NUMERICAL MODELING RESULTS OF CRYOLITHIC ZONE’S THERMAL STATE WHILE EXPLOITING AN UNDERGROUND MULTI-MODULE SMALL NUCLEAR POWER PLANT

The paper gives the 3D numerical modeling results of heat-transfer processes in cryolithic zone with accounting the ice-water phase transfer when placing a multi-module underground small nuclear power plant (SNPP) with two reactor plants and required infrastructure in permafrost thickness. To perform numerical tests there was designed a 3D model of heat-transfer processes through COMSOL software (option Conduction in Porous Media). A methodical approach was described which allows assessment of volume of thawed rocks occurring in the cryolithic zone when installing there thermal-generating sources. The assessment is based on integration of density by the cryolithic zone volume and known difference in ice and water density values.
A set of thermal-physical parameter values for the model’s main materials (rock matrix, lining) has been indicated as well as selected initial and boundary conditions. The calculations were performed at the fixed temperature of 20°С in the SNPP modules, at variation of a coefficient of thermal conductivity for the 1 meter lining of the SNPP modules (from 2.0 to 0.05 W/(m·K)) and at values of permafrost rocks porosity in an interval of 5-10%. For the observed boundary conditions there were determined space-time temperature distributions along the cryolithic zone space with accounting the ice-water phase transfer. Heterogeneity has been revealed in temperature distribution in the cryolithic zone’s modeled object; large areas of thawed permafrost rocks have been noticed in the space between the largest SNPP modules (reactor and machinery rooms). The examples are given of the temperature dynamics in a control point located between the machinery room and a technical service module. It has been established that under the values of the coefficient of lining’ thermal conductivity such as 2.0, 1.0 and 0.5 W/(m.K), the ice-water phase transfer in the control point is forecasted to be in 1.5, 2.1 and 3.1 exploitation years of the SNPP. Under smaller values of the coefficient of lining’ thermal conductivity the transfer through 0°С in the control point for the five-year exploitation lifetime is not forecasted. It has been shown that under the chosen value of temperature in the SNPP modules the efficient coefficient of thermal conductivity of lining material equal to 0.05 W/(m.K) allows providing the integrity of the cryolithic zone close to the object.
The multi-module underground small nuclear power plant has been a basis to implement the approach proposed and estimate (in the form of analytical dependencies) dynamics of thawed rocks volume and velocity of thawed permafrost rocks volume at varied porosity of cryolithic zone. It has been shown that minimal values of porosity agree with maximal values of thawed rocks and this doesn’t conflict with the physics of the process.

1. Iudin M. M. Geomekhanicheskaya model otsenki mekhanicheskikh svoystv merzlykh gornykh porod v massive. [Geo-mechanical model of evaluation of mechanical properties of frozen rocks in massif]. Vestn. YaGU, 2008, vol. 5, nо. 2, рр. 40-45. (In Russian).
2. Melnikov N. N., Amosov P. V., Novozhilova N. V., Klimin S. G. Ekologiya podzemnykh obyektov yadernoy energetiki v usloviyakh kriolitozony. [Ecology of underground nuclear power facilities under permafrost conditions]. Yaroslavl: OOO “Printkhaus-Yaroslavl”, 2015, 119 р. (In Russian).
3. Melnikov N. N., Amosov P. V., Klimin S. G., Novozhilova N. V. Otsenka oblasti ottaivaniya kriolitozony pri ekspluatatsii podzemnoy ASMM na baze chislennogo modelirovaniya v trekhmernoy geometrii. [Estimation of thawing cryolithic area with numerical modeling in 3D geometry while exploiting underground small nuclear power plant]. Vestn. MGTU, 2016, vol. 19, nо. 1/1, рр. 28-34. (In Russian).
4. Melnikov N. N., Amosov P. V., Klimin S. G. Otsenka dinamiki razmera oblasti ottaivaniya kriolitozony integrirovaniyem teplofizicheskikh parametrov. [Dynamics assessment of size of permafrost rocks thawing area]. Mat. metody v tekhnike i tekhnologiyakh. MMTT, 2016, vol. 1, рр. 29-32. (In Russian).
5. Melnikov N. N., Amosov P. V., Klimin S. G. K voprosu otsenki razmera oblasti ottaivaniya kriolitozony posredstvom integrirovaniya teplofizicheskikh parametrov modeli. [On a question to assess dimensions of cryolithic zone’s thawing area through integrating thermal-physical parameters of the model]. Ecological Problems of the Northern Regions and Ways for Their Solution. Proceedings of V All-Russian Conference with foreign participants, 2016, рр. 294-298. (In Russian).
6. Melnikov N. N. Rol Arktiki v innovatsionnom razvitii ekonomiki Rossii [Role of the Arctic region in the innovation-driven economic development of Russia]. Eurasian Mining, 2015, nо. 7, рр. 23-27. (In Russian).
7. Kazakov A. N., Lobanov N. F., Mankin V. I. Dinamika razvitiya teplofizicheskikh protsessov pri podzemnoy izolyatsii teplovydelyayushchikh RAO v mnogoletnemerzlykh gornykh porodakh. [Dynamics in development of thermal-physical processes during underground isolation of heat dissipating radioactive waste in permafrost rocks]. Geoecology, 1997, nо. 2, рр. 36-40. (In Russian).
8. Podzemnyye obyekty v gornykh vyrabotkakh kriolitozony Yakutii. TSN 31-323-2002 Respubliki Sakha (Yakutiya). [Underground objects in mining excavations of the Yakurian cryolithic zone. TSN 31-323-2002 Sakha Republic. (Yakutia)]. Available at: http://www.complexdoc.ru/ntdpdf/481072/podzemnye_obekty_v_gornykh_vyrabotkakh_kriolitozony_yakutii.pdf. (In Russian).
9. Amosov P. V., Novozhilova N. V. Vliyaniye poristosti mnogoletnemerzlykh gornykh porod na glubinu ottaivaniya [Influence of permafrost porosity on thawing depth]. Vestn. KSC RAS, 2014, nо. 2 (17), рр. 58-64. (In Russian).