Reactor coolant systems

REACTOR COOLANT SYSTEM Design of the reactor coolant system 

The reactor coolant system, its associated anxiliary systems, and the control and protection systems shall be designed with sufficient margin to ensure that the design conditions of the reactor coolant pressiue bonndaiy are not exceeded in operational states. Provision shall be made to ensure that the operation of pressure relief devices, even in design basis accidents, will not lead to nnacceptable releases of radioactive material from the plant. The reactor coolant pressure boundary shall be equipped with adequate isolation devices to limit any loss of radioactive fluid. 

The reactor pressure vessel and the pressure tubes shall be designed and constmcted to be of the highest quality with respect to materials, design standards, capability of inspection and fabrication. 

The pressure retaining boundary for reactor coolant shall be designed so that flaws are very unlikely to be initiated, and aity flaws that are initiated would propagate 

The design of the components contained inside the reactor coolant pressure boundary, such as pump impellers and valve parts, shall be such as to minimize the likelihood of failure and associated consequential damage to other items of the primary coolant system important to safety in all operational states and in design basis acciderrts, with due allowance made for deterioration that may occitr in service. 

The RCS forms a pressure retaining boundary for the reactor coolant and is therefore a barrier to radioactive releases in all modes of plant operation. The RCS transports the coolant and thereby heat from the reactor core either to the steam generating systems or directly to the turbine generator. The RCS also forms part of the route for the transport of heat from the reactor core to the ultimate heat sink during shutdown and in all transient conditions that are considered in the design of the RCS. 

Undue challenges to the integrity of the RCS should be prevented. As a minimum, measures should be provided for the following  

The capability for venting should be consistent with the capacity of the make-up system. 

Since gross failure of the reactor pressure vessel would result in severe core damage, special consideration should be given to ensuring that there is an extremely low probability of such a failure. To design the vessel in accordance with established codes and safety standards is one of the approaches to making such a failure extremely unlikely. 

The design considerations for the pressure vessel should include the following  

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Fig. 21. Schematic of a pressurized-water-loop reactor coolant system.

In some BWR transient scenarios, the high pressure injection systems are postulated to fail. To make use of the low pressure injection system, it is necessary to depressurize the reactor coolant system, a function performed by the automatic depressurization system (ADS). In the scenario considered, ADS actuation is manual because the signals for automatic initiation of the system are not present. 

A first attempt to estimate the potential consequences from severe LWRs accidents was the BNL report WASH-740 (1957). The authors of WASH-740, to overcome the lack of information and methods, estimated "Hazard States based on the core state, radioactive inventory, fuel cladding, reactor coolant system, and containment conditions. 

The assumed form of iodine is not substantially retained in early containment failure, but may be retained in the reactor coolant system, where cesium iodide is more strongly retained than the elemental iodine assumed by the RSS. 

An accident sequence source term requires calculating temperatures, pressures, and fluid flow rates in the reactor coolant system and the containment to determine the chemical environment to which fission products are exposed to determine the rates of fission product release and deposition and to assess the performance of the containment. All of these features are addressed in the... 

Overall behav<4 of reactor coolant system, mrilten core, and containmerf... 

Initial blowdown of the coolant from the reactor coolant system. 

Mass Transport at Very Low Concentrations. Heavy Water Plants. The phenomenon of mass transport at very low concentrations is not unique to the reactor coolant systems. It can occur also in the heavy water production plants. Table III compares iron transport in a reactor primary circuit and a GS plant dehumidifier circuit and illustrates the quantities of iron that can be transported each day. While the concentrations in the reactors are typically two orders of magnitude lower, the flow rates are an order of magnitude higher. The lower concentration in the reactors gives a lower driving force for deposition and the efficiency of deposition is considerably lower. 

Two core make-up tanks provide berated make-up water whenever die normal make-up system is unavailable. The tanks are located above die reactor coolant system loop piping and kept at system pressure by steam lines from die pressurizer. These tanks function at any system pressure, using only gravity as a motive force. If the reactor protection system detects a need for make-up water, core make-up tanks discharge and... 

A nuclear power plant is a system in which some of the energy released by nuclear fission is used to generate electricity. Every such plant contains four fundamental elements reactor, coolant system, electrical-power generating unit, and safety system. 

The heatup and cooldown of the reactor vessel and the addition of makeup water to the reactor coolant system can cause significant temperature changes and thereby induce sizable thermal stresses. Slow controlled heating and cooling of the reactor system and controlled makeup water addition rates are necessary to minimize cyclic thermal stress, thus decreasing the potential for fatigue failure of reactor system components. 

On a servicing operation on reactor coolant system argon blowers (RAAO), early dismantling of a terminal box on blower 02 CO led to opening of the intermediate containment barrier (2" bamer) The third bamer is bypassed by this circuit... 

REACTOR COOLANT SYSTEM Cooling Mode Forced Circulation... 

Phenix is France s experimental fast breeder reactor that is located in Marcoule and is operated by the French Atomic Energy Commission (CEA). The CEA, has decided to renovate the aging plant to extend the life expectancy of the reactor. The program reqnires field inventory and inspection of equipment to support the component life-span evaluation studies. The presence of liquid sodium and the high temperature of the reactor coolant system even at cold shutdown - between 150 and 180°C - make inspection of Fast Breeder Reactors (FBR) a difficult technical challenge. Framatome-ANP has in particular conqileted fom innovative nondestructive tests at Phenix ... 

Avoid reduction of boric acid concentration in the reactor coolant system (RCS), makeup and safety injection systems, and systems of organized leakages ... 

Many plants decided to install water level monitoring systems in response to this problem. A typical choice is the ultrasonic level measurement system. Some HI are focused on providing interlocks to automatically terminate the draining upon a low level signal. Other HI concern detection of vortex phenomena and automatic makeup of the reactor coolant system. 

Pressure retaining parts of components in the reactor coolant system Components of or connected to the primary reactor coolant system that are essential for ensuring the shutdown of the reactor and cooling of the nuclear fuel in relevant operational states and in postulated accident conditions ... 

Reactor Coolant Boundary. The reactor coolant boundary means all those coolant-containing components of nuclear reactors, such as pressure vessels, piping, pumps, valves, and heat exchangers, which are part of the reactor coolant system, or connected to the reactor coolant system, up to... 

The reactor coolant system safety and relief valves. 

The totaI heat d i ss i pat i on capab iIi ty of the UHS should include conservative estimates of reactor decay heat, heat stored in reactor coolant system components and structures, and other heat sources removed by the cooling systems. This heat removaI capab i I i ty shouId be avaliable during normal operation and following AOEs. 

The PDHR system is employed for both the hot stand-by and long-term core cooling modes. This system can operate at full reactor coolant system pressure and places the reactor in the long-term cooling mode immediately after shutdown. 

Cooling the reactor coolant system to 473 K (200°C) in about 72 hours. 

The AP600 automatic depressurization system comprises 16 valves divided into four depressurization stages. These valves are installed in the reactor coolant system at three different locations. The valves... 

Anticipated operational occurrences are off-normal events, usually plant transients, which can be coped with by the plant protection systems and normal plant systems but which could have the potential to damage the reactor if some additional malfunction should happen. Their typical frequency of occurrence may be more than 10 year Some of the anticipated occurrences (PIEs - postulated initiating events) are due to the increase of reactor heat removal (as might occur for an inadvertent opening of a steam relief valve, malfunctions in control systems, etc.). Some are due to the decrease of reactor heat removal (such as for feed-water pumps tripping, loss of condenser vacuum and control systems malfunctions). Some are due to a decrease in reactor coolant system flow rate, as in the case of a trip of one or more coolant pumps. Some are connected with reactivity and power distribution anomalies, such as for an inadvertent control rod withdrawal or unwanted boron dilution due to a malfunction of the volume control system for a PWR. Events entailing the increase or decrease of the reactor coolant inventory may also happen, due to malfunctions of the volume control system or small leaks. Finally, releases of radioactive substances from components may occur. 

AR58 Design of the reactor coolant system and associated systems in nuclear power plants. Safety guide,... 

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