What happens if the pressure in a pressurizer exceeds 15MPa?

[matt222]

I have question regarding the pressure of the pressurizer, for PWR the primary pressure is around 15MPa, what will happen if the pressure increase beyond 15MPa let's say around 20 to 50 MPa, in the case of station black out, in my point of view this due to loss of heat sink, what scenario migh occur due to this increase

 

[Astronuc]

There are safey relief valves that are set to limit the pressure in the primary coolant circuit, in order to protect the pressure vessel, piping, steam generators and reactor coolant pumps.

The setpoint is typically about 2500 psia (172 bar or 17.2 MPa) but could be slightly higher or lower. Other components would be designed with margin so their rating might be more like 2800 to 3000 psia (19.3 to 20.6 MPa).

Industry Performance of Relief Valves at U.S. Commercial Nuclear Power Plants through 2007
http://www.nrc.gov/reading-rm/doc-collections/nuregs/contract/cr7037/cr7037.pdf

 

[matt222]

yes I know about this limits in the pressure, so is that mean the reactor vessel would melt down if the pressure of the pressurizer went beyond 20Mpa or so

 

[Astronuc]

yes I know about this limits in the pressure, so is that mean the reactor vessel would melt down if the pressure of the pressurizer went beyond 20Mpa or so

No. The core/fuel would not melt as a result of overpressurization. Rather, an overpressurization event could result in a rupture in the pressure boundary (e.g., reactor coolant pump seals) such that one would have a small (or large) leak, or a loss of coolant.

The objective in limiting the overpressurization is to protect the pressure boundary of the primary system and preclude damage to the system that would compromise the structural integrity of the system and otherwise compromise continued operation of the plant or increase the risk for release of fission products to containment. The safety valves allow for a controlled release of steam and relief of pressure.

If the reactor is operational and the control system receives an over pressure signal, the reactor trips, the unit shuts down, and additional cooling systems are brought online.

 

[jim hardy]

There's a control system with sprays to control pressure as Astro says.
Ultimate releif valve is reactor head bolts stretch.
Compare wetted area of head to cross sectional area of head bolts and their tensile strength. I think you'll find it's designed to lift head slightly before 3206, water's critical point. We hydro'd ours to 3106 just 100 psi shy.

EDIT i know at first that sounds scary. But think a minute - that's a leak that you know where it is and it's well above the fuel. Seems a smart place to build in a control of last resort.

You sure wouldn't want the bottom to blow out first.