Israeli Standard IS 5098

Transcription

1 Hotel Dan Tel Aviv, November 2005 Israeli Standard IS 5098 (Setting limits on the concentration of natural radionuclides in building products) By Tuvia Schlesinger* and Yair Grof** * Scientific secretary of the National Professional Advisory Committee on the Control of Radiation in Israel, 1 Maaleh Brosh, Efrata 90435, Israel. ** Radiation Safety Division, Soreq NRC, Yavneh, Israel

2 2 Background Fly ash and bottom ash produced in the process of burning coal in power stations contain enhanced concentrations of the natural radionuclides 40 K, 232 Th and 226 Ra and their decay products (mainly 222 Rn and its radioactive daughters). The ashes are utilized as constituents of building products (e.g. concrete, masonry blocks, tiles, etc). The residents of buildings constructed of these products are exposed to external ionizing radiation emitted the by these radionuclides and to Radon 222 emanated by the 226 Ra. This exposure can cause detrimental health effects to these residents.

3 3 Israeli Standard IS 5098 What is IS 5098 Radiation Protection Principles The radiation index The activity concentration index criteria Dose estimation External radiation and radon emanation Practical implementation

4 4 Israeli Standard IS 5098 Israeli Standard IS 5098 (the standard) sets limits on the concentration of natural radionuclides in building products in Israel. It was published in November 2002 by the Israel Standards Institute under the title: Israeli Standard - IS 5098 Radioactivity in building products

5 5 Radiation Protection Principles The limitations set by IS 5098 on the concentrations of natural radioisotopes in building products in Israel are based on international recommendations. In 1999 the ICRP recommended setting a maximal dose constraint, in the range msv/y, on the dose to members of the public from practices causing prolonged chronic exposure to natural radiation (ICRP pub.82). The constraint should be set on the dose increment (the additional dose) caused by the introduction of a new practice.

6 6 Radiation Protection Principles (cont.) The ICRP recommended a dose rate range of ( msv/y) for the constraint set on a single practice. The specific dose constraint selected should be the result of an optimization process conducted by the competent authority granting the authorization to the new practice. The exposure caused by enhanced concentrations of natural radionuclides in building materials is a prolonged chronic exposure that can be (and should be) controlled..

7 7. IS 5098 Radioactivity in Building Products Legal Basis The National Professional Advisory Committee on the Control of Radiation in Israel adopted, in the year 2000, the recommendations of the ICRP related to the protection of the public in situations of prolonged exposure to ionizing radiation. The committee recommended setting a dose constraint of 0.3 msv/y or less on the dose increment to members of the public in Israel from any single practice involving prolonged exposure to natural radiation. It decided that the building industry engaged in the production of concrete, masonry blocks, tiles, etc. should be regarded as a single practice.

8 8 IS 5098 legal basis (cont.) In the context of natural radioactive elements in building products the committee interpreted the notion additional dose as the additional dose incurred by inhabitants of buildings due to enhanced concentrations of natural radioactive elements in building materials/products. The reference level for calculating the additional dose is the dose incurred in buildings constructed of conventional building materials used in the building industry in Israel before the early 1990s.

9 9 IS 5098 legal basis (cont.) Based on surveys and laboratory measurements the committee concluded that reference level (i.e. the annual dose to the public due to natural radioactive elements in the majority of conventional building products in Israel was (before the introduction of the novel building products) of the order of 0.15 msv, including gamma radiation and inhalation of radon- 222 originated in the building material. Therefore the committee recommended allowing a total annual dose of 0.45 msv ( msv) to members of the public due to. 40 K, 226 Ra and 232 Th in building materials/products. These recommendations are the basis for IS 5098 (the standard).

10 10 The activity concentration index The limit on the dose increment caused by a building product is presented in the standard as a derived constraint expressed in terms of the activity concentrations of the three major natural radio isotopes 40 K, 226 Ra and 232 Th. An activity concentration index i (the radiation index) for a building product is introduced in the standard. This index is defined as: i = C ( 40 K) / A ( 40 K) + C ( 226 Ra) / A ( 226 Ra) + C ( 232 Th) / A ( 232 Th) Where C ( 40 K), C ( 226 Ra) and C ( 232 Th) are the activity concentrations (Bq/kg) of 40 K, 226 Ra and 232 Th in the building product, respectively and A ( 40 K), A ( 226 Ra) and A ( 232 Th) are limits set (by the standard) on the activity concentrations (Bq/kg) of these radionuclides in the specific building product, respectively.

11 11 The activity concentration index (cont.) The activity concentration limit A ( s X), for a specific radio-nuclide s X, is the activity concentration value (Bq/kg) for the radio-nuclide s X that, if present in a building product, will cause, alone, a specified additional annual dose (in our case 0.45 msv) to the residents of a standard building constructed of this building product. The activity concentration index criteria The activity concentration index criteria for compliance, with the requirements of IS 5098, of a specific building product, containing 40 K, 226 Ra and 232 Th with activity concentrations C ( 40 K), C ( 226 Ra) and C ( 232 Th), is that: C ( 40 K) / A ( 40 K) + C ( 226 Ra) / A ( 226 Ra) + C ( 232 Th) / A ( 232 Th) <= 1

12 12 Dose estimation model and computer code A theoretical model and a computer code were developed for estimating the annual dose to residents of buildings constructed of building products/materials containing 40 K, 226 Ra and 232 Th in order to compare this dose to the dose constraint set by the standard (0.45 msv). The model assumes a room of standard sizes made of 4 walls and a floor and a ceiling. The parameters used by the computer code are: a. The thickness (m) and the density (kg/m 3.) of the walls and of the floor and the ceiling of the building made of the building product. b. The concentrations, in the product, of 40 K, 226 Ra, and 232 Th (Bq/kg) and c. The radon exhalation rate (Bq/m 2 Sec) from the building product in the specified conditions, etc.

13 13 Dose estimation model and computer code (cont.) The computer code estimates the dose rate (due to external gamma radiation and due to inhalation of radon exhaled from the building product), anticipated to a person staying in a specified location in the room (e.g. in the center of the room). Activity concentration limits The standard provides tables of the activity concentration limits A ( 40 K), A ( 226 Ra) and A ( 232 Th) corresponding to an annual dose of 0.45 msv, to a person staying for 7,000 hours in a year at the center of a standard room of dimensions 3X3X2.7 m. The same density and thickness is assumed for all four walls and the floor and ceiling. The air change rate is assumed to be 1 in an hour.

14 14 Activity concentration limits (cont.) The standard presents tables of activity concentration limits A ( 40 K), A ( 226 Ra) and A ( 232 Th) for materials in the density ranges kg/m 3 and thickness range m. For each set of density and thickness values three values of A ( 226 Ra) are presented, corresponding to radon emanation (from the building material into the room) of 0.01, 0.03 and 0.05 (i.e. emanation of 1%, 3% and 5%). Radon emanation The emanation value for a specific building product is defined as the fraction of the 222 Rn produced in the building product that emanates into the enclosure (room) whose walls are made of this product.

15 15 Radon emanation (cont) The emanation e is estimated from the measured values of the exhalation rate, E (Bq/m 2 -h) using the equation E = e C λ ρ d/2 e is the emanation (e.g. if the emanation is 5% e=0.05) C is the concentration (Bq/kg) of 226 Ra in the product λ is the decay constant of radon λ = h -1 ρ is the density (kg/m 3 ) of the building material d is the thickness (m) of the product Activity concentration limits (cont.) Table 1 is an example of the activity concentration limits A ( 40 K), A ( 226 Ra) and A ( 232 Th) calculated by the code and presented in the standard for 20 cm wide building blocks.

16 16 Table 1-Activity concentrations limits A ( 40 K), A (226Ra) and A ( 232 Th) for building products of thickness 20 cm and density in the range kg/m 3 Thickness 0.2 m A (Th-232) A(Ra-226) A(k-40) Density (Kg/m 3 ) e=0.05 e=0.03 e=

17 17 Application of the standard In the following examples we will demonstrate the practical application of the standard to examine the compliance with the requirements of the standard of a new type of thermal building masonry blocks containing 50% and 60% (by weight) of a mixture of fly ash and bottom ash. a. The measured values of the density, thickness and radon emanation of the block containing 50% ash are: Density (effective): 800 kg/m 3. Thickness: 20 cm. Activity concentration: 167 Bq/m 3, 83 Bq/m 3 and 79 Bq/m 3, for 40 K, 226 Ra and 232 Th, respectively. Emanation: 1%

18 18 Application of the standard (cont.) Checking Table 1 we find that activity concentration limits for the three radio-nuclides as specified for these values of the density, thickness and emanation are: 1,896 Bq/m 3, 210 Bq/m 3, and 205 Bq/m 3, respectively. The activity concentration index for this block is therefore: i = C ( 40 K) / 1,896 + C ( 226 Ra) / C ( 232 Th) / 205 Inserting the activity concentrations of the three radionuclides measured in the block in this formula we get: i = 167 / 1, / / 205 = The building product complies therefore with the legal requirements of the standard (i.e. the activity concentration index criteria i <= 1).

19 19 Application of the standard (cont.) b. The data related to the density, the thickness and the radon emanation of the block containing 60% (by weight) of a mixture of fly ash and bottom ash are similar but the radio-nuclide concentration was found to be somewhat higher as follows: 185 Bq/m 3, 98 Bq/m 3 and 95 Bq/m 3, for 40 K, 226 Ra and 232 Th, respectively. The activity concentration index for this block is therefore: i = 185 / 1, / / 205= 1.03 This building product does not comply with activity concentration index criteria (i > 1).

20 IS 5098 References 20 (ICRP 2000) Protection of the public in situations of prolonged radiation exposures, Annals of the ICRP Vol. 29 No , ICRP Pub. 82, (IS 2002) Radioactivity in building products, Israeli Standard IS 5098, Israel Standard Institute, Nov (Grof and Schlesinger 2002) Y. Grof and T. Schlesinger, Radiological aspects of the use of building materials containing enhanced concentrations of natural radionuclides in Israel Theoretical model and computer code to estimate the radiation dose to residents, Proceedings of the European IRPA Congress, Florence, Italy, 2002.