ML11104A048
| ML11104A048 | |
| Person / Time | |
|---|---|
| Site: | University of Maryland |
| Issue date: | 04/13/2011 |
| From: | Al-Sheikhly M Univ of Maryland |
| To: | Document Control Desk, Office of Nuclear Reactor Regulation |
| References | |
| Download: ML11104A048 (4) | |
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UNIVERSITY OF SMARYLAN Building 090 College Park, Maryland 20742-2115 301.405.5207 TEL 301.314.2029 FAX DEPARTMENT OF MATERIALS SCIENCE AND ENGINEERING April 13, 2011 Document Control Desk United States Nuclear Regulatory Commission Washington, D.C. 20555-0001 RE:
Maryland University Training Reactor (MUTR), Docket No. 50-166, License No. R-70 The University of Maryland hereby submits this Summary of Corrective Actions report relative to the radioactive contamination incident of June 3, 2010.
I declare under penalty of perjury that the Summary is true and correct.
Sincerely, Mohamad AI-Sheikhly Professor and Director Maryland University Training Reactor cc:
Robert Briber Johnny Eads Isaac Patrick Enclosure ADDD
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Summary of Corrective Actions This report summarizes the corrective actions taken by the Maryland University Training Reactor (MUTR) personnel as a result of the radioactive contamination incident of June 3, 2010.
The incident occurred when graphite samples, which had been irradiated in the reactor pneumatic transfer system, were being removed from an electric resistance heated container.
Supporting documents are on file at the MUTR office.
The experiment was considered to be a Routine Experiment since a similar experiment had been performed previously. As a Routine Experiment, it did not require review and approval by the Reactor Director. However, after a more detailed (post-incident) analysis, we recognize that there were sufficient differences between the experiments such that it should have been treated as a Modified Routine Experiment.
Radioactive Contamination Incident Graphite irradiation experiments utilizing the heater had been conducted at MUTR starting in January 2010. The heater used in the experiments was essentially a duplicate of one that had been used in a 1997 experiment, with the only difference being the addition of a polyethylene tube for purge gas (which was intended to be used in later experiments, but not used in the current experiment).
After the irradiation ended on May 27, 2010, the heater, still containing the graphite sample, was left in the pneumatic transfer system to allow the intense short half-life radioactivity to decay away. On June 3, 2010, the heater was taken out in order to remove the graphite sample. The team consisted of a Senior Reactor Operator (RSO), a Health Physicist, and a graduate student. In the process of opening up the heater the RSO was contaminated with a gray dust that was found to be radioactive. The dominant radioisotope was subsequently identified as 124Sb, which has a half-life of 60.11 days and a maximum P3 particle energy of 2.37 MeV.
Decontamination of the SRO involved removing and bagging up outer clothing including shirt, pants and shoes. Radioactive dust on forearm and hair was removed by repeated washing.
Dose Assessment The most critical exposure to the SRO is estimated to be on the skin of the forearm. The contaminated area was about 80 cm 2. Based on G-M measurements of the contaminated hair, the total activity of this area is estimated to be 128,000 dpm. The resulting skin dose has been estimated by four different methods, two of them manual and the other two using software. The first manual method simply calculated the maximum dose deposited as 5.23 x10.2 mGy (5.23 mrad). A more refined empirical model for skin dose gave a value of 9.63 x1 02 mGy (9.63 mrad). Using the computer software Varskin 3 gave a dose of2.97 x 10-2 mGy (2.97 mrad).
Finally the MCNP 5 model gave a dose of 1.37 x 10-2 mGy (1.37 mrad). All of these estimates are within the same order of magnitude of about 3 x 10.2 mGy (3 mrad). This is fair below the occupational dose limit and would not be a reportable level for notification to the NRC.
Forensic Inspection of Heater The heater was removed from the pneumatic transfer system on January 3, 2011 and inspected in the reactor hall on January 4, 2011. An image of the overall heater system is given in Fig. 1.
The heater components are contained in the aluminum housing. These include the heater
element itself, a sample chamber for the graphite and a thermocouple. The heater is attached to the control unit on the reactor bridge by a wiring harness which includes the two wires for the heater, the thermocouple leads and two / inch o.d. polyethylene tubes, which were intended to be used to flow an inert gas over the graphite sample.
The region of most interest is at the end of the heater assembly where the wiring harness enters through a port in the end cap. A key feature is the pair of aluminum wings that protected the wiring harness where it entered the cap. The wings were originally held together by a pair of wire ties, which came off during the incident. Pieces of the wire ties from the graphite heater were recovered and showed a significant activity 13.5 mr/hr. These pieces are now black, but they were originally light yellow. This suggests that they became embrittled through exposure to heat and radiation. Since they appear to be in tension from the wings, this embrittlement could have led to mechanical failure and dispersion of the powder.
The insulation of the wires at this location show significant deterioration. The originally white PVC insulation on the heater wires is blackened and embrittled. Several pieces have broken off, exposing the bare copper wire. The braided insulation on the thermocouple leads appears to have been completely lost.
Finally, there is gray discoloration on the end cap itself. This is presumably associated with the gray radioactive powder that contaminated the SRO. This end of the heater also showed the highest relative number of counts.
Figure 1: Overall view of graphite heater Activation Analysis of Heater Components The radioisotope 124Sb is produced by neutron capture in 123Sb. The heater was made of several materials that could possibly contain Sb, which is used in pigments, solder and as a catalyst in making plastics. Therefore, several undamaged pieces of the heater system were irradiated and subsequently measured for gamma-ray activity using an HPGe detector. These
were: 1 Complete power wire with white insulation, 2 Bare power wire, 3 Power wire insulation only, 4 Thermocouple wire, 5 Bare thermocouple wire, 6 Heater mounting tape, 7 Solder used on power wire, and 8 Graphite samples from the experiment.
The samples were inserted into the reactor using the pneumatic transfer system. The reactor was operated at a constant power of 250 kW, and each sample was run individually for 15 minutes. In addition, a piece of charred insulation from the heater's power cable was also measured for gamma ray activity.
Significant 124Sb gamma radiation was detected only on Sample 2, the power wire without its insulation. The level was 0.128 pCi or 4.74kBq. The wires are 14 gauge UL type 1015. This designation indicates that the conductor is 41 strands of 30 gauge tinned copper with a nominal outer diameter of 0.140 in. Since Sb is often used as a constituent of solders, it is inferred that in this case, it is present in the "tinning" of the wire. Analysis of the bare wire using inductively coupled mass spectrometry confirmed that 124Sb was present at a level of 35.2 ppm, which is an order of magnitude larger than in the other samples. It should be noted that the solder sample itself did not show any 124Sb activity.
A wipe test of the blue-gray powder from the heater was performed on April 11, 2011. The sample was both beta and gamma counted. The gamma analysis showed 5.45 x 10-2 pCi. The beta analysis showed results similar to the initial counting and both results were consistent with the 60 day half-life of Sb-1 24.
Corrective Actions The actions taken by MUTR personnel since the incident fall into three main categories:
Future Graphite Experiments The post-incident analysis of why the event occurred what isotopes were produced, and the origins of the isotopes in the experimental setup will result in modifications to the experiment and procedure in order to prevent a reoccurrence of the failure that led to the contamination.
These modifications will be made before any additional graphite experiments are conducted.
Personnel trainingq Emergency procedures are being reviewed and will be revised, as appropriate, to ensure that they are clear and understood completely by individuals working on experiments in the reactor.
These reviews are focused on the immediate actions to be taken in the event of a contamination as well as notification requirements.
Review/Approval of Experiments Modified routine experiments, as defined in the Technical Specifications, will be reviewed by the Reactor Director or his/her designee prior to the experiment being performed. This review and approval will be documented and retained.
The date at which full compliance of the corrective actions was achieved was 7, 1, 2010.