Wood’s Regulatory Support Directorate carried out a study into the effect of temperature on the criticality safety of fissile systems for the Office for Nuclear Regulation (ONR), the UK civil nuclear regulator.

Criticality calculations for fissile material transport packages are typically carried out at room temperature.  However, transport packages may encounter a wide range of temperatures, and the IAEA Transport Regulations state that packages shall be designed for ambient temperatures from ‑40°C to +38°C.  Previous work on temperature effects has mainly looked at the high temperatures found in reactors, but this new study also considers temperatures below freezing.

Calculations were carried out using the Monte-Carlo criticality code MONK over a temperature range from 193 ‑ 1073K, (-80^oC – 800^oC) with 273K (0^oC) being calculated for both water and ice.   The interaction of thermal neutrons in water and in ice has been shown to be significantly different, even at similar temperatures. The crystalline structure that forms when water freezes not only leads to a reduction in density but a change to the nature of the interactions between the atomic bonds within a single H₂O molecule and between adjacent molecules. Therefore, the bound thermal scattering data is fundamentally different between water and ice.

The report presents an investigation into the effect of temperature on the reactivity of fissile systems.  An explanation of the important physical phenomena that may result in changes in the reactivity of a fissile system with temperature is provided.  One of the main effects is the Doppler broadening of resonances in the neutron cross sections.

A range of calculations was performed to estimate how a change in temperature influences the neutron multiplication factor, K, in a number of fissile systems.  The range of materials covered in the study included a number of fissile / fissionable species being one or more of U-235, U-238, Pu-239 and Pu-240 combined with different moderator materials (water/ice, polythene or graphite).   The systems were modelled as either an infinite mixture or an infinite array of pin-cells, therefore the measure of neutron multiplication considered in all cases is K_infinity (no leakage of neutrons from the system).

Results are plotted as difference in k against temperature and against moderator-to-fuel ratio.  The example in the graph above is for 5% enriched uranium oxide rods in light water.  The unit cell size was adjusted to give three different water volume to fuel volume ratios V(H₂0) to V(UO₂).

The discontinuity at 273K (0°C) is clear.  For an under-moderated system, reactivity decreases with increasing temperature; for an over-moderated system, reactivity increases with temperature.

The report compares the major criticality codes MONK, MCNP and SCALE-KENO with respect to their current and developing abilities to model the temperature dependence of neutron multiplication. This includes consideration of the limitations of the available nuclear data.

The study concluded that variation of reactivity with temperature is not always intuitive, and those assessing transport safety cases should be aware that for some systems criticality safety margins may be smallest at temperatures below freezing.

A full report on the findings is available on the ONR website here:
http://www.onr.org.uk/documents/2019/onr-rrr-077.pdf

Contact
Steve Power, Wood
steve.power@woodplc.com

Источник: eurosafe-forum 5.2019

Екидин А. А., к.ф.-м.н. (ekidin@ecko.uran.ru), Антонов К. Л., к.ф.-м.н. (antonov@ecko.uran.ru), Васильев А. В., к.т.н. (alexey.vasiljev@gmail.com), Васянович М. Е. (vasyanovich_maks@mail.ru), Пышкина М. А. (maria1pyshkina@gmail.com) (ИПЭ УрО РАН), Курындин А. В., к.т.н. (kuryndin@secnrs.ru), Шаповалов А. С. (shapovalov@secnrs.ru) (ФБУ «НТЦ ЯРБ»), Антушевский А. С. (antalex1956@mail.ru), Семенов М. А. (semenovmw@mail.ru), Мурашова Е. Л. (katiy77mr@gmail.com) (ФГУП «ПО Маяк»), Капустин И. А., к.х.н. (kapustin@bk.ru), Филатов И. Ю. (fiu@electrospinning.ru) (ОАО «НИФХИ им. Л. Я. Карпова»).

В настоящей работе представлены результаты оценки поступления трития из брызгальных бассейнов Балаковской АЭС в атмосферу в холодный период. Показано отличие условий формирования газоаэрозольного облака тритированной воды над поверхностью брызгальных бассейнов в холодный и теплый периоды года. Проведено сравнение возможных подходов к оценке интенсивности выбросов трития с поверхности брызгальных бассейнов. Выполнена оценка накопления трития в снежном покрове в районе расположения брызгальных бассейнов. Получена консервативная оценка облучения критической группы населения, показывающая необходимость нормирования выбросов трития из брызгальных бассейнов Балаковской АЭС.

Скачать полную версию статьи можно в формате PDF.

Assessment of admission of tritium into the atmosphere from spray cooling ponds by Balakovo NPP in cold period of time

Ekidin А., Ph. D., Antonov К., Ph. D., Vasiljev А., Ph. D., Vasyanovich М., Pyshkina М. (Ural branch of Russian academy of sciences Institute of Industrial Ecology), Kuryndin A., Ph.D., Shapovalov A. (SEC NRS), Antushevskiy А., Semenov М., Murashova Е. (FSUE «Mayak PA»), Kapustin I., Ph. D., Philatov I. (JSC «Karpov Institute of Physical Chemistry»).

The article provides results of assessment of airborne radiological releases of tritium from spray cooling ponds of Balakovo NPP during cold season. The differences in conditions of generation of tritiated water cloud near the surface of spray cooling ponds between cold and warm seasons are demonstrated. The results of comparison of reasonable approaches for assessment of intensity of tritium discharge from spray cooling ponds are provided as well. The issues of accumulation of tritium in snow cover near Balakovo NPP are addressed and demonstrated in article. Results of conservative assessments of radiological impact of the tritium discharges show that Balakovo NPP spray cooling ponds are source for which discharge limits shall be set and properly controlled.