Oifferenrurgans of the body may receive different dose equivalent values due to the nature of the irradiation taking place. There will be varying risks associated with each organ, depending on the amount of dose received by the organ and the particular organ concerned. By considering the risk associated with each organ, it is possible to produce a total risk figure for the whole individual. This total risk is known as effective dose equivalent and is equal to:

T = WT HT

where H у is the average dose equivalent in the organ T
Wj is the weighting factor for the organ T.

The values of the weighting factors are:

gonads	0.25
breast	0.15
red bone marrow	0.12
lung	0.12
thyroid	0.03
remaining 5 organs (each)	0.06

The unit of effective dose equivalent is the same as dose equivalent, i. e., the sievert (Sv).

In the same way that dose equivalent may be committed, effec­tive dose equivalent may also be committed. If the time period chosen or integration is 50 years, then the quantity determined is committed effective dose equivalent, and has the sieved as the unit.

A6 Collective dose

A useful concept in radiological protection is collective dose. If dose equivalent or effective dose equivalent Is assessed for any number of individuals, which for some purposes may be as low as one or, for others, be equivalent to the world population, that assessed quantity is known as collective dose. The unit of collective dose equivalent and collective effective dose equivalent is the man sieved (man Sv).

A7 The old units

Each of the quantities described has been expressed in terms of the

St unit appropriate to each. These units were introduced as a requirement of the The Units of Measurement Regulations 1980, (Statutory Instrument 1070, 1980), which impfement a European Council Directive requiring the introduction of SI units in member states. The regulations require that SI units be used from 1 January 1986.

Previously, the radiological units used to describe activity, absorb­ed dose and dose equivalent were the curie (Ci), the rad and the rem respectively, A special unit, the roentgen (R), which is defined in terms of the amount of ionisation in the air that a particular quanti­ty of X or gamma radiation produces, has been dropped altogether.

Table 4.17 summarises the relationship between the old and SI units.

Table 4.17 Relationship between old units of measurement and Sf units Quantity New named unit and symbol In other SI units Old unit and symbol Relationship between old and new units Exposure — C/kg roentgen (R) 1 R = 2.58 x 10~4 C/kg Absorbed dose gray (Gy) J/kg rad (rad) 1 rad = 0.01 Gy Dose equivalent sievert (Sv) J/kG rem (rem) 1 rem = 0.01 Sv Activity becquerel (Bq) S’1 curie (СІ) 1 Ci = 3.7 x 1010 Bq

[1] Inelastic scattering Strictly, inelastic scattering is an absorption event in that, in accordance with the theory of the compound nucleus, the neutron is firstly absorbed by the target nucleus and a pos­sibly different neutron subsequently ejected in a random direction. After the collision the target nucleus is left in an excited state and reverts to the ground state by emission of a 7 ray. The energy of the ejected neutron is much lower than that of the initial neutron, being the difference between the kinetic energy of the bombarding neutron and the excitation value of the nucleus. It may be deduced that inelastic scattering can only occur if the kinetic energy of the impinging neutron is at least equal to the excitation energy of the target nucleus. For a given material there is, therefore, a minimum threshold energy below which inelastic scattering is not possible, typically 0.1 MeV for the heavy elements and greater than 1 MeV for the light elements.

[2] Uranium 238 capture cross-section, Fig 1.6 (b). oc exhibits the /v dependency in the low neutron

[3] U-235 is a fissile material; it can undergo fission with neutrons of any energy but is much more likely to do so the less energetic, or slower, the neutron, Fig 1.6 (c).

• The bulk of the energy appears as kinetic energy of the fission products. This energy is given up in the form of heat within the fuel.

• Not all the energy release is instant. About 13 MeV comes from fission product decay and is delayed.

• Some of the energy produced is associated with the neutrons and gamma radiation. As neutrons and gamma rays can travel large distances in matter some of the energy is released as heat some distance from the place of fission.

[5] Some of the energy is associated with antineutrinos which have an extremely low probability of inter­acting with matter; this energy is lost from the system.

Using appropriate conversion factors and a value of 200 MeV release per fission it can be shown that 3.1 x 1010 fissions per second will produce 1 watt of power; hence a 1000 MW (thermal) power station requires 3.1 x 1019 nuclei to fission every second. However, this very large number is small compared with the number of nuclei in a cubic metre of solid matter, of the order of 1028.

Fig. 1.30 Reactor layout and containment system of the CANDU/PHWR

pressurised in the acronym above refers to the pres­surised DiO coolant which flows in opposite direc­tions in adjacent tubes and passes its heat to the secondary coolant via the steam generators. System pressure is maintained by a pressuriser on one of the legs of a steam generator.

Adoption of pressure tubes in preference to a pressure vessel has a number of advantages:

• The failure of a pressure tube has not the same significance as the failure of a pressure vessel.

• The moderator can be kept at low temperature, giving a lower thermal neutron energy spectrum. Also, in the analysis of possible major accidents, the potential thermal energy represented by the mod­erator temperature is minimised.

[7] The reactor can, in principle, be made indefinitely larger.

• Access to individual pressure tubes makes on-load refuelling possible.

The fuel used in Candu is natural UO2 clad in zirca — loy. A bundle of fuel rods make a fuel assembly,

[8] A thermal neutron reaction with 10B which is gen­erally present as a grain boundary impurity at an overall concentration of 2 atom ppm.

[9] I he rate ol increase ol power, on w hich depends [he чгаіп rale (hence stress) in the clad and. pos­

[10] is the structural factor

[11] Contamination from added chemicals, make-up water and ion exchange resins.

[12] Some metal cations can be removed by solubilisation,

and although the concentrations will be very

[13] Achievement of a low oxygen concentration can be assisted initially by the use of hydrazine. During power operation, with the specified hydrogen con­

[14] Plant for collecting and treating active liquid waste and cooling pond water treatment.

[15] A store near the cooling pond for shield cooling air and ventilation filters (sealed), sludge, sand and resins from pond and active effluent treatment (in sealed drums) and Ctb filters (contained).

[16] The tensile and compressive stresses in the concrete, under all operating, test and shutdown conditions, are always specified below limits. (These limits are specified in BS4975, the British Standard for con­crete pressure vessels.)

[17] CB — background count derived from fission

products emitted from fuel surface con­tamination.

[18] Fail sate properties, i. e., any failures that do occur should have a high probability of failure in the direc­tion of safety.

[19] Station waste vault for incombustible materials and

[20] Close control of the product is maintained by checks on chemical analysis, neutron absorption cross-

action, material cleanliness, hardness and erain

size.

[21] Mechanical and electrical plant.

[22] The design and layout of other systems significant to safety (including cabling) incorporates segrega­tion and other means of hazard protection comen — surate with their safety role.

[23] To allow access to the fuel for its replacement and to allow access from outside for instrumentation

and control rod drives.

[24] The 3500 kW rated motor is a single-speed (1485 г/ min), drip proof, air cooled, three-phase, squirrel cage induction motor with a Class F thermalastic — epo. xy insulation system. The rotor and stator are of conventional design. The motor houses the RCP assembly thrust bearing, which is a double­acting Kingsbury type (accommodating either up­ward or downward thrust) comprising pivoted seg­mental shoes and a shaft-mounted runner. A high pressure oil lift system provides the initial oil film during start-up and the thrust bearing is self-lubri­cating at speed. Also mounted within the motor is an anti-reverse rotation device, which prevents

[25] Control the feedwater flow, in order to maintain steam generator level within satisfactory limits.

[26] To impose an absolute liability (even if due to unavoidable accident) on licensees for injury to per­sons and damage to property of any person, attri­butable to the radioactive, toxic, explosive or other hazardous properties of nuclear matter. Licensees must provide cover by insurance or otherwise, up to £5 million for any one cover period.’

[27] To control exposure to personnel, the site is zoned into areas defining the radiation and contamina­tion levels. The areas are designated by a Duly Authorised Person appointed for this purpose. The method by which areas are zoned must be included within the Safety Rules. There is a commitment

[28] Fault conditions which may be due directly to break­down of plant or the malfunction of some piece of equipment or system. The fault condition may or may not lead to a loss of generation or shut down

[29] Major overhaul has to be done on some items of plant which form a major component of a system (e. g.. boiler feed pumps, CV pumps and intakes).

[30] A group of plant operators who carry out the acti­vities of running the plant and refuelling the reac­tors, headed by a foreman.

[31] Inspection of reactor internal structures for sound­

[32] Automatic protective devices will shut down the reactor if power diverges at a doubling time of less than about 20 s, equivalent to a net reactivity of about + 200 mN.

[33] Turbine-generator and dump condenser controls to utilise the steam generated in the boilers.

Reactor power output is gien by the formula:

Power = gas flow x specific heat x temperature rise

[34] Control rod position which is related to the re­activity which is available by movement of the control rods. Magnetic synchros or resistance po­

[35] Data processor alarm from the *B’ thermocou­ples for any channel above target temperature. This margin is common to all channels, but since each channel has itv own target the alarm level is channel-specific.

[36] It may identify a leaking boiler so it can be iso­lated.

[37] The fuel dement identification number.

[38] The flask load is selected to ensure that the final heat burden of the flask is not greater than that required by the safety case. The heat burden varies

[39] Direct charge — this is the cost of actually carrying out the work specified and is dependent on the

[40] To provide specialist advice to all levels of manage­ment on nuclear health and safety aspects of siting, and to inspect and approve services in relation to

[41] Thermoluminescence dosemeters (TLD) have been located around each power station at a distance of between 1 km and 10 km and disposed at intervals of 15°. In addition TLDs have been placed at centres of population up to 30 km from magnox power stations and 15 km from AGR sites.

• Integrating electronic dosemeters have been placed at some of the TLD sites close to the power stations.

• Gamma radiation monitors have been installed at 60° intervals round the perimeter fence of each

[42] Liaise with the senior police officer and officers in charge of other emergency services.

• Arrange controlled access to the flask and trans­porter.