Technical Meeting on Research Reactor Ageing Management, Refurbishment and Modernization International Atomic Energy Agency

National Organization of Test, Research, and Training Reactors Brewster, Massachusetts United States of America

5–9 October 2015

“Modernization, Refurbishment and Power Upgrade of Argentine Research Reactor RA-6”

Fabricio Brollo

CNEA - National Atomic Energy Commission - Argentina

Abstract

The RA-6 reactor is owned and operated by the Argentinean National Atomic Energy Commission (CNEA). It is a MTR open pool type reactor, cooled and moderated by light water. Commissioned in 1982, it was designed as a teaching and research reactor to support the Nuclear Engineering career at the Balseiro Institute. In the frame of the Global Threat Reduction Initiative a cooperation agreement was signed between CNEA and US Department of Energy (DOE) for the conversion of the RA-6 reactor from high to low enrichment. That initiative provided an opportunity to increase the reactor’s power in order to optimize its applications, increasing the neutron flux in the core and its irradiation facilities. An upgrade project was implemented to modify or replace some structures, systems and components in order to improve safety and reliability, and to increase the maximum thermal power of the reactor from 0,5 MW up to 3 MW. The cost of the core conversion was covered by DOE and the budget for the upgrade project was provided by CNEA. The main replacements were: the primary and secondary centrifugal pumps, heat exchanger, cooling towers, flap valves and siphon breakers. The main modifications were: the decay tank and piping layout, I&C Systems and Electrical Systems. In addition, the reactor building was completely repaired. An outline of the project schedule and the demanded resources, including financial and human needs, was drafted. It was decided that all the necessary activities were to be done during one large outage in order to minimize the affectations to the reactor users. Some project activities had to be outsourced. The project schedule was modified several times due to consecutive delays in the implementation of different tasks. This paper details the various engineering considerations, experience gained during refurbishment, the various improvements and upgrades that were carried out, and finally some lessons learned.

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Project background The RA-6 reactor had been operated at 500 kW with high enrichment fuel elements (90% U235) since 1982. It had been used mainly for: teaching in Reactor Physics, Neutron Activation Analysis, Boron Neutron Capture Therapy, Neutron Radiography, Instrumentation Testing, etc.

In the middle of the past decade, the RA-6 core conversion project from high to low enrichment, brought about the opportunity to develop simultaneously a power upgrade project. Despite the enthusiasm, many aspects had to be previously analyzed in order to assess the risks, for instance:

-Operational and regulatory aspects -Economic and financial aspects -Safety and security aspects -Environmental and social impact

It is known that such important plant modifications would inevitably disrupt operation. On the other hand, they would produce obvious benefits in terms of staff motivation and allow seeking out new users and applications. In our case, there wasn't a risk of losing important clients due to interrupting the operations for an extended period. Nevertheless, the adoption of a risk management approach was considered in order to reduce the risk of an extended interruption of operation or even a permanent shutdown of our reactor. The main reasons to consider this power upgrade were: the need to cater for new experiments, to improve irradiation conditions, to diversify experimental capacities and to increase reactor availability. Furthermore, there is a single reactor dedicated to radioisotopes production in Argentina (RA-3). It is a 10 MW reactor which has been operated over 50 years. In that sense it was stated the possibility or convenience of using the upgraded RA-6 reactor as a back-up facility for Argentine radioisotope production, for short periods during the year. All these considerations entailed higher neutron fluxes. Therefore, it was necessary to define the maximum achievable power upgrade, taking into account the compatibility of needed

modifications with original reactor specifications and limiting the changes as much as possible.

The expertise acquired in our country over decades, in the design, construction, commissioning and licensing of nuclear research reactors, encouraged the National Atomic Energy Commission (CNEA) to face the challenge. From preliminary studies it was concluded that increasing the reactor power from 0,5 MW to 3 MW would provide the adequate neutron fluxes in the core and in the irradiation facilities, for the intended purposes. From a thermohydraulic point of view, to operate the reactor at this power level, a significant increase of the primary cooling system flow rate (from 150 m3/h to 340 m3/h) would be required. Additionally, other for the new operating conditions. This would be accomplished by refurbishment, replacement and addition of new components and systems to improve safety and increase reliability. As a result, a core conversion and power upgrade project named UBERA6 was initiated on 2005. Affected systems

The primary and secondary cooling systems had to be modified in order to dissipate 3 MW. This involved changes in the piping size and its routing, replacement of centrifugal pumps, heat exchanger, cooling towers, etc. Next figures show the cooling circuit modifications, comparing the previous systems vs. the new systems.

In order to maintain the coolant flow rate through the core in the event of a loss of normal power supply, an inertia flywheel was designed to be attached to the primary pump shaft. Three new cooling towers (1MW capacity each one) were installed at the top of an adjacent building, dedicated to all the auxiliaries services (air compressors, etc).

The common basin (made of concrete) for the new cooling towers had to be increased in order to hold up a greater water inventory in the secondary system shutdown mode.

The design and construction of new concrete foundation for process equipment and their junction with the reactor basement concrete.

Electrical systems refurbishment demanded plenty of economic resources too. New Uninterruptible Power Supplies (batteries) were purchased. A new power transformer was installed, dimensioned for respective higher electrical loads. Switchboards and motor control centres for the new centrifugal pumps and cooling towers were required.

In order to be consistent with modern safety standards and regulatory requirements, it was mandatory provide additional protective instrumentation systems to improve reactor safety. A simplified design to implement a core pressure drop monitoring system was developed. Changes in core components and supporting structures were not necessary, with a simple and reliable operating principle, easy to mount and inexpensive. With this new safety system, any sudden change exceeding 10% of nominal core DP value, will cause automatic shutdown of the reactor. The implementation of a Nitrogen-16 Linear Channel, for complementary power level monitoring and regulation, based in N16 activity measurement in the coolant of the primary circuit, was another safety improvement. Simultaneously, a new operating principle of the syphon breaker and redesign of the natural convection flap valve became imperative.

Both were designed with the single failure criterion (redundancy) for improving the reliability of this systems important to safety.

Higher coolant velocities at the entrance of the decay tank led to the design and installations of a flow distributor in order to reduce the turbulence forces over the internal structures of the tank. Finally, the reactor building was completely repaired. All the main repairs were focused on the confinement boundary leak tightness. Inside walls and confinement penetrations were sealed, etc. Nuclear Engineering Department of CNEA performed the main conceptual and preliminary engineering tasks, i.e: neutronic and thermohydraulic core design, mechanical stress analysis, hydraulic design and process equipment specification, deterministic and probabilistic safety assessment, instrumentation development, etc. Some detailed engineering tasks were completely outsourced (piping & supports, electrical system wiring diagrams and civil works contractors) The licensing process was started from the very beginning of the project, in order to ensure that nuclear and radiological safety related activities were an integral part of the development of the reactor modification.

A licensing group was aimed to coordinate the safety assessment and licensing activities to ensure that the established regulatory criteria were met, including the preparation of SAR Safety Analysis Report.

Required resources RA-6 conversion from HEU to LEU was financed by mean of an agreement signed between Argentina and the USA. In this sense, acquisition of consumables and graphite reflectors, the

fresh LEU supply, the manufacture of the new silicide fuel, conditioning, transport and exportation of spent HEU core, were financed by US-DoE. On the other hand the entire budget and human resources of the power upgrade project were provided by CNEA.

Project Management Power upgrade of a research reactor requires a high degree of expertise regarding the technical management of engineering projects. The owner/operating organization has to respond to the technical and programmatic aspects of the modern research reactor projects, and needs to validate that all modifications are appropriate and safe, in order to accept the prime responsibility for safety when the research reactor is commissioned. A key factor in this project was the clear identification of the role that had to be undertaken by CNEA from the beginning, in relation to its deep involvement on specific and diverse technical aspects such as design, procurement, manufacture, licensing, commissioning and performance demonstration of the reactor. In this way CNEA, as the ultimate Operator of the Reactor, would be in a better position to run the plant in a safe, sustainable and efficient manner.

Accordingly, in order to properly accomplish this strategic objective a dedicated Technical Management Team was established for this project whose main functions were:

0

20

40

60

80

100

120

Piping &Supports

ElectricalSystems (UPS,Transformer,

Switchboards )

ProcessEquipment

(Pumps, HeatExchanger,

Cooling Towers)

Civil Works In poolcomponents

(SyphonBreaker & Flap

Valves)

U$S

(x 1

000)

Main Project Costs

• Monitor the progress of the project during all the stages. • Prepare Technical specifications for the procurement process. • Coordination between working groups and the external suppliers. • Control of Project Documentation. • Licensing issues and interaction with the regulatory body. • Commissioning procedures

The majority of the project tasks were completed using internal resources of CNEA, from silicide fuel design and manufacture, the spent fuel conditioning and shipping to USA, main engineering of the modifications and up to commissioning. Nevertheless, some manufacture and installation activities were outsourced because they are conventional specialties like electricity and civil works, or because of our lack of qualified resources in technical specific topics (e.g. ASME welding, which was outsourced to INVAP).

The general purchasing rules were those mandatory in all government departments (Public Administration). In particular, the legal and commercial aspects were monitored by the relevant administrative services. Project Timeline It had been decided that the project were implemented during one large outage, however at the moment to shipping the HEU spent fuels to USA, most of the engineering & procurement tasks included in the project schedule, had not been finished. Mainly the new core components were not available yet. On June 2007 the RA-6 reactor stopped working and its personnel started to remove the HEU core to the auxiliary pool. The preparation for shipment of the spent fuels to the USA lasted about 4 months. On November 2007 the MTR-type HEU spent fuel assemblies, which had been the RA-6 reactor core for 25 years, were successfully shipped in a NAC-LWT transport cask to Savannah River Site (USA).

On July 2008, the new core components were provided. Design, procurement, manufacture and shipping to Bariloche of new LEU based fuel elements, graphite reflectors and control rods assembly, lasted about 18 months.

All over 2008 main equipment replacements and plant modifications were implemented. The project schedule was modified several times due to consecutive delays in the implementation of different tasks. The drawbacks were mainly related with technical contingencies, e.g:

• Some problems arose during mounting tasks of mechanical components inside reactor pool, because the fabrication tolerances were not adequate.

• Some quality control results from NDT (Non Destructive Testing with Liquid Penetrant inspection methods) for the detection of cracks and surface discontinuities, were unsatisfactory and they forced reworks.

On March 2009, the RA-6 reactor finally achieved criticality again. The commissioning stages lasted about 3 months. The reactor is fully operational since July 2009. (See note) Our experience has shown that these major modification projects can bring about considerable stress, but on the other hand, they help to maintain and develop the staff's competence and motivation.

---------------------------------------------------------------------------------------------------------------------- Note: The reactor have preventively been operated at 1MW nominal power over last years. Some thermohydraulic studies and experimental tests have been made for the assessment of the forced convection coefficients, in the unsteady laminar-turbulent transition regime which is present in the hot channel of the current core configuration. They are aimed to establish the maximum reactor power permitted by confirming that conservative safety margins are guaranteed.

Conclusions and Lessons Learned • The RA-6 reactor staff, with the CNEA support, was fully capable of planning and

accomplishing this large scale and complicated project of refurbishment & power upgrade. Since June 2007 to March 2009 there was a total outage time of 18 months.

• The project schedule was modified several times due to consecutive delays in the implementation of different tasks. However, it was mandatory to assign higher priority to safety and quality over cost and schedule.

• Refurbishment, modernization and power upgrade of research reactor projects require a high degree of expertise on the management of engineering project.

• If different tasks are conducted by different organizations/companies, good coordination and communication is essential for a successful outcome.

• The systematic administration and preservation of technical documentation, in order to support the future traceability of the modifications, is of vital significance.

• Implementation changes, reworks and repairs, etc. entail additional engineering hours and project schedule delays therefore their control is a very important aspect within project management. An important lesson learned on this regard is that even minor changes may cause large cascading effects.

• Early definition and mutual agreement with the regulatory body, about the regulatory framework to be applied, about the scope and depth of safety assessment, and about structure and contents of SAR, allows a straightforward licensing process, saving time and effort.

• Design solutions supported on proven technologies are less challenging, complex, and costly than innovative design solutions.

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Acknowledgments

This work was completed with the financial support of the DOE and CNEA, and the technical support from several groups of CNEA who engaged with the “UBERA6 Core Conversion and Power Upgrade Project”.