Radionuclide Transport in the Repository Near-Field and Far-Field

This re­port is a back­ground re­port of the TURVA-2012 safety case re­port “As­sess­ment of Ra­dionu­clide Re­lease Sce­nar­ios for the Repos­i­tory Sys­tem” (Po­siva 2012a). This re­port gives a com­pre­hen­sive ac­count of the mod­el­ling of ra­dionu­clide re­lease from a de­fec­tive can­is­ter and the sub­se­quent mi­gra­tion to the sur­face ground­wa­ter sys­tem. The fo­cus of this re­port is in the ra­dionu­clide mi­gra­tion both in the repos­i­tory near-field and in the repos­i­tory far-field. Ra­dionu­clide re­leases from the can­is­ter and mi­gra­tion through the repos­i­tory near-field and far-field have also been analysed in the prob­a­bilis­tic sen­si­tiv­ity analy­sis based on the Monte Carlo sim­u­la­tion method. Those sim­u­la­tions are dis­cussed in a sep­a­rate re­port by Cor­men­zana (2013).

Cal­cu­la­tion cases are de­rived from three dif­fer­ent types of sce­nar­ios:
i) The base sce­nario that as­sumes a sin­gle ini­tially de­fec­tive can­is­ter lo­cated in a cau­tiously se­lected can­is­ter po­si­tion, i.e. se­lect­ing the failed can­is­ter lo­ca­tion such that ra­dionu­clide re­lease and trans­port prop­er­ties are con­ser­v­a­tive com­pared to the sta­tis­tics over all can­is­ter lo­ca­tions. Mi­gra­tion processes and pa­ra­me­ter val­ues fol­low the most likely lines of evo­lu­tion. Repos­i­tory safety func­tions are as­sumed to per­form ac­cord­ing to the de­sign ba­sis. Cal­cu­la­tion cases de­fined in the As­sess­ment of Ra­dionu­clide Re­lease Sce­nar­ios re­port (Po­siva 2012a) are also sup­ple­mented by ad­di­tional cal­cu­la­tion cases that are aimed to study vari­abil­ity be­tween dif­fer­ent DFN re­al­i­sa­tions (ad­di­tional BS-ALL cases), lon­gi­tu­di­nal dis­per­sion (BS-RC-ld cases) and al­ter­na­tive re­al­i­sa­tions of the trans­port classes along the re­lease paths (BS-RC-tc cases).
ii) Vari­ant sce­nar­ios that study de­clined per­for­mance of the repos­i­tory safety func­tions. These in­clude en­hanced cor­ro­sion fail­ure and degra­da­tion of the buffer un­der vari­ant geo­chem­i­cal con­di­tions
iii) Dis­tur­bance sce­nar­ios that analyse in­flu­ences of un­likely events on the ra­dionu­clide re­lease and mi­gra­tion.

Analy­sis of the vari­ant and dis­tur­bance sce­nar­ios above are sup­ple­mented by a num­ber of com­ple­men­tary cases that aim to de­velop a bet­ter un­der­stand­ing of the mod­elled sys­tem or sub­sys­tems. Far-field mod­el­ling has not needed to be car­ried out for all com­ple­men­tary cal­cul­tion cases.

The se­lec­tion of cau­tious lo­ca­tions for the analysed de­fec­tive can­is­ter in the base sce­nario is sup­ported by mod­el­ling cal­cu­la­tion cases where all po­ten­tial can­is­ter lo­ca­tions for the de­fec­tive can­is­ter are con­sid­ered.

A thor­ough analy­sis of the re­sults from the cal­cu­la­tion cases is car­ried out in the As­sess­ment of Ra­dionu­clide Re­lease Sce­nar­ios re­port (Po­siva 2012a). The pre­sent re­port in­cludes a cou­ple of ex­ten­sions of the main cal­cu­la­tion cases that are not di­rectly dis­cussed in the As­sess­ment of Ra­dionu­clide Re­lease Sce­nar­ios re­port. These in­clude analy­sis that il­lus­trates in­flu­ence of the sto­chas­tic vari­abil­ity on the se­lec­tion of the cau­tious can­is­ter lo­ca­tion and in­flu­ence of the lon­gi­tu­di­nal dis­per­sion on the re­lease rates. The se­lected cau­tious can­is­ter lo­ca­tion gives very pes­simistic geosphere re­ten­tion prop­er­ties rel­a­tive to the dis­tri­b­u­tions of the trans­port prop­er­ties from the analysed al­ter­na­tive DFN mod­els and re­al­i­sa­tions. The in­flu­ence on the re­lease rates of the dom­i­nat­ing nu­clides caused by the lon­gi­tu­di­nal dis­per­sion and re­al­i­sa­tion of the trans­port classes along the geosphere re­lease path ap­pears to be neg­li­gi­ble.