RRFM 2009 Transactions - European Nuclear Society

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3. Core operation and monitoring
The core management activities include operation of the reactor in accordance with the
design intents and conditions as specified in the OLCs. The core operation should be
performed in accordance with approved procedures, which include precautions that are
necessary for maintaining safe core operation. The operating procedures related to safe core
operation include:
• Core configuration change;
• Reactor start-up, operation, power level changes, and shut down;
• Control rod calibration;
• Determination of the excess reactivity, reactivity shutdown margin, and reactivity
worth of experimental devices and materials to be irradiated;
• Handling of fuel elements (including failed ones) and core components;
• Determination of the reactor thermal power;
• Determination and adjustment of the safety system settings;
• Performance of routine checks of reactor operation and status of the systems and
components.
Monitoring of the reactor core parameters and conditions provides for verifying that the
reactor operation is conducted in accordance with the OLCs. The parameters to be
monitored or verified include:
• Reactor thermal power;
• Reactivity as a function of the control rod positions;
• Control rods drop time, moderator or reflector dump time;
• Reactor water level;
• Pressure difference across the reactor core, coolant flow rate, coolant temperature at
the core inlet and outlet;
• Margins to the thermalhydraulic critical phenomena (derived);
• Fuel temperature (it may be derived from other measured parameters);
• Radioactivity contents in the primary coolant water;
• Physical and chemical parameters of the coolant and moderator.
Integrity of the fuel elements is one of the important parameters to be monitored along
reactor operation. Some research reactors use delayed neutron detectors located in the
primary coolant flow for on-line monitoring of fuel cladding. Other research reactors may use
methods based on detection of the fission products in the coolant or in off-gas from the
coolant. In addition, activities such as checking, testing and inspection within an in-service
inspection programme should be established for early detection of any deterioration (e.g.
bowing, dimensional change, etc.) of the fuel. Failure contingency procedures of the fuel
should be established to ensure identification and removal of failed fuel from service,
determination of the root cause of the failure, and implementation of the necessary corrective
actions that prevent re-occurrences of such events.
4. Core refuelling
The details of the core configurations throughout the reactor lifetime and a schedule for
movement of the fuel elements and core components are defined by the refuelling
programme. This programme should be developed in the design stage and be subjected to
review for further improvement based on the experience acquired from reactor operation and
on the changes in the utilization programme. It involves shuffling of the fuel through the core
in a predetermined pattern to provide sufficient reactivity to compensate for fuel burn-up and
build-up of fission products. The refuelling strategy should be established to achieve a
uniform burn-up of the fuel within the bounds of burn-up limitations, which also enhances the
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