Method of Nuclear Reactor Core Annealing and Nuclear Reactor

Abstract

The method estimates the damaging dose of fast neutrons (dpa) which results in unacceptable degradation of paste-forming properties of steel. Upon achievement of the reactor energy yield, the direction of the coolant flow is changed from the standard direction to the reverse direction. Then an acceptable period of time is set for the annealing of reactor core elements. The temperature of the annealing mode is set and maintained by controlling the power level sufficiently to restore paste-forming properties of steel of the lower core section within the set period of time. At the end of the pre-set annealing period, the direction of the coolant flow is changed from reverse to the standard one.

Claims

1. The nuclear reactor core annealing method applied to a reactor consisting of, at least, a core, at least one steam generator, and at least one electrically-driven axial-flow recirculating pump of the primary circuit; it is characterized in that it estimates a value of the damaging dose of neutrons causing unacceptable degradation of paste-forming properties of steel; changes the direction of the coolant flow from the standard one (bottom-upwards) to the reverse one (top-downwards) at a corresponding value of the reactor energy yield; annealing of brittled steel elements of the lower core section with hot coolant starts, then an acceptable period of the annealing mode and temperature sufficient to restore paste-forming properties of steel of the lower core section within the pre-set time period are set; providing that the temperature is excessively high or low, the time period shall be set respectively and then the temperature shall be reset; then the reactor power shall be controlled to maintain the temperature within the pre-set time period, and, at the end of the annealing process, the annealing mode shall be terminated and the coolant direction shall be changed from the reversed one (top-downwards) to the standard one (bottom-upwards).

2. The method according to claim 1, wherein hot coolant flows through the lower core section at the temperature of 450° C. minimum.

3. The method according to claim 1, wherein it is required to control coolant consumption in the annealing mode in order to maintain the pre-set temperature within the pre-set period of time.

4. A LMC nuclear reactor consisting of a core, at least one steam generator, at least one electrically-driven axial-flow recirculating pump of the primary circuit which is equipped with a power supply circuit to change the rotation direction of the recirculating pump.

5. A feature of the reactor according to claim 4, characterized in that an electrically-driven recirculating pump equipped with a power supply circuit which makes it possible to control rotation frequency.

Description

INVENTION DISCLOSURE

[0009] The purpose of this invention is to develop a core annealing method which is free from the drawbacks of well-known technical solutions of the area under consideration.

[0010] Implementation of this invention will lead to the following technical results, in particular:

[0011] increased safety of high-temperature radiation defect annealing and restoration of paste-forming properties of steel, in particular, ferritic martensitic steel sections of reactor cores;

[0012] possibility to anneal radiation defects at high temperature and to restore paste-forming properties of steel, in particular, ferritic martensitic steel sections of reactor cores, directly within the nuclear reactor;

[0013] lower cost of high-temperature radiation defect annealing and restoration of paste-forming properties of steel, in particular, ferritic martensitic steel sections of reactor cores;

[0014] mitigation of incident risks in the course of reactor refueling owing to better paste-forming properties of steel, in particular, those of ferritic martensitic steel items of NR cores prior to their refueling;

[0015] possibility to restore paste-forming properties of steel sections of reactor cores and to anneal radiation defects at high temperature during operation time, other than refueling period, if required.

[0016] The following invention features contribute to achievement of the technical results listed above.

[0017] The proposed core annealing method is applied to, for example, LMC nuclear reactors which include a core, at least one steam generator (SG) and at least one electrically driven axial-flow recirculating pump of the primary circuit.

[0018] A nuclear reactor core annealing method is proposed. The method implies estimation of the damaging dose of fast neutrons (dpa) which results in unacceptable degradation of paste-forming properties of steel, in particular, ferritic martensitic steel. Then, as soon as corresponding energy yield of the reactor is achieved, it is required to change direction of coolant flow, for ex., LMC, to change the standard direction (bottom-upwards) to the reverse direction (top-downward). Upon that, the system transits to the annealing mode, as hot coolant at temperature 450° C. (minimum), for ex., LMC, flows through the lower section of the core which includes elements made of brittled steel. Then, an acceptable period of time for the annealing mode shall be set which is sufficient to anneal reactor core elements and to restore paste-forming properties of steel of the lower core section. Then, the temperature which is equal to or higher than the temperature required to restore paste-forming properties of steel of the lower core section within the pre-set period of time shall be set. Providing that the temperature is excessively high or low, durability shall be adjusted respectively and the temperature shall be reset. The pre-set temperature shall be maintained within the pre-set period of time by controlling the reactor power level and coolant consumption, if required. At the end of the pre-set annealing period the direction of the coolant flow shall be changed, for example, LMC, from the reverse direction (top-downward) to the standard one (bottom-upward).

[0019] The proposed design of LMC nuclear reactors includes a core, at least one steam generator (SG) and at least one electrically driven axial-flow recirculating pump of the primary circuit. Moreover, an electrical drive of the recirculating pump includes a power supply circuit which makes it possible to switch to the reverse direction of recirculating pump rotation and to control rotation frequency.

EMBODIMENT OF INVENTION

[0020] We propose a technical solution relating to high-temperature radiation defect annealing of engineering materials of the core in order to restore paste-forming properties of steel under environment of LMC and HLMC reactors (for ex., eutectic lead-and-bismuth alloy, lead), including corrosion-resistant ferritic martensitic steel (FMS) with temperature envelope up to 650° C. when exposed to LMC.

[0021] The proposed nuclear reactor core annealing method is applied to LMC nuclear reactors equipped with electrically driven axis-flow recirculating coolant pumps.

[0022] When in the non-isothermal mode and at relatively low power level, it is possible to change the direction of pump rotation by switching the electrical power supply circuit of the pump drive in order to anneal the reactor core at high temperature by means of axial-flow pumps. This changes the direction of coolant flowing through the core. In this case, “cold” coolant downstream the steam generator is supplied to the core outlet, hot coolant at temperature 450° C. flows through the lower section of the core with brittled steel elements. This results in restoration of paste-forming properties of steel. As it is not required to drain SG to run this high temperature annealing method, residual released energy will be removed at the end of the annealing mode and reactor shutdown. Therefore, this annealing mode will be safe. As hydraulic efficiency of the pump wet end decreases when the pump rotates in the reversed direction, coolant consumption at the same number of pump rotations (rotation frequency) also will be lower than that with the pump rotating in the right direction. This will support the annealing mode with relative power exceeding relative consumption, and will maintain coolant temperature 450° C. at the low core section at lower reactor power, i. e., under safer environment. This off-design mode will not reduce pump life, as it does not last long.