Utilization of Crushed Radioactive Concrete for Mortar to Fill Waste Container Void Space

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1 Journal of Nuclear Science and Technology ISSN: (Print) (Online) Journal homepage: Utilization of Crushed Radioactive Concrete for Mortar to Fill Waste Container Void Space Takeshi ISHIKURA, Hiroyuki UEKI, Kazuhiko OHNISHI & Daiichiro OGURI To cite this article: Takeshi ISHIKURA, Hiroyuki UEKI, Kazuhiko OHNISHI & Daiichiro OGURI (2004) Utilization of Crushed Radioactive Concrete for Mortar to Fill Waste Container Void Space, Journal of Nuclear Science and Technology, 41:7, 741-7, DOI: / To link to this article: Published online: 07 Feb Submit your article to this journal Article views: 2 View related articles Citing articles: 3 View citing articles Full Terms & Conditions of access and use can be found at Download by: [ ] Date: 28 December 2017, At: 09:56

2 Journal of NUCLEAR SCIENCE and TECHNOLOGY, Vol. 41, No. 7, p (July 2004) ORIGINAL PAPER Utilization of Crushed Radioactive Concrete for Mortar to Fill Waste Container Void Space Takeshi ISHIKURA 1;, Hiroyuki UEKI 2, Kazuhiko OHNISHI 1 and Daiichiro OGURI 1 1 Nuclear Power Engineering Corp., Toranomon 4MT Bldg., 4-1-8, Toranomon, Minato-ku, Tokyo Mitsubishi Materials Corp., 1-297, Kitaikebukuro, Omiya-ku, Saitama 3-88 (Received December 19, 2003 and accepted in revised form April 19, 2004) Minimizing the volume of radioactive waste generated during dismantling of nuclear power plants is a matter of great importance. In Japan waste forms buried in a shallow burial disposal facility as low level radioactive waste must be solidified by cement or other materials with adequate strength and must provide no harmful opening. The authors have developed an improved method to minimize radioactive waste volume by utilizing radioactive concrete for fine aggregate for mortars to fill void space in waste containers. Tests were performed with pre-placed concrete waste and with filling mortar using recycled fine aggregate produced from concrete. It was estimated that the improved method substantially increases the waste fill ratio in waste containers, thereby decreasing the total volume of disposal waste. KEYWORDS: crushed concrete, recycled sand, waste disposal volume reduction, radioactive concrete, compressive strength of mortar, sand usage ratio, waste usage ratio, concrete recycle I. Introduction There are 52 commercial nuclear power plants (NPPs) in operation in Japan, most of which will cease operation and start decommissioning by the middle of this century. Minimizing the volume of radioactive waste generated during dismantling of NPPs to reduce total disposal volume is a matter of great importance, because dismantling of NPPs will generate a large amount of radioactive waste. In Japan, low level radioactive waste (LLW) has been buried in a shallow burial disposal facility (Rokkasho Low Level Waste Disposal Center). Waste forms are required to be solidified by cement or other materials as a homogeneous solid form or a miscellaneous solid form, providing sufficient strength to bear the load at the disposal facility, and with no harmful opening. Until now containers of miscellaneous solid waste for the disposal facility were loaded with solid waste such as metal and concrete, and then solidified with mortar that contains cement and ordinary fine aggregate. According to past experience in Japan, the concrete waste fill ratio of miscellaneous solid waste forms was approximately vol% for concrete. 1) Thus, before solidification, there are vol% of void space in the waste containers. Most of solid radioactive waste generated during dismantling will be disposed in these forms. In the overseas countries, such as France, Sweden, and Spain, LLW has been solidified in waste containers by mortar or other solidification materials. Some countries may not require the solidification of LLW for their repositories. The authors have developed a method to minimize the total radioactive waste volume by utilizing radioactive concrete as fine aggregate for the mortar to fill void space in waste containers. 2,3) As a result, mortar using crushed concrete as fine aggregate was seen to meet the requirements Corresponding author, Tel , Fax , ishikura@nupec.or.jp specified in the ordinances and related specification when it is properly mixed. Therefore it is expected that this method can substantially reduce the total volume of radioactive concrete waste requiring disposal. This paper describes the development of radioactive concrete utilization for mortar to solidify waste that is implemented as a part of the verification tests on decommissioning technologies for commercial NPPs. II. Objectives The objective of this development is to minimize radioactive concrete waste disposal volume by utilizing radioactive concrete as recycled fine aggregate (recycled sand) for the mortar to solidify radioactive waste in waste containers. Therefore the mortar using recycled sand must meet the specification that is applied for LLW waste forms. The technical criteria for the waste form is stipulated in Article 8 of the Ordinance of Ministry of Economy, Trade and Industry Radioactive Waste Burial Rules and detailed in the Article 4-7 of the Burial Ordinance of the Science and Technology Agency. In more detail, numerical targets are stipulated in the Standard Production Method to Fill and Solidify Waste Forms produced by 10 Utilities in Japan. 4) Therefore, one of the targets of this development is that the recycled sand and its mortar meet the following criteria: Fluidity: between 16 and s of the efflux time of the mortar through a standardized flow cone (p-cone time) given by American Standard of Testing Materials C 939 Fluidity retention of the mortar: more than min with no ingredient separation nor bleeding Compressive strength of mortar: N/mm 2 (0 kgf/ cm 2 ) minimum after 28 days Mortar fill ratio in waste form: 95 vol% minimum Fine aggregate diameter: 2.5 mm maximum. The other target is that the recycled sand ratio in mortar shall be more than that of ordinary aggregate: 741

3 742 T. ISHIKURA et al. Recycled sand usage ratio in mortar: 900 kg/m 3 minimum. III. Experiments and Results The radioactive concrete generated during the dismantling of NPPs is estimated to be more than 1,000 t for a standard 1,100 MWe light water reactor and more than 10,000 t for a gas cooler reactor (166 MWe). 5) This will consist of % of blocks, 20 % of rubble and 3 7% of powder. Until now, LLW concrete has been solidified in waste containers by filling with ordinary mortar after pre-placing the blocks and the rubble, or by mixing the powder with cement and water. The current approach fills approximately vol% of radioactive concrete in each waste container. Therefore authors have developed an improved method to utilize radioactive concrete as recycled sand produced by crushing blocks and rubbles for the mortar to fill void space in waste containers. Demonstration tests on the improved method have been implemented to increase the fill ratio of LLW concrete waste, and to reduce the total LLW disposal volume. Figure 1 shows the process for the improved method. 1. Common Experimental Conditions (1) Concrete Used for the Experiments Two kinds of raw concrete were used. Raw Concrete A was taken from the turbine building wall of the Tokai NPP and Raw Concrete B was the ordinary concrete prepared for these experiments (Table 1). Raw Concrete A and B were crushed under mm by a jaw crusher to prepare Raw A and B respectively. Raw A and B were crushed and graded to less than 2.5 mm for Recycled Sand A and B respectively. Raw Powder A for these experiments was produced by collecting the cutting debris from wire saw cutting of Raw Concrete A. Individual raw material is identified by suffix in tables. (2) Mortar Used for the Experiments Mortars used in these experiments are composed of ordinary Portland cement, fine aggregate and water. The mortars are named in this paper depending on the kind of fine aggregate, i.e., Mortar R for the mortar that uses recycled sand, Mortar P for the mortar that uses recycled sand and powder, Pre-packng of Concrete & Fig. 1 Concrete Crushing & Sieving Recycled Sand Mix Proportion Cement /Water Mortar R or P Filling Dismantling of Nuclear Power Plant Concrete Mortar Mixing Powder Waste Container of Radioactive Concrete Dehydrating Slurry Method of radioactive concrete utilization for mortar Mortar M for the mortar that uses recycled metal dross produced by thermal cutting of metal and Mortar O for the mortar that uses ordinary fine aggregate. (3) Disposal Container Disposal containers for LLW generated during dismantling of NPPs are assumed to be the standardized steel disposal container designed by utilities in Japan with a Table 1 Raw concrete Raw concrete Aggregate Mix proportion Compressive Fine Coarse W=C aggregate aggregate (%) Cement Water Unit weight (kg/m 3 ) strength Fine Coarse (N/mm 2 ) aggregate aggregate 28 d Concrete A a) Sand Stone 56 b) 2 b) 145 b) 1,989 b) 22.2 c) Concrete B 1 Sand Crushed , Concrete B 2 Sand Stone , Concrete B 3 Sand Stone , a) Concrete taken from the Tokai NPP Turbine building wall for these experiments b) Presumption test of mix proportion c) Core test JOURNAL OF NUCLEAR SCIENCE AND TECHNOLOGY

4 Utilization of Crushed Radioactive Concrete for Mortar to Fill Waste Container Void Space 743 Table 2 Features of crushers Crusher Rod mill Cone crusher Jaw crusher Hammer crusher Sketch and outline specification Media: Rod 25 Φ Gap: 2mm Gap: 1-4mm High speed Basket rotation Roll motion Reciprocal motion Rotation: 3,000rpm Waste Fill Ratio[vol.%] Fig. 2 10~ 10~ 15~ ~ (shaking) Particle Size[mm] Relation between waste fill ratio and particle size capacity of 5 m 3 (1:6mW3:2mD1:0mH)(5m 3 disposal container). 6) (4) Method to Mix Mortar All the mortars in the tests were prepared in accordance with JIS R 5201 using a mechanical mixer. 2. Demonstration Tests on Pre-Placement of Concrete pre-placement fill ratios in small containers were measured to evaluate those for the 5 m 3 disposal containers. (1) Test Method Raw A was pre-placed into small cylindrical containers (2 mm220 mm) and the rubble fill ratio was measured by replacing void space in the container with water, compensating water adsorption of rubble. particle distribution patterns were changed within the range of 10 mm, as they affect the rubble fill ratio. (2) Test Results Figure 2 shows the result of the tests. The fill ratio of rubble pre-placement into an empty container was found to be 55 vol% depending on the rubble particle distribution patterns. 3. Selection Tests on the Recycled Sand Production Method As the technique for crushing concrete blocks and rubble for recycled sand is a conventional one that has been used for producing crushed aggregate from rocks or concrete blocks, there is no difficult issue to solve. In these tests, VOL. 41, NO. 7, JULY 2004 methods to produce recycled sand for the mortar that meets the required characteristics for solidifying waste are compared to select the proper crushing process and the operating conditions. The target for the recycled sand usage ratio is set as 900 kg/m 3. (1) Test Method Four kinds of crushers with different crushing principles, (i.e. rod mill, cone crusher, jaw crusher, and hammer crusher), were investigated for producing recycled sand. Representative models for each type of crusher were selected for these tests. Main features and specifications are shown in Table 2. Particle distribution was measured in accordance with JIS A1102 by the sieves provided by JIS Z8801. Tests were implemented under 2 different conditions from Raw B 2. While p-cone time for the standard mix proportion of mortars currently used for 200 drums has been 16 s, in these tests p-cone time is conservatively set as 25 s because the mortar flows long distance within the large container (5 m 3 ). In order to attain recycled sand usage ratio of 900 kg/m 3, the sand cement ratio (s=c) was set at 1.. Mix proportion for mortar is shown in Table 3. Bleeding, fluidity, and compressive strength of the mortars were measured. a;b Using the recycled sand produced by the most appropriate crusher selected in the tests above, the characteristics of the mortar with the recycled sand usage ratio of 9 kg/m 3 were measured. Then, in order to investigate the proper particle distribution pattern on the finer part of particles, the characteristics of the mortar using the recycled sand that was fur- Table 3 Mix proportion of mortar W=C Unit weight (kg/m 3 ) High eff. s=c (%) Water Cement Recycled sand AE agent % of cement a Bleeding: Free water at the solidifying state of concrete or mortar which is measured by free water volume after certain time of settling certain quantity of mortar in certain polyethylene bag (JSCE-F ). b P-cone time: A test method of fluidity of concrete or mortar at fresh state which is measured by time of certain volume of mortar to flow through p-type flow cone (JSCE-F ).

5 744 T. ISHIKURA et al. Sieve Passage Raito [wt.%] Jaw Crusher/1Cycle Jaw Crusher/2Cycle Rod Mill /0rpm Rod Mill/1000rpm Hammer Crusher/Whole Size 20 Hammer Crusher/Coarse Size Cone Crusher/Whole Size 10 Cone Crusher/Coarse Size Sieve Size [mm] Fig. 3 Table 4 Particle distribution of recycled sand Characteristics of crushed sand mortar p-cone time Bleeding Compressive strength Item (s) (%) (N/mm 2 ) 0 min min after 3 h 28 d Target value 16 s <1:0 < Rod mill Cone crusher Jaw crusher Hammer crusher used Table 5 Mix proportion and characteristics of mortar p-cone time Bleeding Compressive strength (s) (%) (N/mm 2 ) 0 min min after min 28 d B B ther crushed were measured. Surface area of the particles was measured by the Blaine method prescribed in JIS R (2) Test Results The particle distributions of the recycled sand produced by the 4 different crushers with 2 different operating conditions are shown in Fig. 3. Because the operating conditions caused no big difference in particle distribution, mortar characteristics were measured by mortar using the coarser fine aggregates as shown in Table 4. The results are summarized as follows: Rod mill: The recycled sand that had finer particles exceeded the fluidity target in a short time. Cone crusher: The recycled sand that had coarser particles exceeded the bleeding target. Jaw crusher: The performance of recycled sand mortar was good in fluidity and bleeding. Hammer crusher: The performance of recycled aggregate mortar was better in bleeding and compressive strength than that of the Jaw crusher. The hammer crusher was selected as the most appropriate crusher from the results above. Particle distribution is good for solidification performance when it is equally distributed among , , , mm in size and has mm in medium diameter. The performance of the mortar R met the target value (Table 5), keeping p-cone time within 16 s after min of mortar mixing and compressive strength of 59 N/mm 2. As for proper particle distribution pattern by further crushing of a finer aggregate, it was seen that mortar compressive strength increased up to 5 10% higher at particle surface area (5,000 cm 2 /g) than the original (3,000 cm 2 /g) when particles below 0.15 mm were further crushed. (3) Summary The hammer crusher was the most appropriate crusher as a crushing process for the recycled sand and attained 9 kg/ m 3 of the recycled sand usage ratio. While the further crushing of the finer part of the aggregate increased the compressive strength of the mortar, it does not seem effective when considering the cost increase for the processing. 4. Filling Performance Tests on Recycled Aggregate Mortar In order to increase the waste usage ratio in waste containers more than that given in previous section III-3, the margin of compressive strength was cut down to that least required for solidification material. These tests cover not only Mortar R but also Mortar P as recycled sand mortar. (1) Test Method Recycle Sand B 3 was mixed under mix proportion condition as shown in Table 6 to produce Mortar R. After A (15 mm in size) was pre-placed into small cylindrical containers (2 mmh220 mm), Mortar P was poured from the upper side of the container. pre-placement fill ratio and mortar fluidity were measured to find the proper fluidity for more than 95 vol% of mortar fill ratio. Mortar P performance tests using Recycle Sand A and Raw Powder A were implemented under the similar condition as the tests above (Table 6). In the mix proportion of Mortar P, the powder ratio in sand can be a parameter other than the water cement ratio (W=C) and sand cement ratio (s=c). In most of the tests, however, the powder ratio in sand was fixed to a specific value to simplify the result. Mortar fill ratio was calculated by the void volume, the mortar density, and the mortar weight derived from the container weight before and after filling. Table 6 Mix proportion of mortar R, P, M and O Mortar W=C Unit weight (kg/m 3 ) High eff. s=c (%) Water Cement Recycled sand AE agent R , % of cement P ,100 a) 3.0% of cement a) M , % of cement O % of cement a) Including powder (Powder=0 kg/m 3 ) JOURNAL OF NUCLEAR SCIENCE AND TECHNOLOGY

6 Utilization of Crushed Radioactive Concrete for Mortar to Fill Waste Container Void Space 745 Upper Limit of Mortar Fluidity P Cone Time [s] 20 Mortar R Mortar P Mortar Fine Aggregate Size High Eff.AE Agent (% X Cement) R Crushed Concrete < 2.5mm 1.0 P Crushed Concrete + < 2.5mm 3.0 Powder 10 µ m (Median Dia.) ( % X (Cement+Powder)) Pre placement Ratio [vol.%] Fig. 4 Relation between mortar and rubble fill ratio Waste Usage Ratio (Waste Volume/Mortar Volume) Fig Mortar O Mortar R Mortar P : Good : No Good Water Cement Ratio(W/C) Relation between waste usage ratio and W=C of mortar (2) Test Results The upper limit of the fluidity for Mortar R and P to meet the required filling performance (more than 95 vol% mortal fill ratio) is summarized in Fig. 4. This figure shows that p-cone time for Mortar R should be less than 25 s at a 45 vol% rubble pre-placement ratio, taking into account of 20 s loss due to passing through rubble fill zone in actual container (5 m 3 ) that was given by the other experiment. The proper waste usage ratio for Mortar R and P obtained by the tests above is summarized in Fig. 5. Plots on the figure marked by an open circles and squares (, ) show the mortar meets the targeted performance including fluidity and compressive strength as well as bleeding and material separation criteria. For a reference, suitable area on Mortar O 7) was shown in the figure. The bold line shows the assumed upper limit of the proper mix proportion. Mortar R met the target performance, when s=c was increased to 2.10 from 1. as W=C was increased to % from %, which attained 48 vol% of waste usage ratio (1,100 kg/m 3 ). Mortar P met the target performance, when s=c was increased to as W=C was increased to 80 90%, which also attained 48 vol% of waste usage ratio (1,100 kg/m 3 ). In these tests, a small amount of high efficiency air entraining (AE) water reducing agent was added to maintain the mortar fluidity. The result suggests that the powder has a water adsorption effect to decrease material separation. The strength was more than N/mm 2, meeting the target value ( N/mm 2 ) even considering of margin for the strength fluctuation at concrete construction. (3) Summary Proper mortar fluidity (p-cone time) was made clear to get good filling into the containers after pre-placing rubble. The limit of waste usage ratio in the Mortar R and P was seen to be approximately vol%. 5. Filling Tests Using an Actual Scale Model A remote rubble pre-placement test and mortar filling test in containers simulating the 5 m 3 disposal containers were performed to confirm the performance gained in the small scale tests in the previous section III-4. (1) Test Method A model container simulating a configuration of concrete waste in the 5 m 3 disposal container was prepared for a remote operation test and Raw B 3 (15 mm) was remotely pre-placed into the model container through a hopper and a guide as shown in Fig. 6. Another model container Check of Scattering, ing, Spalling Fig. 6 Hopper Guide Concrete Distance between Containerand Actual scale filling test by remote operation VOL. 41, NO. 7, JULY 2004

7 746 T. ISHIKURA et al. IV. Discussion Mortar P Filled Hopper Guide Concrete Until now little knowledge of the solidification characteristics of mortar using recycled sand produced by crushing concrete has been acquired. In this chapter, characteristics of the recycled sand mortar are discussed for general understanding. It also expands discussion on recycled aggregate using crushed concrete rubble as well as metal dross. This generalization is still in a preliminary stage in depth, therefore further discussion is expected in the future. Partial Model Fig. 7 Actual scale filling test using partial model simulating 1/3 of the 5 m 3 disposal container was prepared for an actual scale fill test and Mortar P was poured from the upper side of the container to check the filling performance as shown in Fig. 7. In these tests, concrete blocks were pre-placed before pre-placing the rubble. (2) Test Results In the remote pre-placement test, the rubble was remotely pre-placed into the model container without rubble dispersion at the guide angle of 55 and 1 mm of distance between the container and blocks. The rubble fill ratio, however, was decreased to 45 vol%, which is 5 vol% less than that of the manual operation because remote operation tends to leave more openings. In the mortar filling test, after rubble pre-placement into the container, Mortar P with 25 s of p-cone time was poured at a rate of 25 /min which is same condition as current practice. The fill ratio was 96.5 vol% meeting the target value as shown in Fig. 8, because the mortar went well into narrow gaps, i.e., void space among rubbles. (3) Summary Remotely operated concrete rubble pre-placement into containers is slightly inferior in the fill ratio to the manual operation. Recycled sand mortar filling performance in the real scale container was confirmed consistent with the result of small container tests. 1. Compressive Strength of Recycled Sand Mortar Based on the data obtained by the tests above and others, 5) the relation between the mix proportion (Table 6) and the compressive strength is shown on Fig. 9. Compressive strength F of Mortar R, P, M and O is in proportion to the reciprocal number of the water/cement ratio (W=C), well known as I. Lyse s Theory shown in formula (1). Both of a and b in the formula are constant derived from experiment: F ¼ a þ b=ðw=cþ: ð1þ Mortar R that uses recycled sand from concrete has relatively higher compressive strength than that of Mortar O that uses ordinary sand. One of the factors to cause this difference is supposed to be the effect of surface roughness of Compressive Strength [N/mm ] Mortar M Mortar O Mortar R Sb=0.0 Mortar P Sb= Water Cement Ratio (W/C) Fig. 9 Relation between strength and W=C of mortar Concrete Mortar P Mortar P Concrete Fig. 8 Partial model waste container filled with recycled sand mortar JOURNAL OF NUCLEAR SCIENCE AND TECHNOLOGY

8 Utilization of Crushed Radioactive Concrete for Mortar to Fill Waste Container Void Space 747 the aggregate, because it would be the only significant difference among such factors to affect the concrete strength as W=C ratio, air ratio, cement strength, and surface roughness. 8) Mortar P that contains water-adsorptive powder with approximately 10 mm medium diameter tends to increase the strength, because water consumed for cement solidification lessens due to adsorption by the powder. Therefore, with the assumption that strength depends on not only the commonly accepted I. Lyse s Theory but also a powder ratio (Sb) given by formula (2), the relation between compressive strength and W=C was analyzed by multivariable analysis or actually two variables, i.e., W=C and Sb: Sb ¼ Unit powder usage in weight ð2þ /Unit water usage in weight: The result is shown in Fig. 9 and following formulas including the other Mortar R, M and O: Mortar P: F(N/mm 2 ) ¼ 4:97 þ 19:7ð1 þ 0:767SbÞ =ðw=cþ Mortar R: F(N/mm 2 ) ¼ 17:5 þ 41:1=ðW=CÞ Mortar M: F(N/mm 2 ) ¼ 32:6 þ 38:5=ðW=CÞ Mortar O: F(N/mm 2 ) ¼ 21:8 þ 29:8=ðW=CÞ: The difference in the compressive strength between Mortar R with no powder (Sb ¼ 0:0) and Mortar O with river sand are also judged to be caused by roughness of the aggregate surface, because the recycled sand produced from concrete must have a rough surface and adhesive cement paste. Mortar M also has rough surface, because the metal dross is produced by thermal cutting of metal. This is apparently the reason why the compressive strength of Mortar M is higher than that of Mortar O. It should be noted that metal dross has a higher density (approximately 6 g/cm 3 ). Therefore, W=C of Mortar M should be limited to less than a relatively small value (approximately %) to avoid material separation after mixing. The formulas above enable preliminary estimation of proper mix proportion for recycled sand mortar that meets the required compressive strength. 2. Proper Mix Proportion of Recycled Aggregate Mortar Proper mix proportion of Mortar R in relation to W=C and s=c is shown in Fig. 10. Because compressive strength decreases as W=C increases, maximum W=C is calculated to be 0.71 when the lowest compressive strength N/ mm 2 is substituted for F in formula (4). High W=C was seen to cause bleeding on the one hand, and low W=C at high s=c tends to be short of water on the other. An upper limit of s=c as shown in a bold line in Fig. 10 is regarded to have a linear relation with W=C. A straight line gives the following formula: Upper Limit of s=c ¼ 0:95 þ 4:48 ðw=cþ: ð7þ The upper limit of recycled sand ratio in mortar given by s=ðs þ C þ WÞ is drawn from formula (7) as shown in formula (8). Actual data and the relation is shown in Fig. 11: ð3þ ð4þ ð5þ ð6þ Sand Cement Ratio (s/c) Waste Usage in Mortar [kg/m ] Fluidity: Low P Cone Time(Initial) sec Material Separation: Large Water Cement Ratio (W/C) Fig. 10 1,0 1,0 1,200 1,100 1, Fig. 11 Upper limit of s=ðs þ C þ WÞ ¼f 1:58 þ 5:03 ðw=cþg =f 0:2 þ 2:97 ðw=cþg: 3 1.1t/m 3 1.0t/m 0.9t/m 3 0.8t/m3 Relation between s=c and W=C of Mortar R Fluidity: Low P Cone Time(Initial) sec Material Saparation: Large Water Cement Ratio (W/C) Relation between waste usage and W=C of Mortar R The proper mix proportion for Mortar R, P, M and O given by filling performance tests and others 9) in relation to W=C and s=ðs þ C þ WÞ is shown in Fig. 5. Maximum waste usage ratio in Mortar R was seen to have a similar trend with Mortar P and M, which may support the result of the speculative analysis above. V. Applicability Study for Actual Waste Solidification In order to apply this technique to actual decommissioning waste solidification, nuclide confinement characteristics, process design, and the effect of volume reduction on total waste disposal volume need to be studied. 1. Nuclide Confinement Characteristics When mortar is used to solidify radioactive waste into a waste container, it is common to measure the distribution coefficients of each nuclide. For the recycled sand mortar, the effect on the distribution coefficients must be checked. Be- ð8þ VOL. 41, NO. 7, JULY 2004

9 748 T. ISHIKURA et al. Relative Distribution Coefficient Ratio (1=current value for Rokkasyo Center) Co Sr Ni Eu Nuclide Mortar Fine Aggegate High Eff.AE Agent Materials Size ( %X Cement) R Crushed Concrete Crushed Concrete + <2.5mm <2.5mm P Powder 10µ m (Median Dia.) ( % X (Cement+Powder)) O Silica Sand <2.5mm 0.9 Remarks (Simbol in Fig.) Fig. 12 Nuclide confinement of mortar cause recycled sand mortar consists of ordinary aggregate and concrete paste originated from ordinary Portland cement, there must be no significant difference between ordinary mortar and the recycled sand mortar. In detail, however, adsorption of carbon dioxide in air into concrete, known as neutralization, and the increase of cement ratio due to cement paste attached to recycled sand can be considered. Therefore nuclide confinement characteristics of recycled sand mortar were measured in accordance with AESJ-SC- F003:2002. The result is that there are some differences in nuclide distribution coefficients between the recycled sand mortar and ordinary mortar except for that of strontium as shown in Fig. 12. These results must be considered for detail design of the waste disposal facility. 2. Study of Process Design The improved method requires two additional areas to the current solidification process, i.e. an area for rubble production and pre-placement, and an area for recycled sand production. In the detail design, protection against radiation exposure from massive radioactive concrete and from inhalation of micro particles generated by the crushing process shall be taken into account. Hoods and compartments are recommended for separating from operators. 3. Effect of the Process on Volume Reduction The effect of the process on the waste volume reduction was estimated for NPP decommissioning. As for pre-placement of blocks and rubble into waste containers, the shape of the 5 m 3 disposal container was taken into account including projections of the twist lock attachment and the L-steel on the bottom plates. As a result, the pre-placement ratio of blocks and rubble is estimated as approximately vol% and vol% respectively as shown in Fig. 13. In the case of Mortar P or R to fill the residual void space of the disposal containers after pre-placement of concrete blocks and rubble, the radioactive waste usage ratio was estimated to attain approximately 76 vol% (Fig. 14). Fig. 13 Mortar The 5 m 3 disposal container after pre-placement Waste Usage Volume Ratio in Container [vol.%] Cement and Water Recycled Sand and Powder Waste (Mortar Fluidity) Recycled sand and Powder small (9%) Low big (17%) Volume Ratio constant (51%) big (16%) high small (9.6%) Fill Ratio [vol.%] Fig. 14 Relation between waste usage ratio and rubble fill ratio JOURNAL OF NUCLEAR SCIENCE AND TECHNOLOGY

10 Utilization of Crushed Radioactive Concrete for Mortar to Fill Waste Container Void Space 749 Relative Number of Waste Forms Cement/ Water/Sand Powder Recycled Sand Scenario Current A B block Partially Crushed Pre placemente Pre placement rubble Pre placement fine aggregate powder Homogeneous Solid Recycled Sand for Mortar Fig. 15 Disposal volume of each disposal scenario Since the radioactive waste usage ratio of 76 vol% is approximately 1.5 times larger than that of the current technique, the radioactive waste disposal volume is estimated to decrease below 2/3. Therefore the newly developed method can provide a large waste disposal volume reduction for the radioactive concrete waste generated during NPP decommissioning (Fig. 15). VI. Conclusions Various tests confirmed applicability of the technique of radioactive concrete utilization for solidifying radioactive waste in waste forms. As the result: (1) The mortar using the recycled sand produced from concrete was confirmed under the conditions simulating actual waste containers to achieve the target of required solidification characteristics under proper mix proportions. (2) Relations between the mix proportions and the compressive strength as well as the maximum waste usage ratio of mortars using not only crushed concrete but also metal dross were discussed. (3) The developed method was estimated to drastically increase the waste usage ratio in waste containers, which is quite effective for decreasing waste disposal volume of radioactive concrete generated during dismantling of NPPs. The authors expect the developed technique will be actually applied as a measure to decrease the total radioactive waste disposal volume generated from NPP decommissioning in the future. Nomenclature C: Cement weight in unit mortar (kg/m 3 ) F: Compressive Strength (N/mm 2 ) s: Sand weight in unit mortar (kg/m 3 ) Sb: Powder ratio, given by powder weight/water weight in unit mortar W: Water weight in unit mortar (kg/m 3 ) Abbreviations LLW: Low Level Radioactive Waste Mortar R: Mortar that uses recycled sand for aggregate Mortar P: Mortar that uses recycled sand and powder for aggregate Mortar M: Mortar that uses metal dross produced by thermal cutting of metal for aggregate Mortar O: Mortar that uses ordinary fine aggregate NPP: Nuclear Power Plant p-cone time: Efflux time of mortar through a standardized flow cone (ASTM C 939) 5m 3 disposal container: Standardized steel disposal container with 5 m 3 designed by utilities in Japan with a capacity of 5 m 3 JIS: Japan Industrial Standard JSCE: Japan Standard of Civil Engineering AESJ-SC: Atomic Energy Society in Japan Standards Committee Acknowledgement This development work was implemented under contract between the Ministry of Economy, Trade and Industry (METI) and Nuclear Power Engineering Corp. (NUPEC) regarding verification tests for nuclear power plant decommissioning techniques. The authors wish to extend their sincere gratitude to Prof. K. Ishigure, Saitama Institute of Technology, Chairman of the steering committee of the verification tests and concerns. References 1) Radioactive Waste Management Research Center, Radioactive Waste Data Book, p (1998), [in Japanese]. 2) H. Ueki, et al., Verification test of decommissioning waste processing system A technique on recycling concrete waste; utilization of radioactive concrete for filling material, Proc. Int. Conf. on Environmental Remediation, Tokyo, Japan, Sept. 26, 1999, (1999), (CD-ROM). 3) T. Ishikura, et al., Utilization of radioactive waste for solidifying material to fill waste forms, Proc. Int. Conf. on Environmental Remediation, Brugges, Belgium, Sept. Oct. 4, 2001, (2001), (CD-ROM). 4) Ten Utilities in Japan, Standard Production Method to Fill and Solidify Waste Forms Rev. 2, Sept. 2000, [in Japanese]. VOL. 41, NO. 7, JULY 2004

11 7 T. ISHIKURA et al. 5) Institute of Applied Energy, Report of Decommissioning Technology on Commercial Nuclear Power Plant, Heisei 11 Fiscal Year, (2000), [in Japanese]. 6) S. Karigome, Measures for radioactive waste on nuclear power plant decommissioning, Proc., Issues on Radioactive Waste, Japan Nuclear Energy Information Center Co., p. 80 (2001), [in Japanese]. 7) Radioactive Waste Management Research Center, Waste Form Production Technique for LLW Disposal Various Solid Waste, p. 100 (1998), [in Japanese]. 8) M. Shirakawa, et al., Development of recycling techniques for nuclear power plant decommissioning waste using dross-filled packaging technique, Proc. Int. Conf. on 11th Nuclear Engineering, Tokyo, Japan, Apr , 2003, ICONE11-386, (2003), (CD-ROM). 9) Japan Concrete Institute, Essence of Concrete Technology, Japan Concrete Institute, Tokyo, p. (2002), [in Japanese]. JOURNAL OF NUCLEAR SCIENCE AND TECHNOLOGY