RIAR CRITICAL ASSEMBLIES: STATUS, UTILIZATION, …...RIAR CRITICAL ASSEMBLIES: STATUS, UTILIZATION, PROSPECTS A.L. Izhutov, V.V. Kalygin, A.P. Malkov, ... physics of the MIR reactor

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RIAR CRITICAL ASSEMBLIES:

STATUS, UTILIZATION, PROSPECTS

A.L. Izhutov, V.V. Kalygin, A.P. Malkov, A.L. Petelin, D.V. Fomin

Bariloche, Argentina, November 17-21, 2014

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RIAR operates two critical experimental facilities that are

the physical mockups of the most powerful research reactors in

Russia SM and MIR.

The report presents the main physical and design

parameters of the critical experimental facilities , areas of

research and application of the obtained results. The prospects

of continued operation of the existing SM and MIR facilities are

shown. The recent experience and operational issues are

presented.

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SM Critical Assembly

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The first critical assembly of SM was assembled in March,

1961. By the reactor startup in November, 1961 the main core

physical parameters had been studied at the critical assembly .

Over the operational period numerous studies have been

performed in support of the SM reactor reconstructions, on

determining the parameters of the core and test rigs.

The geometry and material composition of the critical

assembly core and reflector correspond to the reactor ones.

The core and reflector are accommodated in the experimental

tank filled with water as moderator before bringing the critical

assembly to a critical state. After the test completion the water

is drained from the tank to ensure safety.

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6

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The core cross-section is a 420420 mm square (66 cells along the

square grid spaced at 70 mm). Four central cells are used to accommodate

the central moderator – neutron trap; four corner cells are used for shim rods.

All in all, up to 28 FAs can be installed in the core (when inserting shim rod

clusters in the core during the experiment the total number of the FAs can be

32). Four safety rods are accommodated in the central moderator corners.

Enclosed the FA shroud there are fuel rods that take a shape of a cross

at the cross-section (the circumscribed diameter is 5.15 mm) twisted along

the axis. The fuel is uranium dioxide powder dispergated into a copper matrix.

The fuel rod active part height is 350 mm. The FA design is dismountable. To

simulate fuel burnup the rods with 100% - 40% 235U nominal content are

used. Absorbing fission products are simulated by absorbers in a fuel rod

bundle.

The control and safety rod design corresponds to the reactor one.

The core is surrounded by a reflector consisting of 48 Beryllium blocks

with holes to accommodate experimental channel mockups.

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MIR Critical Assembly

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MIR critical assembly was constructed in 1966. Over the forty years

of its operation a large scope of R&D has been conducted on studying the

physics of the MIR reactor and its test rigs.

The critical assembly core and reflector are accommodated in the

experimental tank filled with distilled water.

The core and reflector are made of hexagonal Beryllium blocks with the

width across flats of 148.5 mm and height of 1100 mm. The blocks are

located in the hexagonal grid spaced at 150 mm. The central rows of the

blocks serve as a moderator and medium for neutron diffusion, the external

rows serve as a reflector. In the axial holes of the first four rows of the blocks

the channels with standard FAs and the test rig mockups are installed. Twelve

cells to accommodate test rig mockups are in the second and third stacking

row so that each is surrounded by six cells with operating channels.

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In the experiments performed at the critical

assembly the operating FAs containing six

and four fuel rods are used. Each fuel rod

is a three-layer tube: a fuel layer is

enclosed on the both sides into aluminium

alloy. A fuel rod is 2 mm thick, the gap

between the fuel rods is 2.5 mm, the core

height is 1000 mm, the external fuel rod

outer diameter is 70 mm. Uranium-

aluminium alloy is used as fuel in the FAs

containing six fuel rods and UO2

encapsulated in the aluminium matrix is

used in the FAs containing four fuel rods.

The fuel enrichment in 235U is 90%. The

nominal mass of 235U in the FAs of the both

types is 350 g. The fuel burnup in the

operating FAs is simulated using the

assemblies of the same design but with the

reduced 235U content.

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calculating neutronic parameters of the experimental channels and rigs of

the SM and MIR reactors;

selecting a means of irradiation mode creation and agreeing the set test

modes of the test rigs that are simultaneously irradiated in the reactor;

feasibility of the SM and MIR safe nuclear operation;

study in support of the core upgrading concepts and design solutions;

testing the methods to calculate the reactor neutronic parameters;

education and training of the staff, students and postgraduates.

Areas of Research

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Some Results of the Experiments Performed at the SM Critical Assembly

Reactor neutronic parameters have been tested in support of all the

reacor reconstructions. The latest reconstruction was performed in 1991-1992.

During the reconstruction a reactor vessel was replaced, a pattern of the

coolant supply to the reactor was modified, neutron trap arrangement was

changed as well as the number and location of the experimental channels in

the reflector. About 90% of the experiment scope required under the physical

startup programme was completed. The following issues have been studied:

reactivity effects induced by the introduced changes; limits and regularities of

safety and control rod performance change; power density distribution in the

core, the maximum possible values of the power peaking factors both for the

reactor and for the fuel rods of the FAs of different types for similar cells in the

core; neutronic parameters of the experimental channels depending on their

mutual filling and control and safety rod moving.

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The reactor neutronic parameters were studied

in changing the neutron trap arrangement

Loop channel Transuranium target

central Be-block

Separator

arrangement

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Some neutronic parameters of the SM reactor with various neutron trap arrangement

Parameter Channel-type Be-block Separator-type

Maximal thermal neutron density in the

target dummies, N/m2s

2.61019 1.71019 2.51019

Reactivity margin (related to the channel-

type), eff

0 +1.5 +0.2

Temperature reactivity effect

(from 15 С to 70 С), eff

-0.33 - 0.65 - 0.56

Power reactivity factor, (k/k)/%Nnominal -4.510-5 - (31)10-5 - (3 1)10-5

Power peaking factor factor by:

- core height

- core section

-section of a FA on the boundary of the

central moderator

- core volume

1.25

1.65

2.92

6.0

1.25

2.16

2.06

5.60

1.25

1.88

2.27

5.33

Control and safety rod worth, eff:

Core

Central shim rod

Shim rod

Control rod

0.59

4.09

2.04

0.05

0.5 – 1.5

3.0 – 4.5

1.3 – 3.5

0.01 – 0.4

0.4–1.5

2.5–4.5

1.3–3.5

0.01–0.4

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Feasibility of SM conversion to a new fuel with the increased loading of U-235

irradiation rig designs were selected and justified to test fuel rods in the

loop facility;

the reactivity effects and power density distribution when testing new

type FAs in the reactor core;

simulating the reactor conversion to a new fuel during the current

reactor operation;

the reactivity effects induced by the introduced changes were

determined;

the limits and regularities of control and safety rod worth variation were

established.

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Some Results of the Experiments Performed at the MIR Critical Assembly

The studies were conducted on determining a possibility for formation of a

local critical region during the core refueling that resulted in increasing the

number of control and safety rods. In 1990 the reactor was equipped with six

compensators with fuel suspenders (CF) and four shim rods in addition to the

existing ones. At present, the total number of the compensators with fuel

suspenders in the reactor is 12, and the number of shim rods is 27. The limits on

the core loading have been stated.

РК

РК

РК

РК

РК

РК

ПК

КС

КС

КД

КД КД

КС

КС

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The limiting values and variation ranges for reactor control and safety rod worth

Rods Group No.

(pcs)

Rod No. in a

group (pcs)

Group worth,

eff

Reactivity insertion

rate when cocked,

eff/s

Safety 6 1 0.065 3.2 0.001 0.052

Control 2 1 0.1 0.7 0.007 0.05

Shim 21 1 0.065 3.2 0.001 0.052

CF 12 1 0.08 7.0 0.0002 0.018

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The regularities of the positive reactivity effect change have been studied when lowering water density in the loop channels

induced by various factors

0,2

0,4

0,6

0,8

1,0

0,6 0,8 235U mass, rel. units

Re

activity e

ffe

ct, r

el. u

nits

1,0

Number of control rods, pcs

Re

activity e

ffe

ct, r

el. u

nits

0

0,2

0,4

0,6

0,8

1,0

0 3 6 9

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The conditions for safe conducting dynamic experiment in the MIR reactor were determined on simulating accident and transient

conditions of the fuel rod operation: AVR rig, loss of integrity, cycling

AVR

CYCLE

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Recent Operation Experience and Difficulties

Maintenance of equipment, systems and components of the SM-2 and

MIR.M1 critical assemblies is performed on a scheduled basis (checkups,

repairs, maintenance, audits, calibration) as well as technical inspection,

service lifetime extension and licensing.

The main enhancements of the critical assembly engineering systems in

recent years are related to the area of MPC&A.

The difficulties of the critical assembly operation in recent years are as

follows: operational and research equipment aging and lack of the funds for its

upgrading, lack of qualified staff.

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Prospects for the SM and MIR Critical Assemblies Usage

Testing neutronic parameters of the new test rigs for the SM and

MIR reactors (fuel and structural components of various-purpose

reactors, research reactor advanced fuel, radionuclide accumulation

rigs);

Feasibility of the SM reactor conversion to new low poisoning fuel

rods and the MIR reactor conversion to low-enriched fuel;

Feasibility of the SM and MIR reactor core arrangements that allow

extending the experimental capabilities and improving the fuel usage

efficiency;

Verification of the calculation codes;

Lab research engaging the students of Dimitrovgrad Engineering

and Technical Institute NRNU MEPhI.

Thank you for your attention!