3.3.1 Standard Construction Practices
The NPP concrete, which serves as a structural component, is also used as a material for radiological protection. For radiological protection of the NPP heavy concrete is used with volumetric mass density from 1.8 to 2.5 t/m3 and particularly heavy concrete with density of more than 2.5 t/m3.
Heavy concretes for radiological protection, which can be employed at temperatures up to 50 oC, are called ordinary heavy concretes. Concretes, which are used at temperatures from 51 to 350 oC, are called concretes for abnormally high temperatures. Concretes, which are used at temperatures higher than 350 oC are called heat resistance concretes.
Concrete is a composite material, which consists of a binder, fine and coarse aggregates. In most cases portland cement is used as binder, sand is used as fine aggregate. For preparation of coarse aggregate used in protection concretes, metamorphic rocks (serpentinite) are used, as well as metallic ores (magnetite, chromite, hematite),and artificial materials (chamotte, boron carbide).
For radiological protection of the Ignalina NPP ordinary heavy concrete with a density of 2.2 t/m3 is used most widely, and to a smaller degree, particularly heavy concrete with a density of 4.0 t/m3 [2]. Special protective concretes or solutions with complex chemical composition are used in very small amounts. At the Ignalina NPP serpentinite solution is used as a filling of top metallic structures. Density of the serpentinite solution is about 1.7 t/m3 [2].
Use of concrete for radiological protection of nuclear reactors is possible provided the following conditions are met:
3.3.2 Material Properties Used
All initial concrete properties are determined at normal temperature (20 oC) and depend on concrete consistency. Protective properties of concretes are determined by two main factors: chemical composition and nuclear density. Composition and protective properties of ordinary and particularly heavy as well as serpentinite concretes are shown in Tables 3.8 and 3.9.
Variation of structural and material properties caused by the influence of ionizing radiation, depends on two main factors, namely, composition and the radiation load on the material. After radiation all concretes (except chromite) increase in volumetric dimensions, and the density decreases. The influence of radiation on strength of concrete is shown in Table 3.10.
Compression strength of ordinary heavy concrete depends on the type of cement used and can reach 50 MPa. Heat conductivity and the coefficient of thermal expansion range up to 700 oC is 1.14 W/(mK) and 3.5× 10-6 oC-1, respectively. Limiting permissible temperature of ordinary heavy concrete is 1200 oC [31]. Concrete exposure to neutron flux leads to a storage of radiation defects in the concrete, which can be a cause of variation of their mechanical and thermo-physical properties. The exposure of ordinary heavy concrete to a neutron fluence up to 5× 1019 n/cm2 does not change properties significantly. Increasing the fluence to 1.45× 1020 n/cm2 leads to visible variation of properties: decreasing of density to 7-11 %, thermal conductivity to 30-35 %, coefficient of linear thermal expansion by 5-10 times, compressive strength and elastic properties to 10-20 % [32].
Compressive strength of particularly heavy (chromite) concrete depends on the type of cement used and may reach 40 MPa. Heat conductivity and coefficient of thermal expansion at 700 oC is 1.57 W/(mK) and 3.4x10-6 oC-1, respectively. The permissible temperature operating limit of particularly heavy (chromite) concrete is 1700 oC [31].
Compressive strength of serpentinite concrete is 40 - 62.5 MPa Elastic modules of serpentinite concrete varies from 18200 to 6800M at temperature variation from 20 to 500 oC, respectively. Serpentinite concrete is a heat resistant material and has a coefficient of heat conductivity, which linearly decreases from 0.91 to 0.74 W/(mK) with temperature increase from 20 to 450 oC. Coefficient of linear expansion is constant in temperature interval 100-450 oC and equals 4.2× 10-6 oC-1. Limit of permissible temperature for serpentinite concrete is 500 oC [31]. Maximum change of coefficient of linear expansion of serpentinite concrete is 1.3-1.7 % and is affected by exposure dose of (1.3-1.7)× 1021 n/cm2. Strength of serpentinite concrete with increase of exposure dose (1.3-1.7)× 1021 n/cm2 reduces to 40 % of the original strength. Variation of the modules of elasticity is nearly the same as the strength variation. Thermal conduction coefficient decreases by 13 % upon exposure of 1.7× 1021 n/cm2. Coefficient of linear thermal expansion at exposed and nonexposed serpentinite concrete by repeated heating is the same (6-7)× 10-6 oC-1 [33].
Table 3.8 Chemical compositions of different concretes [30]
|
Concrete |
Chemical elements, kg/m3 |
||||||||||||
|
H |
O |
B |
C |
Na |
Mg |
Al |
Si |
Ca |
Fe |
S |
F |
Cr |
|
|
Ordinary heavy |
8 |
1275 |
- |
- |
- |
- |
110 |
774 |
137 |
46 |
- |
- |
- |
|
Particularly heavy (chromite) |
- |
1200 |
- |
2 |
29 |
194 |
175 |
145 |
119 |
263 |
2 |
21 |
1119 |
|
Particularly heavy (magnetite) |
10 |
1150 |
- |
- |
- |
- |
30 |
661 |
164 |
1245 |
- |
- |
- |
|
Serpentinite |
25 |
1000 |
12 |
- |
- |
332 |
42 |
309 |
132 |
86 |
9 |
- |
- |
Table 3.9 Neutron and gamma quantum attenuation parameters of concrete [29,30]
|
Concrete |
Cross-section of extraction |
Storage coefficient for X = 1.0 - 1.5 m |
Attenuation of gamma radiation with E = 3 MeV coefficient |
||||||
|
S r |
Ki |
Kh |
Kg |
m , |
m /r , |
||||
|
m-1 |
(W/m2)/(n/cm2) |
m-1 |
10-3m2/kg |
||||||
|
Ordinary heavy |
8.0 |
10.2 |
105 |
130 |
8.52 |
3.63 |
|||
|
Particularly heavy (chromite) |
10.02 |
1000 |
100 |
900 |
11.84 |
3.63 |
|||
|
Particularly heavy (magnetite) |
9.5 |
18.0 |
8 |
100 |
11.90 |
3.63 |
|||
|
Serpentinite |
6.5 |
20.0 |
82 |
93 |
7.01 |
3.64 |
|||
Ki, Kh - storage coefficients of intermediate and heat neutron, respectively, (W/m2)/(n/cm2),
K - storage coefficients of gamma radiation capture, (W/m2)/(n/cm2),
S r - macroscopic extraction of protection material, m-1,
m - linear coefficient of attenuation of gamma quantum flux density, m-1,
m /r - mass coefficient of attenuation of gamma radiation, m2/kg,
X - thickness of protection, m.
Table 3.10 Radiation influence to strength of concretes [29]
|
Concrete |
Neutron flux, n/cm2 |
Temperature, oC |
Compressive strength, MPa |
|
Ordinary heavy |
0 |
20 |
12.5 |
|
(0.4-0.6)× 1020 |
100 |
12.5 |
|
|
(1.2-1.4)× 1020 |
150 |
10.0 |
|
|
(3-4)× 1020 |
180 |
4.5 |
|
|
0 |
200 |
9.0 |
|
|
Particularly heavy (chromite) |
0 |
20 |
12.0 |
|
(16-24)× 1020 |
550-650 |
4.8 |
|
|
0 |
550 |
7.9 |
|
|
Serpentinite |
0 |
20 |
9.0 |
|
(1.3-1.9)× 1020 |
100-150 |
9.5 |
|
|
(5-6)× 1020 |
200-250 |
7.0 |
|
|
(13-17)× 1020 |
250-300 |
4.0 |
|
|
0 |
300 |
11.0 |