Main Steam Line Break

Main steam line break accident Power range neutron flux high 13.8 2.31... 

Example of a category 4 accident main steam line break... 

Figure 4-9. Main steam line break at full power with external electric power supply available primary pressure as a function of time.

The original test programme, including three AP-600 major design basis transients, small break loss of coolant accident, steam generator tube rupture and main steam line break, was concluded in October 1994. Two new tests were also performed to verify the facility repeatability and the effect of a more severe cooldown. 

The main steam line flow limiter, a flow restricting venturi built into the RPV MSL nozzle of each of the two main steam lines, limits the coolant blowdown rate from the reactor vessel to a (choke) flow rate equal to or less than 200% of rated steam flow at 7.07 MPa (1025 psig) upstream gauge pressure in the event a main steam-line break occurs anywhere downstream of the nozzle. 

An integral steam flow structure is installed in the steam nozzle to limit the quantity of steam flow in the event of main steam line break. 

The plant designer shall therefore define the mass and energy input to containment to include flow of emergency feedwater to the affected steam generator following a main steam line break. It should be assumed that the operators will not take action to terminate the flow of emergency feedwater to the affected steam generator within 30 minutes of the break, based upon current industry recommendations. 

Typical environmental conditions (temperature, pressure, humidity, integrated radiation dose, and exposure to chemicals) are given in CESSAR-DC Appendix 3.11A and cover the 60-year design lifetime. Conditions are tabulated for normal operation in and outside of containment, and for loss-of-coolant accident (LOCA) and main steam line break inside containment (MSLB). 

The containment spray system is a safety-related system designed to reduce containment temperature and pressure from a main steam line break or a loss-of-coolant accident. It is a single function system separated from other systems to minimize the potential for inadvertent operation. 

Steam system piping failure (main steam line break). This fault models the failure of one core makeup tank discharge valve as the limiting single failure. This is fault 4.2.5 in the fault sehedule. 

The only fault which would result in a radionuclide release is the main steam line break. The only releases of significance are iodine and the alkali metals that become airborne and are released to the environment as a result of the accident. Noble gases are also released to the environment, but their impact is secondary because any noble gases entering the secondary side during normal operation are rapidly released to the environment. 

Results of the analysis for the postulated feedwater line rupture show that the capacity of the PRHR heat exchanger is adequate to remove decay heat, to prevent overpressurising the reactor coolant system, and to maintain the core cooling capability. Radioactivity doses from mptures of the postulated feedwater lines are less than those presented for the postulated main steam line break and meet US regulatory criteria. Fiuther consequence analysis will be conducted to demonstrate that the results meet the relevant UK criteria. 

Main steam line break upstream of isolating valve (3.51x10 )... 

Main steam line break downstream of MSIV lEV-SLB-D 5.96E-04 9.15E-12 9.07E-12... 

A Core damage with RCS at high pressure following anticipated transient without scram or main steam line break inside contaimnent... 

The maximum flow rate capability of the two start-up feed water pumps is physically limited, so as to avoid any possibility of overcooling the reactor coolant system or of overfilling the steam generators or of inputting excessive steam into the containment following a main steam line break. 

The start-up feed water piunps deliver feed water to the steam generators using a route diverse from that of the main feed system. It is possible to isolate this supply from a single steam generator by manually closing an isolation valve in the event of a tube mpture or main steam line break affecting the said steam generator (Section 10.4.9.1.2 of Reference 6.1). 

Design basis accident LOCA Fuel handling accident Main steam line break outside containment Control rod drop accident. 

The following ECC system impairments are analysed in turn failure of injection, failure of loop isolation, failure of steam generator secondary side crash cooldown. The combination of a main steam line break with a loss of... 

Similarly a main steam line break with an LO ECC has additional or changed acceptance criteria relative to those of Section 7.9.3 as follows ... 

The following containment system impairments (for main steam line breaks inside containment) are analysed in turn loss of air coolers, loss of dousing, open ventilation dampers, deflated airlock door seals, open airlock doors. 

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