Pressure–temperature operating limits

Operating limit curves for normal plant operation are developed using approximately the same methodology in all countries. Key features that must be defined are:

— Assumed reference flaw size and shape;

— Safety factors on pressure and thermal stresses;

— Reference fracture toughness curve and safety factor to be used.

Table 27 illustrates the various definitions of these features as used in the different calculative approaches.

The fracture mechanics approach in the USA is contained in the ASME Code Section III [132], Appendix G and Section XI [20], Appendix G. The rules for Japan are specified in JEAC 4206-2000 [146], Appendix 1. Note that the German methodology uses a combination of two methods, one of which is fracture toughness-based [152].

6.4.1. Assumed reference flaw

The reference flaw size and shape used for calculating P–T operating curves are generally quite large compared to current non-destructive inspection capabilities. In the methodologies employed in the USA, Japan, Germany (Method-2) and France (Method-1), the reference flaw is assumed to be ¼-thickness in depth with a length of 1.5 times the thickness. The Russian approach uses a reference flaw of ¼-thickness, but the length is ¾-thickness rather than 1.5-¾-thickness. The French approach has a second method (Method-2) that uses a smaller, more realistic flaw size indicative of the size that could exist in a vessel. This smaller size is about one third that of the ¼-thickness flaw.

TABLE 27. PRESSURE–TEMPERATURE OPERATING CURVE DEVELOPMENT SI 

method Methodology Reference flaw Safety factor(s) Fracture toughness

curve Comments

6.4.2. Safety factors on stresses

Most of the methodologies rely on a generic safety factor of two applied to pressure stress, with the safety factor on thermal stress set at unity. For leak and hydrostatic tests, the safety factor on pressure generally is reduced to 1.5. The Russian approach and the French Method-2 use a safety factor of unity for pressure stress, but the fracture toughness curve either has a safety factor included (Russian approach) or additional safety factors are applied to the fracture toughness curves (French Method-2).

6.4.3. Reference fracture toughness curve and safety factors

In the methods in which a safety factor of two is applied to the pressure stress, there is no additional safety factor applied to the reference fracture toughness curve. However, the reference fracture toughness curve used can vary. In the USA, the current ASME Code approach uses the K_Ia (K_IR) curve, but an approved Code Case [153] allows use of the K_IC curve instead of the more conservative K_IR curve; a similar approach is being adopted in Japan. The French Method-1 and the German methods only allow use of the K_IR curve. The Russian approach uses a specific K_IC curve for normal operation that has a safety factor included in the curve since the safety factor on stress is unity. Method-2 of the French approach uses a combination of both the K_IC and the K_Ia curves for the smaller assumed flaw size. Also, note that the French method allows for different flaw sizes from those defined if properly justified.

6.4.4. Attenuation of damage into the RPV wall

In order to calculate P–T curves, values of toughness are needed at the ¼-thickness and ¾-thickness locations in the RPV wall. Calculations are usually performed to determine the attenuation of neutron flux/

fluence from the inside surface of the RPV into the wall. The best method for making these projections is to use dpa as the measure of fluence change [154], since dpa takes into account the change in neutron spectra that occurs as the neutrons are attenuated. Regulatory Guide 1.99, Revision 2 [140] uses this approach and specifies a generic exponential decay function. ASTM E 900-02 [142] recommends general use of a calculated dpa function with the exponential decay function as a backup position since it is generally conservative [154]. The dpa change through the RPV wall is then used to adjust the correlation parameter, , used in the predictive embrittlement correlations since dpa was not used in developing the embrittlement correlations identified in Table 26.

6.4.5. Low temperature overpressure protection

The US regulations require the use of a system to assure that inadvertent over-pressurization cannot occur during normal heat-up and cool-down such that the P–T limits are violated. This requirement can be met using different system approaches, the most common of which is to use a safety relief valve in the residual heat removal system. Some systems only have a single safety valve set-point, such that the operators must snake around an imposed knee based upon the limitations of the P–T curves and a lower limit from pump seal and/or cavitation restrictions. ASME Code Case N-641 [153] provides recent redefinition of calculation procedures for P–T curves and LTOP that are currently being implemented at operating plants.

6.4.6. Unanticipated transients

Unanticipated or non-normal operation transients sometimes occur that can exceed the P–T limits and generally require an integrity assessment to allow continued plant operation. The overall safety margin is known to be quite high due to conservative assumptions and applied safety factors within the generation methodology of P–T curves. In the ASME Code, Section XI [20], Appendix E, a procedure exists that allows a quick check on structural integrity; if that quick check is not adequate, a more detailed analysis can be performed, as specified.

The purpose of Appendix E is to provide plant operators with a simple method to assess the severity of any unanticipated transient and to quickly document the severity so that plant operating time can be maximized.