RADIATION HAZARDS FROM REACTORS .1 General

In general, reactors are a lesser radiation hazard when they are operating than when they are shut down. The shielding is designed to give acceptable radiation levels at working positions and this is confirmed by thorough surveys during the commissioning of the plant and subsequently at regular intervals. Systems are provided which permit safe means of sampling coolants and other radioactive effluents. During shut-down periods, a great variety of non-routine jobs may be undertaken, some of them on highly radioactive systems. It is during such periods that exposure of personnel to radiation and radioactive contamination must be carefully controlled.

A serious fault or maloperation could cause considerable damage to the plant and give rise to dangerously high levels of radiation or radioactive contamination. If the hazard is confined to the reactor site, it is often called a site emergency, but if it extends off site then it may become a public emergency. Accidents of this type are discussed in Chapter 16.

11.5.2 Sources of radiation

The main sources of radiation from a reactor at power are the core and the coolant. The radiation from the core includes fission neutrons, fission g-rays, fission product decay g-rays, neutron capture g-rays and activation product decay g-rays. The last two arise predominantly in the core structure and shield. The radiation from the coolant is mainly g-rays arising from activation of the coolant, activation of impurities and fission-product contamination of the coolant. The sources are illustrated in Figure 11.7.

Radiation from the core

The neutrons produced in a reactor as a result of the fission process are fast neutrons in the range of 0.1–15 MeV, with an average energy of about 2 MeV. Those emerging from the surface of the biological shield have undergone varying degrees of moderation and so neutrons of all energies from thermal to fast may be present.

Fission g-rays are those emitted immediately after the fission fragments and vary in energy from 0.25 to about 7 MeV. The g radiation resulting from the decay of fission products in the fuel elements is small compared with the fission g radiation but, whereas the latter ceases on shut-down of the reactor, the fission products continue to emit radiation for many years after the fuel has been withdrawn from the reactor.

Neutron capture in the structural materials of the reactor and in the shield results in the emission of capture g-rays and makes these materials radioactive. The radiation from the decay of the radioactivity, as in the case of fission products, continues to be emitted

Core

Figure 11.7 Sources of radiation in a nuclear reactor system.

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when the reactor is shut down. Neutron scattering also leads to g-ray emission but does not, in general, induce radioactivity.

Radiation from the coolant

As noted previously, in both water-cooled and carbon dioxide-cooled reactors, the reaction

16O(n, p)¹⁶N

is important because nitrogen-16 decays with a half-life of 7.2 s and emits very penetrating (6.1-MeV) g-rays. This means that the coolant circuit of the reactor is a significant source of g radiation and, in most cases, must be shielded.

Air is present as an impurity in the coolants of both gas- and water-cooled reactors.

Argon, which is present to the extent of 1.3 per cent in the air, is activated by neutrons via the reaction:

40Ar(n, g)⁴¹Ar

Argon-41 decays with a half-life of 1.8 h and emits g radiation with an energy of 1.29 MeV.

It can contribute to operator dose during online sampling and immediately after reactor shutdown.

In LMFBRs, which use sodium as a coolant, the important activation product is sodium-24 (²⁴Na), which is produced by an (n, g) reaction on sodium-23.

Although reactor coolants are very pure by normal standards, they always contain some impurities. When subject to the very high neutron flux in the reactor core, these impurities become radioactive to an appreciable extent. Impurities also arise as a result of corrosion or erosion from the core and the walls of the coolant system. In water-cooled reactors, iron, nickel, cobalt and manganese are common impurities because of corrosion of the coolant system. Deposition of this corrosion material in the core causes the build-up of a film of corrosion products on fuel surfaces. As a result of irradiation by neutrons, these films become highly radioactive. The continuous release of material from the core and subsequent deposition in the out-of-core regions causes a build-up of radioactivity in the coolant system which is known as crud in PWR systems. In Magnox and AGRs, graphite dust (which itself contains impurities) collects around the cooling system.

Reactor coolants usually contain readily detectable levels of fission-product contamination arising from:

1. uranium contamination on the fuel element surface;

2. uranium impurity in the fuel cladding material; and 3. release from any damaged fuel elements.

The coolant is being continually cleaned up by the coolant treatment system and so the long-lived fission products do not build up appreciably. The predominant fission product activities are usually krypton-88 (⁸⁸Kr) and xenon-138 (¹³⁸Xe), which are inert gases, their particulate daughter products rubidium-88 (⁸⁸Rb) and caesium-138 (¹³⁸Cs), and the three isotopes of iodine, ¹³¹I, ¹³³I and ¹³⁵I. A seriously damaged fuel element could lead to considerable fission product activity being spread around the cooling system. In water-cooled reactors, the presence of fission products in the coolant is detected by sampling and radiochemical analysis. This is carried out routinely while the reactor is at power using

special sampling facilities. The samples are tested for the presence of fission products, including nuclides of iodine, caesium and strontium. Most gas-cooled reactors are fitted with a system known as ‘burst can detection’, which continuously and automatically

‘sniffs’ each channel in turn and gives warning to the operators if an increase in the fission product activity should occur. The channel containing the damaged fuel is then unloaded and the faulty element is replaced.

11.5.3 Sources of radioactive contamination Beta emitters

Almost all of the radioactive nuclides mentioned in the preceding paragraphs decay by b emission. Beta radiation is so easily absorbed that the shielding designer does not even need to consider it, concentrating instead on the associated g emission. On the other hand, if radioactive contamination occurs because of a leak of radioactivity from the reactor system, the b radiation is often of prime importance.

A radionuclide of considerable importance that is produced in all reactor systems is tritium (³H). This nuclide has a half-life of 12.3 years and decays by low-energy b emission only. It is produced by fission, by an (n, g) reaction on deuterium (hydrogen-2) and by various reactions on lithium and boron. In reactors cooled or moderated by heavy water, large amounts of tritium build up. In light water systems, because of the much lower concentration of deuterium and the frequency of water change (heavy water is much too expensive to change), build-up of tritium is usually less significant. Lithium and boron are present in most systems, either as additives, neutron absorbers or impurities, and can contribute to tritium production by a number of reactions, including:

6Li(n, a)³H and ¹⁰B(n, 2a)³H Coolant leaks

Contamination can, of course, occur because of a coolant leak. In pressurized water systems, the leak may be direct to the atmosphere or via a heat exchanger into the secondary system. In the latter case, radioactivity, mainly the gaseous activities ⁸⁸Kr, ¹³⁸Xe and ⁴¹Ar, would be carried over with the steam into the turbines and then to the atmosphere via the condenser air ejector. As noted above, the fission product gases ⁸⁸Kr and ¹³⁸Xe decay to their particulate daughters ⁸⁸Rb and ¹³⁸Cs. Note that a leak in the heat exchanger of a gas-cooled reactor would normally cause steam to leak into the primary system because of the higher secondary pressure. It should also be noted that, in a BWR, the steam from the reactor carries with it ¹⁶N from the reactor water into the turbines and associated plant. Even though its half-life is only 7.2 s, the high-energy g radiation results in significant radiation levels in the vicinity of the steam systems which require the provision of shielding around the steam pipes, turbine and other major items.

Containment

The core of a reactor at power contains about 0.2 TBq of fission products per watt of thermal power. Thus a reactor operating at, say, 1000 MW contains about 2 × 10⁸ TBq of fission products. This vast inventory of radioactivity is contained within the fuel can or cladding, which provides the first level of containment. The second level of containment is the boundary of the primary systems, that is, the pressure vessel and the coolant system.

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This boundary also contains the radioactivity of the coolant which, in a 1000-MW reactor, may amount to some hundreds of terabecquerels. Provided that the primary and secondary containments remain intact, there is little risk of serious contamination. In practice, some contamination does occur during the operation of most reactors. For example, it is usually necessary to sample the coolant periodically and there is often some radioactive effluent. A contamination hazard could arise in both cases but the risk is minimized by good design of facilities.

In its most common usage, the term ‘reactor containment’ refers to the structure within which the whole system is housed. The main function of this containment is to protect the general public by limiting the release of fission products in the event of a serious reactor accident, although its secondary function, to protect the reactor systems from external hazards and terrorist threats, has become increasingly important over the past decade or so (see section 16.4).

11.5.4 The shut-down reactor Maintenance

A reactor represents a large capital investment, and shut-down periods, whether scheduled or not, are costly. Reactor systems are comparatively simple, well engineered and normally very reliable. The majority of maintenance, either corrective or preventative, is on ancillary or secondary equipment. A major overhaul may include decontamination and refitting of coolant circulators, control rod mechanisms, inspection of heat exchangers and various other jobs on radioactive systems. At such times, the need to keep to a tight schedule can lead to a general reduction in standards of safety, both radiation and conventional, because it may slow down the work. To prevent this state of affairs, all major work must be planned in consultation with interested parties and sometimes it is desirable to make a mock-up to allow particularly difficult jobs to be rehearsed under non-active conditions. Personnel should receive instruction in general safety matters and be familiar with the particular hazards associated with their own work.

External radiation

When the reactor is shut down, the primary shield gives adequate protection against the fission products in the core. The radiation hazard to personnel working on the primary system is caused by radioactivity within the system. The dose rate in the vicinity of the primary system tends to decay rapidly in the first 24 h after shut-down, mainly because of the decay of coolant activities or their clean-up by the treatment system. Thereafter, the levels do not change significantly from day to day. The half-lives of most of the radioactive corrosion products are in the range of 1 month to about 5 years. The dose rates vary considerably from reactor to reactor but, in systems with corrosion problems, levels of 10–100 mSv/h can be encountered on certain components. If the dose rate is excessive, it is sometimes possible to provide additional shielding on ‘hotspots’. An alternative approach is to decontaminate the component, but this would be done only during major shut-down periods.

Careful control is required of personnel working in areas of high dose rate. This often takes the form of a manned control point at the entrance to the area. Personnel entering the area are given a ‘working time’ based on a radiation survey of the area. In addition to their normal personal dosimeter they are required to wear some form of direct reading device

such as an electronic dosimeter. The times of entry and exit and the dosimeter reading are logged. Some electronic dosimeter systems can be read and logged automatically and remotely.

Contamination

As noted earlier, reactor coolants normally contain measurable amounts of radioactive fission products and activated corrosion products. Contamination is likely to occur during maintenance operations that involve breaching the primary coolant system. It is obviously essential to depressurize the primary circuit before attempting to breach it, and it is general good practice for the personnel involved to wear full face masks when first breaking into any part of the system.

The standards of protective clothing required on a particular plant are evaluated from experience. As well as the presence of contamination, there are often other factors such as temperature, humidity and the possible presence of toxic gases which affect the choice of protective clothing and equipment. Personnel cannot be expected to wear impervious clothing, such as PVC suits, in temperatures of 40–50°C unless the suits are fully ventilated.

This in turn causes difficulties in confined spaces because of the required air lines. These and other considerations mean that all maintenance operations on the primary circuit must be planned and executed carefully. It is essential that the necessary changing, monitoring and data-logging facilities are established before the maintenance activity begins. If significant levels of contamination are present, there should be an attendant to assist in the removal of contaminated clothing. Frequent monitoring of levels of contamination both inside and outside the area should be undertaken to ensure proper control.