Atomic Energy of Canada Limited National Research Universal Reactor Safety System Upgrades and the Canadian Nuclear Safety Commission's Licensing and Oversight Process
General Description of the NRU Reactor and Safety Upgrades
General Description of the NRU Reactor
The NRU reactor is one of the largest and most versatile research reactors in the world, and is used for a wide variety of irradiations, including fuels and materials testing, small-sample irradiations, neutron scattering research, and isotope production. The NRU reactor site has a large variety of experimental facilities that provide engineering research and development support for the other AECL programs. The NRU reactor produces a significant share of the world's supply of medical radioisotopes, the most important of which is Mo-99, used in millions of medical treatments annually.
NRU is a heterogeneous reactor, operating at power levels up to 135 megawatts (thermal). It is heavy water cooled and moderated, and surrounded by an annular light water reflector. The reactor is fueled at power. The core consists of a vessel cylinder approximately 3.7 meters in diameter and 3.5 meters high. It is made up of 227 vertical lattice sites arranged in a hexagonal array. Control rods and enriched uranium fuel rods occupy about half of the lattice sites; most of the remaining sites are used for low-temperature/low-pressure experiments and isotope irradiations, or are vacant. Two high-pressure /high-temperature loops, U-1 and U-2, supply coolant to three test sections. A number of horizontal beam facilities are also available.
The primary coolant and moderator are the same process system. Low temperature and low pressure (35°C and 0.76 MPa) heavy water is pumped via eight parallel circuits, each consisting of a pump, heat exchanger, and associated valves and piping to a common header below the reactor. Coolant is distributed to each cooled position, flowing upwards through the assembly and exiting into the upper vessel.
The reactor is controlled using 18 control/safety rods, each consisting of a drive unit and neutron absorber.
There are two independent trip systems in the reactor, the First Trip System and the Second Trip System, which activate the shutdown system when monitored operating parameters in the reactor or facilities exceed allowable limits.
The Main Heavy Water Primary Cooling System transfers the fission heat generated in the fuel to river water secondary coolant (Process Water System) when the reactor is operating, and provides decay heat removal under reactor shutdown conditions. Eight parallel branches provide primary cooling.
The Main Heavy Water Pumps (MHWPs) in the main coolant/moderator system are powered by Class 4 power from the off-site power grid. Four of the eight pumps have two-speed motors supplemented by a separate DC motor. On loss of Class 4 power, the reactor will trip and DC power is provided to two MHWPs from Class 1 power battery banks, supported through rectifiers from Class 3 diesel generators. The power to the MHWPs is also backed up from the Emergency Power System. This ensures that forced cooling is always available to the fuel rods in the core.
The Reactor Protection System is comprised of neutron absorbing rods and their release circuits, the First Trip System and the Second Trip System.
Engineered Safety Features that are provided in the reactor are Reactor Protective System, Emergency Core Cooling Functions, Main Pump Emergency DC Drive System, Emergency Secondary Cooling Functions, Rod Monitoring System, Emergency Filtration System and the seven reactor safety upgrades. The seven safety system upgrades included an independent second trip system, qualified emergency response center, new emergency core cooling system, qualified emergency water system, main pump flood protection, liquid and gaseous confinement boundary, and emergency power system.
General Description of the NRU Reactor Safety Upgrades
The following is a general description of the seven major upgrades installed under the NRU Upgrades Project. All the upgrades are required to be seismically-qualified and environmentally-qualified. They are all designed to “modern” codes and standards.
Secondary Trip System: The Secondary Trip System is an independent second trip system that safely shuts down the reactor based on trip units detecting: seismic events, Class 4 power failure, major process water flood, excess neutron power, or excess log rate neutron power. The safety design requirements include separation, redundancy, and signal buffering.
Qualified Emergency Response Centre: The Qualified Emergency Response Centre is an all hazards-qualified alternative location to ensure the reactor can be placed in a stable shutdown state with adequate fuel cooling. It provides initiation and monitoring of all engineered safety features in the event that the main control room is unavailable. It houses equipment used for the other safety upgrades, which are also hazards-qualified, thereby providing separation of these systems from the other process and safety related systems in NRU.
New Emergency Core Cooling: The New Emergency Core Cooling system upgrade ensures that water is automatically made available to the primary cooling pumps of the emergency cooling circuits in the event of a Loss of Coolant Accident. Combined with the Liquid Confinement/ Vented Confinement upgrade, it provides for collection and recirculation of heavy water discharged from a break.
Emergency Power System: The Emergency Power System supplies electrical power independently and separately from the original electrical distribution system, providing Class 1, 2 and 3 electrical power to the upgrades components. It also provides back-up Class 1 power to existing DC motor starters, for emergency cooling Main Heavy Water Pump motors #4 and #5.
Qualified Emergency Water System: The Qualified Emergency Water System incorporates an independent water reservoir and redundant pumping system for post-shutdown heat removal via emergency cooling circuits 4 & 5. The cool qualified emergency water system water is pumped through the secondary sides of the Main Heat Exchangers, absorbing the primary coolant heat load and returning to the reservoir.
Main Pump Flood Protection: The Main Pump Flood Protection is principally a passive system designed to divert water from major leaks in process piping. It ensures the proper operation of the main primary coolant pumps, including the emergency DC drive system, in the event of a major failure of the process water piping within the NRU building. The flood level detectors associated with this system are designed to trip the reactor and trigger an automatic shutdown of all four large process water supply pumps at the powerhouse.
Liquid Confinement/Vented Confinement: The Liquid Confinement/Vented Confinement provides a confinement boundary surrounding the reactor and a variety of rooms in the immediate area. It confines gaseous fission products, tritiated vapor, and released light and heavy water.
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