Inlet Thermocouples
Figure 8 Schematic of test rig used for corrosion studies of both fuel and materials specimens
30-t
25-I 20
M(/I
2 15
^-source
10-&
~ 10' 20 k) io. , &>'," £ 0 ) 0
Top Axiol position, cm Bollom100 10
Figure 9. Interim oxide thickness measurements on a BWR rod; It can be seen that the presence of a local 3-source in the coolant region surrounding the cladding produced
appreciable corrosion enhancement.
ttSXT PAGE(S)
!J
RBMK FUEL ASSEMBLIES: CURRENT STATUS AND PERSPECTIVES A.I. KUPALOV-YAROPOLK, V.A. NIKOLAEV, YU.M. CHERKASHOV Research and Development Institute of Power Engineering, Moscow
A.K. PANYUSHKIN """""""XA9846755"
Machine Construction Company, Ehlectrostal, Moscow region A.M. FEDOSOV
Russian Scientific Center Kurchatov Institute (RSC KI), Moscow Russian Federation
Abstract
The safety enhancement measures implemented since 1986 have led to substantial burnup reduction in the RBMK fuel assemblies and consequently to economical losses. With the purpose to compensate the losses, computer analysis and experiments were performed during the last decade.
The works were aimed at the RBMK fuel charge perfection to reduce void reactivity effect and to increase fuel burnup.
The paper presents principle results of the studies which are currently under implementation or are supposed to be implemented in the nearest future.
Approach to fuel assembly design modification
As one result of the safety enhancement measures implemented after 1986, in particular, by installing a large number of additional absorbers to the core, the fuel burnup of the RBMK fuel assemblies (FAs) was reduced by 30-35 % compared to the design value. To compensate the losses, computer analysis and experimental works aimed at modifying the RBMK fuel charge were performed during the last decade with the purpose to reduce void reactivity effect and increase the fuel burnup. The activities on fuel assemblies modification were performed in two lines:
• with change in geometrical dimensions of fuel assemblies involved;
• without changing basic overall dimensions.
One RBMK FA for both the 1000 MW reactors and RBMK-1500 contains 18 fuel element of 13.6 mm diameter, all are located over two circumferences: the first radius for 6 fuel elements and the second one - for 12 fuel elements (Fig. 1).
The first line, aimed at alternating the FA design, envisaged development of two options, i.e. with fuel elements of ether diminished or enlarged diameters. The fuel elements designs 10.2 mm and 14.8 in diameter are worth noting. In the first case, the FA incorporated fuel elements of three radii, in the second case, it had two, the same as in the standard case. The reactor process channel was also envisaged to get larger in diameter where in a fuel assembly of 80 to 86 mm diameter was installed.
The analysis showed that the change for 10.2 mm fuel elements practically did not change the void reactivity effect. Thus, it makes up is ~0.9 P for the 2 % enriched fuel in the core with 80 additional absorbers, whereas for the 10.2 mm fuel elements it is ~1.0 (3.
The fuel elements with diameters enlarged from 13.6 up to 14.8 mm allows additional absorbers to be discharged from the core and reduce the cost by ~15 %. In this case the fuel assembly uranium charge gets higher by -20 %. However, bearing in mind, that actual change for channels with large
FIG. 1. Draft of the RBMK-1500 fuel assembly.
diameter may be realised only when the core is updated as well as the RBMK units service life design terms, this line did not get progression.
Therefore, the investigations were reoriented during last several years such that application of burnable absorbers should allow the geometry of the fuel assembly and fuel channel to remain unchanged
Development and implementation of burnable absorber fuel
A potentiality for application of different burnable absorbers B, Dy, Gd, Hf, Er was subjected to analysis. Also different options of their location in fuel assemblies (in structure components, partially in fuel elements, etc.) were considered. The analysis performed showed that erbium may get to be better absorber for the RBMK reactors.
The options considered for Er location in FAs allowed to conclude that the highest effect may be attained when Er is added directly to the fuel of all FA fuel elements. The calculations demonstrated that 0.4 % of Er added to the 2.4 % enrichment fuel enables to practically have the same voidage effect in the core with no additional absorbers (AA) as in the reactor with 80 AAs (Table 1).
At the same time, application of additional absorbers permits to substantially reduce (-10 %) the FA maximum power which realises at the initial stage of the FA operation (Fig. 2). This margin can be used for increasing the fuel enrichment. The calculations illustrated that Er application m RBMK allows to pass to 2.4 % enrichment without exceeding the FA maximum power limits. The higher enrichment and discharge of additional absorbers from the core lead to a fuel burnup higher by 30 %.
The fuel burnup for the RBMK-1000 reactors get higher by 18 % as the enrichment is changed for 2 6%.
The presence of erbium in fuel results in a substantial (by 10-15 %) reduction in the fresh fuel reactivity as compared to standard which allows FA reload that are performed in RBMKs on power by more "mild" control. This does not require significant movement of control rods to suppress local peaks in power density.
TABLE 1. BASIC CALCULATED CHARACTERISTICS OF RBMK REACTOR CORES WITH NEW FUEL
Characteristics Enrichment, %
Erbium content by mass, % Number of AA in core Discharged fuel burnup, MW day/kgU
Average-over-service life voidage, P
Annual economic effect due to reduction in fuel
component per 1 power unit, mln. USD
Possible date of FA serial production commencement