B.J. Lewis

Fission product release mechanisms during reactor accident conditions

During postulated accidents the reactor fuel is estimated to be exposed to a variety of conditions. These conditions are dynamic and, during the course of an accident, the fuel may experience a wide range of temperatures and conditions from highly oxidizing to mildly reducing environments. The exposure of the reactor fuel to these environments and temperatures may affect its stoichiometry and release performance. In this paper a review of the important fission product release mechanisms is presented. The results of three out-of-pile experimental programs are also summarized.

Fuel performance and fission product release studies for defected fuel elements

Results from numerous in-reactor experiments with uranium dioxide fuel elements that contain defects in the Zircaloy cladding are reviewed. The various factors that influence the rate of physical deterioration of a defected element are examined. Experimental and theoretical investigations into the release behavior of radioactive nobel gases and iodine are considered for both the steady-state and transient situation, focusing on the relationship between the release behavior and the state of deterioration of a fuel element. Application of this work to power reactor operation is discussed. 95 ref.

Low volatile fission-product release and fuel volatilization during severe reactor accident conditions

An analytical model has been developed to describe the release behavior of low-volatile fission products from uranium dioxide fuel under severe reactor accident conditions. The effect of the oxygen potential on the chemical form and volatility of fission products is determined by Gibbs-energy minimization. The release kinetics are calculated according to the rate-controlling step of diffusional transport in the fuel matrix or fission product vaporization from the fuel surface. The effect of fuel volatilization (i.e., matrix stripping) on the release behavior is also considered. The model has been validated against several out-of-pile annealing experiments performed at high temperature in various oxidizing and reducing conditions.

A model for the release of radioactive krypton, xenon, and iodine from defective UO2 fuel elements

The steady-state release of active noble gas and iodine from defective fuel elements is described either in terms of a kinetic or a diffusion model. Both models assume a diffusional release in the fuel. Transport of fission products in the fuel-to-sheath gas is represented either by a first-order rate process or diffusion process, and is characterized with an escape-rate constant or diffusion coefficient, respectively. The kinetic model predicts a release dependence on the decay constant of lambda /sup -1/2/ to lambda /sup 3/2/. The diffusion model predicts a dependence of lambda /sup -1/. Observed release data from inpile loop experiments, for a wide range of defect states, confirm the predictions of the models. A fitting of the model to the measured data yields estimates of the empirical diffusion coefficient in the fuel matrix, and the scape-rate constant or diffusion coefficient in the fuel-to-sheath gap. Evaluation of the fitted parameters enables the various rate-controlling processes to be deduced as a function of the defect size.

An examination of uranium levels in Canadian forces personnel who served in the Gulf War and Kosovo.

A uranium bioassay program was conducted involving 103 active and retired Canadian Forces personnel. The total uranium concentrations in each of two 24-h urine collections were analyzed separately at independent commercial laboratories by inductively coupled plasma mass spectrometry (ICP-MS) and by instrumental neutron activation analysis (INAA). The mean and median concentrations were determined to be 4.5 ng L−1 and 2.8 ng L−1, respectively, from ICP-MS and 17 ng L−1 and 15 ng L−1, respectively, from INAA. The total uranium concentrations were sufficiently low so that isotopic (238U:235U ratio) assays could not be performed directly from urine samples. Isotopic assays were performed on hair samples from 19 of the veterans participating in the testing. The isotopic hair assays were scattered around the natural 238U:235U ratio of 137.8, ranging from 122 ± 21 to 145 ± 16 (1&sfgr;). Due to concern expressed in the media over possible depleted uranium exposure and long-term retention in bone, a single bone sample (vertebrate bone marrow) from a deceased member of the Canadian Forces was also analyzed for total uranium content and isotopic ratio by ICP-MS. The sample was shown to have 16.0 ± 0.3 &mgr;g kg−1 uranium by dry weight and a 238U:235U isotopic ratio of 138 ± 4, consistent with natural uranium.

A model for fission gas release and fuel oxidation behavior for defected UO2 fuel elements

This paper is based on a number of in-and out-of-reactor experiments at the Chalk River Nuclear Laboratories, a physically based model developed to predict the activity release of radioactive noble gases from defected UO{sub 2} fuel elements during steady-state reactor conditions. This model has been interfaced with the ELESIM fuel-performance code, and verified against all-effects experiments in the National research Experimental reactor with defected elements containing various sizes and types of sheath failure, and operating at linear powers ranging from 22 to 67 kW/m up to a maximum burnup of 278 MW {center dot} h/kg U.

Advances in Measuring and Modeling the Atmospheric Radiation Environment

New radiation monitors based on solid-state detectors have been developed to perform wide-ranging measurements of the atmospheric environment and provide warnings of sudden increases during solar particle events. Results have been obtained during the current deep solar minimum across the full range of latitudes and from sea level to 13 km altitude. Results for ambient dose equivalent agree very closely with Tissue Equivalent Proportional Counters carried on the same flights. Values of 10 ¿Sv/hr are being reached at 12 km altitude and high latitude. Comparisons are made with the QinetiQ Atmospheric Radiation Model and the need to include cosmic-ray heavy ions is demonstrated.

Characterisation of neutron-sensitive bubble detectors for application in the measurement of jet aircrew exposure to natural background radiation

A survey of the natural background dose equivalent received by Canadian Forces aircrew was conducted using neutron-sensitive bubble detectors (BDs) as the primary detection tool. Since this study was a new application for these detectors, the BD response to neutron dose equivalent (RD) was extended from thermal to 500 MeV in neutron energy. Based upon the extended RD, it was shown that the manufacturers calibration can be scaled by 1.5 +/- 0.5 to give a BD sensitivity that takes into account recently recommended fluence-to-neutron dose equivalent conversion functions and the cosmogenic neutron spectrum encountered at jet altitudes. An investigation of the effects of systematic bias caused by the cabin environment (i.e., temperature, pressure and relative humidity) on the in-flight measurements was also conducted. Both simulated and actual aircraft climate tests indicated that the detectors are insensitive to the pressure and relative humidity variations encountered during routine jet aircraft operations. Long term conditioning tests also confirmed that the BD-PND model of detector is sensitive to variations in temperature to within +/- 20%. As part of the testing process, the in-flight measurements also demonstrated that the neutron dose equivalent is distributed uniformly throughout a Boeing 707 jet aircraft, indicating that both pilots and flight attendants are exposed to the same neutron field intensity to within experimental uncertainty.

A Prototype Expert System for the Monitoring of Defected Nuclear Fuel Elements in Canada Deuterium Uranium Reactors

This paper reports on a prototype expert system for fuel failure monitoring in Canada deuterium uranium (CANDU) power reactors. Based on a coolant activity analysis, the system is able to provide information in an operating reactor on the number of fuel failures, the average defect size, and the amount of tramp uranium deposited on the in-core surfaces of the primary heat transport system. The fission product release model used in the system is based on results from an in-reactor experimental program at Chalk River Nuclear Laboratories. The expert system is validated against fuel failure data from a number of CANDU power reactors.

Source term of iodine and noble gas fission products in the fuel-to-sheath gap of intact operating nuclear fuel elements

Abstract To support “source-term” studies, a series of in-reactor tests have been performed at the Chalk River Nuclear Laboratories to determine the behaviour of short-lived fission products (Xe, Kr, I) in operating UO 2 fuel elements. In this paper, a methodology has been developed to present the noble gas activity available for release in the fuel-to-sheath gap of intact fuel during normal reactor operation, including reactor shutdown and startup. Although iodine was not directly observed in these experiments, a model is described to estimate the available gap activity of the short-lived iodine species based on the release kinetics of noble gas. The model is shown to be in excellent agreement with indirectly measured values of 133 I and 135 I. The fission gas measurements are used to assess the standard ANS 5.4 fission product release model.