BWR Technical Specification Temperature Limit Evaluation
SIB-96-128

For boiling water reactors (BWRs), there are several plant Technical Specifications that establish limits on how the plant must be operated. These limits include the following:

• Plant Cyclic Operations: Limits established to assure that the reactor pressure vessel (RPV) is operated within the number of cycles that were considered in the original vessel design.
• Pressure-Temperature (P-T) Conditions: Limits established to assure that the plant is operated within certain heatup and cooldown rates such that the requirements of ASME Code, Section XI for vessel fracture toughness are met.
• Recirculation Pump Restart Conditions: Limits established to limit thermal shocks on the recirculation system and RPV bottom head components.

These limits have been difficult to maintain for many plants during operation associated with reactor scrams and accompanying trips of the recirculation pumps. In this type of event, the bottom head region tends to cool off rapidly due to cold control rod drive (CRD) cooling water flow entering the bottom head region. The situation is especially severe if there is an accompanying isolation of the reactor water cleanup (RWCU) system, such that the cold flow is not withdrawn from the bottom head region through the bottom head drain. This phenomena is shown schematically below.

If the recirculation pumps can not be brought back on-line relatively soon, the measured temperature drop in the bottom head region may exceed 100°F/hour. In addition, if the RPV top-to-bottom temperature differential exceeds 145°F, the recirculation pumps may not be restarted, forcing a reactor shutdown to cold conditions prior to restart.

Issues
In order to justify relaxation of RPV operational temperature limits, the following issues need to be addressed:

• Transient revision introduces additional cyclic stresses on the bottom head components. This requires analysis to show that ASME Code fatigue usage limits can still be satisfied.
• Since stresses potentially increase, a new fatigue analysis is required for affected components.
• More severe bottom head transients require establishing alternate P-T limitations for the bottom head region of the reactor vessel.
• The revised transient definition must allow restart of the recirculation pumps at temperature differentials significantly exceeding 145°F.
• A strategy must be developed for restart of the recirculation pumps if bottom drain flow temperature is not available.

Evaluation
SI has recently completed an evaluation for a domestic BWR to justify relaxation of existing Technical Specification RPV temperature limits by addressing the issues identified above. Specifically, the 145°F steam dome-to-bottom head fluid temperature limit (discussed in GE SIL No. 251), and the 50°F idle recirculation loop restart limit were addressed. This work was initiated in part by an inadvertent scram event which followed the description provided above. Plant procedures required 20-30 minutes to restart the recirculation pumps. During this time, the 145°F limit was exceeded, preventing restart. This sequence of events also resulted in heatup/cooldown and P-T limit violations, as shown in the figure below.

SI's work was focused on thermal, stress, fatigue, and fracture toughness assessments to justify alternate cooldown transients. Specifically, the bottom head penetrations, shroud support plate and legs, RPV support skirt, and the recirculation pump and piping were addressed from a fatigue standpoint.

The results of SI's evaluations significantly relaxed the previously established Technical Specification limits. Implementation of these relaxed limits can have significant economic benefits since complete cooldown can be eliminated prior to restart.

SI has also developed an automated P-T curve spreadsheet for EXCEL. This product, known as the Reactor Pressure Vessel Integrity Monitoring System (RPVIMS), provides P-T related fracture toughness assessment for transients that may have been experienced, or for assessing existing heatup/cooldown limits. If you are interested in these services or the RPVIMS spreadsheet, please contact SI.