Worker Protection - Nuclear Criticality Monitoring

Current monitoring equipment inside the shelter has shown occasional increases in the neutron count rate, suggesting that uncontrolled nuclear chain-reactions might be taking place. This could be caused by the accumulation of water inside the shelter, which could moderate the neutrons--causing an increase in the number of neutrons and their reaction rates. On the other hand, the monitoring equipment may be faulty. Water may have caused corrosion on the cable ends. Water also may have changed the neutron energy spectrum, which in turn could affect the monitors' measurements.

Existing monitoring equipment at the shelter has been inadequate to determine the cause of the high neutron count rate (also called the "flux rate"). Specialists at the Pacific Northwest National Laboratory built and tested a prototype neutron monitoring system to be installed in a portion of the shelter early in 1998. Personnel from the shelter and Ukrainian scientific organizations examined the system, which was shipped in November 1997. The system consists of eight detector modules, power supplies, and a computerized control and data acquisition system. U.S. specialists will conduct training, additional testing, and troubleshooting on site.

The prototype monitoring system is designed to determine whether the occasional increased neutron flux rate is indeed caused by fission reactions. If so, work to assess the safety implications and produce a permanent monitoring system will be conducted with international funding under the Shelter Implementation Plan. Data collected by the prototype system would support that work. Cost of the equipment to date totals about $1.3 million.

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