Injection grout for deep repositories: Subproject 1: Low-pH cementitious grout for larger fractures, leach testing of grout mixes and evaluation of the long-term safety

Con­struct­ing an un­der­ground dis­posal fa­cil­ity for spent nu­clear fuel deep in bedrock re­quires low-pH ce­ment-based in­jec­tion grout, be­cause as­sured data of the ex­tent of a pos­si­ble high-pH plume in sat­u­rated bedrock con­di­tions is lack­ing.

In this work low-pH grout mixes of new de­sign were sub­jected to leach test­ing. Be­fore cho­sen to leach test­ing the grout mixes had to ful­fil cer­tain tech­ni­cal re­ruire­ments. Leach test­ing was per­formed in or­der to es­tab­lish that the pH re­quire­ment (=11) set for the leachates was met. For com­par­i­son rea­sons also one con­ven­tion­ally used ce­ment based grout ma­te­r­ial was in­cluded in the tests. Two kinds of low-pH grout ce­ment mixes were tested; mixes with added blast fur­nace slag (4 mixes) or added sil­ica (6 mixes). All the mixes were not com­pletely tested ac­cord­ing to the test plan, be­cause for some mixes dur­ing leach test­ing fac­tors detri­men­tal to the long-term safety of a repos­i­tory were ob­served, e.g. too high pH or leached sul­phide, which is harm­ful for cop­per.

Leach test­ing of the grout mixes was per­formed in a glove-box (N2 at­mos­phere) in or­der to avoid the in­ter­fer­ence of at­mos­pheric CO2 on the al­ka­line leachates. Two sim­u­lated ground­wa­ter so­lu­tions, saline OL-SO and fresh ALL-MR, were used as leachates. Two leach tests were ap­plied; equi­lib­rium and dif­fu­sion tests. In the equi­lib­rium test at each mea­sur­ing point only a part of the leachate was ex­changed, whereas in the dif­fu­sion test the en­tire leachate was ex­changed. The pH value of each leachate sam­ple was mea­sured, but to­tal al­ka­lin­ity was de­ter­mined only for some leachates. Na, K, Ca, Mg, Al, Fe, Si, SO42-, STOT, and Cl were analysed in the leach so­lu­tions col­lected in the dif­fu­sion test of four grout mixes cho­sen. Also the cor­re­spond­ing solid spec­i­mens were analysed (SEM, XRD, EPMA, MIP, TG) in Japan. A few grout pore fluid pH val­ues were mea­sured in Spain, as well.

The sim­pli­fied ther­mo­dy­namic model cal­cu­la­tions were suc­cess­ful in qual­i­ta­tively re­pro­duc­ing the ex­per­i­men­tally ob­served re­sults. Leach rates or/​and dif­fu­sion co­ef­fi­cients were cal­cu­lated for Ca, K, STOT, Si, Al, and Na us­ing two dif­fer­ent Fick­ian dif­fu­sion mod­els.

Amongst the tested new in­jec­tion grouts the most promis­ing char­ac­ter­is­tics (from chem­i­cal point of view) were found in the sil­ica mod­i­fied mixes when the con­tent of sil­ica fume was in­creased and et­trin­gite-ac­cel­er­a­tion added. Two of these mixes, f63 and w1, had sim­i­lar com­po­si­tions; the ra­tio of sil­ica fume to ce­ment was 0.69 and only the type of ce­ment used (Or­di­nary Port­land Ce­ment vs. Egypt­ian White Ce­ment, re­spec­tively) was dif­fer­ent. In ad­di­tion to ful­fill­ing the pH re­quire­ment of the leachate at the end of test­ing these two mixes had also demon­strated promis­ing tech­ni­cal char­ac­ter­is­tics (Kro­n­löf 2004). Of these two mixes mix w1 showed bet­ter chem­i­cal char­ac­ter­is­tics. In saline leachate in the cho­sen equi­lib­rium con­di­tions the dis­solved amounts were lower for K, SO4 (al­most an or­der of mag­ni­tude) and Si. Also in ALL-MR at the end of dif­fu­sion test­ing the leachate con­tents of Na, K, Ca and Si were lower for mix w1.