United States Nuclear Regulatory Commission - Protecting People and the Environment

IE Circular No. 78-05, Inadvertent Safety Injection During Cooldown


                                UNITED STATES
                         NUCLEAR REGULATORY COMMISSION
                     OFFICE OF INSPECTION AND ENFORCEMENT
                           WASHINGTON, D.C.  20555

                                  May 23, 1978

                                                    IE Circular 78-05

INADVERTENT SAFETY INJECTION DURING COOLDOWN

Salem-1, a four-loop Westinghouse PWR, was being cooled from Mode 3 (hot 
standby) to mode 5 (cold shutdown) on January 23, 1978.  With one reactor 
cooling pump operating, the atmospheric relief valves on all four steam 
generators were used to remove heat from the reactor coolant system.  When 
reactor coolant pressure and average temperature were at 1500 psig and 403 
degrees fahrenheit, an inadvertent safety injection occurred due to low 
pressure in the steamline from one of the steam generators.  Water at 61 
degrees fahrenheit was transferred from the refueling water storage tank to 
the reactor coolant system resulting in thermal shock to the safety 
injection nozzles.  Operator action limited the duration of the safety 
injection to 2 to 4 1/2 minutes. 

The nuclear steam system supplier had informed the licensee that 50 safety 
injections using refueling water at 40 degrees fahrenheit would not result 
in excessive stress at the safety injection nozzle.  Because Salem-1 has had 
an unexpectedly large number of inadvertent safety injections, the licensee 
is taking actions to reduce the frequency with which they occur.

There are a number of contributing factors which lead to inadvertent safety 
injection when the reactor is being cooled from Mode 3 to Mode 5.  These 
factors include: (1) operation of a single reactor coolant pump instead of 
reactor coolant pumps in opposed cooling loops, (2) lack of pressure 
recording instruments for the steamlines, and (3) use of atmospheric relief 
valves instead of steam dump valves.

The reactor vessel inlet nozzles for the four reactor cooling loops are not 
equally spaced on the circumference of the reactor vessel.  Two pairs of 
inlet nozzles are located on opposite sides of the vessel.  The azimuthal 
separation between the inlet nozzles of each pair is approximately 45 
degrees.  With one reactor coolant pump in operation, backflow occurs in the 
other three cooling loops and is expected to be greatest in the loop with 
the nearest adjacent inlet nozzle.  Because of the different flow rates in 
the four cooling loops, the task of controlling the cooling rate in each 
steam generator is difficult.  If the atmospheric relief valves on each 
steam generator are positioned alike, the cooling rate will 

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May 23, 1978                                           IE Circular 78-05

be greatest in the steam generator receiving the least reactor coolant flow 
and thus the least heat.  Therefore, the saturation pressure in that steam 
generator falls more rapidly than in the other steam generators.  When the 
pressure in any steamline is 100 psi less than the pressure in any two of 
the other three steamlines, a safety injection occurs.

So that the flow rates will be more nearly equal in the four reactor cooling 
loops, the licensee has changed the procedure for cooling from Mode 3 to 
Mode 5.  The revised procedure requires that two reactor cooling pumps in 
opposing reactor coolant loops be in operation when available. 

While cooling from Mode 3 to Mode 5, the Salem-1 operator monitors the 
pressure in each steamline and adjusts the position of each atmospheric 
relief valve as necessary to maintain an equal rate of pressure decrease.  
The operator's task is more difficult than necessary because steam generator 
pressures are indicated but not recorded on the control panel. 

The steam generators can also be cooled using the steam dump valves when the 
condenser is available.  The steam dump valves are in steamlines connecting 
the mixing bottle to the condenser.  The mixing bottle is a large diameter 
manifold which receives steam from all four steam generators.  Use of the 
steam dump valves and the mixing bottle, instead of the atmospheric relief 
valves, causes heat transfer in each steam generator to be essentially self- 
regulating and to tend to maintain the same pressure in each steamline.  For 
this reason the licensee has changed the procedure for cooling from Mode 3 
to Mode 5 to require that the steam dump valves be used when the condenser 
is available.  

Holders of Operating Licenses for PWRs which have experienced problems with 
inadvertent safety injections during cooldown and holders of construction 
permits for PWRs should consider actions which would minimize the frequency 
of those occurrences.

No written response to this circular is required.  If additional information 
is needed regarding this matter, please contact the Director of your NRC 
Regional Office.












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