Information Notice 1987-23, Loss of Decay Heat Removal During Low Reactor Coolant Level Operation

Loss of Decay Heat Removal During Low Reactor Coolant Level Operation

ML031130662
Person / Time
Site:	Beaver Valley, Millstone, Hatch, Monticello, Calvert Cliffs, Dresden, Davis Besse, Peach Bottom, Browns Ferry, Salem, Oconee, Mcguire, Nine Mile Point, Palisades, Palo Verde, Perry, Indian Point, Fermi, Kewaunee, Catawba, Harris, Wolf Creek, Saint Lucie, Point Beach, Oyster Creek, Watts Bar, Hope Creek, Grand Gulf, Cooper, Sequoyah, Byron, Pilgrim, Arkansas Nuclear, Three Mile Island, Braidwood, Susquehanna, Summer, Prairie Island, Columbia, Seabrook, Brunswick, Surry, Limerick, North Anna, Turkey Point, River Bend, Vermont Yankee, Crystal River, Haddam Neck, Ginna, Diablo Canyon, Callaway, Vogtle, Waterford, Duane Arnold, Farley, Robinson, Clinton, South Texas, San Onofre, Cook, Comanche Peak, Yankee Rowe, Maine Yankee, Quad Cities, Humboldt Bay, La Crosse, Big Rock Point, Rancho Seco, Zion, Midland, Bellefonte, Fort Calhoun, FitzPatrick, McGuire, LaSalle, 05000000, Zimmer, Fort Saint Vrain, Shoreham, Satsop, Trojan, Atlantic Nuclear Power Plant, Skagit, Marble Hill
Issue date:	05/27/1987
From:	Rossi C Office of Nuclear Reactor Regulation
To:
References
IN-87-023, NUDOCS 8705200749
Download: ML031130662 (6)

SS!PNS No.: 6R35 IN 87-23 UNITED STATES

NUCLEAR REGULATORY COMMISSION

OFFICF OF NUCLEAR REACTOR REGULATION

WASHINGTON, D.C. 20555 May 27, 1987 NPC INFORMATION NnTICE NO. P7-23: LOSS OF DECAY HEAT REMOVAL DURING

LOW REACTOR COOLANT LEVEL OPEPATION

Addressees

permit for nyessurited- All holders of an operating license or a construction

water reactor facilities.

Purpose

decay heat removal

This notice provides information regarding the loss of the loss of PHR pump

capability at pressurized water reactors resulting from levels. It is ex- suction during plant operations with low reactor coolantfor applicability to their

pected that recipients will review this information to prevent similar

reactor facilities ard consider actions, if appropriate, notice do not constitute NRC require- problems. Suggestions contained in this is required.

ments; therefore, no specific action or written response

Description of Circumstances

a loss of decay

On April 10, 3987 the Diablo Canyon Unit ? reactor experienced system had been

heat removal capability in both trains. The reactor coolant preparation for the

drained down to the mid-height of the hot-leg piping S5inminute period that the

removal of the steam generator manways. During the from 87° F to

heat-removal capability was lost, the reactor coolant heated was spilled from

water

boiling, steam was vented from an opening in the head, levels in the

the partia'ly unsealed manways, and the airborne radioactivityof noble gases

ccntainment rose above the maximum permissible concentration its first refueling, allowed by 10 CFR "0. The reactor, which was undergoing containment equipment

had been shut down for seven days at the time and the

hatch had been opened.

pump suction head/flow

Erroneous level instrumentation, inadequate knowledgeof ofthe air/water mixture in

requirenents, incomplete assessment of the behavior

operations and contain- the system and poor coordination between control room the conditions that

ment activities all contributed to the event. Under in the reactor vessel

existed, the system that indicated the level of coolant level. In addition, read "high" and responded poorly to chances in the coolant was later deter- the intended coolant level, established for this operation, due to vortexing was

pined to be below the level at which air entrainment plant staff believed that

predicted to commence. At the time of the event, the

the level that would allow

the coolant level was six inches or more above

vortexing.

C 870520007l4 )

AhD 4 ce 87-r

AYlZ.

IN 87-23 Pay 27. 1987 Paqe 2 of ;

for a

The event began at about 8:43 pm, when a test enqineer in preparation a section

planned containment penetration local leak rate test, begar drainingte be

of the reactor coolant pump leakoff return line, which he this believed

isolated. However, because of a leaking boundary valve, action caused the

test section to

volume control tank fluid to be drained through the intended not aware

who were

the reactor coolant drain tank. The control room operators,Increased flow to

that the engineer had begun conducting the test procedure, later the

stop the fluid reduction from the volume control tank. A few minutes was increas- operators were informed that the reactor coolant drain tank level

the leakage. Although the

ing but they could not determine the source of droppirC below the

actual level of coolant in the reactor vessel was apparently

of level in the vessel remained within

minimum intended level, the indication of the active

the desired control band. At 9:25 p.m. the electrical current

was started

RHR pump (No. 2-2) was observed to be fluctuating. The 2-1 pump pump also

the current on the 7-1 and the 2-? pump was shut down. However, fluctuated, so it was immediately shut down as well.

The operators did not inmediately raise the water level in the true reactor because

the vessel

they still did not know either the source of the leakage, Operators

level, or the status of the work on the steam generator manways.

vented at 10:(3 were sent to vent the RHP pumps. One pump was reported to be

this RNP pump, but the current

p.m. At 10:21 p.m. an attempt was made to start did

this period the operators

fluctuated and it was shut down again. During the core

the reactor vessel because

not know the temperature of the coolant in planned refuel- in prepare,' for the

exit thermocouples had been disconnected

t

ing. By 10:30 p.m. airborne activi y levels in thp ' r.t;inment were increasing

and personnel began to evacuate from the containnient buildine.

manways had

At 10:3A p.m. when the operators learned that the steam generator

vessel water level

not been removed, action was initiated to raise the reactor

10 minutes later

by adding water from the refueling water storage tank. About

stopped it. Ry

the test engineer identified the source of the leakage and

to restart one of the

10:51 p.m., the vessel level had been raised sufficiently immediately rose to

RHP pumps. The indicated RHP pump discharge temperature

atmospheric pres-

2200 F. At this time the reactor vessel was slightly above

vessel head.

sure and steam was venting from an opening in the reactor

Discussion:

decay heat removal

The NRC has documented numeroas Instances in the past where

was lost while the plant was

systems have been disabled because pump suction Potice

levels. ?E Information

being operated at low reactor coolant water NRC Case

occurred in 1Q85 trnd 1986.86-101 describes four such events that 1QR4, five

sit such events that occurred in

Study Report AFOn/C503 describes Potice

that nccurrPd in 198?. IE Informationr

that occurred in 1983, and seven case study report

PI-A9 described an event At Beaver Valley in March 1981. The

'rom 197E throuoh

further indicates that a total t' i? such events occurred

ne4. The documentation includes descriptions of a total of 42 events that

capability

have occurred since 1981 involvinot less of decay heat removal

pump suc eon while operating as rrduced water levels.

resultit.c from a loss of

IN 87-23 pay 27, 1987 For all but four of these 23 events the primary cause of the loss of pump

capability was attributed to incorrect.

suction and loss of decay heat removal Two events were attributed to loss

inaccurate, or inadequate level indication. simultaneous operation

of pump suction because of vortexing brought on by the errors were also a contrib- of both pumps. In many of these events procedural

uting factor. In at least nine of the cases, the redundant pump was lost

the cause of

because air was entrained when the operators, not understanding

repeated references to

the problem, switched to the second pump. There are air binding had occurred

difficulties In getting the pumps vented quickly after

and to the operators' inability to take inmediate action to raise reactor

vessel levels until the safety of personnel working or the primary systems

could be assured. The length of time that decay heat removal was completely

lost varied from eight minutes to two hours and averaged almost an hour. In at

least three previous cases, boiling is known to have occurred.

A number of actions have been recommnended previously to prevent the loss of RAR

pump suction during low vessel level operations. These include:

vessel water

Providing accurate level instrumentation designed for reduced

level operations.

flow and

Providing alarms in the control room for low decay heat removal

low water level.

monitoring

Including in the procedures specific requirements for frequent

and strict limits on level.

Considering in the procedures the possibility of vortex formation and air

a second RER pump

entrainment, including a precaution against starting determined and corrective

until the cause of the loss of the first pump is

actions have been taken.

pump

Traininq the operators on the correlation between water level and

speed at the onset of vortexing and air entrainment.

Careful planning, coordination, and connunication with control room

which could affect the primary

personnel regarding all ongoing activities

system inventory.

and air

The NRC review of the Diablo Canvon event indicated that vortexing In addition, entrairment may occur at higher water levels than anticipated. conditions which may

operation at rid-hot-leg levels can lead to unanticipated

procedure

not have been adequately considered in instrumentation design and

preparation.

the potential

The NRC staff's initial assessment of this event has idertifiedboth from a total loss

for a Significant loss of decay heat removal capability heat sink due to air

of the RHR system and from a loss of the steam venerator then become

blanketina of the steam generator tubes. Correct operator actions

critical for plant recovery.

-

IN S7-?3 May 27, 1987 failures to

NRC communications in the past have expressed serious concern with 81-09 maintain adequate decay heat removal capability. IE Information Notice

had been found to be a

pointed out that loss of shutdown cooling capability and other

potentially significant contributor to the total risk. AEOD/C503 before

sources indicate that the time available to restore shutdown coolina shutdown

large. At four days after

core uncovery can occur is not necessarily the RHP suction

from long-term power operation, with the vessel drained down toabout 1/2 hour.

loss level, the vessel water can heat to the boiling point in

core uncovery level can occur in less than

Under such conditions boiloff to the

two hours.

at Diablo

Following the loss of decay heat removal capability on April 10, 1987 during low

Canyon, PGME took a number of actions to prevent loss of RHP suctionThese

level operation and to improve recovery should such a loss occur.

actions included the following: .

the

Evaluation of the reactor vessel level indicating system to determine on the

level at which vortexing would occur and the effect of vortexing

level measurement.

Enhancements of the instrumentation to include accurate during level measurement, temperature measurement low level

alarm capability and core exit

operation.

RHR

Enhancement of procedures to Include requirements for verifying proper are

pump suction before starting the second RHR pump. Also included flow.

precautions specifying minimum vessel levels as a function of PHR

a

Improvements in work planning, control and communication to Include to

restriction of the work scope to items that do not have the potential

reduce RCS inventory.

Improvement of operator training including a discussion of the potential

causes of RHR flow loss, as well as recovery procedures.

issue.

The NRC is currently considering additional generic action on this

IN 87-23 May 27, 1987 action or written response.

This information notice requires no specific please contact the Regional

If you have any questions about this matter, or this office.

Administrator of the appropriate regional office

Charles E. Rossi, Director

Division of Operational Events Assessment

Office of Nuclear Reactor Regulation

Technical Contacts: Donald C Kirkpatrick, NRR

(301) 492-8166 Warren C. Lyon, NRR

(301) 492-7605 Attachment:

1. 1ist of Recently Issued NRC Information Notices

Attachment 1 IN 87-23 May ?7, 1087 LIST nF RECENTLY ISSUEn

INFnRMATION NOTTCES 1987 Pate of

Infomr, .lon Issuance Issued to

Notice No. Sub.ect

5/22/87 All research and

87-2? nperator Licensing Requali- facilitiew sreactor

fication Examinations at

facilities.

Nonpower Reactors

All nuclear power

87-21 Shutdown Order Issued Because 5/11/87 facilities holding

Licensed Operators Asleep an 0L or CP and all

While on Duty licensed operators.

4/20/87 All nuclear power

87-20 Hydrogen Leak in Auxiliary facilities tolding

Building an nL or CP

4/20/87 All PJP facilities86-108 Degradation of Reactor holding an OL or CP.

Sup. I Coolant System Pressure

Soundary Resulting from

Boric Acid Corrosion

4/20/87 All nuclear power

86-64 Deficiencies in Upgrade facilities holding

Sup. 1 Programs for Plant a CP or OL.

Emergency Operating

Procedures.

4/15/87 All licensees

85-61 Misadministrations to authorized to use

Sup. 1 Patier'% Undergoing Thyroid byproduct material

Scans

All Westinghouse

87-19 Perforation and Crackina of 4/9/87 power PWR facilities

Rod Cluster Control Assemblies holdinq an nL or CP

4/8/87 All P.RC licensees

87-18 Unauthorized Service on authorized to use

Teletherapy Units by Non- radioactive material

licensed Maintenance Personnel in teletherapy units

6/7/87 All GE RwP facilities

87-17 Response Time of Scram holdinq an nL or CP

instrument Volump Level

Detactors

OL c Operatirc License

CP a Construction Permit