Mobile measurements

IN THE HIGHLY UNLIKELY EVENT of a release of radioactive material from a nuclear power plant the immediate concern would be to protect people from radiation dose. That is why EDF Energy, the operator of the UK’s generating nuclear power stations has developed a new piece of equipment that would improve the speed and accuracy of radiation readings during an off-site event.

The off-site plan advises the most appropriate countermeasures that would prioritise protecting the public. This is an optimisation exercise that has to be carried out in real time, as the plume of radioactive material disperses on the wind, and it requires an understanding of the distribution of off-site dose rates.

Assessing dose rate distribution is a specialised field that relies on predictions about how the release rate might change, assessments of how the radioactive material will disperse with the wind and where it might settle, the dose implications of this dispersed radioactivity, and the time required to implement countermeasures and their potential effectiveness.

At the start of the event very little will be known about any of these parameters. The dose assessors will work with a safety case prediction that makes assumptions about the event in which the reactor damage and leak rates are chosen from a wide range of possible values. The uncertainties are significant and field data is required to improve the dose predictions.

The first real data comes from fixed monitors around the site fence. These can confirm that a release is underway and the direction in which it is leaving the site, and they can measure external dose. However, most of the dose to a person is likely to come from inhalation of radioactive dust and gases and there is little correlation between external radiation dose and inhalation dose. This is because the release may be a mix of noble gases and particulate fission products and actinides. Noble gases, such as Argon-40, give external dose but no inhalation dose, whereas alpha- emitting actinides give very little external dose (some also emit x-rays) but a give a high inhalation dose. Unless you know the isotopic composition of a plume, the external dose rate is a poor predictor of total dose rate to an individual.

To improve their understanding of the situation, operators send off-site survey vehicles to sample radiation levels within the plume. Each EDF Energy reactor site deploys two off-site teams at short notice should a release of airborne radioactivity have occurred or be likely to occur. In the event of a serious accident there are arrangements for off-site emergency vehicles from their other sites to support the affected site.

These vehicles can measure external beta or gamma doses, which can be used to confirm where the plume is — and where it is not. They can also measure the airborne particulate concentration by drawing air through a filter and measuring what is captured but until recently they could not assess the isotopic composition.

Getting an assessment of the isotopic composition used to mean sending exposed filters to a laboratory for detailed assay — with all the organisational complexities and delays. In-vehicle gamma spectroscopy was impractical because of the need to use cryogenic liquids to cool detectors. Advances in solid state detectors and on-board

computing power have changed that. EDF Energy has developed a new system using cadmium zinc telluride (CZT) detectors in purpose-built shielding enclosures that can analyse the trapped dust. Gamma energy is deposited in the crystal and generates pairs of charge carriers. Under an electric field they induce a current pulse, where the amplitude is proportional to the gamma energy (which identifies the isotope). The pulse frequency is proportional to the activity level of the radionuclide.

Although the largest crystal size that can currently be manufactured is only about 1cm3 its density gives it a high stopping power for most gamma energies at room temperature. The detector software locates peaks in the spectrum and relates them to isotopes using its purpose- built library. It determines the airborne concentrations (Bq.m-3) from the amount of air passed through the filter and the collection and detection efficiencies for those isotopes. It then uses MIDA (Maypack Inhalation Dose Assessment) software to estimate the inhalation dose rate for an adult, child and infant standing in that location. It provides this as a dose rate estimate (mSv/hr) and as a colour-coded representation of the time at Emergency Reference Level — a parameter of great interest to those determining countermeasure advice. If the time to ERL is less than the expected time to release termination, after correcting for expected changes in the release rate, then the countermeasure is indicated.

Knowledge of the isotopes coming out of the reactor can also give important information on the state of the reactor, including whether or not it shut down and a rough estimate of the peak fuel temperature. This can be sent to control centres and shared with other organisations, such as public health representatives.

This system has taken six years to develop. Optimising the counting time and the spectrum analysis software required detailed work, since a reactor accident could release a complex mixture of isotopes. Research effort has also been invested in better understanding the retention of volatile isotopes in the filter material which contains activated charcoal in addition to physical filters. Roll out to operational sites will be completed by the end of 2019.

“EDF Energy is very proud of this system which uses state of the art technology to provide key information to decision makers significantly more quickly and more accurately than was possible before,” Rhonda Dubouchet, Radiation Protection Adviser and EDF Energy’s lead for this project, told Nuclear Engineering International.

Author Details: Keith Pearce is Owner and Principal Consultant at Katmal Limited