ML20148A246

Forwards Update on Univ of Virginia Neutron Radiography Facility Improvements in Light of 870702 Personnel Overexposure Incident

ML20148A246
Person / Time
Site:	University of Virginia
Issue date:	03/04/1988
From:	Copcutt B, Mulder R VIRGINIA, UNIV. OF, CHARLOTTESVILLE, VA
To:	NRC OFFICE OF ADMINISTRATION & RESOURCES MANAGEMENT (ARM), NRC OFFICE OF ENFORCEMENT (OE)
References
NUDOCS 8803170236
Download: ML20148A246 (8)

Text

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UNIVERSITY OF VIRGINIA r ,g,1 del' ART 31ENT OF NUCLEAR ENGINEERING AND ENGINEERING l'IIYSICS NUCLEAR REACTOR FACILITY SCilOOL OF ENGINEERING AND APPLIED SCIENCE

( J CIIARLOTTESVILLE. VA 22901

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March 4, 1988 Telephone: 801-924 71?6 Director Office of Enforcement U.S. Nuclear Regulatory Commission ATTN: Document Control Desk Washington D.C. 20555

Subject:

University of Virginia Reactor Facility Docket No. 50-62 and License No. R-66 Follow-up Report on Neutron Radiography Facility Improvements Made following July 2, 1987 Incident Gentlemen:

Please find enclosed a supplementary report on recent operational data taken and additional safety related improvements made in regards to the Neutron Radiography (NR) facility, which is operated in conjunction with the University of Virginia Reactor (UVAR). This report appears to be the definitive and final document sent to the NRC to close out the potential personnel overexposure incident of July 2, 1987. Also, this report sheds light on a second possible origin for that incident, The information provided could be of benefit to other non-power reactor licensees operating similar radiography facilities.

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Brian G. Cbpeutt U. Va. Rad. Safety Officer

Enclosure:

Update on Neutron Radiography Facility cc: U.S. NRC Region II Regional Administrator Mr. Al Adams, U. Va. Project Manager, U.S. NRC, Wash. D.C.

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UPDATE ON UVA NEUTRON RADIOGRAPHY FACILITY Introduction -

The water shield in the north beamport nosepiece and associated drain / fill l system have been closely monitored since July 2, 1987, when a partial drain of  !

i the shield was discovered and a potential for personnel overexposure created.  ;

Previous U.Va. Reactor Facility reports to the NRC listed inadvertent actuation 1 of the drain / fill pump as the most likely cause of that event. The purpose of  ;

j the close monitoring of the neutron radiography (NR) beamport fill status was  ;

to check for the possible existence of another mechanism for self drainage. As .

a result of this vigilance, a second possible explanation for the July 2 event  !

was discovered. However, it is now impossible to ascertain which of the two mechanisms was the true cause of the incident.

Construction on the NR facility was finished in the early fall of 1987.

On November 9, 1987, a spontaneous slow small drain of water from the nosepiece to the reservoir tank was observed. At this time, the NR facility was 1 available for routine use, but had not been operated in the preceding three ,

j weeks. Because of prior improvements made to the NR facility, and the adoption '

of detailed experimental methods, this partial drain did not result in a potential for accidental personnel overexposures, violations of procedures or ,

l of federal regulations, as on July 2,1987.

The Reactor Director was notified and his personal lock was put on the NR blockhouse to prevent entry without his knowledge. Next, at his direction and with the assistance of the neutron radiography princips.1 investigator, the '

reactor staff developed and reviewed written methods for a series of tests, to l find out the mechanism for the second nosepiece self-drain. The findings are  !

summarized below. Other minor details are recorded in memos to the Reactor  ;

Safety Committee.  !

i i

New Facts  ;

Information about the set up of NR facility and the north beam tube  !

nosepiece's fill / drain system were furnished to the NRC inspectors at the time  !

of the July and August 1987 NRC inspections. A diagram of the drain / fill ,

system as it was in November of 1987 is attached to this report, but reference  ;

may have to be made to prior reports, for a background to the July 2 event. In l brief, draining of water from the north beamport's nosepiece is by way of the  !

lower aluminum exit line which is connected to a clear rubber hose extending to  ;

the water and helium cover gas reservoir tank. This flexible hose passes  ;

through the drain / fill pusp, which operates on the basis of a rolling  !

mechanism. Depending on the position of the rollers, a drain path can exist  !

when a perfect seal at the point at which the rollers pinch the flexible hose is not made. However, given that the reservoir is located at a level higher f than that of the beamtube, it had previously been thought that a dtain not ,

involving the pump would in all cases be from the reservoir to the beamtube l nosepiece, i.e. a "fail safe" situation. ,

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2 Nevertheless, a drain against gravity from the nosepiece to the r6;ervoir was observed to develop in November, aftor the NR facility had not been operated for about three weeks. (It is interesting to note thet this period corresponds approximately to the period that this facility was not operated prior to the July 2 event). As the pump had not bee:,t.perated, it was conjectured that a driving force was working against gravity, to displace water out of the beamport nosepiece, by way of the lower line passing throu6h the fill / drain pump and its rollers. Cas formation inside the nosepiece seemed the most likely reason, however, it was puzzling that degassing was not observed with the south beamport.

After careful consideration and planning, a valve located at the beamport nesepiece's lower exit line was closed. This permitted the observation of the d>:ainage rate. In this new configuration, water could be displaced from the n>sepiece to the reservoir only through the beamport's upper exit line. The a:tached schematic indicates that a second transparent TYGON hose links the baamport nosepiece's upper line with the reservoir tank. The amount of wster displaced from the nosepiece was then measurable from the movement of the r,as/ liquid interface within the TYCON hose, until the moment when the interface reached the reservoir tank. The closing of the valve on the lower line clearly had no safety significance, for any expansion caused by gas in the beamtube nosepiece could be relieved to the reservoir tank.

In this fashion, it was determined that gas was indeed forming in the north beamport nosepiece, at a rate of 3 ml/hr with the UVAR secured (not operating), and a rate of 30 ml/hr with the UVAR at 2 MWth. Clearly, it appeared that gamma radiation from the UVAR, both at shutdown and at power, was associated with the phenomena.

Two possibilities were identified to explain gas fornation in the nosepiece. (1) Radiolysis is known to occur when water is irradiated. The free radical intermediates H+ and OH* are produced, which in turn recombine to give the primary molecular products H2 , H22 0 , and H20. After some time the H2 gas concentration in a closed system stabilizes (saturation is reached), for H2 O is regenerated by recombination reactions. Appreciable quantities of oxygen gas are not produced with radiolysis, an important fact of safety significance. The literature suggests that the radiolysis rate is strongly affected by the impurities in the water. (2) Since helium is used as a cover gas in the reservoir tank and a displacement gas in the nosepiece, heliun gas transfer between the reservoir and the beamport nosepiece, by dissolution and dif fusion in the water in the lines linking the reservoir und the beamport, was also considered as a potential but likely insignificant source for gas formation. It was suggested that thermal gradients resulting in degassing within the nosepiece could account for a driving force.

The puzzle as to why gas formation was being observed in the north beamport nosepiece and not in the south beamport nosepiece was resolved with a water change of both beamports, made by reactor staff according to written and approved methods. When fresh demineralized pool water (at a pH of 6.4 and conductivity of 3.0 umho/cm) was inserted, the level of gas formation in the north beamport nosepiece was reduced. The remaining small displacement of water from the beanport nesepieces was observed to be largely a function of

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water temperature (thermal expansion). The south and rorth beamports now behaved identically,

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The approximately three week time lag between last nosepiece filling and the first observation of radiation in the NR blockhouse can also be explained.

Frequent operation of the betaport results in complete nosepiece filling and i transfer of gas in the nosopiece to the reservoir. Therefore, a period of non-  :

use is required, for enough gas to form and to displace water for the effect to r manifest itself.

Samples of the original water from both beemport nosepiecos were taken and compared. The water from the north beamport, which had not been changed since 1982, had a pH of 6.0 and a conductivity of 25 umho/cm. The water from the -

south beamport had never been cycled, and its pH was 6.2 and conductivity was 10 umno/cm. It appears that cycling of water in the closed system increases r the Icvels of dissolved solids, and consequently results in increased amounts of gas to be produced in radiolysis, t 1

i lt was concluded that water in the north beamport nosepiece should be i changed periodically, to avoid a repeat of the situation. An annual frequency ,

i has been adopted by the reactor staff, and its suitability will be determined from a close observation of the future beanport behavior.

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1 Additional Measures 4  ;

The installation of an area gamma radiation monitor inside the FR )

, blockhouse was considered, to serve as a sec-ond indicator of "beamport open"  ;

status (in addition to toe present sightglass indicators monitored by '

i photosensors). Plans were made to hook the cr# .or into the NR blockhouse i personnel access control cystem. It was hope. . hat this monitor could be

. installed in the NR blockhouse at a location crf the neutron beam axis, and calibrated in this position to the mixed neutron and gamma field existing

! whenever the beamport is open. Details for the proposed installation, together ,

! with the appropriate 10 CFR 50.59 safety analysis and installation and testing 1 procedures, were submitted to the Reactor Safety Committee (RSC) for review and I approval, Meanwhile, until a final resolution of the new issues had been reached, the Reactor Director maintained personal control, by virtue of lock  :

f l and key, over access to the interior of the NRF blockhouse.

l At the time of installation of the area gamma radiation monitor in the NR blockhouse, it was determined that the neutron to gamma radiation field ratio depended on the amount of water shield present in the nosepiece, the variable .

scattering off of the experimental apparatus located in front of the beam, and ,

i the monitor's position relative to the tightly collimated neutron beam. It

! bcease evident that calibration of this monitor was simply impossible, and that its use, if attempted, would likely result in future NFC violation citations.

' The RSC was informed of this, and the attempt to install the area radiation l monitor was aborted.  ;

I Not satisfied with this outcome, the reactor staff next considered the  !

l viability of placing a neutron sensor in toe neutron beantube's annulus, i  !

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Placed in this position. the uansor can funct!on as a highly sensitive detector of a draining beamport nosep3 vee, and not be aficcted by experimed al conditions in the blockhouse. , an important feature of the neutron sensor idea i is that its intended use did not requiro its calibration to a range of dose rates. In fact, given the non linear response of the sensor, an available BF 3 tube, and its location within the beamport (no personnel eccese possible), this j would not be reasonable. The experimenter'c BF-3 counter, which was had formerly been employed to measure the presence of neutrons in the NR blockhousa, was no long2r needed and could be removed. (Information about the radiation field inside the blockhouse is obtained with calibrated Survey instruments, when needed.) t 1

Afccr a written method hed been developed for a test of this idea, it was '

deterained by experiment that a BF ^ tube placed within the beamport would

indeed respond to neutrons utch a high degree of sensitivity, and would slert werktrs of a draining beam long before a high radiation area in the blockhouse would be d.veloped. This was checked with the simult.nneous placement of gamma

• and neutron survey metets in front of the beamtube during remotely video '

1 monitored drainings, at which time a comparison of neutron count rates and total dose rates was done, to determine what could be considered a gross "one. -

4 point calibration" point, corresponding to a doce rate below that of a high l radiation area, beyond which a reactor scram, warning light and audible alarm

) would be enabled.

Reactor Safety Committet approval wap obtained for the nautt'on sensor  !

l proposal. The unit has now been Jr. stalled, hooked into the reactor scram  !

3 system, and tested. The BF 3 counter readout is mads on a source range drawer

commonly used in research reactor consoles. t i An additional device was installed to further preclude unintentional drains. Since the drain / fill pump would permit leakage from the noseplece to the reservoir by way of the lower line, whenever a tight seal was not made by j the pump's rollers on the flexible rubber hose, approval was obtained for the ,

> installation in this line of a pneumatically operated ball valve. This valve  !

. is normally closed, and opens only when the pump is energized. With the j additional safety features in place, the Reactot Director returned the noutrou [

L i radiography facili+y ta routine use.

i Discussion The verified displacement of water against gravity, from the beamport nosepiece to the roservoir tank, suggests a second pos.siblo explanation for the (

July 2 unanticipated partial drain, which has been assumed to have occurred due l

to an inadvertent actuation of the fill / drain pump. This second scenario i- ,

consistent with the state of the beamport during both occasions (not opetnted l for about 3 weeks). Until there is sufficient displacement of water from tho l nosepiece, so that a single large gas bubble exists along upper part of the i entire nosepiece barrel, little radiation exits through the beasport. After l that point is reached, radiation levels can rise quickly, i

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5 Despite the new data, the circumstances for the July 2, 1987 partial drain can never be tot. ally deterintned, and the inadvertent actuation of the drain / fill pump is not ruled out as a posu'ble cause. In our recent response to the NRC, we indicated this to be the,likely .eason for the incident.

The Reactor Director has documented the new findings, maintained the RSC infortned, and personally supervised the NR facility improvements. This (final) follow-up report to the nit 0 is sent now that all safety related measures taken for tha NRF have been comp'leted, and all outstanding issues have been resolved.

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