Reactor pressure boundary isolation

Reactor coolant pressure boundary isolation These are lines fhat connect directly to the reactor vessel and penetrate the dr3rwell and containment barrier. 

The functional requirement for the overall containment system derives from its major objective, which is, in conjunction with other safety systems, to restrict the release of radionuclides resulting from accidental conditions to acceptable limits. The basic functional requirement that stems directly from this objective is to envelop (and thus isolate from the environment) certain systems, as the failure of these particular systems could cause an unacceptable release of radionuclides. This applies to all components of the reactor pressure boundary that cannot be safely isolated from the reactor core. 

Because the HTR-10 test reactor is designed on the inherent safety philosophy, safety classifications of systems and components departure from the way it is done for water cooled power reactors For example, primary pressure boundary is defined to the first isolation valve Steam generator tubes are classified as Class II component Diesel generators are not required to be as highly qualified as those used for large water cooled power reactors, since no systems or components with large power demand would require an immediate start of the diesel engines at a plant black-out accident... 

Reactor coolant pressure boundary penetrat i ng conta i nment Primary containment isolation Closed system isolation valves... 

Criterion 57 - Closed system isolation valves. Each line that penetrates primary reactor containment and is neither part of the reactor coolant pressure boundary nor connected directly to the containment atmosphere shall have at least one containment isolation valve which shall be either automatic, or locked closed, or capable of remote manual operation. This valve shall be outside containment and located as close to the containment as practical. A simple check valve may not be used as the automatic isolation valve. 

ISOLATION OF LOW PRESSURE SYSTEM CONNECTED TO THE REACTOR COOLANT PRESSURE BOUNDARY... 

Nuclear system isolation Isolates the reactor vessel and all connections of the primary pressure boundary that penetrate the containment barrier Engineered safety feature actuation Actuates engineered safety feature systems... 

Closed system isolation These are the lines that penetrate the containment. However, they are neither part of fhe reactor coolant pressure boundary nor are they connected directly to the containment atmosphere. 

PC 2 Adequacy of the isolation of low pressure systems connected to the reactor coolant pressure boundary... 

The thimble tubes for the PWR incore instrumentation, over most of their length, serve as a portion of the reactor coolant system (RCS) pressure boundary. Thus, the wear of the thimble tubes results in degradation of the RCS pressure boundary and can also create a potentially non-isolable leak of the reactor coolant. Furthermore, thimble tube thinning could result in multiple thimble tube failures beyond a facility s design basis during flux mapping operations or a transient event. 

Excessive wear of the thimble tubes results in degradation of the reactor coolant system pressure boundary and could lead to a non-isolable small loss of coolant accident. Multiple thimble tube failures could result in a beyond design basis accident situation. 

Reactor coolant system boundary isolation failure could result in coolant blowdown and overpressurization of the low pressure piping. This can lead to a loss of coolant accident and containment bypass that, if combined with failures in the emergency core cooling systems, would result in a core-melt accident with significant off-site radiation releases. 

When a BWR is not in a scrammed state, the scram valves are held closed by control air pressure and reactor coolant is retained on the upstream side of the closed valves. In this state, the scram valves perform reactor coolant boundary and primary isolation functions. During and immediately following a scram the SDV system assumes a reactor boundary function and a primary containment isolation function. It is during this fully pressurized state of the SDV system that there exist a potential safety concern associated with a break in the SDV system piping. 

Freeze seals are used to isolate components (such as inboard isolation valves) for maintenance in locations that cannot otherwise be isolated. The seal is created and maintained by applying a cooling agent such as liquid nitrogen to the exterior of the pipe. The cooling agent freezes the water within the pipe section, thus sealing the pipe. When used in the reactor coolant system (RCS) pressure boundary, these freeze seals become a temporary part of the pressure boundary. 

The reactor coolant pressure boundary meets the ASME IE code requirements for Class 1 components. These requirements are relaxed for components which are connected to the reactor coolant pressure boundary but can be isolated from die reactor coolant system by two valves in series (both closed, both open, or one closed and die other open), widi automatic actuation to close such components are designed to ASME El Class 3. 

The safety functions provided by the chemical and volume control system are limited to containment isolation of chemical and volume control system lines penetrating containment, termination of inadvertent reactor coolant system boration, isolation of makeup on a steam generator or pressuriser high level signal, and preservation of the reactor coolant system pressure boundary, including isolation of normal chemical and volume control system letdown from the reactor coolant system. 

The potential for release of activity from a break or leak in the chemical and volume control system is minimized by the location of the purification subsystem inside containment and the design and test of the isolation valves. Chemical and volume control system leakage inside containment is detectable by the reactor control leak detection function as potential reactor coolant pressure boundary leakage. 

The second potential over-pressurisation pathway for the normal residual heat removal system is by way of the discharge branch lines, which each connect to a direct vessel injection line. Each line contains two normally closed check valves, which, as reactor coolant pressure boundary valves, are designed forthe reactor coolant system design pressure. The branch line cormectsto a common header, which penetrates the contaimnent. The header contains two containment isolation valves. 

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 PWS consists of six primary coolant loops that circulate moderator to cool the reactor. Each loop consists of a main coolant pump, two parallel heat exchangers, expansion joints, and valves. All branch lines connected to the primary water system up to the first normally closed isolation valve require seismic qualification to assure pressure boundary integrity. This qualification has been demonstrated by various dynamic analyses or by the previously mentioned interim verification program procedure. 

Lines that penetrate the primary containment boundary and lines that are connected to the reactor coolant pressure boundary should be provided with adequate isolation. The isolation devices could be either open or closed in operational states and in accident conditions, depending on their design requirements and required safety functions. 

The RCS pressure boundary comprises those components whose failure could cause a loss of coolant from the reactor core and that cannot be isolated from the core by means of an appropriate interface. 

For all water cooled reactor types, the pressure retaining boundary of the RCS extends up to and includes the first passive barrier or first active isolation device (as viewed from the core). For indirect cycle reactors, such as pressurized water reactors (PWRs), the pressure retaining boundary of the RCS includes the primary side of the steam generators (see Annex II). For direct cycle reactors, such as boiling water reactors (BWRs), the pressure retaining boundary of the RCS also includes the primary coolant recirculation system and the steam and feedwater lines up to and including the outermost isolation valve. 

Boundary conditions were set as follows room temperature of 293 K (20°C) in each part of the facility and atmospheric pressure of 1 bar. The level in the pools is at the elevation of 10 m, which is the equivalent of 2.85 m measured from the bottom of the pool. The initial velocity of the fluid is 0 m/s everywhere in the facility. The input heating power was 99 kW in equal radial distribution in the 3 heated channels of the core. Upper plenum was filled with water totally. Table II shows the performed scenarios and status of the isolation valves. The initial condition of the experiments did not fully correspond the condition of the reference reactor in a LOCA situation after the depressurization period. The initial temperature is close to room temperature when density differences, which create the natural circulation, doesn t exist yet. 

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