Крючков Фундаменталс оф Нуцлеар Материалс Пхысицал Протецтион 2011

Categorization of nuclear material in Russia (NP–03 0–05 [1])

Categories of nuclear material are defined depending on the NM mass, the form of the NM-containing product and the NM type (Tables 2.3 – 2.6). Depending on the quantity of nuclear material, the MBA facility may fall into category I, II, III or IV. The next parameter is the form of the NMcontaining product. As evidenced by Tables 2.3-2.6, this demonstrates the attractiveness of nuclear material. Attractiveness is assessed by how rapidly a nuclear material can be converted to a form fit to produce nuclear weapon.

Noteworthy is that irradiated fuel is the least attractive. The reason is simple. Such fuel is inherently safe which makes it an essentially unlikely target for theft or misuse. Same is true for low-enrichment uranium that needs to be highly enriched to be used directly in nuclear weapons production.

Cases are possible when the area of interest contains different products and diverse NM. To categorize the existing NM in these conditions, one is expected to proceed from the total mass of NM in each product and the quantitative mass limits, as specified in Tables 2.3 – 2.6, for the nuclear material in products falling under a higher category.

Russia defines NM physical protection, accounting and control requirements subject to the following categorization of nuclear material.

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NM categorization practice

The existing material should be categorized if the core contains:

∙fresh FAs of a research reactor core (the reactor grid is composed of 16 FAs, each FA containing 90% of uranium in the form of a uranium and aluminum alloy. One FA contains 300 g of uranium-235);

∙one fresh FA of a research reactor;

∙fresh FAs of a light-water power reactor numbering a third of the overall reactor load (a 4.4%-enriched UO2 fuel is used, one FA contains 19 g of uranium–235. There are 160 FAs in the reactor. The overall reactor load is 80 t of UO2);

∙one spent FA;

∙metal waste with 250 g of plutonium and a container with PuO2 of the net weight 6.5 kg;

∙450 kg of natural uranium.

2.2. Forms of nuclear material. Batch of nuclear material

Forms of nuclear material

All nuclear material may have one of the following two forms.

Item NM in the form of canned material. This has a unique identification number of its own. Examples of item NM are fuel elements and fuel assemblies.

Bulk-form NM is any NM other than canned NM, i.e. having an unclad form (gases, liquids, powders, pellets). This material cannot be permanently identified as its quantity and composition in containers can be changed.

Depending on the existing NM form, various instruments and identifiers are used to account for this NM. So, bulk nuclear material can be tested destructively with sampling and subsequent sample analysis. These methods are not applicable to item NM. On the other hand, this NM is often measured nondestructively.

Batch of nuclear material

An exclusively important concept used in the new system (as, though, in the old system as well) is nuclear material batch. This concept is used to record item and bulk NM.

A batch of NM is a collection of uniform items that contain NM the parameters whereof are determined in a single series of measurements.

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Uniformity means that, e.g., one batch cannot contain items of different enrichments.

Each NM batch has five data elements:

∙name of NM batch. Batch name is the identifier composed of a number of symbols. The identifier shows the manufacturer code, the batch serial number and so on. Therefore, name is used as the unique identifier of NM batch;

∙number of uniform items in batch. The number of items is often equal to 1. “Item” is the term often used to denote an NM batch. Examples of batches: fuel element (the number of items in batch is 1), container with UF6 (the number of items in batch is 1), five containers with UO2 powder (a batch is understood as the given set of containers of which each is viewed as an item);

∙NM ligature mass (Мlig) which is the mass of a material that contains a chemical or physical NM composition with some inert substance. For example, the mass of the U-Al alloy is understood as the ligature mass of NM in the U–Al alloy used as fuel in some research reactors (of the IRT type). The ligature mass of NM in the form of a chemical compound of uranium dioxide (fuel of light-water power reactors) is understood to be the mass of the UO2 compound;

∙element mass (Мel). For example, for the UO2 compound we have the element masses of Мel = МU, Мel = МO;

∙isotope mass (Мis). Accounting deals primarily with fissile isotopes: U–

235 isotope for uranium and Pu–239, 241 isotopes fo r plutonium. Enrichment, a similar parameter, is considered occasionally instead of the U–235 mass.

The five noted NM batch parameters are central to NM accounting and balancing closing. These parameters are applicable both to item and bulk NM. For material in the form of items, however, all data that accompanies unirradiated material is the manufacturer data, while data for bulk material normally change over time.

2.3. Material balance area (MBA). Organization of accounting and control of NM in MBAs

Material balance area

Material balance area (MBA) is the central concept to the new NM accounting and control system. MBA is an elementary structural unit of the state NM accounting and control system.

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There is no such notion in bookkeeping where NM balances are operated for the whole of the enterprise. As a concept, MBA was introduced in 1959 by US experts and “legalized” by the IAEA in the Guidelines for States’ Systems of Accounting for and Control of Nuclear Materials (1980 [2]) going later into the Concept of the State Nuclear Material Accounting and Control System as approved by the Russian Federation Government in October 1996 [3].

Definition of MBA: A territorially and administratively area established within a nuclear facility or an NM storage point to account for and control nuclear material, the quantity of nuclear material therein determined by measurements each time it is transferred into or out of the given area with the nuclear material balance closed for the respective time period.

Therefore, controlled inputs and outputs are defined clearly in an MBA, and the NM inventory is periodically subject to determination by NM physical inventory takings (PIT).

Construction of MBA. Proper selection of the NM balance area structure is essential to ensuring sound operations of the corporate NM accounting and control system. However, MBAs are often hard to size and set boundaries for. For example, Figure 2.1 shows a simplified flowchart of a power reactor fuel fabrication plant.

Fig. 2.1. NPP fuel fabrication. Flowchart of major processes 138

questions in this enable one to get a set of data required to define the MBA boundaries.

Determination of NM flows and inventories in MBAs

Incoming and outgoing NM flows, as well as NM inventories in MBAs are determined at key measurement points (KMP).

Key measurement point (KMP) is where nuclear material can be measured to have NM flows and inventories determined.

Inventory determination methods for canned articles (FAs, individual fuel elements (FE) and others) are accounting of items (identification of items plus counting thereof) and nondestructive NM measurements.

For bulk material these are a set of measuring procedures used to determine the inventory with potential sampling taken into account. Chemical and isotopic compositions are measured on the samples taken. This gives significance to making so that NM samples to be representative.

Figure 2.2 demonstrates measurements for different NM forms (bulk and items) at key measurement points for the fuel element loading department in an NPP fuel fabrication cycle. It can be seen that both destructive (for incoming NM flow) and nondestructive NM analyses are used to account for NM.

Attributes are the data that determines unambiguously if NM has any feature.

Examples of attributes:

∙Is there NM in the given item or not?

∙Does the tamper indication device verify to the absence of unauthorized access to NM or not?

∙Is NM of the type shown in certificate data or not?

∙Does the NM gross weight differ from certificate data in the measurement error limits or not?

∙Does the number of items in the container coincide with documented data or not?

∙Is the item with the given identifier at its specified location or not?

Attributes are particularly useful with basic accounting procedures. This is the case, say, when an NM transfer requires incoming material to be rapidly received. Incoming tests of material are based predominantly on attributes.

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