The Free Surface Contamination Limit in Controlled Areas

Who this page is for, and what it answers

The free surface contamination limit in a controlled area is two numbers, not one: 0.37 becquerels per square centimetre for alpha emitters, and 3.7 for beta and gamma emitters. Those are the figures in clause 1 of article 1. Table 1 in article 9 then splits alpha between group one and the groups after it, giving the latter 3.7 in the surfaces row and the skin row. That is a divergence between two places in the same decision; we transcribe both and reconcile neither.

Five words are needed on this page. A controlled area is a place where radioactive material is handled under entry and measurement controls. Free contamination, also called loose contamination, is contamination that transfers by wiping or by contact, and fixed contamination is contamination that does not. The becquerel is a unit of radioactive activity, and the becquerel per square centimetre measures contamination spread over an area. The curie is an older unit, and a microcurie is a millionth of it, so a table cell gives the figure in microcuries and then its becquerel equivalent. Peak energy is the highest energy reached by a beta particle emitted from the nuclide, measured in kilo electron volts, written keV.

The page is for the radiation protection officer in a laboratory. It is for whoever prepares a periodic survey plan. And it is for whoever reads a survey report and wants to know whether the number is acceptable.

Where this sits in the text: clause 1 of article 1 of the instructions in decision 3 of 2007, together with table 1 in article 9. Both articles are in the instructions attached to the decision, not in its two enacting articles.

The most important caution: the number is an upper limit, not a target. Reaching it is not a success. The duty is to reduce contamination as far as practicable, and the limit then acts as a ceiling that is not crossed.

Three things change the judgement on a single reading. The first is the type of emitter: alpha, or beta and gamma. The second is where the reading was taken: a surface, skin, or clothing. The third is the state of the contamination: free or fixed. So three questions precede the judgement, not one.

The four radiotoxicity groups are a precondition for reading the two alpha columns in this table. They are groups of radioactive materials, not of alpha emitters alone. They are explained on a separate page in this batch. Read that page if you do not know your material's group.

The limit as the decision states it

Alpha emitters carry the lower of the two limits. The text sets it as ten to the minus five microcuries per square centimetre, and writes the equivalent beside it as 0.37 becquerels per square centimetre.

Beta and gamma emitters carry a limit ten times higher. The text sets it as ten to the minus four microcuries per square centimetre, and writes the equivalent as 3.7 becquerels per square centimetre.

Why does the text distinguish them? Because an alpha particle deposits its energy over a very short distance. Its effect in tissue is therefore greater per unit of activity. That is our technical explanation, not a reason given in the text.

Fixed contamination has a narrow allowance, stated as a figure. The text permits slightly higher contamination where the surface contamination is fixed, provided the increase does not exceed ten per cent. So the allowance is ten per cent only. Its condition is that the contamination be fixed and not loose. It is not said that fixed contamination has no limit. The increase is worked out on the limit stated in its own place, and where that place is one of divergence the increase follows the divergence and is not settled.

The limit also differs by place, and that is in table 1. Its title names the upper limit allowed for free contamination in controlled areas. It covers three places: surfaces, walls, ceilings and solid materials; skin; and clothing. The clothing limit is higher than the skin and surface limit in every column.

Table 1 has two beta columns, not one. One is for a peak energy above 200 keV, the other for a peak energy below it. The divider is a peak energy of 200 keV, and where the peak is lower the limit is higher. There is no gamma column in this table. Clause 1 sets gamma at 3.7 with no split by energy. So gamma is not measured against the lower energy beta column.

What falls outside this limit

The limit in this place is for controlled areas. The clause says so in terms. We do not extend it elsewhere by our own analogy.

Limits by the nature of the material are a separate matter, in table 2 of article 10. Calculating the rate by area is a third matter, in table 3 of article 11. Each has its own page in this batch. Numbers from one place are not imported into another.

The places we have read in these instructions contain no penalty and no transition period. So it is not said that exceeding the limit carries a particular sanction. What we have not read, we do not judge.

Contaminated skin is not the subject of this page except as a number. Personal contamination and the referral that follows it are stated in another article of the instructions, which is a later batch in this series. Contaminated skin is not handled by analogy to a surface. And this place does not state what follows an exceedance.

Table 1 restated, with its values transcribed

The table below gathers the three rows and four columns of table 1. Skip it on a first reading; the explanation above carries the point.

The cell values are transcribed with both the exponent and the stated equivalent. We have converted no unit and rounded no figure. The branches of the table that sit above the clause 1 figure are places of divergence from it, as set out above, so compliance is not declared on a table figure alone. The table has four columns: two for alpha and two for beta. It has no gamma column, so gamma is not measured against the lower energy beta column.

A worked case, and four practical steps

Suppose free, that is loose, contamination on the top of a bolted bench in a controlled area. The calculation area is 300 square centimetres applied separately, from the row for solid materials, equipment and tools in table 3. The reading is beta, with a peak energy above 200 keV, at 3 becquerels per square centimetre. The limit in that column is 3.7, so the reading is below it. Had it been 4, it would have exceeded it. Had the peak been below 200 keV, the limit in the table would have been 37, while clause 1 gives beta and gamma 3.7 with no split by energy. That is a second place of divergence between the two, so we do not declare compliance there. Had the contamination been group one alpha, the limit would have been 0.37, and a reading of 3 would be about eight times the limit. A reading close to the limit is not declared compliant without allowing for the limits of the instrument's accuracy, which is a caution of ours and not a condition in the text. The bench also falls within the scope of table 2, where the beta and gamma exponent is the same exponent. Both headings are read and the stricter is applied, which is our reading.

Step one: establish the type of emitter before judging the number. If it is beta, establish its peak energy, above or below 200 keV. If it is gamma, its limit comes from clause 1 with no split by energy. The same figure is acceptable under one heading and unacceptable under another.

Step two: establish where the reading was taken. A solid surface, skin, or clothing. Each has its own column.

Step three: record whether the contamination is free or fixed. The ten per cent allowance is conditional on its being fixed. Without that record it cannot be relied on.

Step four: keep the reading with its date, its location and the instrument used. A report without those four cannot be read a month later. We train people to record those four. Any exceedance is raised with the licence holder and his protection officer, and they decide the handling under the relevant provisions.

Questions people ask

Notes on transcription and reading

What we work from is a written transcript, not a scanned image. The values of table 1 are arithmetically consistent across its three rows, so nothing inside it is held in reserve. Set against clause 1, however, there is a divergence in three places. The clause gives alpha 0.37 with no split by group, while the table gives groups two to four 3.7. The clause gives beta and gamma 3.7 with no split by energy, while the table gives the lower peak 37. And the clothing row sits above the clause figure for its own type of emitter in all four of its columns. The shape of the divergence is the same in the alpha and beta places: the clause states one figure without subdivision, while the table subdivides and raises one branch. The clothing row diverges by place rather than by subdivision, and there both branches sit above the clause figure. We do not correct the text and we do not reconcile the two places. In training we work to the stricter figure, 0.37, which is our reading and not a provision of the decision.

Table 2, however, has one line on beta and gamma that we hold in reserve as a possible transcription problem. It is set out on its own page in this batch, and we build no rule on it here.

A point the text leaves open: the clause mentions fixed contamination without defining it. It states no test by which to distinguish it from free contamination. We have not raised it with the competent authority, so no view of theirs is reported here.

What the institute offers, its limits, and the reference

This page is about the free surface contamination limit in controlled areas. The institute trains radiation protection officers to read this limit and to document the reading.

ALDuha Institute for Training and Environmental Consulting works in training and consulting. Its training is licensed. It does not run a laboratory analysis service for smears. It does not calibrate radiation survey instruments. It does not test equipment or certify it as safe. It does not carry out decontamination in a laboratory or on a site.

Surveying and decontamination are the work of the licence holder and his radiation protection officer. Where outside expertise is needed, it is arranged through specialist partners. We train people to read, measure and document. We do not do the work in place of the licence holder.

Neither our training nor our consulting gives any undertaking that a licence will be issued. There is no undertaking about how long an approval will take. That decision belongs to the competent authority alone.

Reference: decision 3 of 2007 of the chairman of the Supreme Council for the Environment and Natural Reserves, on radioactive decontamination instructions. Location: clause 1 of article 1 and table 1 in article 9 of the attached instructions. The decision rests on decree law 31 of 2002, in particular its article 17, and on the executive regulation issued by decision 4 of 2003, in particular its article 97.

A note on the names of authorities: the decision was issued by the Supreme Council for the Environment and Natural Reserves. The authority addressed today is the Ministry of Environment and Climate Change, which houses the Radiation Protection Department. The name changed and the duty stands.

This page is an introductory explanation of a published text, and is not a legal opinion. It does not replace reading the decision itself. It does not replace checking with the competent authority.