Reactor core management starts with working out the scheme for initial fuel loading in the core with control rods in appropriate positions and the required concentration of neutron poison in the reactor moderator to ensure that the prescribed level of sub-criticality is maintained all the time. The next step is to compute the core configuration for its first criticality and to prepare the procedure for achieving first criticality. Special instrumenta­tion is installed in the core for measuring the very low neutron flux in the core during the approach to first criticality. It may also be necessary to install a neutron source to ensure that the reactor startup instrumentation is comfortably on-scale before starting to reduce the boron concentration in the moderator water or withdrawing control rods for making the reactor critical.

After the reactor is made critical, the predicted and the actual core con­figuration for criticality are compared and any significant differences or anomalies are resolved. Thereafter various tests are conducted with the reactor at low power, such as measurement of reactivity worth of control rods and various coefficients of reactivity. Other checks such as measuring neutron flux at different axial and radial locations in the core, establishing the relation between reactor thermal power and neutron power, effective­ness of radiation shielding and response of the NPP control system during situations like partial or total load throw-off are carried out at different power levels during the reactor power ascension stages.

Thermal hydraulics computations are done to assess the power delivered from fuel to coolant and various important thermal parameters such as the fuel rod linear power, the fuel centre temperature, the fuel clad surface temperature and the temperature gradient across the fuel thickness, both for steady-state condition and under transients such as tripping of main coolant pumps and movement of control rods. Computations using coupled neutronic and thermal hydraulic codes facilitate obtaining a better and more holistic assessment of the reactor core.

After fuel burn-up proceeds to a level at which there will not be sufficient reactivity available to operate the reactor, the core will have to be refuelled. Refuelling is generally done by removing high burn-up fuel from the central zone of the core, moving the low burn-up fuel from the outer zone to the central zone and loading fresh fuel in the outer zone. As refuelling is done in the reactor shutdown state with the reactor vessel head open, care needs to be taken to prevent open vessel criticality of the core, especially when control rods are removed for maintenance work.

For performing the above activities a strong reactor physics group has to be developed at the NPP with proficiency in use of neutronic and thermal hydraulic computational codes and a good understanding of the reactor core behaviour. Based on the results of various measurements and operat­ing experience, the computational codes will have to be fine-tuned or upgraded. This group will advise the plant management on the refuelling of the core and will work out the detailed refuelling scheme. For reactor designs with on-power refuelling, this task has to be performed on a day — to-day basis. This group will also analyse reactivity anomalies and other reactivity-related events as and when they occur and advise the plant man­agement on the corrective actions. As the neutronic and thermal hydraulic behaviour of the core is one of the most important areas of reactor safety, the staff of the regulatory body also need to have proper understanding and appreciation of effective safety regulation. Many regulatory bodies have standing expert groups to advise them in these areas. Such advisory groups may comprise experts from within the regulatory body, the technical support organization and academic institutions and even personnel from the utility headquarters who are not directly involved in managing the reactor core. While assistance from the reactor vendor may be obtained during the initial few years for managing the core, it is absolutely essential that a high level of expertise in this field be progressively developed in the operating organization, the technical support organization and the regula­tory body for managing the reactor in the long term as also for future expansion of the nuclear power programme.