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

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Discussion Overview

The discussion revolves around the implications of exceeding the pressure limit of a pressurizer in a pressurized water reactor (PWR), specifically focusing on scenarios where the pressure might rise from the typical 15 MPa to levels between 20 and 50 MPa, particularly during a station blackout. Participants explore potential outcomes and safety mechanisms in place to handle such situations.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant questions the consequences of exceeding 15 MPa in a pressurizer, suggesting that a loss of heat sink could lead to significant issues.
  • Another participant explains that safety relief valves are designed to limit pressure in the primary coolant circuit, typically set around 17.2 MPa, with some components rated higher.
  • There is a concern raised about whether the reactor vessel would melt down if the pressure exceeds 20 MPa, which is met with a rebuttal stating that overpressurization would not directly cause core meltdown but could lead to ruptures in pressure boundaries.
  • A later reply emphasizes that the primary objective of limiting overpressurization is to protect the integrity of the pressure boundary and prevent coolant loss, detailing the safety measures in place, including controlled steam release.
  • Another participant mentions a control system with sprays to manage pressure and discusses the design considerations regarding reactor head bolts as a last resort for pressure relief.

Areas of Agreement / Disagreement

Participants express differing views on the consequences of overpressurization, with some asserting that it could lead to structural failures rather than core meltdown, while others question the implications of such pressure increases. The discussion remains unresolved regarding the specific outcomes of exceeding the pressure limits.

Contextual Notes

Participants reference specific pressure limits and safety mechanisms, but there are uncertainties regarding the exact scenarios that could unfold during overpressurization, as well as the definitions of terms like "meltdown" and "rupture." The discussion does not resolve the potential risks associated with extreme pressure increases.

matt222
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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
 
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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
 
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
 
matt222 said:
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.
 
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.
 
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