GB2211834A

GB2211834A - Radiation shielding concrete composition

Info

Publication number: GB2211834A
Application number: GB8725798A
Authority: GB
Inventors: James Edward Brannan
Original assignee: Individual
Current assignee: Individual
Priority date: 1987-11-04
Filing date: 1987-11-04
Publication date: 1989-07-12
Anticipated expiration: 2007-11-04
Also published as: GB2211834B; GB8725798D0

Abstract

A radiation shielding concrete composition comprises cement, magnetite, coarse aggregate, e.g. limonite coarse aggregate, and water together with water insoluble calcium borate.

Description

Concrete Composition This invention relates to an all purpose radiation shielding and structural concrete composition. In particular it relates to a concrete composition which provides protection against both gamma rays and neutrons, and also possesses high structural strength.

Concrete compositions that have been used in the past to provide radiation shielding have not been able to provide adequate protection against neutrons. In previous experiments borax has frequently been included in concrete compositions in an attempt to capture the slow neutrons.

However this form of material has proved unsuccessful in that the borax has prevented the concrete composition from setting.

It has now unexpectedly been found that by adding water insoluble calcium borate (boron) to concrete compositions, excellent results can be achieved in providing overall protection against gamma rays and neutrons.

Furthermore, the concrete formed therefrom also possesses high structural strength.

According to the invention, there is provided a radiation shielding concrete composition comprising cement, magnetite, coarse aggregate and water together with water insoluble calcium borate.

Preferably the concrete composition comprises from 330 - 550 kg/m3 cement, from 764 - 1222 kg/m3 magnetite, 1485 kg/m3 coarse aggregate, water insoluble calcium borate present in a borate/cement ratio of 6% and water which is present in a water/cement ratio of 0.38.

Most preferably the concrete composition comprises 500 kg/m3 cement, 188 kg/m3 sand, 814 kg/m3 magnetite, 90 kg/m3 boron, 1485 kg/m3 limonite coarse aggregate and 190 kg/m3 water.

The cement used in the composition may be ordinary Portland Cement, or alternatively a sulphate resisting cement may be employed if constructing in sulphate bearing strata.

The sand is preferably limonite-bearing sand, from 764 to 1222 kg/m3 of which may be included in the concrete composition. Alternatively, the sand may be omitted completely and replaced by magnetite. The amount of magnetite can, therefore, vary and may increase as the quantity of cement and/or sand is decreased.

The limonite coarse aggregate used which is a naturally occuring iron ore being a hydrous form of ferricoxide, preferably has a maximum size of 14mm down.

A plasticising agent, such as Conplas 211, may be used in the composition, the quantity of the agent is preferably included within the water content in accordance with the manufacturers instructions. A plasticising agent is employed to counteract the coarse and sometimes gapgraded characteristics of the limonite coarse aggregate.

The concrete composition of the present invention is mixed as a normal concrete in a standard pan mixer and compacted by standard forms of vibration when placed or poured to form the required structural configuartion.

This concrete composition has been found to have higher structural strength and greater density than that of an ordinary Portland Cement Concrete (OPCC), as well as providing outstanding radiation shielding values. For a given transmission factor the concrete of the present invention can be 33.33% less in thickness than OPCC against any form of gamma ray exposure, and 16.66% less in thickness than OPCC when exposed to Americum neutrons.

The composition of the present invention may be used for all purposes of nuclear protection, radioactive waste disposal and/or storage, radiological installations in hospitals, industrial x-ray structures and all structural purposes in the nuclear/radiation fields, or even applied in a gunite type operation.

The invention will now be further illustrated by the following non-limitative examples.

Example 1 Materials kg/m3 Moisture Wet Water Content Weight Weight Portland Cement 500 Lirronite-bearing Sand 188 3.0 194 6 Magnetite 814 1.5 826 12 Boron 90 - Limonite (14mn down coarse aggregate) 1485 Water/cement ratio = 0.38 (190 kg/cm3) including 0.5 plasticising agent (Conplas 211).

The above composition was mixed as a normal concrete in a standard pan mixer, and compacted by external vibration when placed to form the required structure.

Example 2 Comparative Tests For the purposes of the following comparison the concrete prepared in Example 1 was tested in accordance with BS 1881 against an ordinary Portland Cement Concrete (OPCC) with a compressive strength of 26 N/mm2 after 28 days, and with a density of 2.35 T/m3 (as interpreted from BS.4094).

(a) Structural Strengths & Densities (i) Compressive strength after 28 days: ex.l composition = 62 N/mm2 OPCC = 26 N/mm2 (ii) Plastic density after 28 days: ex 1. composition = 3.125 T/m3 OPCC = 2.35 T/m3 These comparisons prove that the concrete composition of Example 1 possesses a structural value 238% greater than that of OPCC after 28 days together with a density 133% higher than that of OPCC (b) Shielding Values Tests were carried out by the Radiation Protection Advisory Service and provided the following results: (i) Against gamma rays, when using a 50cm thickness of both materials exposed to the following isotopes::60Co - the concrete of Example 1 proved to be 10 times better than OPCC 137Cs - the concrete of Example 1 proved to be 36 times better than OPCC 1921r - the concrete of Example 1 proved to be 70 times better than OPCC (ii) Against fast neutrons, when using a 60cm thickness of both materials exposed to the following isotope, the result was:241AmBe - the concrete of Example 1 proved to be twice as good as OPCC. (This was a comparison of the concrete of Example 1 between OPCC and water).

(NB) Co = Cobalt, Cs = Caesium, Ir=Iridium, AmBe = Americium Beryllium.

(iii) Against slow (thermal) neutrons. The slow neutron region is considered to consist of two componenets; thermal neutrons peaking at 0.3 eV and epithermal neutrons peaking at 0.3 eV to 2.0 eV.

Measurements were carried out using bare and lmm thick cadmium-covered indium foils, utilising the 115IN(n *) 116IN reaction.

The beta-gamma emission was detected by a scintillation copunter mounted in a lead castle.

A source of slow neutrons was created by water moderating 241AmBe fast neutrons. Thin slabs of the concrete of example 1 were placed between the source and detector and the cadmium ratio on the activated foils was obtained.

It will be seen from the following Table 1 that llcm thickness of the composite concrete Example 1 has a similar 1,000 attenuation to thermal neutrons as 0.5mm of cadmium.

For ordinary concrete of density of 2.4T/m3 and barytes concrete of a density of 3.1 T/m3, a 1,000 attenuation factor of thermal neutrons is only obtained with a thickness of 56cm.

In the thermal neutron energy region, cadmium is superior to boron in attenuating neutrons, by virtue of its factor of 4 higher cross section.

In the epithermal region however, the cadmium cross section falls rapidly and at 2.0eV, the boron cross section is a factor of 10 greater than cadmium and is still a factor of 6 greater at 10.0 eV.

This superior efficiency of the composite concrete of Example 1 for attenuating epithermal neutrons, is clearly shown on the following Figure 1.

Table 1 shows slow neutron attenuation through slabs of the composition Example 1. Figures 1 to 4 showing the attenuation curves when exposed to the aforementioned isotopes.

TABLE 1 SLOW NEUTRON ATTENUATION THROUGH LIMACRETE SLABS

1 Thickness Epithermal Thermal of Neutrons Neutrons Limacrete cm 0 100 100 3.7 54.4 7.1 7.4 42.4 1.5 11.1 27.8 x0.1

Claims

CLAIMS 1. A radiation shielding concrete composition comprising cement, magnetite, coarse aggregate and water together with water insoluble calcium borate.
2. A concrete composition as claimed in claim 1 comprising from 330-550 kg/m3 cement, from 764 - 1222 kg/m3 magnetite, 1485 kg/m3 coarse aggregate, water insoluble calcium borate present in a borate/cement ratio of 6%, and water which is present in a water/cement ratio of 0.38.
3. A concrete composition as claimed in claim 1 or claim 2 wherein the coarse aggregate is limonite coarse aggregate.
4. A concrete composition as claimed in claim 2 comprising 500 kg/m3 cement, 188 kg/m3 sand, 814 kg/m3 magnetite, 90 kg/m3 boron, 1485 kg/m3 limonite coarse aggregate and 190 kg/m3 water.
5. A concrete composition as claimed in any one of claims 1 to 4 wherein the cement is ordinary Portland Cement or sulphate resisting cement.
6. A concrete composition as claimed in any one of claims 1 to 3 or 5 which also comprises sand.
7. A concrete composition as claimed in claim 5 wherein the sand is limonite-bearing sand and is present in from 764 to 1222 kg/m3.
8. A concrete composition as claimed in any one of claims 3 to 7 wherein the limonite coarse aggregate has a maximum size of 14mm down.
9. A concrete composition as claimed in any one of claims 1 to 8 which also comprises a plasticising agent.
10. A radiation shielding concrete composition substantially as hereinbefore described with reference to the accompanying examples.