Thermal Shock to Reactor Pressure Vessels (Generic Letter 81-19)

                               UNITED STATES 
                       NUCLEAR REGULATORY COMMISSION 
                          WASHINGTON, D. C. 20555 

                               April 20, 1981 

TO ALL LICENSEES OF OPERATING PWR NUCLEAR POWER PLANTS 

SUBJECT: THERMAL SHOCK TO REACTOR PRESSURE VESSELS (GENERIC LETTER 81-19) 

For a number of years the NRC and industry have been studying the effects of
thermal shock to reactor pressure vessels. Efforts have focused on the 
identification and characterization of thermal repressurization transient 
scenarios and their probabilities and on the development of material 
properties data and improved analytical tools for assessing vessel 
integrity. A number of on-going research programs are supportive of this 
effort and continue to provide information intended to quantify actual 
thermal/mechanical vessel behavior. 

In the event of an overcooling system transient resulting in a cooldown of 
the reactor vessel, followed by repressurization of the pressure boundary 
above a critical level during the cooling period, vessel integrity could be 
jeopardized. The likelihood of a vessel cracking upon experiencing a 
thermal/repressurization transient depends upon its material properties, 
which degrade with increased irradiation; (2) the severity of the thermal 
shock which is a function of the degree of mixing of primary water in the 
system and relatively cold water injected by the high pressure pumps making 
up part of the Emergency Core Cooling System; and (3) the magnitude of the 
pressure transient occurring during repressurization. 

Earlier this year a number of analyses sponsored by the Commission research 
program were completed and results became available to the staff.  These 
analyses were directed at providing a better understanding of the severity 
of overcooling transients which combine operational experience and expected 
reactor vessel material properties. In the same time frame, as a response to
post-TMI requirements, the staff initiated its review of thermal/mechanical 
reports from licensees of Babcock and Wilcox (B&W) operating reactors 
intended to further evaluate the effect of high-pressure safe injection on 
vessel integrity for small-break loss-of-coolant accidents item II.K.2.13 of 
NUREG-0737, Clarification of TMI Action Requirements, November 1980). It was 
as a result of a review of these on-going efforts that the staff decided to 
accelerate its evaluation of possible thermal shock to reactor pressure 
vessels. 

On March 31, 1981, the NRC staff met with the PWR Owners Group and 
representatives of NSSS vendors to discuss the effects of potential thermal 
shock to reactor pressure vessels by overcooling transients and the 
potential consequences of subsequent repressurization at relatively low 
temperature. A copy of the minutes of that meeting is enclosed for your 
information. 
.

                                    - 2 -

This letter is to confirm the intent of the owners groups to perform a study
of this concern including developing alternatives to protect the vessel by 
avoiding repressurization with cold water. 

As indicated in the enclosed minutes of the meeting, we expect to receive a 
letter report from each of the owners groups summarizing their efforts not 
later than May 15, 1981. We expect that you are familiar with the activities
of the appropriate owners group, actively participating in their 
discussions, and will provide a docketed response by May 22, 1981, 
identifying the specific actions you propose to take for your facility. 

                              Sincerely, 


                              Darrell G. Eisenhut, Director 
                              Division of Licensing 
                              Office of Nuclear Reactor Regulation 

Enclosure: 
Minutes of PWR Owners Groups 
  Meeting with NRC on March 31, 1981