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REGDOC-3.6, Glossary of CNSC Terminology- Glossary - U

U (U)

See uranium or use factor.


unacceptable; see safety performance rating methodology

ultimate heat sink (source froide d’ultime secours)

A medium into which the transferred residual heat from nuclear fuel can always be accepted, even if all other means of removing the heat have been lost or are insufficient. This medium is normally a body of water or the atmosphere.

unacceptable (UA) (inacceptable [IN])

See safety performance rating methodology.


Underwriters Laboratories (UL) is a global independent safety science company offering expertise across five key strategic businesses: product safety, environment, life and health, university, and verification services.


Underwriters Laboratories of Canada (ULC) is an independent product safety testing, certification and inspection organization.

uncertainty analysis (analyse d’incertitude)

The process of identifying and characterizing the sources of uncertainty in an analysis, evaluating their impact on the analysis results, and developing, to the extent practicable, a quantitative measure of this impact.

uncertainty in the bias (incertitude liée au biais)

A measure of both the accuracy and the precision of the calculations and uncertainty of data.

unconditional clearance level (niveau de libération inconditionnelle)

In respect of a bulk quantity of material, except for surface contaminated material, in which the radioactive nuclear substance is uniformly distributed, means an activity concentration that

  1. in respect of a radioactive nuclear substance set out in column 1 of Schedule 2, is the corresponding activity concentration set out in column 2;
  2. in respect of a radioactive nuclear substance that is not set out in column 1,
    1. is 1 Bq/g, if the atomic number of the substance is equal to or less than 81,
    2. is 1 Bq/g, if the atomic number of the substance is greater than 81 and the substance, or its short-lived radioactive progeny, does not emit alpha radiation, and
    3. is 0.1 Bq/g, if the atomic number of the substance is greater than 81 and the substance, or its short-lived radioactive progeny, emits alpha radiation; or
  3. in respect of more than one radioactive nuclear substance – except for thorium-232, uranium-235, uranium-238 and their radioactive progeny mentioned in paragraph 4.3 of the IAEA Safety Standard RS-G-1.7 – is the quotient obtained by dividing the total activity concentration by the sum of quotients obtained by dividing the activity concentration of each radioactive nuclear substance by its corresponding activity concentration as referred to in paragraph (a) or (b).

(Source: Nuclear Substances and Radiation Devices Regulations)

Note: Bq/g means becquerels/gram.

unified uranium (uranium unifié)

A category of uranium, used for nuclear material accounting and reporting purposes under the Safeguards Agreement, through which all uranium (natural, depleted and enriched) is included in a single (unified) account.

unirradiated thorium (thorium non irradié)

As defined in the IAEA Regulations. (Source: Packaging and Transport of Nuclear Substances Regulations 2015)

Note: In the IAEA Regulations, unirradiated thorium means containing not more than 10‑7 grams of uranium‑233 per gram of thorium‑232.

unit 0 operator (opérateur de la tranche 0)

The person in a multi-unit nuclear power plant (NPP) who is responsible for operating and monitoring a group of safety and process systems common to all reactor units from the main control room unit 0 panels, in accordance with the NPP licence, policies and procedures.

unsealed source (source non scellée)

A source other than a sealed source. (Source: Nuclear Substances and Radiation Devices Regulations)

Note: These nuclear substances are in a physical form where dispersion of the radioactive material is possible during use or handling. Usually a liquid, they may also be in solid, powder or gaseous form. Unsealed sources are commonly used in medical diagnostic and therapeutic treatments, as well as in laboratory research applications. Also called open source.

update training (formation de mise à jour)

See continuing training.

upper subcritical limit (USL) (limite supérieure de sous‑criticité [LSSC])

The maximum allowed value of the calculated effective multiplication factor (keff) or of a single-parameter subcritical limit, under both normal and credible abnormal conditions, including allowance for the bias, uncertainty in the bias, and an administrative margin of subcriticality.

uprate (augmentation de puissance)

The action of increasing a nuclear power plant’s output capacity.

uranium (U) (uranium)

See depleted uranium, enriched uranium or natural uranium.

uranium concentrate (concentré d’uranium)

A product with a high abundance in uranium obtained by physical and chemical treatments, requiring further refinement before it is suitable for nuclear use. Uranium concentrate from a mill is upgraded by refining and converting it to uranium trioxide (UO3), and subsequently into uranium dioxide (UO2) (used in Canada) and uranium hexafluoride (UF6) (exported). Also called yellowcake.

uranium ore concentrate (concentré de minerai d’uranium)

See uranium concentrate.

urine specimen collector (personne chargée du prélèvement des échantillons d’urine)

A trained person who instructs and assist workers at a urine collection site, receives the specimen provided by each worker and performs an initial inspection of that specimen, and initiates and completes a custody control form for that specimen.

usability (capacité d’utilisation)

The extent to which a product can be used by specified users, to achieve specified goals, with effectiveness, efficiency and satisfaction in a specified context of use.

use (utiliser)

Manipulate or handle nuclear substances and radiation devices; operate radiation devices. Note: This definition applies to nuclear substances and radiation devices.

used fuel pool (piscine de combustible usé)

See wet storage bay.

used nuclear fuel (combustible usé)

Used fuel assemblies removed from a reactor after several years use and treated as waste. Also called irradiated nuclear fuel; spent fuel. Note: In Canada, “irradiated nuclear fuel” or “used nuclear fuel” is a more accurate term for “spent fuel” (a term used internationally and in some Canadian reports with the same meaning in this definition, to align with international agreements and conventions), because discharged fuel is considered a waste material even when it is not fully spent.

use factor (U) (facteur d’utilisation [U])

The fraction of the total workload during which a radiation field is present at a particular location. This factor (≤1) is multiplied by the workload and occupancy factor at that location, to derive the maximum personal dose any single person would be expected to receive.

use type (type d’utilisation)

The purpose for which a licence has been issued.


See upper subcritical limit.


See U.S. Nuclear Regulatory Commission.

U.S. Nuclear Regulatory Commission (U.S. NRC) (Nuclear Regulatory Commission des États‑Unis [NRC des États‑Unis])

The agency of the U.S. federal government responsible for regulating commercial nuclear power plants and other uses of nuclear materials through licensing, inspection and enforcement of its requirements.

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