Primary system pressurizer

Figure 4.27 RELAP-5 modeling of primary system pressure variations during Transient 3 (Table 4.4). (From Fletch and Bolander, 1986. Reprinted with permission of U.S. Nuclear Regulatory Commission, subject to the disclaimer of liability for inaccuracy and lack of usefulness printed in the cited reference.)... 

The PNP-3000 reactor plant was designed for the generation of process heat for a steam reformer (1071 MW) and of electricity with 540 °C / 19.5 MPa turbine steam (1929 MW). The helium coolant is heated up from 250 to 950 °C at a primary system pressure of 4 MPa. Nuclear power density is 5 MW/m. Four (or more) loops are connected to the reactor either one with circulator, steam generator plus two steam reformers. Eight steam reforming furnaces are to be operated at 825 °C with a methane conversion of about 65 % [69]. 

As motioned in Chapter 19, the name implies that a pressurized water reactor is cooled by hot high pressure water, either H2O (PWR, VVER) or DjO (PHWR). In the PWR and VVER types the coolant is also us as moderator whereas a separate D2O containing moderator tank is normally used in the PHWR type. These power reactor types have several things in common primary — secondary coolant circuits separated by heat exchangers (steam generators), a pressurizer to adjust primary system pressure and often diemical shim control for adjustment of the excess reactivity with fresh fuel. 

Figure 4-7. Spurious opening of a safety valve on the pressurizer calculated primary system pressure trend.

This accident might happen if one of the control rod drive housings circumferentially breaks and is projected into the containment by the primary system pressure. In this scenario, the control rod drive and the control rod itself would be expelled (in a few hundredths of a second) and the rod would be completely and rapidly expelled from the core. 

The combination of opening the rehef valve with the arrest of the chain reaction (as if the valve on the pressure cooker was opened and the burner shut ofi) causes a quick decrease of the primary system pressure. However, the automatie eontrol system of... 

Since the steam generators in an integral reactor are located inside the RPV and can be isolated individually (if there are more than one in the design), there is no need to reduce the primary system pressure in the event of a steam generator tube rupture, unlike the situation in current large loop-type PWRs. 

The interfacing system LOCA (ISL) is presumed to result from exposing low pressure piping (design pressure 400-700 psi) of the interfacing system to high primary system pressure (about 2250 psi). The initial plant response to an ISL is the same as the response to an equivalent sized LOCA inside containment. However, RCS inventory is discharged outside containment and is not returned to the containment sump for recirculation. In addition, an ISL will provide a path, which bypasses the containment, for release of radioactive materials. 

Also, an adequate emergency feedwater supply is available to allow the plant to remain at hot standby for 8 hours followed by an orderly cooldown to the primary system pressure and temperature at which the Shutdown Cooling System (SCS) can be initiated to continue cooldown to cold shutdown conditions. Level instrumentation and a low level alarm are provided on each EFWST to help the operator align the EFWST from the other train to preclude the tank from being emptied before the changeover to SCS cooling can be effected. 

Selected safety/relief valves are associated with the automatic depressurization of the primary system under assumed LOCA conditions. These valves have two independent logic channels powered from different power sources, either of which can initiate depressurization. Valves open automatically and remain open imtil the pressiue falls to a preset closure pressure. These valves open automatically upon signals of high drywell pressure and low reactor water level and confirmation of one LPCI function of the RHR system or LPCS system running. Initiation signals need not be simultaneous. The valves remain open until the primary system pressure is reduced to a point where the LPCI function of the RHR system and/or the LPCS system can adequately cool the core. The initiation of automatic depressurization is delayed from 90 to 120 s to allow the operator to terminate the initiation should the HPCS system initiation and acceptable reactor vessel level have been confirmed. 

Safety relief valve. The plant safety relief valve used in an overpressure scenario is designed not to reseat until primary system pressure has fallen below the set pressure for activation of the passive ADS. 

The turbine trip fault is analysed using LOFTRAN. LOFTRAN computes pertinent plant variables, including the nuclear power transient, the flow coast-down, the primary system pressure transient, and the primary coolant temperature transient. FACTRAN code is then used to calculate the heat flux based on the LOFTRAN analysis results for nuclear power and flow. Finally, VIPRE-01 is used to calculate the DNBR during the transient, using the heat flux from FACTRAN and the flow Ifom LOFTRAN. 

The primary system pressure at the time of initial core damage (high or low). 

Primary system pressures are available locally for supply and return lines at the heat exchanger inlet and outlet but are neither displayed nor recorded at the console. 

Primary system pressure control Is shown In Figure C-60. Some of the features included to Increase system reliability are duplicate pressure transsd.tters summing point monitors on all critical as lifiers automatic transfer to manual control following component failure use of servo followers and transfer units to pexmlt a smooth transition from manual to automatic to manual control and duplicate electric-pneumatic converters. 

The anticipated primary system pressure loss of this LFR is about 1.5 bar thus a free level difference between the cold and the hot collector of only about 1.5 m is sufficient to feed the core. 

Molten Salt Reactor (MSR). The MSR [3] uses a liquid molten-fluoride salt as fuel and coolant. The uranium or plutonium fuel is dissolved in the molten salt. Two test reactors were built. In the 1950s, the Aircraft Reactor Experiment operated normally with molten salt exit temperatures of 815 C with peak operating temperatures up to 860 C and very low primary system pressures. Work continued on MSR technology for power applications until 1976. The reactor can be built in large sizes with passive safety systems. 

TMI-2 scenarios Primary system pressure and temperature nearly constant 2.4-28... 

Figure 2.4-16 TMI-2 primary system pressure reaches a minimum as pressurizer temperature returns to saturated conditions. Liquid in system maintained during depressurization by high-pressure injection and core fiood tanks.

Figure 2.4-17 TMI-2 scenario Steam generators blocked by hydrogen. Reactor vessei ievei decreasing. Primary system pressure increasing. Minimal make-up flow.

Figure 5.1. LOFT LP-FP-2 Primary System Pressure Predicted by MELCOR,...

---