisms can contribute to the deterioration processes in metallic structures (corrosion) [15] of the nuclear reactor.In this way, the present proposal related to the chemical control of the cooling water of IPR-R1 Triga can help to avoid such inconvenient.

CONCLUSIONS
The water used in IPR-R1 Triga research reactor acts as coolant, secondary neutron moderator and biological shielding.The entire tank, as well as the core and the main reactor components, are in direct contact with the coolant.Therefore, it is extremely important to keep the physical and chemical water characteristics within the recommended standards, enabling a safe and efficient operation of the reactor.
In normal operations of research reactors, it is necessary to continuously evaluate coolant quality in real time [4].IPR-R1 Triga has been operating for 60 years, and as the natural aging of the reactor progresses, the control and monitoring of physicochemical parameters of the coolant becomes more necessary to keep operations at the highest possible safety levels, maintaining the integrity of reactor components and minimizing corrosion processes.
So, the present work is relevant, as it addresses a proposal for monitoring and controlling such parameters, pH and electrical conductivity of cooling water, in all operations of the IPR-R1 Triga reactor.The measurements of these variables will be saved in a Data Acquisition System (DAS), allowing future consultation and analysis by operators and authorities.As a result of this proposal and the adoption of procedures for new management of water quality of Triga research reactor, its useful life may be extended and there will be greater reliability and safety in its operation.