ASME Code, Section XI, Nonmandatory Appendix G (ASME-G) provides a methodology for determining pressure and temperature (P-T) limits to prevent non-ductile failure of nuclear reactor pressure vessels (RPVs).

 

Low-Temperature Overpressure Protection (LTOP) refers to systems in nuclear power plants that are designed to prevent inadvertent challenges to the established P-T limits due to operational events such as unexpected mass or temperature additions to the reactor coolant system (RCS).  These systems were generally added to commercial nuclear power plants in the 1970s and 1980s to address regulatory concerns related to LTOP events.  LTOP systems typically limit the allowable system pressure to below a certain value during plant operation below the LTOP system enabling temperature.

Major overpressurization of the RCS, if combined with a critical size crack, could result in a brittle failure of the RPV. Failure of the RPV could make it impossible to provide adequate coolant to the reactor core and result in a major core damage or core melt accident. This issue affected the design and operation of all pressurized water reactors (PWRs).

 

This paper provides a description of an investigation and technical evaluation regarding LTOP setpoints that was performed to review the basis of ASME Code, Section XI, Paragraph G-2215, “Allowable Pressure,” of ASME-G includes provisions to address pressure and temperature limitations in the development of pressure-temperature (P-T) curves for nuclear power plants that have LTOP systems.

This paper does not provide a comprehensive summary of the entire LTOP issue.  LTOP was a significant issue for pressurized water reactors (PWRs) starting in the 1970s, and there are many reports available within the U.S. Nuclear Regulatory Commission (NRC) documentation system for this topic, including Information Notices, Generic Letters, and NUREGs.  Rather, this paper describes a particular aspect of LTOP as related to ASME-G requirements for LTOP.  This paper was developed as part of a larger effort to document the technical bases behind ASME-G.


Background on Low Temperature Overpressure Protection System Setpoints

Division / Sponsors: 
Pressure Vessels & Piping (PVP)


Paper Proceedings: https://asmedigitalcollection.asme.org/PVP/PVP2020/volume/83815

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