Reactor Shutdown and Reactor Restart

The power referred to most frequently in reactor physics is neutron power. Neutron power is essentially the fission rate multiplied by the average prompt energy released and recovered per fission (see Section 2.1.2). It is also called “prompt” power, as it appears very quickly following fission. We cannot measure neutron power directly, but we do monitor the neutron flux with ion chambers located outside the calandria and in-core flux detectors. These neutronic signals are calibrated to the thermal-power measurement which allows neutron power to be derived.

Measurement with electret ion chambers of absorbed dose outside the treated volume, during external-photon radiation therapy

The capabilities of electret ion chambers to measure non-target absorbed dose for distances greater than 20 cm from the irradiated volume during radiotherapy treatment was investigated for the first time. During radiotherapy, nontarget doses can be classified as one of three approximate dose levels: high doses, intermediate doses and low doses. Low doses (<5 % of the prescription dose) are not generally considered during treatment planning, due to the fact that is difficult to measure, characterize, or model them in the planning system. In this work were performed measurements with electret ion chambers of absorbed dose outside the treated volume (<5 % of the prescription dose), during external photon radiation therapy in an Elekta Infinity Linear Accelerator of ?Theagenio? Cancer Hospital of Thessaloniki, Greece. The absorbed dose values for distances greater than 20 cm from the irradiated volume varied from 0.3 to 17 mGy which corresponds to 0.01% up to 0.6% of the prescription dose (2660 mGy). Near the irradiation volume the absorbed dose values were greater than the upper detection limit of the electret ion chambers (threshold 40 mGy). The results are compared with the calculated ones by the Monaco Treatment Planning System (Elekta Monaco 5.11.03). In the non-target radiation region where Monaco Treatment Planning System calculates rather precisely (within uncertainties of less than 10%) the absorbed dose, measured and calculated doses are the same within experimental uncertainties. On the contrary, when leakage radiation becomes the dominant source of out-of-field dose the differences are up to 31%.

PROCEDURES TO MEASURE MEAN AMBIENT DOSE EQUIVALENT RATES USING ELECTRET ION CHAMBERS

The capabilities of electret ion chambers (EICs) to measure mean ambient dose equivalent rates were investigated by performing both laboratory and field studies of their properties. First, EICs were ‘calibrated’ to measure ambient gamma dose equivalent in the Ionizing Calibration Laboratory of the Greek Atomic Energy Commission. The EICs were irradiated with different gamma photon energies and from different angles. Calibration factors were deduced (electret’s voltage drop due to irradiation in terms of ambient dose equivalent). In the field studies, EICs were installed at eight locations belonging to the Greek Early Warning System Network (which is based on Reuter-Stokes ionization chambers) for three periods, averaging 5 months each. In the same locations, in situ gamma spectrometry measurements were performed with portable germanium detectors. Gamma ambient dose equivalent rates were deduced by the in situ gamma spectrometry measurements and by soil sample analysis. The mean daily electret potential drop (in Volts) was compared with the mean daily ambient dose equivalent, measured with a portable HPGe detector and Reuter-Stokes high-pressure ionization chambers. From these measurements, ‘field’ calibration factors (electret’s voltage drop due to gamma radiation in terms of ambient dose equivalent) were deduced and found in very good agreement with the values deduced in Laboratory. The influence of cosmic radiation and the intrinsic voltage loss when performing long-term environmental gamma measurements with EICs, was estimated.